turbocompressor antisurge control, new solution for an old problem

92-GT-428THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS345E. 41St., Now York, N.Y. 10017

The Society shall not be responsible for statements or opinions advanced in papers or in dis-cussion at meetings of the Society or of its Divisions or Sections, or printed in its publications.Discussion is printed only if the paper is published In an ASME Journal. Papers are availablefrom A$M£ for fifteen months after the meeting.

Printecf in USA.

Turbocompressor Antisurge Control, New Solutionfor an Old Problem

JOHN R. GASTONDresser-Rand Company

Olean, NY

1)ABSTRACT

The performance of many antisurge control systems is degraded becausethey do not adequately compensate for variable gas properties (primarilymolecular weight). The consequences can range from inefficient operationto serious compressor damage due to surges.

To eliminate deficiencies previously experienced, a new, relatively simple(requires measuring only three process variables) system was developedfor a machine which normally compresses a 24.2 molecular weight gas, butis also required to operate on other gas mixtures of approximately 40%lower molecular weight. The new method is explained and the resultscompared with two other widely used concepts.

2) NOMENCLATURE (S.I. Units in parenthesis)

ACFM Actual Cubic Feet Per Minute (M 3 / Hr = 1.699 ACFM)Differential head produced by flow element

Hp Polytropic head, Hp =Ft. Lbs/Pound ( Kilojoules / Kilogram)Ratio of specific heat at constant pressure to specific heat atconstant volume

• Proportionally constant• Mass flow rate, Pounds/Minute (Kilograms/Hour = 27.216

Pounds/Minute)M /Hr Cubic meters per hourMW Molecular Weight• Rotative speed, RPM• Pressure, PSIA (KPa)a Volumetric flowrate at flowing conditions, ACFM (M 3 /Hr)• Gas constant, 1545.3/MW (8.3145/MVV)Rc Ratio of Compression, Pd/Ps• Temperature, ° R ( °K)• Compressibility factor to correct for the deviation from the ideal gas

law.• Density, Pounds/Cubic Foot (Grams/Liter)

SUBSCRIPTS

Discharge• Suction

INSTRUMENT SYMBOLS

ASIC Antisurge Indicating ControllerFE

Primary Flow ElementFT

Flow TransmitterFV

Flow Control ValveGain Constant

PDT

Pressure Differential TransmitterPT

Pressure TransmitterST

Speed TransmitterTT

Temperature Transmitter

3) INTRODUCTION

Effective antisurge control is dependent on various factors such as thedynamic response characteristics of the process, compressor, controlvalves, measuring and control instruments etc, and especially the accuracyof the measurement system. Adequate, or even good, antisurge control issometimes possible even if the performance of one or more of the above(e.g., control valve, controller etc.) is less than optimum, but not if themeasurement system is deficient. The results of measurement errors canrange from inefficient operation to serious compressor damage due tosurges. Variable gas properties (e.g., MW, T, etc.) change the operatingpoint at which the compressor will surge, and also cause measurementerrors in conventional flow measuring systems. Gas properties often vary,consequently many antisurge control systems are deficient because theydo not accurately measure the compressor operating point. The focus ofthis paper is on a new concept that was developed to eliminate errors dueto variable molecular weight, temperature, compressibility foctor, pressureand rotative speed. Its key features are the Pd/Ps vs. MVM1Z/P universalperformance curve, and the concept for measuring MVT-TrZ/P. The newsystem will be explained and its advantages over two other widely usedmethods will be pointed out. In order to make the primary subject (i.e. TheMeasurement Concept) more meaningful a brief discussion on surge andhow to prevent it will be given first.

4)SURGE AND HOW TO PREVENT IT

Surge is an unstable, pulsating condition that can occur in anyturbocompressor that is improperly operated.

If the flowrate to the compressor is reduced by a restriction in thecompressor inlet or discharge system, the ratio of compression, Pd/Ps forthe gas will increase.

If the restriction is large enough, flow will decrease and the compressionratio will increase to a point where a momentary flow reversal occurs insidethe machine, which is called "Surge". Surge is usually evidenced by anaudible boom, piping vibrations, and pressure pulsations. The unstable(surge) region is shown on performance curves for turbomachinery, suchas in Fig. 1, and is all of the area to the left of the surge line.

Mild surging will not normally damage the compressor. However, operationunder surge conditions should be avoided. Violent surging may causethrust-bearing failure that, in turn, could result in rubbing and severedamage to the compressor internals. Overheating due to prolongedsurging also causes damage.

