condenser performance monitoring guide

Condenser Performance Monitoring Practices

1007309

EPRI Project Manager J. Stallings

EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 • [email protected] • www.epri.com

Condenser Performance Monitoring Practices

1007309

Technical Update, September 2002

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John L. Tsou Consulting Service

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CITATIONS

This report was prepared by

John L. Tsou Consulting Service 56 Williams Lane Foster City, CA 94404

Principal Investigator J. Tsou

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner:

Condenser Performance Monitoring Practices, EPRI, Palo Alto, CA: 2002. 1007309

iii

ABSTRACT

Steam surface condensers and associated systems cause significant loss of generation and heat rate degradation in both nuclear and fossil-fired power plants. The purpose of this report is to provide the engineering and operating personnel of the power industry with a guide for selecting and using the practices available for monitoring condenser performance.

Strictly speaking, only condenser backpressure needs to be monitored. However, the cause of high condenser backpressure cannot be determined without monitoring other operating parameters. Common causes for high condenser backpressure include the following:

• High inlet cooling water temperature

• Low cooling water flow

• Partially filled waterbox

• Excessive heat load

• Fouled tubes

• Excessive air in-leakage

• Vacuum equipment problem

• Tube bundle design problem

It is not the intent of this paper to address the causes and remedies of high condenser backpressure. This report does provide details of specialty instruments used in condenser performance monitoring. References for in-depth studies and sources for obtaining the specialty instruments, services and cost are also included.

v

CONTENTS

1 INTRODUCTION....................................................................................................................1-1

Background.............................................................................................................................1-1

Purpose...................................................................................................................................1-2

Scope .....................................................................................................................................1-2

Organization of the Report........................................................................................................1-3

Reference................................................................................................................................1-3

2 TEMPERATURE MONITORING PRACTICES .....................................................................2-1

Background.............................................................................................................................2-1

Instruments .............................................................................................................................2-1

Cooling Water Inlet Temperature ..............................................................................................2-2

Cooling Water Outlet Temperature ............................................................................................2-2

Condenser Shell Temperature ...................................................................................................2-4

Hotwell Temperature ...............................................................................................................2-4

Condensate Temperature ..........................................................................................................2-4

References ..............................................................................................................................2-4

3 PRESSURE MONITORING PRACTICES ..............................................................................3-1

Background.............................................................................................................................3-1

Instruments .............................................................................................................................3-1

Cooling Water Inlet Pressure ....................................................................................................3-3

Condenser Backpressure...........................................................................................................3-3

Reference................................................................................................................................3-4

4 FLOW MONITORING PRACTICES ......................................................................................4-1

Background.............................................................................................................................4-1

Instruments .............................................................................................................................4-1

Periodic Cooling Water Flow Measurement ...............................................................................4-2

vii

Dye Dilution Test................................................................................................................4-2

Velocity Traversing Method.................................................................................................4-3

CW Pump Curves and Total Dynamic Head Method ..............................................................4-5

Heat Balance Method ..........................................................................................................4-5

Continuous Cooling Water Flow Measurement...........................................................................4-6

Differential Producer Method ...............................................................................................4-6

Ultrasonic Time of Travel Method.............................................................................................4-9

Air In-Leakage Flow Monitoring.............................................................................................4-11

Rotameter.........................................................................................................................4-11

Orifice Plate ..........................................................................................................................4-12

Electronic Air In-Leakage Monitor ..........................................................................................4-14

Summary ..............................................................................................................................4-15

References ............................................................................................................................4-17

5 FOULING MONITORING PRACTICES ................................................................................5-1

Background.............................................................................................................................5-1

Instruments .............................................................................................................................5-1

Off-Line Microfouling Monitor.................................................................................................5-2

On-Line Microfouling Monitor .................................................................................................5-3

Bridger Scientific Continuous Side-Stream Reduced-Scale On-Line Microfouling Monitor .......5-3

Bridger Scientific Continuous Small On-Line Microfouling Monitor ............................................5-3

Conco Systems Continuous Side-Stream Reduced Scale On-Line Microfouling Monitor ................5-5

Periodic In-Situ On-Line Microfouling Monitor..........................................................................5-6

EPRI Continuous In-Situ On-Line Microfouling Monitor ............................................................5-6

Taprogge Continuous In-Situ On-Line Microfouling Monitor ......................................................5-7

On-Line Macrofouling Monitor.................................................................................................5-9

Intake On-Line Macrofouling Monitor ..................................................................................5-9

Tubesheet On-Line Macrofouling Monitor ...............................................................................5-10

Summary ..............................................................................................................................5-12

References ............................................................................................................................5-13

6 LEVEL MONITORING PRACTICES.....................................................................................6-1

Background.............................................................................................................................6-1

Instruments .............................................................................................................................6-1

Hotwell...................................................................................................................................6-2

viii

Waterboxes .............................................................................................................................6-2

Intake .....................................................................................................................................6-2

Summary ................................................................................................................................6-5

References ..............................................................................................................................6-6

7 DISSOLVED OXYGEN MONITORING PRACTICES............................................................7-1

Background.............................................................................................................................7-1

Instruments .............................................................................................................................7-1

References ..............................................................................................................................7-2

LIST OF FIGURES

Figure 2-1 Circulating Water Discharge Temperature vs. Distance from Inside Edge of Pipe ...............2-3 Figure 2-2 BBC Circulating Water Outlet Temperature Measurement ...............................................2-3 Figure 3-1 Condenser Pressure-Sensing Basket Tip Source: ASME PTC 12.2 ...................................3-2 Figure 3-2 Condenser Pressure-Sensing Guide Plate Source: ASME PTC 12.2 ..................................3-2 Figure 3-3 The Four-Tap Selector Connects the Basket Tips to the Pressure Transducer and

Purge Supply........................................................................................................................3-4 Figure 4-1 Recommended Velocity Traverse Locations Source: ASME PTC 12.2 .............................4-4 Figure 4-2 Water Flow Through Condenser ....................................................................................4-8 Figure 4-3 Flow Through an Abrupt Contraction .............................................................................4-8 Figure 4-4 Elbow Differential Pressure Method...............................................................................4-9 Figure 4-5 Diagram of Four-Path Ultrasonic Flow Meter................................................................4-10 Figure 4-6 Schematic of Rotameter ..............................................................................................4-11 Figure 4-7 Orifice Plate Installation..............................................................................................4-12 Figure 4-8 Schematic for Differential Pressure Transmitter Installation ...........................................4-13 Figure 5-1 Conco Heat Transfer Testing Unit ..................................................................................5-2 Figure 5-2 DATS Heat Exchanger Cross-Section View ....................................................................5-3 Figure 5-3 ProDATS Probe and its Schematic .................................................................................5-4 Figure 5-4 Conco Portable Test Condenser .....................................................................................5-5 Figure 5-5 On-Line Fouling Monitor ..............................................................................................5-7 Figure 5-6 Taprogge Monitoring System ........................................................................................5-8 Figure 5-7 Typical Cooling Water Intake System ............................................................................5-9 Figure 5-8 Typical Ultrasonic Microfouling Monitoring System .....................................................5-10 Figure 5-9 TVA Tubesheet Macrofouling Monitor.........................................................................5-11 Figure 6-1 Typical Installation of Capacitance Level Monitor ...........................................................6-3 Figure 6-2 Typical Installation of Submersible Differential Pressure Level Monitor............................6-3 Figure 6-3 Typical Installation of Bubbler Level Monitor .................................................................6-4

xi

LIST OF TABLES

Table 4-1 Circulating Water Flow Monitors ..................................................................................4-15 Table 4-2 Air In-Leakage Flow Monitor ......................................................................................4-16 Table 5-1 Microfouling Monitor ..................................................................................................5-12 Table 6-1 Remote Level Monitors ..................................................................................................6-5

xiii

1 INTRODUCTION

Background

Steam surface condensers and associated systems cause significant loss of generation and heat rate degradation in both nuclear and fossil-fired power plants. The loss of generation due to condenser backpressure increase can be estimated from a plant turbine thermal kit. The deviation of heat rate due to condenser backpressure increase is 204 Btu/kWh/in Hg on the utility average based on an earlier EPRI report[1-1]. Simply speaking, only condenser backpressure needs to be monitored. However, the cause of high condenser backpressure cannot be determined without monitoring other operating parameters. Common causes for high condenser backpressure include the following:

• High inlet cooling water temperature

• Low cooling water flow

• Partially filled waterbox

• Excessive heat load

• Fouled tubes

• Excessive air in-leakage

• Vacuum equipment problem

• Tube bundle design problem

Some of the causes of high condenser backpressure, such as high inlet cooling water temperature and excessive heat load, cannot be controlled. Other causes require remedial action. It is not the intent of this paper to address the causes and remedies of high condenser backpressure. However, any remedial action involves additional cost. To avoid unnecessary cost, it is important to isolate the true cause of high condenser backpressure. This is the reason that it is essential to monitor the performance of the condenser on a routine basis to ascertain that it is performing properly based on current operating conditions. If a condenser is not performing properly, the true cause of the deficiency most likely can be determined from condenser performance monitoring.

