flow meter selection strategies

7/30/2019 Flow Meter Selection Strategies

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Figure 1. In order to understand the characteristics of different

types of flowmeters, it is important to first review some basic facts

about how gas flows through a closed piping system.

Part I I : Flow met er Selection Str ategies

How to Choose the Right Technology for Gas Measurements

By John Frederick

In this second installment of a two-part article, we address the topics typically covered in a comprehensive gas flow measurement

course. The following describes common types of gas flowmeters and provides guidelines for their selection and usage. It also discusses

the influence of basic gas properties on meter performance and examines methods and equipment for accurate calibration. Part one of

this article covered similar topics in liquid flow measurement (Aug. 2010, pages 18-22).

Basic Gas Law s

In order to understand the characteristics of different types of

flowmeters, it is important to first review some basic facts about how

gas flows through a closed piping system. One of the key

characteristics that can help in this regard is the Reynolds Number,

which takes into account the relationship between the velocity of the

flow, the size of the pipe, and the density and viscosity of the gas.

Identifying the Reynolds Number relationship helps to understand

how a gas flows through the pipe and how various flowmeters are

influenced by the flow (Figure 1).

Gas flow measurement is somewhat more complicated than liquid

flow measurement because a gas is a compressible fluid and a liquid

is a non-compressible fluid. When we say that a liquid is non-

compressible, we mean that when the pressure is increased, the

density changes only a negligible amount. A gas is compressible, so

when the pressure changes, the density also changes. When the

absolute pressure on a gas is doubled (if the temperature is held

constant), the density will double. When a gas flows through a

system, the pressure continually changes resulting in density

changes. These density changes must be taken into account in gas

flow calculations.

It is also important to make accurate measurements of pressure and

temperature at the appropriate locations in the system in order to

determine the true flow characteristics.

The relationships among pressure, temperature, density, and flow are based on gas laws, which describe the relationship of pressure,

temperature and volume for a fixed quantity of gas in a closed container. Two gas laws that are useful in understanding gas flow are

Boyles Law (Robert Boyle) and Charless Law (Jacques Charles), named for the early experimenters who discovered their underlying

principles. The gas laws are based on using absolute pressures and absolute temperatures.

Boyles Law states that at a constant temperature, the volume occupied by a given quantity of gas varies inversely with the absolute

pressure (Figure 2).

Charless Law states that at a constant pressure, the volume occupied by a given amount of gas varies directly with the absolute

temperature (Figure 3).

Page 1 of 6Part II: Flowmeter Selection Strategies

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Figure 2 . Boyles Law for gas. Figure 3. Charless Law for gas.

These two laws can be combined into a single law, called the Perfect Gas Law, as follows:

where:

P = Absolute pressure

T = Absolute temperature

Q = Volume

Subscripts 1 and 2 represent two different conditions.

The previous equation can be rearranged to solve for each of the three quantities:

Popular Flowm eter Designs

Modern gas flowmeters can be classified into two general categories: quantity meters and rate meters. Quantity meters are those in

which the gas passes through the meter in successive, isolated quantities. Rate meters are those in which the gas passes though the

primary element in a continuous stream. The flowrate (amount of flow per unit of time) is derived from the interaction between the

flow stream and the primary element.

The most popular gas flowmeter designs in use by industry include:

Differential- Pressure Flowmet ers: When a gas flowing through a pipe encounters a restriction reducing the area of the pipe, the




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Figure 4. Turbine fl owm eters have a large installed base and remain a high

accuracy solut ion for gas applications.

gas must go faster past the restriction. This increase in speed results in a difference between the pressure upstream of the restriction

and the pressure downstream of the restriction. There is a definite relationship between this pressure difference and the flowrate of the

gas passing the restriction, which can be used to create a flow measuring device by purposely installing a restriction with known

characteristics into the pipe. Establishing the relationship between flowrate and differential pressure enables measurement of an

unknown flowrate in the pipe.

Differential pressure or differential head flowmeters fall into three general categories: orifice, nozzle and venture. The basic principle of

operation of each of these is the same, but the basic shapes are different.

Pitot Tube Flow meters: A pitot tube flowmeter is a small-diameter tube with one end bent 90 degrees to the stem of the tube. The

tube is inserted into a pipe so the bent end faces into the flow. A pressure gauge connected to the other end of the tube reads the total

or stagnation pressure of the flow impacting on the open end of the tube. This total pressure is greater than the static pressure, which

is measured at a pressure tap on the side of the pipe. The difference between the total pressure and the static pressure is called the

velocity pressure and is related to the flow velocity at the tube opening.

Turbine Flow meters: Turbine flowmeters are employed

extensively for high-accuracy flow measurements. They

can be built for use at very high pressures, as well as very

high and low temperatures. They are made in sizes from

to over 24 inches in diameter. The meters signal can

be used to measure the flowrate at any given time or thetotal amount of flow over a period of time (Figure 4).