A control system must be used to prevent surge by maintaining a minimumflow through the compressor, at a value safely away from the capacity atwhich surge occurs. This is done by allowing some gas to recirculatethrough an antisurge valve and recycle line from the compressor dischargeto its inlet. For air, and occasionally other contaminant-free gases such asoxygen and nitrogen, the antisurge valve vents gas to the atmosphere toprevent surge.

Presented at the International Gas Turbine and Aeroengine Congress and ExpositionCologne, Germany June 1-4, 1992

Copyright © 1992 by ASME

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TO PROCESS

705o so 80 90

FLOW, %

110 120100

120

90

so

FIGURE 1. Process operating points, 60% (A) and 100% (D), surge andcontrol points B and C.

FIGURE 2. Gas flow paths when process flow requirement is less thanthe compressor surge capacity.

Figures 1 & 2 illustrate a typical example of reduced capacity operation for acompressor that has a minimum flow limit of 80%.

Fig. 1 illustrates surge and control points B and C, respectively, andprocess operating points A and D at 100% discharge pressure. Theantisurge control maintains 80% flow through the compressor even thoughthe process requirement is less than 80%. For example, if the processrequires only 60% flow (Point A), the antisurge control maintains 20% flowthrough the recycle line. Hence, flow through the compressor is equal tothe process flow (60%) plus the recycle flow (20%), or 80%, as shown inFig. 2. Recycle flow will be zero whenever the process is using 80% flow,or more.

Hopefully the above description makes it apparent that an accuratemeasurement system is an essential part of any antisurge control system.Unfortunately, variable gas conditions often cause measurement errors andchange the location of the compressor surge line. Conventionalperformance curves for the compressor will be used next to illustrate this,followed by an explanation of the universal curve and how the new conceptprovides an accurate measurement regardless of gas pressure,temperature, molecular weight and rotative speed.

5) CONVENTIONAL PERFORMANCE CURVES FOR VARIABLE GASCOMPOSITION

A performance curve is made to illustrate one variable at the compressordischarge for a defined set of inlet conditions and rotative speed. Thedischarge variable is sometimes shown as adiabatic or polytropic head, butmore commonly units of pressure or pressure ratio (Pd/Ps) are used. Gasconditions affect the discharge variable, therefore a curve for eachspecified gas condition is usually required to properly document thecompressor performance when gas conditions are expected to vary.Figure 3 is a typical example. It shows the predicted performance of a sixstage centrifugal compressor for three different gas mixtures (Figures 3A,3B, and 3C).

Of particular significance is the different shapes and locations of the surgelines for the three different conditions, I.e. Normal 24.2 MW (Fig. 3A) andtwo alternate 14.0 MW gas mixtures (Fig. 3B & 3C). To better illustrate this,the surge lines for the three conditions are shown on a single plot, Fig. 3D.For added clarity the speed lines are omitted from Fig. 3D, and speedpoints of 50%, 70% and 105% are marked on the surge lines at points 1, 2and 3. Numerical data from detailed compressor calculations is used tofurther illustrate the magnitude of shift in the compressor surge line causedby a change from normal gas to the alternate, H2/N2 gas mixture. A surgepoint calculated for the normal gas (24.2 MW) at 75% RPM indicates thatthe compressor will surge at a compression ratio of 2.176 and a flow rate of13,442 ACFM (22838 M3/Hr). The horizontal line (Figure 3-0) at 2.176Pd/Ps intersects the 14.0 MW, H2/N2 surge line at slightly below maximumspeed (105% RPM), and at a flow rate of approximately 18,300 ACFM(31092 M3/Hr). Thus, at a compression ratio of 2.176, surge duringoperation on the alternate gas (14.0 MW, H2/N2 mixture) will occur at a flowrate approximately 36% greater than for the normal gas (24.2 MW).

•While the 36% shift in the surge lines is very significant, it would not cause acontrol error of equal magnitude in most antisurge control systems. Thereason for this is that most control systems utilize a head producing typeflow element that is also affected by the same gas variables that change thecompressor performance, thus producing an inherent, self-compensatingeffect. For some compressors this self-compensating effect results in verysatisfactory antisurge control, (Ref. 3, Pgs. 161-162), but for others,including the compressor for this application, a better measurement systemis needed. After two other widely used antisurge control concepts wereevaluated, and deemed unsatisfactory, the new system being explainedhere was developed. The basis for the new system is the method ofdocumenting the compressor performance on a single, universalperformance curve. The new system, utilizing the universal performancecurve, is virtually unaffected by any variations in compressor speed, suctionpressure and/or gas composition. The universal curve will be explainednext, followed by the method of measuring the flow coefficient (MNFT-22P).