1-1

Introduction

Unfortunately, performance analysis of condenser problems is complicated by the difficulty in measuring critical parameters, such as heat rejection rate, and cooling water outlet temperature and flow rate. Of the three parameters, only two are required for analyzing the performance of a particular condenser. The heat rejection rate can be calculated from the turbine heat balance. If the heat rejection rate is known, the outlet temperature can be calculated from the flow rate, or the flow rate can be calculated from the outlet temperature. If the heat rejection rate is not known, then both outlet temperature and flow rate are required. This paper will address common practices used in the power industry to monitor and analyze condenser performance.

The performance factor, a.k.a. cleanliness factor or fouling factor, is generally calculated. However, it can also be measured directly with on-line or side-stream specialty instruments. This paper will address various devices available for this purpose.

Local instruments provide adequate data for most of the analysis. Data transmitted to a remote location, such as a control room, provide opportunities to record and to trend the data. Trending provides additional insight into the causes of performance deficiency.

Purpose

The purpose of this paper is to provide the engineering and operating personnel of the power industry with a guide for selecting and using the practices available for monitoring condenser performance.

It is not the purpose of this paper to evaluate or to recommend practices required for proper condenser performance monitoring. In fact, it is neither necessary nor economically feasible to use all the monitoring practices. However, sufficient information will be provided to allow for choosing and specifying suitable monitoring practices based on one’s needs.

Scope

This paper covers a range of practices used by the power industry to monitor condenser performance. The monitoring practices for each parameter discussed in this paper are as follows:

• Temperature: cooling water inlet and outlet, hotwell and condensate.

• Pressure: cooling water inlet and outlet, condenser backpressure.

• Flow: cooling water flow, air in-leakage.

• Fouling: microfouling and macrofouling, on-line and side-stream.

• Level: hotwell, waterboxes and intake.

• Dissolved oxygen: condensate.

1-2

Introduction

This report will provide details of specialty instruments used in condenser performance monitoring. This report will not provide details of commonly used instruments, such as for temperature and pressure monitoring. A brief evaluation based on the author’s own opinion will be provided. References for in-depth study and sources for obtaining the specialty instruments, services and cost are also provided.

Since this report is not about performance analysis or diagnosis, detailed procedures for these will not be included.

Organization of the Report

This report contains seven sections. After the introduction, the subsequent six sections each cover one monitored parameter.

Each of the sections 2 through 7 contain subsections covering background, instruments, monitoring practices, and references. The objective and purpose of monitoring each parameter will be provided in the background subsection. Instruments and methods used to monitor the parameters will be briefly discussed in the instrument subsection. Detailed discussion including instrument type, monitoring location, source, cost, and author’s comments will be provided in the monitoring practice subsections. References cited in the text will be provided in the reference subsection.

Reference

[1-1] Heat Rate Improvement Guidelines for Existing Fossil Plants, EPRI Report CS-4554, EPRI May 1986.

1-3

2 TEMPERATURE MONITORING PRACTICES

Background

Cooling water inlet temperature has the greatest impact on condenser performance. It is also the easiest parameter to be monitored, and every plant does so. If no other problem exists, normal condenser backpressure can be found from the thermal kit at the particular operating load. There is not anything a plant can do if the cooling water source is from a natural body. However, if the cooling water is from a cooling tower, the cooling tower should be investigated for possible problems.

Cooling water outlet temperature is used to establish terminal temperature difference (TTD), which is the temperature difference between the steam saturation temperature corresponding to the backpressure and the outlet cooling water temperature. TTD is a starting point used in diagnosing many condenser problems. Outlet temperature is also used to establish the differential temperature (∆T) between the inlet and outlet cooling water. Higher than normal ∆T indicates insufficient cooling water flow. Cooling water outlet temperature is used to calculate heat load and fouling factors. Unfortunately, accurate cooling water outlet temperature is very difficult to determine. A workable solution is recommended by the ASME Performance Test Code on Steam Surface Condensers (PTC 12.2 –1998)[2-1].

Condenser shell temperature is not normally measured but is inferred from condenser backpressure. As mentioned before, it is used to calculate TTD.

Hotwell temperature is compared to shell temperature. A large differential between shell temperature and hotwell temperature may indicate an excessive steam pressure drop, which suggests a condenser design deficiency. Hotwell temperature is also compared to condensate temperature. A larger than normal difference indicates condensate subcooling, which causes higher dissolved oxygen and heat rate degradation.

Instruments

Instruments used in condenser temperature monitoring are the same as those used elsewhere in power plants. Thermometers are used for local temperature monitoring. Thermocouples (TCs), thermistors, and resistance temperature detectors (RTDs) are used remote temperature readings. The ASME Performance Test Code cited above recommends type E thermocouples, 100-ohm platinum RTDs, and thermistors with a nominal impedance of greater than 100 ohms at 32oF (0oC). For RTDs and thermistors the four-wire method is recommended. All temperature monitors should be calibrated initially and periodically to within 1oF (0.6oC) using traceable

2-1

Temperature Monitoring Practices

standards. All monitors should be protected from debris and other potentially damaging elements. If a thermal well is used, it should have sufficient length to extend into the flow stream to measure the actual temperature. Heat conducting gel and spring-loaded elements should be used to insure accurate readings.

Cooling Water Inlet Temperature

Cooling water inlet temperature is generally uniform, and its measurement does not present a special challenge. A monitor mounted in any convenient location will be sufficient. Redundancy may be desirable to insure accuracy and backup.

Cooling Water Outlet Temperature

Cooling water outlet temperature is difficult to determine accurately, because the temperature of the cooling water that exits from the tube bundle is not uniform. Tubes located in the outer tube bundle will transfer more heat than the tubes in the inner tube bundle. Consequently, the cooling water that exits from the outer tubes will have a higher temperature than that from the inner tubes. The degree of non-uniformity depends on the mixing effect of the condenser waterbox, its outlet configuration and the piping configuration. Figure 2-1 shows temperature readings from a thermocouple traverse immediately below the discharge waterbox of a condenser[2-2]. Note that the temperature will be more uniform if the measuring point is further away from the condenser outlet waterbox.

The measuring point should be located as far downstream from the condenser as possible, where the discharge is well mixed and the temperature is uniform. It may be necessary to traverse the discharge pipe to confirm temperature uniformity. The location is acceptable if there is no significant heat loss and there are no other significant flows mixed in.

If an acceptable, well-mixed downstream location cannot be found, then the outlet temperature has to be determined from an array of temperature measurements. PTC 12.2[2-1] recommends one temperature measurement point for every 1.5 square feet (0.14 m2) of pipe flow area. These points are located at the centroid of each area, and the measured temperatures are averaged with equal weight.

In practice, the number of measuring points may be judiciously reduced as long as the accuracy of the outlet temperature is within the tolerance. The number and length of thermal wells may be determined by traversing the pipe. Figure 2-2 shows a temperature probe arrangement by one of the condenser manufacturers[2-3]. It consists of a total of nine probes. Three longer probes extend 1/3 diameter of the pipe into the pipe and six shorter probes extend 2/15 diameter into the pipe. The readings from the nine probes are averaged for the true water temperature.

2-2


Figure 2-1 Circulating Water Discharge Temperature vs. Distance from Inside Edge of Pipe

Figure 2-2 BBC Circulating Water Outlet Temperature Measurement

2-3


Condenser Shell Temperature

As mentioned earlier the condenser shell temperature cannot be measured accurately because the incoming steam may contain superheat. The shell temperature is generally inferred based on the backpressure.

Hotwell Temperature

Hotwell temperature can be measured above the hotwell condensate level with conventional temperature measuring devices.

Condensate Temperature

Condensate temperature is normally measured at the suction side of the condensate pump.

References

[2-1] Performance Test Code on Steam Surface Condensers, ASME PTC 12.2 –1998. ASME, New York, 1998.

[2-2] Bell, R.J., Mussalli, Y. G., “Instrumentation and Techniques for Condenser Performance Monitoring”, presented at the Joint ASME/ANS Nuclear Engineering Conference, Portland, Oregon, July 1982.

[2-3] The High Reliability Condenser Design Study, EPRI Report CS-3200, July 1983.