A turbine flowmeter has a rotor suspended on low-friction

bearings in a meter body in such a way that the rotor can

spin freely as the gas passes through the device. A pickup

assembly mounted on the outside of the meter body

generates an electrical signal when the rotor turns and

produces one electrical pulse as each rotor blade passes

the pickup. The pickup may be a self-generating magnetic-

type or a carrier-excited nonmagnetic type.

Laminar Flow Elements: Laminar flow elements are

made with one or more small passages through which gas

flows. Pressure taps are provided to measure the

differential pressure developed by the flow going through

these small passages. The passages are designed to

produce a Reynolds Number small enough (less than 2000)

to always produce laminar flow. Under laminar flow

conditions, the relationship between the differential

pressure and volumetric flowrate is not the square-root

relationship found in other differential-pressure flowmeters, but a linear relationship.

Vortex-Shedding Flowm eters: Vortex-shedding flowmeters are made by installing a metal bar, called a bluff body, across a

diameter of the pipe. The bluff body creates an obstruction so that the flow must go faster in order to pass. This results in the creation

of swirls or vortices in the flow, which form first on one side of the bluff body and then on the other side. These vortices, called vonKrmn vortices (Theodore von Krmn), are generated at a frequency proportional to the flowrate of the gas past the bluff body. The

frequency can be measured in several ways, including pressure sensing, temperature sensing, and by detecting modulation of an

ultrasonic beam. Bluff bodies are made in a variety of shapes to produce the desired operating characteristics.

Ultrasonic Flow meters: Ultrasonic flowmeters utilize piezoelectric transducers mounted on the pipe in contact with the gas, or can be

mounted on the outside of the pipe (clamp-on transducers). The transducers generate high frequency sound waves that travel across

the pipe at the speed of sound in the gas.

Two basic ultrasonic meter principles are used: transit-time and Doppler. Transit-time meters have two or more pairs of transducers

that alternately send signals downstream and then upstream. The transit-time in both directions is measured and the time difference is

proportional to the average flow velocity along the sonic path. Transit-time meters are only suitable for relatively clean liquids, since

dirt, particles and gas bubbles can disperse the sound signals. Doppler meters send a sound beam into the flow stream where it is

bounced or scattered by a particle of dirt. The reflected signal will experience a frequency shift, called the Doppler shift, and the change

in frequency is proportional to the velocity of the particle. Doppler meters require some dirt to reflect the sound waves. They will not

work with very clean gases.




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Crit ical Flow Ventur is: In 1839, from studies of Daniel Bernoullis and Giovanni Battista Venturis works, Jean Claude Barr de Saint

Venant and Pierre Laurent Wantzel developed a general equation of the discharge of fluids from apertures by which the existence of a

sonic-flow limit could be inferred. In 1866, Julius Weisbach observed the phenomenon that the mass rate of a gas through a nozzle

reaches a maximum directly proportional to the inlet. In recent years, the sonic-flow Venturi has been used as a reference meter, as a

transfer standard, and as a control for regulating the flow of gas (See Fig. 5 & 6).

Figure 5. Smooth Approach Orifice (SAO). A form of a Venturi that offers

excellent stability while maintaining its original calibration, unless mechanically

abused. Frequently employed as standards for measuring critical gas flow,

SAOs provide high-efficiency recovery, resulting in less pressure loss. SAOs are

commonly used in the automotive industry for measuring exhaust flow.

Figure 6. Sonic Nozzles. Also known as a Critical Flow Venturi, these

instruments are, by design, a constant volumetric flowmeter. By using

a regulated pressure supply, the Sonic Nozzle becomes a precision

mass flowmeter. Sonic Nozzles are used primarily in the aerospace

industry and for flow transfer standard calibration systems.

Therm al Flow meters: Thermal flowmeters function on the principle of measuring heat transfer, where the Delta T is a function of

flow. Nature wants everything to exist in a state of equilibrium. If two objects of different temperatures are placed in contact with each

other, there will be a heat energy transfer until the two bodies obtain a temperature equilibrium. An analogy would be the human body.

If you stand in the wind and bare your arms by removing your jacket, you will feel chill on your arms. This chill is caused by the

temperature of the wind and by the mass of wind molecules taking heat energy away as the cooler air crosses your skin surface, which

is a heat transfer of energy from your warm body to the air molecules that impinge on your skin surface. Your brain controls your body

temperature at a constant 98 F. When air passes over your arms, your skin temperature is reduced by the thermal transfer of body heat

to the air molecules, and your brain, which instantly senses the skin heat loss and tells your body to burn more calories to maintain a

constant temperature. If your brain included a calorie meter, it could provide an indication of air flowrate, because skin heat loss is

proportional to the number of air molecules crossing your skin surface.