2


750

50.0

FIGURE 4. Universal Performance Curve

3.0

20

15

1.0100

(170)

.'

x,r4.1

90%

100A.6156

105N

RPM

"----_

A : . _' . - " - - - • • --...._' ---...._

' ------ 60%50%

. ,,,...70%

80 k

. . . ,0.0 OS 10 15 20 25 30 3.5

(00) (0.85) (1.70) (2. 5) (3.40) (4.25) (5.10) (5.95)

INLET FLOW. ACFM x le (P43/ Hr. x le)

FIGURE 3A. Normal gas MW = 24.2 k = 1.158

,.. "--■................___.

*... .-.

90%

100% NIM%RPM

-

70%

80%

125 2.00 1.75 210 225 253 275 3.00(2.12) (2.55) (2.97) (3.40) (3.82) (4.25) (4.67) (5.10)

INLET FLOW - ACFM x 1Cr4 (843/ Hr. x 104 )

FIGURE 3B. Alternate gas (H2/ iC4) MW= 14.0 k = 1.217

5.0

4 0

° 3 0

20

1.0

OD

3.0 50

4.0

25

f 30

20

320

15

1.0

1.0 0.0110 125 150 1.75 2W 225 250 205 00 050 1.00 150 200 2.50 3.00 / 3.50

(1.70) (2. 2) (2.55) (2.97) (3.40) (3.82) (4.25) (4.67) (5. 0) (00) (0.85) (1.70) (2.55) (3.40) (4.25) (5. 0) (5.95)

INLET FLOW - ACFM x 1114 (M3/ Sr. x 104 )

INLET FLOW • ACFM 104 (I43/ Hr. x 104

FIGURE 3C. Alternate gas (H2 / N2) MW = 14.0 k = 1.399

FIGURE 3D. Surge lines for normal & alternate gas mixtures.

----■.,....„.............„.......

90%

6i56

105k

RPM

" -------.

70%

80%

SPEED POINTS 3 MW . 24.2 , NORMAL1 .. 50% RPM

- 2 . 70%3 .105%

RPMRPM

2.176 Pd / P,MW

3.•r 14.0 (Fl /x I • C, ) , ALTERNATE

(Hx/ N 2) ALTERNATE

5.re"

R..SS

g;.;

e'm

M

FIGURE 3. CONVENTIONAL PERFORMANCE CURVES PRESSURE RATIO VS. VOLUMETRIC FLOW RATE

6) THE UNIVERSAL PERFORMANCE CURVE FOR VARIABLE GASCOMPOSITION

Instead of volumetric flow units, the universal performance curve (Figure 4)utilizes a flow coefficient WV-71/P. This single, universal curve representsthe performance of the same six stage compressor as illustrated by Figure3. The same three gas mixtures used in the calculations for Figure 3 wereused to produce Figure 4. Except for a minor deviation for the H2/N2 gasmixture, the three surge lines on Figure 3 merge into a single surge line onthe universal curve (Figure 4). The surge lines for the normal gas and theH2/iC4 mixture overlap between compression ratios of 1.4 and 2.342, withno discernible difference in location. However, the H2/N2 surge line (solidline, Figure 4) is at a flow rate greater than for the other two gases. Thisslight shift in the surge line is due to a very significant difference in "k" value(1.399 - 1.158 = 0.241) between the alternate (H2/N2) and normal gases.The relative locations of the two 14.0 MW surge lines on Figure 4 isapproximately the same as on Figure 3D. This indicates that this newmeasurement concept does not inherently compensate for changes in "k"value. However, for most applications, the change in "k" value would besimilar to or less than the difference between the H2/iC4 and normal gasexplained above and the error would not be discernible. For major changesin "k" value such as the H2/N2 example, the control is set to recognize thesurge line located at the highest flow rates. This will assure that thecompressor is safely protected from surge even though a large change in"k" causes only a very minimal control error.