2-4

3 PRESSURE MONITORING PRACTICES

Background

The most important pressure to be monitored is the condenser backpressure, which determines the condenser performance. Backpressure varies with heat load, cooling water temperature, cooling water flow rate, tube fouling, and other factors, and is monitored by every plant. Condenser backpressure is not difficult to monitor. However, the measured pressure is not always accurate. Backpressure is used in conjunction with cooling water outlet temperature to establish TTD.

Another pressure parameter to be monitored is the cooling water pressure. Low cooling water pressure indicates deficient cooling water flow due to poor cooling water pump performance or low water level in the intake bay. High cooling water pressure indicates blockage of the flow pass.

Monitoring the differential pressure between the inlet and outlet can indicate blockage of the tubes or tubesheet due to macrofouling. Differential pressure is also used to monitor cooling water flow. The details will be discussed in the fouling and flow section.

Instruments

Basket tips (Figure 3-1) and guide plates (Figure 3-2) are the most common condenser backpressure-sensing instruments. Because the guide plates have to be installed parallel to the steam flow direction, which may be difficult to predict, the author believes the basket tips are more forgiving and more accurate on average. The pressure measurement device includes non-mercury manometers and electronic absolute pressure transmitters.

Cooling water pressure measurement devices include mechanical pressure gages and electronic pressure transmitters.

Differential pressure measurement devices include mechanical differential pressure gages and electronic differential pressure transmitters.

3-1

Pressure Monitoring Practices

Figure 3-1 Condenser Pressure-Sensing Basket Tip Source: ASME PTC 12.2

Figure 3-2 Condenser Pressure-Sensing Guide Plate Source: ASME PTC 12.2

Basket tips and guide plates can be ordered from turbine manufacturers and parts vendors. The basket tip costs around $500. The guide plate can probably be made in most plant maintenance shops at considerably less cost. Due to the construction of the basket tip, orientation with steam flow may be less critical with this device. The guide plate, on the other hand, may be less accurate if it is not oriented correctly. However, the guide plates are less likely to be damaged in highly turbulent steam flow areas. The electronic transmitters can be ordered from most instrument manufacturers.

3-2


Cooling Water Inlet Pressure

Pressure taps for cooling water pressure measurement should be located in a convenient spot between the cooling water pump and the inlet waterbox. Care should be taken to avoid locating the measurement point in areas of local high velocity or local flow separation zones. The tap should be deburred and cleaned on the inside. Isolation valves and fill lines should be installed between the sensing point and the gauge or transmitter. The sensing point should be periodically back flushed to purge any debris from the sensing point.

Condenser Backpressure

Most plants are equipped with basket tips or guide plates as part of the turbine package. As such, they are located in the transition between the turbine and the condenser. Their location may be subject to higher velocity and pressure variation. ASME PTC 12.2[3-1] recommends that the sensing elements be located at least one foot (30 mm) but no more than three feet (90 mm) above each tube bundle. A tube bundle is considered to be all tubes connected to a single-inlet waterbox. For single-shell and multiple-shell condensers, there should be at least three measurement points per tube bundle in each shell. For single-shell multi-pressure condensers, there should be at least two pressure measurement points per tube bundle. If tube bundles are arranged one on top of the other, measurement points need only be provided for the uppermost bundle. Where three measurement points per tube bundle are required, they should be located lengthwise near the quarter-points of the tube bundle. Where two measurement points per bundle are required, they should be located lengthwise near the third-points of the tube bundle. In any case, the lateral position of the measurement points should be as close to the lateral midpoint of the bundle as possible.

ASME PTC 12.2[3-1] requirements are intended to establish rules for performing condenser acceptance tests. For routine performance monitoring, the code requirements may be unnecessary. If the existing pressure-sensing elements produce acceptable results, no modification is required.

The second problem with many existing plant pressure-sensing elements is their installation. PTC 12.2[3-1] recommends that basket tips be installed at an angle of 30 to 60 degrees from the mean flow direction. The guide plates should be oriented so that the steam flow is parallel to the guide plates. Pressure-sensing piping for pressure measurement should conform to the general requirements of subsection 4.3 of PTC 19.2[3-2].

The third problem with many exist plant sensing elements is the piping leading to the transmitter. If there are any water pockets caused by condensation, they will influence the accuracy of the reading. It is essential to slope the piping from the sensing element to the transmitter in a continuous upward slope to allow any accumulated condensations to drain naturally to the basket tip.

To further ensure the complete absence of water pockets in the sensing line, EPRI suggests using automatic air purging before every reading. The schematic of the setup is shown in Figure 3-3[3-3].

3-3


Figure 3-3 The Four-Tap Selector Connects the Basket Tips to the Pressure Transducer and Purge Supply

Reference


[3-2] Performance Test Code on Instruments and Apparatus: Pressure Measurement, ASME PTC 19.2 –1987. ASME, New York, 1987.

[3-3] MARK I Performance Monitoring Products, EPRI Report GS/EL-5648, September 1989.

3-4

4 FLOW MONITORING PRACTICES

Background

As mentioned previously, the cooling water (CW) flow rate is required to analyze condenser performance. CW flow measurement is the most difficult task because of the large conduit and the turbulent flow profile. The conduit can be as large as eight to ten feet (2 to 3 m) in diameter. The flow pattern exiting the CW pump is turbulent. The limited straight length of conduit is not long enough to establish laminar flow. To further complicate the measurement problem, most of the conduit is buried underground, and access to sensing locations is very limited.

CW flow measurement methods are either periodic or continuous. Periodic CW flow measurement methods include the dye dilution test and the velocity traversing method. Periodic methods are frequently used to calibrate continuous flow measurement instruments. Periodic water flow measurement is also used to determine the circulating water pump performance.

Over the years, a number of direct and indirect continuous flow measurement methods have been developed. The direct flow measurement methods include the differential producer method and the ultrasonic time-of-travel method. The indirect flow measurement methods include the circulating water pump motor load method and the heat balance method.

Air in-leakage may cause high backpressure and increased dissolved oxygen in the condensate, which in turn can increase corrosion potential and oxygen-scavenging chemical consumption. Generally, air in-leakage flow monitoring is performed at the ejector or vacuum pump discharge. The measured flow includes both non-condensible gas and moisture, and needs to be corrected to indicate dry air flow rate.

Instruments

Rhodamine WT fluorescing dye is most commonly used for the dye dilution test. A precision positive displacement pump is used to inject the concentrated dye solution, and a calibrated precision fluorometer is used to measure the dye concentration.

The Fechheimer and Keil Pitot static-type probe, the insertion-type fiber-optic laser Doppler velocimeter, and the insertion low-drag turbine flow meter are used for velocity traversing.

A precision differential pressure transmitter is used for the differential producer method. Multiple-channel ultrasonic flow meters are employed for the time-of-travel flow measurement.

4-1

Flow Monitoring Practices

Total dynamic head readings are entered into the pump characteristic curve to determine the flow.

The instruments used for air in-leakage monitoring include rotameters, orifice plates, and electronic air in-leakage monitors.

Periodic Cooling Water Flow Measurement

Dye Dilution Test

The principle of the dye dilution test is that an unknown flow rate can be determined by adding a known quantity of an easily identifiable tracer, mixing fully, and then measuring the resulting concentration of the tracer in the flow. If this dye is injected at a constant rate, the relationship between the concentration and the flow is as follows:

Q1 C1 = Q2 C2

Where: Q1 = dye injection rate C1 = concentration of injected dye Q2 = flow rate to be determined C2 = concentration of diluted dye in water stream

Thus: Q2 = Q1 x C1/C2 C1/C2 is known as the “dilution factor” (DFt)

Rhodamine WT fluorescing dye is the most commonly used dye because it is non-toxic and exhibits a minimal tendency to be adsorbed onto organic and in-organic surfaces. The dilution factor of a solution cannot be measured directly, but it can be determined by comparing its fluorescence (which is proportional to its concentration) with that of a specially prepared “standard solution” of precisely known dilution. This standard solution is prepared by diluting a sample of the injected dye by approximately the same amount as it will undergo when injected into the system. The fluorescence levels of a test sample and of the standard solution are measured in a fluorometer, and the dilution factor of the test sample is determined as follows:

DFt = DFs x Fs/Ft

Where: DFt = dilution factor of test sample DFs = dilution factor of standard solution Ft = fluorescence level of test sample Fs = fluorescence level of standard solution

The flow to be measured is then:

Q2 = Q1 x DFs x Fs/Ft

4-2


The principle seems simple, but the accuracy of this method depends greatly on the experience and accuracy of the tester. A detailed test procedure is given in Reference [4-1]. Numerous companies provide this type of service.