Common Application Crit eria

The choice of a flowmeter for a particular gas process involves an analysis of many factors. After evaluating the major application

criteria, the number of possibilities can be reduced and the final selection can be made based on the remaining factors.

Some or all of the following considerations should be taken into account when choosing what flowmeter to use:

1. What t ype of gas is to be met ered?

Clean or dirty

Corrosive or noncorrosive

2. What are the flow conditions?

Measuring flowrate or total flow or both

Normal flowrate

Minimum and maximum flowrate

Minimum and maximum temperature




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Minimum and maximum pressure

3. What are the install conditions?

Pipe size

Reynolds Number

Is there room for adequate length of piping?

Are flow conditioners needed?

Is pipe vibration a problem?

Is flow steady or pulsating?

4. What are the performance requirements?

Accuracy

Is accuracy required in all conditions?

5. What are the cost factors?

Initial cost of primary and secondary instrument

Cost of accessories

Installation cost

Reliability vs. maintenance cost

Energy cost for pumping

Availability of parts and repair service

Compatibility with existing equipment

Choosing a Calibration Solution

Many gas flowmeters require individual calibration to meet the required accuracy. Flowmeters are recalibrated periodically for quality

assurance and are also recalibrated after being repaired. The precision of a flowmeter is limited to the accuracy of the calibration

equipment used. In some industrial applications, accuracy is not critical, however, in many instances, meters must be calibrated to the

very best possible accuracy. Some of these applications are found in military, aerospace, electronics, and whenever flow measurement

is related to the sale of product.

Major independent flow laboratories offer flowmeter calibration services meeting both end-user and OEM production requirements.

These labs verify flowmeter accuracy by comparing the meter with a primary flow standard traceable to the U.S. Nati onal Inst itu te

of Standards & Technology (NIST). Their calibration services take into account key factors affecting flowmeter performance, such as

orientation of the meter, operating temperature and pressure, and the type of the flow medium and viscosity.

When it comes to flow calibration equipment, the volume vs. time principle of operation is used for most high-accuracy gas meter

calibrations. Some weight vs. time gas calibrators have been made, but they are highly specialized and are not very practical for most

calibration work.

The most common volume vs. time gas flowmeter calibrators are the Bell Prover and the Glass-Tube Piston Prover. (Figures 7 & 8).

Both calibrators are designed to collect a precise volume of gas for a measured length of time, after the gas has passed through the

flowmeter. The volume and time measurements establish the measured volumetric flowrate of the gas. Measurements of the absolute

pressure and temperature of the collected volume of gas inside the calibrator allow calculations of all required flow parameters (Figure

8).




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Figure 7. High Volume Bell Prover provides primary

standard calibrations from 0.1 SCFM to 1,000 SCFM with an

uncertainty of +/-0.2 percent as stated on NVLAP

Certificate of Accreditation to ISO/IEC 17025:2008 register

to Flow Dynamics NVLAP Lab Code: 200668-0.

Figure 8. Glass Tube Low Flow Piston Prover provides primary standard calibrations from

0.000035 SCFM to 1 SCFM with an uncertainty of +/-0.2 percent as stated on NVLAP

Certificate of Accreditation to ISO/IEC 17025:2008 register to Flow Dynamics NVLAP Lab

Code: 200668-0.

Another type of volume base primary flow standard for gas is the Pressure Volume Temperature Time, or PVTt, calibrator. NIST and

other laboratories have used PVTt systems as primary gas flow standards for more than 30 years. The PVTt system consists of a flow

source, valves for diverting the flow, collection tank, vacuum pump, pressure and temperature sensors, and a critical flow Venturi

(CFV), which isolates the meter under test from the pressure variations in the downstream piping and tank.

The vacuum pump is used to remove the gas from the collection tank. The starting pressure and temperature are recorded. The

diverting valves are switched, allowing gas to enter the collection tank. The gas is collected and the resulting temperature and pressure

are recorded. Then, using the Real Gas Law, the volume of gas may be calculated with reasonable accuracy.

John Frederickhas worked in precision metrology for the past 27 years, holding senior technical positions with the U.S. Navy,

Lockheed Martin, and Flow Dynamics Inc. For the past 14 years, John has focused on flow measurement in various positions at Flow

Dynamics, including Calibration Laboratory Manager, Vice President of Engineering, and Vice President of Business Development. John

has authored numerous papers on flow measurement, holds several patents on flow measurement technologies, and teaches flow

measurement and measurement uncertainty courses. John earned a bachelors degree in Physical Science from the State University of

New York and a masters degree in Business Administration from Central Michigan University. John can be reached at

j oh n. f rederick@flo w -dynamics.com or 480 948-3789, ext. 16.

www.flow-dynamics.com


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