3


RCONTROLLERPSFU IC9Titt W1).12K FB 13

P = A - B BIAS36FB 36

ADDER / SUBTRACTOR

_4:11

• F(Pd / Ps)

44

FB 24 VEXTRACTOR

45

FB 44CHARACTIZER

A/C .111P34 3

A/C=P /Ps

FB 34MULTIPUER / DIVIDER FR 35

MULTIPLIER I DIVIDER

A

SUCTION DISCHARGEPRESSURE (Ps) PRESSURE (Pd)

\

FLOW ELEMENT\ DIFFERENTIAL (h)

PROCESS MEASUREMENT SIGNALS

While not really relevant to understanding the new measurement concept,the difference between the multiple curves on Figures 3 & 4 should beexplained. On Figure 3 the curved lines represent the compressorperformance in 5% increments of speed from 50 to 105% RPM. On Figure4, instead of actual RPM the lines represent %N

To summarize, and re-emphasize the differences, most antisurge controlalgorithms are based on conventional performance curves using Pd, Pd-Ps, Pd/Ps, Hp etc. vs. volumetric flow rate squared. Depending to a greatextent on the compressor characteristics, control systems based on theabove perform very effectively in many applications, even when gascomposition varies. However, for many other machines (e.g. Multistagecompressors that must handle extreme gas variations) such controlsystems usually have significant measurement errors that can result ininefficient compressor operation, or worse yet, they may fail to preventsurge. By utilizing the universal performance curve the new concepteliminates virtually all errors due to variable gas composition, regardless ofhow extreme the variations are.

One measurement parameter (Pd/Ps) is easily measured, and sometimesused in more conventional type antisurge control systems. The othervariable (MNITIT7/P) at first glance might appear difficult to measure, but inreality is quite simple. It can readily be measured with conventionalinstrumentation as will be explained next.

7) MEASURING THE FLOW COEFFICIENT MN/1=7/P

MN/FITZ/ P 1.117715 (1)

Equation (1) states that the flow coefficient MNrRTZ/P is equal to VI-W. Thefollowing explains this equality:

M=Ki (2)

e. K2 PRTZ

by substitution:

3FR—F -

P RTZ P

by cancellation of terms:

MNFFZ_ K3INFF_ K3 Vi (5)

Thus, by measuring h and P a signal can be generated that is proportionalto the flow coefficient MV171-1-7/P. These two parameters (h and P) can bereadily measured with conventional instrumentation.

8) THE CONTROL SYSTEM

The new control scheme (System 1) and the two alternates that wereevaluated (Systems 2 & 3) are illustrated on Figure 5. A comparison of thethree systems will be made following this explanation of the new controlscheme.

The system consists of a primary flow element (FE), control valve (FV),three transmiters (FT, PT1 & 2) and a controller (ASIC). The control valve(FV) regulates the recycle gas flow in response to a signal from theantisurge indicating controller (ASIC). The flow element (FE) produces adifferential head signal (h) which is proportional to flow squared in thecompressor suction line. The flow transmitter transmits a control signal thatis proportional to the differential head (h), and transmitters PT1 and PT2transmit signals proportional to compressor suction and dischargepressures respectively.

The heart of the system is the controller (ASIC). It is a single loop digitalcontroller which utilizes a function block principle. It has a total of 71function blocks that can be configured to perform the numerous functionsdefined on the control schematic. Thirty six of the seventy one availablefunction blocks were used for this application. Of the thirty six functionblocks used, only five are really pertinent to this discussion. These are thefive blocks used to compute the signals that represent the X and Yparameters (WM-2/P & Pd/Ps) of the universal performance curve (Figure4) and also produce the process signal (P). Figure 6 illustrates the schmaticarrangement and functions of these five blocks. Muffiptier/divider blockFB34 and‘7R,It,zilare root extractor FB24 compute the N'_) signal which isequal to M P. Multiplier/divider FB35 computes the compression ratio

FIGURES. Comparison of new antisurge control scheme (System 1) to•two other widely used methods (System 2&3)

FIGURE 6. Antisurge controller partial schematic (Function blocks usedto compute process signal.)