The dye injection point for the test should be at the circulating water pump inlet. The sampling point should be as far downstream as possible. A CW piping system with many twists and turns provides better mixing and more accurate results. If the sample can be taken at the CW discharge, then there will be no need to tap into the pipe. Samples from various cross-sections of the sampling point should be taken to ensure the dye concentration is uniform. Chemical injection, such as chlorine, into the CW must be stopped before the test. The dye should be injected until the fluorescence level in the sample reaches a steady state before flow determination data can be taken. The test sample should be allowed to reach the same temperature as the standard solution prior to analysis, as the fluorescence intensity of the dye varies with temperature. A well-conducted test can achieve accuracy within 2%.

Care must be taken to ensure that no flow is introduced or removed during the test. This method may not be suitable for a CW system with high concentrations of organic growth and silt. Additional information and applications can be found in references [4-2] and [4-3]. The method is widely used in the hydroelectric industry to test the turbine efficiency. Typical costs range from $10,000 to $20,000, depending on geographical location, number of tests, required support for test preparations, and desired details of test reports.

Velocity Traversing Method

The velocity traversing method of CW flow measurement actually measures the local velocity of the water in the conduit. The average velocity of the entire flow is then calculated based on the local velocity on a volumetric weight basis. Since the diameter of the conduit is known, the average velocity is used to calculate the water flow rate using the following equation:

Water Flow Rate = Water density x Velocity x Conduit diameter

The PTC 12.2[4-2] recommends the traverse be taken along at least three equally spaced diameters. The traverse locations should follow the Chebyshef weighing scheme with at least ten points along each diameter (Figure 4-1).

4-3


Figure 4-1 Recommended Velocity Traverse Locations Source: ASME PTC 12.2

The water flow emerging from pumps, elbows, or piping diameter changes is very turbulent. As a result, the velocity profile can be irregular and cannot be accurately measured. The flow generally will straighten itself out over a straight length of pipe. PTC 12.2[4-2] recommends that the traverse point be located with at least ten diameters of straight, unobstructed piping upstream and five diameters of piping downstream. In practice, these restrictions may be relaxed somewhat if the repeatability is good.

The measurement procedure depends on the traverse instruments used. Each instrument has its limitations. Some work better than others in certain situations. PTC 12.2[4-2] lists some of these limitations. Encor-America uses a low-drag insertion turbine flow meter that is believed to work well without fully developed flow profiles[4-4].

4-4


Comparing the dye dilution test with the velocity traverse method, one can draw the conclusion that the dye dilution tests work well with a complex piping system that increases the mixing factor. On the other hand, the velocity traverse methods work well with a fully developed flow profile. The dye dilution tests work better with a relatively clean piping system, while organic growth and silt have no negative impact on velocity traverse methods. The dye dilution test will have minimum intrusion into the piping system, but the velocity traverse method requires tapping into the pipe. The velocity traverse method also requires that the pipe be full of water at the measuring point. The dye dilution test does not have this requirement. Typical costs for velocity traversing range from $3,000 to $10,000, depending on geographical location, number of tests, required support for test preparations, and desired details of test reports.

CW Pump Curves and Total Dynamic Head Method

Pump total dynamic head (TDH) is the measure of energy added to the flow by the pump. It is the algebraic sum of the static discharge head, the velocity head at the measurement point, and the vertical distance from the measurement point to the water level in the pump bay (lift). TDH is usually expressed in feet of water.

The static discharge head can be measured with piezometers, manometers, or calibrated pressure gauges. If any of these instruments are located above or below the measurement point, the distance should be subtracted from or added to the static discharge head. The velocity head is calculated based on the estimated velocity of the water at the measurement point. The lift can be measured with any suitable method, such as measuring tape or a fixed ruler. The computed TDH in feet of water is entered into the pump characteristic curve to obtain the cooling water flow.

This simple method has a minimal need for additional instruments. It accuracy depends on the accuracy of the pump curve. If the pump impeller is worn, the pump curve will not be accurate. It is recommended that the pump curve accuracy be verified by one of the more accurate flow tests, such as a dye dilution test.

It is possible to make this method a continuous cooling water flow monitoring method by automating its measurements. The static discharge head can be measured with a pressure transducer. The velocity head can be estimated. The lift can be measured with a remote-reading level gauge (see Section 6 for details). All the data can be fed into a suitable computer or processor, and the TDH will be automatically calculated and compared to the built-in pump curve to obtain the flow rate. If one automates the system, the costs should be between $2,000 and $5,000. Otherwise, the costs are negligible. The verification of pump curves is not included in this estimate.

Heat Balance Method

This method is not a direct measurement of cooling water flow. Instead, a heat balance is performed on the turbine cycle to determine the heat rejection rate to the condenser (condenser duty). With the known condenser duty, and the inlet and outlet cooling water temperatures, the circulating water flow rate can be calculated.

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Since the circulating water flow rate is calculated from the condenser duty and the cooling water outlet temperature, the accuracy of the flow rate depends on the accuracy of these two measurements. ASME PTC 12.2[4-2] recommends that the determination of the condenser duty should be performed according to applicable sections of ASME PTC 6[4-5] for the testing of turbines. Because of the complexity of this test, it is recommended that the condenser be tested at the same time as the turbine. The advantage of this method is that the condenser duty is determined directly without measurement, and therefore no instrument is required for the cooling water flow. If the flow rate determined from this method is deemed accurate, this method may be used to calibrate other continuous cooling water flow measurement devices. The cost is minimal, assuming one does not include the turbine test and the cooling water outlet temperature measurement.

Continuous Cooling Water Flow Measurement

Differential Producer Method

Differential-pressure flow meters have been used for decades. They provide a simple, reliable method of measuring fluid flow with good results. The simplest meter is the orifice flow meter. The pressure drop across an orifice is measured to determine the flow, which is proportional to the square root of the differential pressure produced, according to the following equation:

____ W = C x √(∆H) Where: W = flow rate

C = flow coefficient ∆H = pressure difference

The flow coefficient C depends on the geometry of the flow restriction. If C and the pressure difference ∆H are known, the flow rate can be calculated.

The pressure difference ∆H can be determined by installing pressure taps upstream and downstream of the flow restriction and measuring the pressure difference between them. The pressure difference can be measured with any suitable device, such as a manometer or an electronic differential pressure transmitter.

The flow coefficient C must be determined from calibration. To calculate C, one can pass a known flow rate through the meter and measure the pressure difference ∆H, using the following equation:

____ C = W √(∆H)

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In condenser applications, the flow rate is determined using either the dye dilution test or the velocity traverse method.

The traditional differential producer method of flow measurement uses orifice plates, flow nozzles, venturi meters, or weir plates. However, none of these are suitable for condenser applications because of the large size of the conduit and the low pressure head available for measurement.

Two types of differential producers have been developed for condenser application. The first uses the condenser outlet waterbox as the location for the differential producer. The second uses any elbow in the conduit. Both types are based on the same operating principle, and only the sensing points are different.

The TVA Engineering Laboratory first investigated the outlet waterbox as a location to produce differential pressure for CW flow measurement[4-6]. The schematic plan view and end view of the setup are shown in Figures 4-2 and 4-3. The approach uses the existing abrupt contraction created by the changing configuration from waterbox to pipe at the condenser outlet waterbox. Because of the contraction, there is a flow separation in the pipe joining the condenser waterbox. At this point the water pressure will be lower than the pressure in the waterbox. Tapping into these two points and connecting them to a differential pressure (DP) transmitter provides the flow measurement. The flow will be proportional to the DP between these two points. The flow coefficient can then be determined on-line using either the dye dilution test or the velocity traverse method.

The second method is to determine the DP across an elbow. When fluid moves around the curved path of an elbow, it is subjected to an angular acceleration. The resulting centrifugal force creates a DP between the inner and outer radii. The high-pressure tap is on the outside of the elbow and the low-pressure tap is on the inside, as shown in Figure 4-4[4-4]. This flow measuring device must also be calibrated on-line.

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Figure 4-2 Water Flow Through Condenser

Figure 4-3 Flow Through an Abrupt Contraction

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Figure 4-4 Elbow Differential Pressure Method

Tests show that the elbow-flow DP method and the waterbox-to-pipe DP method track each other closely. Both methods require that the measuring point be full of water. The elbow-flow DP method requires that the elbow bend in only one plane. According to one vendor, the elbow-flow DP method has several advantages over the waterbox DP method: (1) a higher DP is available for measurement, and (2) the elbow-flow DP method may be applied in the event the waterbox approach is not feasible. For example, if the condenser has one conduit supplying water to two waterboxes/bundles in parallel, it would be easier to take the DP across the elbow before the separation. In another case, when the circulating water downstream of a condenser is below atmospheric pressure, the outlet waterbox is not a suitable location to measure DP, because it may not be full of water. The elbow in the supply line would be a better location.