(3)

(4)

4


NORMAL RATEDN

4.0

14

5

, , 1

Pd/Ps. Characterizer FB44 modifies the Pd/Ps signal as required to matchthe shape of the compressor surge line. These two signals ( Vfi71) andmodified Pd/Ps signal) are transmitted to an adder-subtractor FB36. Theoutput from FB36 is the process signal (P) to a proportional-integralcontroller function block FB13. The process signal (FB36 output) will beconstant at any point on the surge line and other lines (e.g. control lines)running parallel to the surge line. The actual value of the process signal forthe surge line or any desired control line can be determined from theuniversal surge and control line curve,Egure 7. The process signal for anyline will have a value equal to the %VTli value at the point where the lineintersects the setpoint reference line. For example, the surge lineintersects the setpoint reference line at approx. 57% and the control line at63%. Therefore, a process signal of 57% indicates that compressor surgeis imminent, regardless of the pressure ratio. Likewise a signal of 63%would indicate that the compressor is operating at some point on thecontrol line. For the controller to maintain a minimum flow at the control line,a setpoint of 63% would be used.

30.0 404 510 60.0 704

900

PERCENT FLOW COEFF. - 07.-/ 100% 41.024

FIGURE 7. Universal Surge & Control Line Curve

The fact that, based on the single process signal, a particular operatingpoint cannot be located may seem to be a disadvantage. However, inaddition to the process, setpoint and valve signal readouts, the controlleralso includes X and Y displays. Based on these two displays (X =YIVT) and Y= Pd/Ps) any operating point can be located on the universal performancecurve.

Also the X and Y outputs can be transmitted to continuously show thecompressor operating point on a universal performance curve displayed ona CRT screen.

9) EVOLUTION & EVALUATION OF SYSTEMS 2 & 3

System 2 is commonly referred to as the flow/delta-P control scheme (Ref.1, 2 & 3). The name is derived from the measurements used, i.e. Flow (flowelement differential) and Delta-P (compressor Pd-Ps). Even though theconcept is old (developed approx. 35 years ago) it is still used extensivelybecause of its low cost, simplicity and effectiveness, even for many variablegas composition applications (Ref. 3, pgs.161 & 162).

System 3 evolved during the eighties (Ref. 4). It utilizes a specializedcontroller designed specifically for variable speed compressors handlinggases of variable composition. This more complex and expensivecontroller computes polytropic head vs. inlet flow. The computation isbased on measurements of flow element differential, compressor suctionand discharge pressure and temperature with correction factors based onrotative speed. Like the Flow/Delta-P system, it is used extensively and isoften specified by contractors and compressor users.

As for any application, our evaluation considered cost, complexity andeffectiveness. Compared to system 1 the total transmitter/controller costfor system 2 is approx. 20 to 25 percent less. System 3 is much morecomplex and the total transmitter/controller cost is 2.5 to 3 times more thanfor the new system.

Cost obviously is important, however, in this case the potential control errordue to the variable molecular weight gas is the number 1 factor that wasconsidered. Our evaluation revealed that neither system 2 or 3 wassuitable because of measurement errors. Also, it is worthy of note thatwhen evaluated for the 14 MW (H2/N2) gas the complex and costly system3 deviated more from a true measurement than the simplest, leastexpensive system 2. For the evaluation, constants for each controlalgorithm were calculated to match the surge line at the normal 24.2 MWconditions Figure 3A (Note 1). The control algorithms, using the sameconstants, were then used to calculate 14 MW surge line locations ascomputed by each system (*). The results are plotted on the compressorperformance curve, Figure 8B (Note 2) and tabulated below in table no. 1.

(*) Ref. 3, pgs. 172 & 173 "Evaluating the Effects of Variable GasConditions".

Table 1 - Control errors at 14.0 MW (H2/N2) gas conditions (variablespeed, 70% through 105%)

System No. 1 (Note 3) 2 3

% RPM Deviation from Actual Surge, %

105 0 -1.6 +21.4

100 0 -2.4 +13.4

95 0 -3.3 + 7.1

90 0 -4.1 + 6.0

85 0 -6.2 + 4.4

80 0 - 7.0 + 3.9

75 0 -7.9 +3.4

70 0 -10.2 + 0.9

Notes:•

1) For this evaluation system 2 was calibrated for zero error at 70% and105% RPM points (Figure 8A) for the 24.2 MW normal gas. Figure 8Aillustrates another shortcoming of system 2. The surge line measured bysystem 2 inherently is shaped like a parabola, but the surge line in this casetends to curve in the opposite direction. The result is an excessively highmeasurement (approx. 7%) at the normal compression ratio (3.037 Pd/Ps).

2) The surge lines on Figure 8B (actual and as computed by each system)should not be confused with control lines. The control lines, where recycleflow would begin, would be located approx. 5% to 10% farther to the right.

3) As explained earlier, system 1 is calibrated for zero error with the 14.0MW, H2/N2 gas. With this calibration a control error of from 1% to 2% ispredicted for operation on the 14.0, H2/iC4 gas.

Another point worthy of mention is that the simple, least expensive system2 would have been a good choice if this had been a constant speed, motordriven, compressor operating at 105% speed. For a constant speedmachine a bias would not be used in the setpoint computation. The errorfor 14.0 MW, H2/N2 gas operation would be less than 1%, in contrast togreater than 21% for system 3.

100.0

750

50.0

25.0

0.0

5


- 00 - MOLE WT 24246, - MOLE WT 14.0 (H2 I - C4 )• - MOLE WT 14.0 (H2/ N2)

110. 120.

125 120 175 200(2. 2) (2. 5) (2. 7) (3.40)

INLET FLOW • ACFM1 x 104

ACTUAL

AS MEASURED BY SYSTEM 1

225 230

2.75

3.00(3.82) (4.25)

(4.67)

(5. 0)

(M3/ Nr. Si 1O)

- AS MEASURED BY SYSTEM 2

- ID- AS MEASURED BY SYSTEM 3

I,_

AT

IAPPROX. 7% ERRORNORMAL CONDITIONS

,

(ASSURGE LINE

MEASURED)

3.037 F'4/ Pi

SURGE(ACTUAL)

LINE ---z-51

•.7........'-'7....„....„....„

7---..........,..N..6156

90%

100% NRPM

70%0%

25 05 1.0 15 2.0 25 3.0 3.5(00) (0.85) (1.70) (2.55) (3.40) (4.25) (5.10) (5.95)

INLET FLOW • ACFM x le (613/ Sir. Si t0)

FIGURE 8A. Surge line relationships, actual and as measured byFlow/Delta - P System 2

FIGURE 9. Surge lines for normal and alternate gas mixtures.

4.0

'47 3.0

"

1.0

DA

Table 2 - Control errors at 14.0 MW (H2/N2) gas conditions(Constant speed, 105% N)

./....S

A,

A

• la'■.,

'..".:•.-___" 171-7------\...........

90%

100.6156

4 NRPM

- EL....,•-••••••.... .,

70%

80%

FIGURE 8B. Surge lines, actual and as measured by Systems 1, 2 & 3,14.0 MW altermate (H2/ N2) gas, k = 1.399

The reasons for the extreme differences can be explained by themeasurements used and the compressor characteristics. System 2computes a setpoint based on Pd-Ps which is approximately 62% less atthe 105% N surge point for the 14.0 MW gas. System 3 computes asetpoint based on measurements more directly related to polytropic head.This results in a significant error because there is less than 1% difference inhead produced by the compressor for the two different gases. Thepolytropic head relationship is illustrated by Figure 9. Control errors, head,flow and setpoint values are given on Table 2.

1 Gas Normal Gas H2/N2 GasMolecular Weight, % 100.00 57.85

2 Measured VariableFlow Rate @ Surge, % 100.0 81.49Flow Signal 0 Surge, % 100.0 (a) 38.42

3 Setpoint Variable (System 2)Pd-Ps, % 100.0 37.95Setpoint, % (Computed) 100.0 (b) 37.95Setpoint, % (Required) 100.0 (a) 38.42Controlled Flow Error, % 0.0 (d) - 0.61

4 Setpoint Variable (System 3)Head, Polytropic, % 100.0 99.13Setpoint, % (Computed) 100.0 (b) 6.57Setpoint, "Ye (Required) 100.0 (a) 38.42Controlled Flow Error, % 0.0 (d) 21.34

(a) Row Signal, °/0= °/0MW (0/01002 (6)

(b) System 2 Setpoint, % = G (Pd-Ps) (7)

(c) System 3 Setpoint, % = (100) f (N) (K) (Ps) (Td/Ts-1) LOG (Pd/Ps) ,81LOG (Td/Ts)

NOTE: For this example, G = 1.0 in Equation (7)f (N) (Ps) = 1.0 in Equation (8)

(d) Controlled Flow Error, % = 100 Setpoint (Computed) 100Setpoint (Required)

10) SURGE LINE CHARACTERISTIC

Many variable speed centrifugal compressors have a surge line shaped likea parabola. This characteristic is based on the "fan law" (Ref. 5) which statesthat capacity is proportional to rotative speed and head is proportional tospeed squared. This designation is derived from the fact that a fan is a lowhead compressor, normally handling air, a light gas. At the small headproduced by a fan the volume ratio effects are extremely small, andexcellent accuracy can be obtained by the dimensional approach on whichthe fan law is based. The accuracy diminishes with increasing head,molecular weight and backward lean of the impeller blades.