The maintenance required for these two DP methods includes routine blowdown of the pressure-sensing line and the periodic calibration of the pressure transmitter. The cost for each of these systems is in the range of $20,000 to $30,000, including instruments and calibration.

Ultrasonic Time of Travel Method

There are a number of ultrasonic flow meters available on the market. They include the time-of-travel system, the sing-around system, the leading-edge system, and the Doppler system. Only the multiple-pass time-of-travel system provides sufficient accuracy for measuring water flow

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through large conduits and open channels such as a circulating water system. This method involves the measurement of travel times of acoustic energy transmitted forward and backward along a number of chordal paths (typically four) positioned on an angled acoustic plane across a pipe, as shown in Figure 4-5.

Figure 4-5 Diagram of Four-Path Ultrasonic Flow Meter

This system uses a high-frequency sonic wave that is beamed at an acute angle across the pipe, as depicted in Figure 4-5. When the wave is transmitted through the water in the direction of the flow, its velocity increases. When the wave is transmitted through the water against the direction of the flow, its velocity decreases. Given the speed of the sound wave in water and the angle between its direction and the flow pass, the average water velocity on the flow path can be calculated. Since the velocity in the pipe is not uniform, a four-path system provides an average water velocity. The flow rate is calculated from the average velocity.

The ASME PTC 12.2[4-2] recommends that the metering section should be preceded by at least ten diameters and followed by at least three diameters of straight pipe. In practice, this restriction may be relaxed somewhat. In a case where there is a very short straight run (less than five pipe diameters) upstream of the measuring section, it is likely that the flow will be very turbulent. The manufacturer recommends that a second “crossed path” be installed at 90 degrees to the first crossed path to eliminate the cross-flow error.

To calibrate this system, precise measurements of the distance between the transducers, the angle of the transducers with respect to the centerline of the pipe, and the physical dimension of the pipe are required. No on-line calibration, such as in the dye dilution or velocity traverse tests, is required. An accuracy of 1% can be accomplished with this system. Entrained air bubbles in the

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water, silt and other particulate in the water, and a partially full water pipe can all affect the accuracy.

Periodic cleaning of the transducers is required to prevent algae growth, which in turn affects the measurement. The transducers can be installed from inside the pipe. However, for ease of maintenance, tapping into the pipe and mounting the transducers on a flanged, removable plate is desirable.

EPRI tested one ultrasonic flow meter widely used in the hydroelectric industry[4-7]. Even though it was installed in a rectangular channel, favorable results were reported[4-8]. The cost for this system is in the range of $100,000 to $150,000.

Air In-Leakage Flow Monitoring

Rotameter

Most plants are equipped with a rotameter for air in-leakage monitoring. The rotameter is a variable-area flow meter. It is composed of a tapered metering tube, mounted vertically with the small end at the bottom, and a float that is free to move up and down in the tube. Flow enters the rotameter at the bottom, passes around the float and leaves the meter at the top. Figure 4-6 shows a schematic of a typical rotameter.

Figure 4-6 Schematic of Rotameter

Under no-flow conditions, the float rests at the bottom of the tapered tube. When flow enters the tube, it passes through an annular space between the float and the tube wall, creating a pressure drop across the float. This pressure drop raises the float to increase the flow area between the float and the tube to reach dynamic equilibrium. At this equilibrium the upward forces on the float are balanced by the weight of the float. Any further increase in flow causes the float to rise higher in the tube; a decrease in flow causes the float to drop. For a fluid of a given density and viscosity, the float position corresponds to a unique flow rate. Flow rate is determined by direct

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observation of the position of the float in the metering tube. The tube is marked to indicate the flow rate. The calibration is completed in the factory.

In power plant air in-leakage applications, the rotameter is located at the steam jet ejector or vacuum pump discharge on a bypass line. Normally, the exhaust is vented through the main line. When taking rotameter readings, the main line is closed with a butterfly valve.

Fluctuations in temperature change the fluid density and viscosity, which in turn affect the accuracy of the rotameter. Therefore, the rotameter provides only an approximate indication of the air in-leakage rate. To improve accuracy, temperature and pressure readings may be taken at the condenser vent connection, and these can be used to correct the air in-leakage rate, according to Appendix H of ASME PTC 12.2[4-2].

The second drawback of the rotameter is that it creates an additional pressure drop when taking the reading, and thus affects the accuracy of the reading. The third drawback is that the rotameter is read manually. Continuous reading of a rotameter is very difficult, although a magnetic technique to determine float location has been used to take continuous readings. The advantage of the rotameter is that it is inexpensive. There will be no cost incurred if the unit is already equipped with a rotameter.

Orifice Plate

The principle of using an orifice plate for flow measurement is similar to using the condenser outlet waterbox. A differential pressure transmitter may be installed across the orifice plate for continuous reading. A schematic of the installation is shown in Figures 4-7 and 4-8.

Figure 4-7 Orifice Plate Installation

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Figure 4-8 Schematic for Differential Pressure Transmitter Installation

ASME PTC 12.2[4-2] recommends using an orifice plate for non-condensible flow measurement. The orifice plate should meet the specifications described in ASME-MFC-3M[4-9]. Detailed installation requirements are given in ASME PTC 19.5[4-10].

The orifice plate air in-leakage system offers better accuracy and is more suitable for continuous flow monitoring. To improve accuracy, temperature and pressure readings may be taken at the condenser vent connection to correct the air in-leakage rate, according to Appendix H of ASME PTC 12.2[4-2]. Installation of the orifice plate air in-leakage monitor will require some piping modifications and will be more expensive than a rotameter. The cost is estimated to be in the $7,000-to-$10,000 range per vacuum equipment discharge location.

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Electronic Air In-Leakage Monitor

The electronic air in-leakage monitor consists of one or more insertion probes, a digital processor, and interconnect cables. The probe is an assembly of four primary sensors that measure flow, temperature, pressure, and relative saturation. The total mass flow of the water-vapor/air mixture is measured. Actual water vapor flow rate is subtracted from the total mixture, yielding the precise air in-leakage flow rate.

Because the electronic air in-leakage monitor measures all four parameters, and the digital processor is capable of subtracting water vapor from the total mixture in these measurements, it is a more precise instrument for air in-leakage monitoring. The drawback is that it is very expensive. Since the monitor includes many sensing instruments, the maintenance costs may also be high. The installed cost is approximately $20,000 to $25,000 per measurement point. While small units usually have three measurement points, a large unit can have up to twelve.

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Summary

Table 4-1 Circulating Water Flow Monitors

Monitor Type Cost Pro Con

Dye dilution test $10K-

$20K

Suitable for complex piping system.

No tapping into the pipe required.

Pipe need not be full of water.

Organic growth and silt affect accuracy.

No flow introduced or removed.

Not suitable if dye mixing is incomplete.

Not suitable for continuous monitoring.

Velocity Traversing $3K-

$10K

Not affected by organic growth or silt. Not suitable for complex piping system.

Tapping into the pipe required.

Pipe must be full of water.


Pump Curves & TDH $5K Minimum additional instruments required.

May be made into a continuous flow monitor.

Accuracy depends on accuracy of the pump curve and condition of the pump.

Periodic testing is required to verify the accuracy of the pump curve.

Heat Balance $0 Inexpensive if tested with steam turbine testing.

No additional instruments required.

May be used to calibrate other monitor.

Accuracy depends on the accuracy of condenser duty and the outlet water temperature.


DP outlet waterbox $20K-

$30K

Inexpensive.

Suitable for continuous monitoring.


On-line calibration required.

DP Elbow $20K-

$30K

Inexpensive.

Higher DP than outlet waterbox method.

Suitable for multiple waterbox/bundle.

More choice of monitoring point.

Suitable for continuous monitoring.


The elbow must bend in one plane.

On-line calibration required.

Ultrasonic Flow meter $100K Provides continuous monitoring.

No on-line calibration is required.

Entrained air bubbles, silt, and partially filled pipe may affect accuracy.

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Table 4-2 Air In-Leakage Flow Monitor

Monitor Type Cost Pro Con

Rotameter $0 - $2K Inexpensive. Less accurate.

Additional pressure drop.


Orifice Plate $7K-$10K More accurate.

Continuous reading.

Additional pressure drop.

More expensive.

Electronic Monitor $20K-$25K

Most accurate.

Continuous reading.

Most expensive.

Maintenance cost may be high.

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References

[4-1] Calibration of Dye Dilution Method of Flow Measurement, CEA No. 320 G 396, December 1987


[4-3] Schagunn, J., Missimer, J., “Demonstration of Dye Dilution for Determining of Circulation Water Flow Rate”, Presented at the EPRI/ASME Heat Rate Improvement Conference, Knoxville, TN, 1989.

[4-4] Diaz-Tous, I. A., Leggett, M., Hill, D., Low-Drag Insertion Turbine Flow Measurement Technology for Circulating Water Systems Without Fully-Developed Flow Profiles, EPRI Report TR-106781, August 1996.

[4-5] Performance Test Code on Steam Turbines, ASME PTC 6 –1996. ASME, New York, 1998.

[4-6] March, P. A., Almquist, C. W., Technique for Monitoring Flowrate and Hydraulic Fouling of Main Steam Condenser, EPRI Report CS-5942-SR, September 1988.

[4-7] MARK I Performance Monitoring Products, EPRI Report GS/EL-5648, September 1989.

[4-8] On-Line Condenser Cooling Water Flow Measurement, EPRI FS-9102, 1991.

[4-9] Measurement of Fluid Flow in Pipe Using Orifice, Nozzle and Venturi, MFC-3M-1989, ASME, New York, 1990.

[4-10] Instruments and Apparatus: Part II of Fluid Meters, ASME PTC 19.5, ASME, New York, 1972.

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5 FOULING MONITORING PRACTICES

Background

Fouling reduces the condenser performance in a number of ways. Fouling is further classified as microfouling and macrofouling. Microfouling consists of biological fouling, precipitation fouling, particulate fouling, and corrosion fouling. Microfouling forms deposits inside the condenser tubes. It mainly increases the heat transfer resistance between the steam and water and in turn decreases the amount of heat that can be dissipated through the cooling water.

Macrofouling consists of marine animals and plants, biological growth, and debris carried over by water. The macrofouling can foul the cooling water system, including the intake system and the flow conduit, and block the tubesheet flow area. It mainly increases the flow resistance, which in turn reduces the cooling water flow rate and again decreases the amount of heat that can be dissipated through the cooling water.

Reducing heat dissipation in the condenser in turn increases the condenser backpressure and turbine cycle heat rate. For this reason, it is important to monitor fouling. When the monitor indicates that the condenser is excessively fouled, appropriate remedial action can be taken.

Using other condenser performance data, including backpressure, heat rejection rate, cooling water inlet and outlet temperatures, and cooling water flow rate; the condenser performance factor (also known as the cleanliness factor or fouling factor) can be calculated. This is a method to monitor condenser performance. However, the reduction in condenser performance may or may not be solely due to fouling. For this reason, it is desirable to use fouling monitors to monitor the actual fouling of the condenser tubes or other components in the system.

Instruments

Microfouling monitors can be classified as on-line or off-line monitors. On-line fouling monitors include both in-situ and side-stream monitors. The in-situ fouling monitor also consists of periodic and continuous monitors. The side-stream fouling monitor includes full-scale and reduced-scale monitors, and is very useful in simultaneous evaluation of treatment options. Off-line fouling monitors are also used to evaluate treatment options and effectiveness. All the microfouling monitors are specialty products. The operating principles, construction details, and sources will be discussed in the text.

Macrofouling monitors are all on-line. One uses the differential water level between the intake and the pump bay to monitor macrofouling of the traveling water screen. The other uses the

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Fouling Monitoring Practices

differential water pressure between the inlet and outlet waterboxes to monitor the tubesheet fouling.

Off-Line Microfouling Monitor

Conco Systems (www.concosystems.com) offers an off-line condenser fouling test apparatus (Figure 5-1). This test apparatus consists of four removable tubes (36 inches [91 cm] long) in a shell. Cooling water is circulated through the tubes. Steam is generated in the shell using an electric heater. Tubes to be evaluated are sent to Conco from the condenser in question. These tubes are loaded into the test apparatus. Testing is conducted at the customer specified heat flux, water velocity, and range of water temperature. Comparisons can be made between the as received tubes (fouled) and the new, mechanically cleaned or chemically cleaned tubes. Results of each test are expressed as overall heat transfer coefficients, fouling factors, and cleanliness factors. The costs of the tests are $1,900 for start-up of the apparatus and the first U coefficient, and $500 for each additional U coefficient determination.

Figure 5-1 Conco Heat Transfer Testing Unit

Strictly speaking, this apparatus is an off-line fouling test apparatus rather than a fouling monitor. As such, it is ideal for evaluating the effectiveness of cleaning methods. The drawback of this method is that it requires removal of a tube from the operating condenser. The removed tube must be representative of the fouling condition, and must also be carefully preserved and packed for shipping to Conco to prevent a change in fouling condition, such as drying out.

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On-Line Microfouling Monitor

Bridger Scientific Continuous Side-Stream Reduced-Scale On-Line Microfouling Monitor

Bridger Scientific’s (www.bridgersci.com) DATS side-stream fouling monitor has been in existence for a long time. This monitor consists of a tube wrapped with an electric heater block (five inches [13 cm] long) (Figure 5-2). A side-stream of cooling water is fed through the tube section at a controlled rate. The electric heater block surrounding the tube simulates the actual heat load of the condenser tube. Calibrated temperature probes measure both the fluid and heater temperatures. These data, the flow rate and heat flux are used to automatically calculate the heat transfer resistance. All heat transfer data are then provided as analog output. Changes over time accurately reflect the accumulation of the fouling deposit. The cost for the system is $9,500.

Figure 5-2 DATS Heat Exchanger Cross-Section View

The advantage of this monitor is that it is relatively small and inexpensive. Since it is off-line, it can also be used to evaluate other treatment programs. The drawback of this monitor is that the heated section is very small. It is not possible to simulate the fouling of the entire condenser tube.

Bridger Scientific Continuous Small On-Line Microfouling Monitor

Bridger Scientific’s (www.bridgersci.com) new ProDATS insertion fouling probe is in the final testing stage (Figure 5-3). This device consists of two independent sensing assemblies in a single probe configuration. The probe may be directly inserted into any pipe two inches (5 cm) or greater via a “hot tap” assembly, or it can be operated as a side-stream system with the addition of a clamp-on assembly. The primary sensing element is a heat transfer sensor, which is complemented by a secondary magnetic flow sensor. Mounted below the flow sensor are two identical 0.25 inch (0.64 cm) tubular sections, each electrically isolated from the other and wound with electrically heated wire. Cooling water flows through these two tube sections. Electric current flowing through the wound wire causes the temperature of the tube to rise, which can be accurately monitored by measuring the wire resistance. Cooling water flowing through the tubes removes heat from the tubes thereby reducing their temperature rise. Since clean tubes

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will transfer heat to the cooling water with higher efficiency for the same flow rate, the sensing elements will experience a relatively small temperature rise compared to the fouled tube. By measuring the temperature rise of the sensing elements above that of the cooling water, and knowing the flow passage dimensions, a heat transfer value can be calculated which over time is related to the fouling deposit buildup on each heated surface. The total fouling can be determined this way.

Figure 5-3 ProDATS Probe and its Schematic

The second use of this probe is to measure the difference between these two tube sections and in turn determine fouling caused by different mechanisms. There are several ways to make the two sensing sections foul in different modes. One way is to periodically inject chemicals to prevent one tube section from microbial fouling. The second way is to generate chlorine electrochemically in a seawater-cooled system by applying electric currents to one tube section. The two sections will experience the same abiotic fouling (scaling) but different microbial fouling. If one subtracts this component from the untreated sensing element, one will have a measure of just the microbial component. By using this approach, one can quantify the total fouling, microbial fouling and abiotic fouling occurring, which can be very useful in developing an overall optimal treatment or control program. The cost for the system is approximately $2,500 to $3,000.

The advantages of this monitor are that it is very small, inexpensive and can be inserted on-line. It can also be used to differentiate microbial fouling from scaling. The primary drawback is that the heated section is very small. It is not possible to simulate the fouling of the entire condenser tube.

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Conco Systems Continuous Side-Stream Reduced Scale On-Line Microfouling Monitor

Conco Systems (www.concosystems.com) offers a portable test condenser (Figure 5-4). The portable test condenser is a single-tube (36 inches [91 cm] long), self-contained model condenser. The portable condenser is installed alongside the unit condenser.

Figure 5-4 Conco Portable Test Condenser

The portable test condenser is a shell-and-tube heat exchanger with an integral electric heater. Demineralized water and water vapor occupy the shell space. Air is removed from the shell prior to startup with a vacuum pump. Cooling water is passed through the test tube section, and the electric heater is energized to generate steam. Steam is condensed on the tube to transfer heat to the tube wall and cooling water.

Fouling detection is accomplished by monitoring the initial temperature difference (ITD), which equals steam temperature minus inlet water temperature. When a tube is fouled, the heat transfer capability of the tube will decrease, causing the steam temperature and, in turn, the ITD to increase. As with an actual surface condenser, ITD will vary with inlet water temperature. This necessitates the preparation of a calibration curve. A comparison of measured ITD to the calibration curve allows for detection of fouling and/or relative degree of fouling.

Foulant mass can be quantified by removing the deposit with a mechanical cleaner and determination of weight. Fouling can also be determined by comparing the ITD before and after cleaning with a mechanical cleaner or with chemical cleaning. The portable test condenser can also be used to evaluate the chemical treatment program. The cost for the system is $12,000 to $18,000, depending on the configuration.

The advantage of this test condenser is that it is portable. A utility can move the monitor around and test the fouling tendency in more than one condenser. The foulant mass can easily be

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quantified. It can also be used to evaluate corrosion control optimization and the replacement tube material. The drawback of this test condenser is that the tube is relatively short. It may not duplicate the fouling occurring in the actual condenser.

Periodic In-Situ On-Line Microfouling Monitor

ASME PTC 12.2[5-1] recommends an in-situ on-line fouling monitor. This monitor uses a pair of condenser tubes. During an outage, it is either cemented to the tubesheet or installed on a tube extension that attaches RTDs or thermocouples to the outlet of this pair of tubes. One tube is thoroughly cleaned. By comparing the performance of these two tubes, the fouling factor can be determined. Details of the installation and other requirements are contained in PTC 12.2[5-1]. Strictly speaking, this is a fouling test rather than a monitoring method.

The advantage of this method is that it is simple and effective with minimal additional investments. It determines the current fouling condition of the entire tube. The major drawback of this method is that it is not continuous monitoring. It requires an outage to install the temperature sensors and to clean the reference tube. The existing plant instruments are relied on for backpressure and cooling water flow rate measurements. These data may or may not be accurate. If these data were inaccurate, the accuracy of the fouling factor would be inaccurate, although the impact would be proportionally less. The cost of conducting this test depends on what needs to be done and whether the tester can complete the test in one trip.

Thermal Engineering Consultants (www.tecus.com) offers a service to conduct fouling tests with this method.

EPRI Continuous In-Situ On-Line Microfouling Monitor

EPRI funded Bridger Scientific, Inc. in the development of this in-situ on-line fouling monitor (Figure 5-5). This monitor uses a pair of actual adjacent condenser tubes. One tube is designated as an “active” tube, and the other is designated as an “inactive” tube, which is plugged on both ends. The monitor is mounted on the outlet tubesheet of these two tubes. The part of the monitor connected to the active tube contains the ultrasonic flow sensor and the discharge cooling water temperature sensor. The part of the monitor connected to the inactive tube contains an inlet cooling water temperature sensor and two spring-loaded steam temperature sensors, which are inserted into the tube to measure the steam temperature in the first half and the second half of the tube. The measurements are digitized locally and transmitted to the outside via an RS-485 link to a remote computer through an appropriate opening on the condenser waterbox. A critical feature of this monitor is that the inside diameter of each monitor assembly is machined to precisely match the condenser tube. Typically, four monitors are used for each condenser to provide diversity and redundancy.

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Figure 5-5 On-Line Fouling Monitor

Knowing cooling water flow velocity, inlet and outlet cooling water temperature, and steam temperature, the heat transfer coefficient can be calculated. The calculated heat transfer coefficient is compared to the HEI or ASME theoretically achievable heat transfer coefficient based on the operating conditions. The fouling factors and cleanliness factors are deducted from this comparison. The detailed design of the monitor and the calculation procedure are contained in an EPRI report[5-2]. The cost for the system is approximately $55,000, including four probes and software.

The advantage of this monitor is that it directly measures the heat transfer of an entire operating condenser tube. The inlet and outlet cooling water temperatures are measured directly. The steam temperature is measured directly rather than from pressure measurement. Because there is no cooling water flow in the blocked-off tube, the wall temperature comes into equilibrium with the surroundings and represents the steam temperature at that position of the condenser. The cooling water velocity is also measured directly and represents the actual cooling water flow through that tube. The drawbacks of this monitor are that it is more expensive, and it requires plugging of some tubes and a short outage to install the monitors.

Taprogge Continuous In-Situ On-Line Microfouling Monitor

The Taprogge (www.taprogge.com) condenser monitoring system (Figure 5-6) was developed mainly for monitoring and controlling the Taprogge tube cleaning system, which uses sponge balls circulating through the tubes to clean them. Since the sponge ball will wear out over time, the need to replace these balls is determined by measuring the effectiveness of the sponge ball wiping action. This monitor is also used to measure and control ball circulation.

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Figure 5-6 Taprogge Monitoring System

The monitoring system utilizes a row of six tubes in the operating condenser. The two outer tubes are used to anchor the sensor probe. One tube is plugged on both ends with spring-loaded thermocouples to measure the steam temperature. Six thermocouples are located at the inlet and outlet of three tubes to measure the inlet and outlet cooling water temperature. One of these tubes is used to measure the cooling water velocity. This is accomplished by periodically injecting hot water at the tube inlet end. Measuring the time required for the hot water to travel to the outlet end determines the water velocity. The other two tubes are used to measure the effectiveness of the sponge balls. This is accomplished by comparing the outlet cooling water temperature of these two tubes when a sponge ball enters one of the tubes. The corresponding software calculates the heat transfer coefficient of the individual tubes from the measured temperature difference and, in turn, determines the effectiveness of the sponge ball. The cost for the system, including software and four probes, is $150,000 to $200,000, depending on the configuration.

The advantage of this system is that it monitors not only the condenser performance but also the effectiveness of the sponge ball cleaning system. The major drawback of this system is that it is very expensive. It would not be cost effective if the condenser in question did not have a sponge ball cleaning system. It also requires plugging some tubes and a short outage to install the monitors.

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On-Line Macrofouling Monitor

Intake On-Line Macrofouling Monitor

A typical cooling water intake system is shown in Figure 5-7. Here the trash bars and traveling water screens are used to protect the condenser and the circulating water system from macrofouling. However, when the trash bars and traveling water screens are overloaded, the water level in the pump bay will decrease, providing less net positive suction head (NPSH) and reducing the circulating water pump efficiency, which results in less cooling water flow through the condensers. Generally, the macrofouling situation is monitored manually by operator observation. The purpose of the on-line macrofouling monitor is to provide early warning about any potential problem.

Figure 5-7

Typical Cooling Water Intake System

The principle of the on-line macrofouling monitor is to monitor the water level in the water source, in the fore bay behind the trash bar, and in the pump bay. Whenever the difference in water levels between these three places exceeds a preset limit, a warning signal will alert the operator to take appropriate action. There are a number of ways to measure the water level remotely, including capacitance, ultrasonic, differential pressure, and bubbler level detectors. The details of these level detectors will be discussed in Section 6. A typical application using an ultrasonic level detector is shown in Figure 5-8.

5-9


Figure 5-8 Typical Ultrasonic Microfouling Monitoring System

Tubesheet On-Line Macrofouling Monitor

The tubesheet on-line fouling monitor measures the differential pressure between the inlet and outlet waterbox. Pressure taps are installed at the inlet and outlet waterbox and connected to a manometer or pressure transducer. Any significant increase in differential pressure indicates macrofouling of the tubesheet. Since the differential pressure is also a function of the flow rate, additional pressure taps are installed to monitor the flow rate. This setup was tried at TVA in conjunction with a flow measurement investigation[5-3]. The TVA installation is shown in Figure 5-9.

5-10


Figure 5-9 TVA Tubesheet Macrofouling Monitor

5-11

Summary

Table 5-1 Microfouling Monitor

Monitor Type Cost Pro Con Conco Tube HT Tester $3.4K Ideal for evaluating cleaning method and

effectiveness. Not a continuous monitoring method. Requires removal of tube. Removed tube must be representative of fouling condition. Removed tube must be preserved during shipping.

DATS $9.5K Relatively small and inexpensive. Can be used to evaluate water treatment program.

Heated section is very small.

ProDATS $2.5K –$3K Very small and inexpensive. Can differentiate microbial fouling and scaling.

Tube diameter and heated section are very small.

Portable Test Condenser

$12K- $18K

Portable. Corrosion and replacement tube evaluation.

Test tube is short.

ASME Method N.A. Minimal additional investment. Measures entire tube.

Not a continuous monitoring method. Requires outage to install sensor and to clean tube. Relies on plant instrument for backpressure and flow measurements.

EPRI On-line Monitor $55K Direct measurement of fouling of an entire tube. Accurate cooling water inlet/outlet and steam temperature measurement. Accurate cooling water velocity measurement.

More expensive. Requires plugging of some tubes and a short outage to install.

Taprogge On-Line Monitor $150K- $200K

Direct measurement of fouling of an entire tube. Accurate cooling water inlet/outlet and steam temperature measurement. Accurate cooling water velocity measurement. Also monitors sponge ball effectiveness.

Most expensive. Requires plugging of some tubes and a short outage to install.

5-12

References

[5-1] Performance Test Code on Steam Surface Condensers, ASME PTC 12.2 –1998. ASME, New York.

[5-2] On-line Condenser Fouling Monitor Development, EPRI Report TR-109232, EPRI, December 1997.

[5-3] March, P. A., Almquist, C. W., Technique for Monitoring Flowrate and Hydraulic Fouling of Main Steam Condenser, EPRI Report CS-5942-SR, September 1988.

5-13

6 LEVEL MONITORING PRACTICES

Background

In a condenser system, the water level needs to be monitored in four places: the condenser hotwell, the inlet and outlet waterboxes, and the intake.

In the hotwell, the condensate level needs to be maintained to provide net positive suction head (NPSH) for the condensate pump.

In the inlet and outlet waterboxes, the water level needs to be monitored to make sure the waterboxes are full of water. If the waterboxes are not full, the tubes on the top portion of the tube bundle will not have water flowing through them, and those tubes will not condense steam. Condenser backpressure will suffer as a result.

In the intake, especially the pump bay, the water level needs to be monitored to make sure there is sufficient NPSH for the circulating water pump. If the NPSH is not sufficient, the pump will cavitate, resulting in a reduction in water rate and possible damage to the pump. See Section 5 on Macrofouling Monitoring for more details.

Instruments

Sight glasses are most commonly used to monitor water level locally. Flexible transparent tubes are used to monitor water level in the waterboxes. These are simple devices and need not be elaborated on.

There are a number of ways to measure the water level remotely, including capacitance, ultrasonic, differential pressure, and bubbler level monitors. All of these are suitable for monitoring the water level in the intake.

The capacitance level detectors measure the changes in electric capacitance that occur between the sensing conductors as the water level changes. A capacitance level measuring system consists of a probe lowered into the bottom of the water body and connected to a transmitter mounted on a platform above the highest anticipated water level.

Operation of the ultrasonic level detector is based on the time required for a sonic wave to travel from a transducer to the water level being measured and back to the transducer. The total travel time is proportional to the water level. The transducer is mounted on a platform above the highest anticipated water level.

6-1

Level Monitoring Practices

Operation of the pressure differential level detector is based on measuring the pressure difference sensed by the unit between its high- and low-pressure sides. The high-pressure side of the unit is connected to the bottom of the bay and the low-pressure side is open to the atmosphere. The measured hydraulic head is directly proportional to the water level. The differential pressure transmitter is mounted on a platform above the highest anticipated water level.

The bubbler level detector is a differential pressure level detector with an air purge system. Operation of this instrument is based on the differential pressure required for an air flow stream to overcome the liquid head of the water. The pressure reading is proportional to the water level. The air supply transmitter is mounted on a platform above the highest anticipated water level.

Details of these instruments are contained in Reference [6-1].

Hotwell

The hotwell is usually equipped with a sight glass and a float or other suitable liquid level controller.

Waterboxes

Waterboxes are usually equipped with a sight glass. However, many sight glasses do not reach the top of the waterbox. If the waterbox is not completely full, the sight glass will not provide the necessary indication. This situation is especially common with outlet waterboxes, because water pressure in many outlet waterboxes is below atmospheric pressure. It is recommended that a transparent flexible tube be used to monitor the water level in the inlet and outlet waterboxes. The top connection for the transparent flexible tube should be at the highest point of the waterbox.

Many sight glasses installed on condenser waterboxes are fouled to the point that the water level is impossible to observe due to algae growth. It is difficult to clean the sight glass. In comparison, a transparent flexible tube is inexpensive, and one can simply replace it when fouled. It is essential to make sure the flexible tube connection is air tight to prevent air from leaking into the system, especially if the waterbox is under negative pressure.

Intake

Typical cooling water intake systems, using ultrasonic level indicators and macrofouling monitoring systems, are discussed in Section 5. Additional typical installations using other level detectors are shown in Figures 6-1, 6-2, and 6-3.

6-2


Figure 6-1 Typical Installation of Capacitance Level Monitor

Figure 6-2 Typical Installation of Submersible Differential Pressure Level Monitor

6-3


Figure 6-3 Typical Installation of Bubbler Level Monitor

The accuracy of the monitoring system is not that important. The cost for each of these systems varies from approximately $3,000 to $5,000. The choice depends on personal preference, availability of process supply (electricity and air), and local conditions. For example, the bubbler detector requires compressed air. If compressed air were not available at the intake, it would be expensive to run a compressed air line to that location. Water level turbulence (barge traffic), foaming, floating debris, and biofouling often make one detector more favorable than the other. Refer to the summary subsection for details.

6-4

Summary

Table 6-1 Remote Level Monitors

Monitor Type Pro Con

Capacitance No moving parts, minimum maintenance.

Not affected by foam and floating debris.

Grease buildup can affect accuracy, needs cleaning.

Barge traffic will give false reading.

Ultrasonic No moving parts, minimum maintenance. Affected by foam and floating debris.


Differential Pressure No moving parts, minimum maintenance.

Not affected by foam and floating debris.


Bubbler Not affected by foam and floating debris.

Not affected by barge traffic.

Air supply required.

Pressure regulator, flow controller and flow tube require maintenance.

Fouling will affect accuracy.

6-5

References

[6-1] Instrumentation Handbook for Integrated Power Plant Water Management, EPRI Report CS-5873, July 1988.

6-6

7 DISSOLVED OXYGEN MONITORING PRACTICES

Background

Dissolved oxygen (DO) in the condensate causes corrosion of power plant components. Oxygen-scavenging chemical consumption increases as the amount of dissolved oxygen increases. It is, therefore, important to monitor oxygen content in the condensate and take appropriate action when needed.

The condenser is the major source of dissolved oxygen in the condensate. The oxygen comes from air in-leakage and make-up water. For boiling water reactor power plants, additional oxygen may come from the reactor. A properly designed condenser and non-condensible removal system should be capable of producing condensate with the desired oxygen content (from 7 ppb to 42 ppb) as defined in Reference [7-1]. A sub-cooled condensate will contain more oxygen. Therefore, a properly designed condenser should reheat the condensate before reaching the hotwell.

ASME PTC 12.1[7-2] recommends that the sampling point for DO be located at the condensate outlet piping as close to the hotwell as possible. At this point, the condensate is under vacuum, and a pump is required to extract condensate from the pipe. There is also the potential for air to leak into the condensate if the sample line from the pipe to the pump is not completely airtight. To make sample extraction easier, many utilities elect to take the sample at the discharge of the condensate pump. In this case, the potential for air ingress exists if the condensate pump seal is not completely airtight.

DO monitoring methods include wet chemical analysis and in-line DO analyzers. Wet chemical analysis will not be discussed here. The reader can refer to References [7-3] and [7-4] for more information.

Instruments

Several companies make DO analyzers. They all have similar construction and are based on the same operating principle. The analyzer consists of a probe containing the oxygen measuring cell and the electronics, which are housed in a waterproof case. The probe may be installed in the pipe or in its own flow-through chamber. The probe is covered with an oxygen permeable membrane. Inside the probe is the actual measuring cell. The cell consists of an insulating substrate that supports two types of noble metal electrodes immersed in an electrolytic solution.

7-1

Dissolved Oxygen Monitoring Practices

In operation, the oxygen diffuses through the membrane into the electrolyte. Electric potential is applied between the electrodes to reduce the oxygen. The electric current required to reduce the oxygen is interpreted as specific units of measurement on the instrument display.

One should follow the manufacturer’s instructions for installing and calibrating the analyzer. Frequent maintenance, including electrolyte and membrane change and recalibration, is required to insure accuracy.

References

[7-1] Standards for Steam Surface Condensers, Heat Exchange Institute, Ninth Edition, 1995.

[7-2] Performance Test Code on Steam Surface Condensers, ASME PTC 12.2 –1998. ASME, New York.

[7-3] Deaerators, ASME PTC 12.3-1997. ASME, New York.

[7-4] Test Method for Dissolved Oxygen in Water, ASTM Standard D888-87.

7-2

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