3.0

25

J.'

20

15

6


The shape of a control line produced by the Flow/Delta-P concept veryclosely matches the shape of a compressor surge line that follows the fanlaw principle. For such a compressor it is an ideal control system. However,many compressor surge lines deviate, to a great extent, from the fan law.For that type compressor the surge and control lines are curved in oppositedirections, and the Flow/Defta-P method is less than optimum, or in samecases unsuitable.

System 3 also takes advantage of the fan law characteristic, but alsoincludes a speed measurement correction factor f (N) to adjust for surgelines that deviate from it. Further, system 3 is based on the theory that atany given rotational speed, each compressor surge limit will correspond tofixed values of polytropic head and volumetric suction flow. However, thecompressor for this application, as well as many others, deviates from thattheory. Table 3 shows the flow rate at surge to be only 81.5% for the lowMW gas, but the polytropic head is 99.1% of normal. This difference in thehead - flow relationship accounts for the large error in the flow ratecontrolled by system 3. The magnitude of the error confirms that thesystem computes polytropic head correctly, but the theory is incorrect,primarily because it does not take into account the volume ratio effect. In amultistage compressor, such as this one, changes in molecular weight,temperature, and/or rotative speed can change the stage which initiatessurge. This is due to the volume ratio effect (Ref. 3,pgs. 163 & 164), andchanges the shape of the surge line. In this case, stage 2 initiates surge atall gas conditions for speeds 70% through 95% RPM, but stage 6 initiatessurge with the normal MW gas at 100% RPM and higher.

The universal performance curve takes into account all of the variables thataffect the surge line. This enables the new concept (system1) to havevirtually zero measurement error regardless of the parameter(s) that varies.

11) APPLICATION GUIDELINES

The new control system is suitable for any fixed geometry centrifugal oraxial flow compressor, constant or variable speed and gas composition.Minor changes in the algorithm make it also suitable for the following:

1) Axial flow or centrifugal compressors with variable angle stators or guidevanes.

2) Flow element at the compressor discharge.

For application 1, a vane position signal would be added.

Error due to variations in "k" value can be significant when the flow elementis located at the compressor discharge. To eliminate the error atemperature ratio correction factor is added. The flow coefficient on theuniversal performance curve would then be (Ts/Td) M P,71/Pd and themeasurement would be (Ts/Td)hcr7171.

12) RESULTS, THEORETICAL AND ACTUAL

At the time of this writing the compressor and control system have beenbuilt and tested individually, but not operated together as a system.Therefore, the data and results presented here are theoretical. However,the new concept has been implemented by modifying another type systemin the field. That system and compressor has to cope with a situation muchmore severe than the application described earlier. Molecular weight andsuction pressure vary in any combination of 16 to 35 MW and from avacuum to 30 PSIG (207kPa) pressure. The modified system solved theproblem that the original was unable to handle.

13) CONCLUSION

It is virtually certain that a standard antisurge control suitable for allturbocompressors would be welcomed by compressor manufacturers andusers. The Pd/Ps vs. MVTITZ/P concept Figure 6, appears to come closerto fulfilling that objective than any method used now or in the past.However, as already mentioned, it would require modifications for a limitednumber of applications. This leads me to essentially the same conclusionquoted verbatim from a "Centrifugal Compressor Control" paper published18 years ago (Ref. 6, pg. 9)

"There is no so-called "universal best method" of centrifugal compressorcontrol. The key to good control is to first; "DEFINE THE OBJECTIVES INTERMS OF WHAT DOES THE PROCESS REQUIRE", then 'WORK OUTAN APPROPRIATE CONTROL SYSTEM THAT ADEQUATELYPROVIDES FOR THE NEEDS AND LIMITATIONS OF THE PROCESS ANDCOMPRESSOR". Use as much existing control system design informationas possible, but not to the extent of assuming an existing system designwit work simply because it was used before."

14) ACKNOWLEDGEMENTS

I would like to thank my good friends and colleagues Bob O'Shei and GaryColby for their contributions. Bob wrote the computer program used togenerate data for the universal performance curve (Figure 4), and theroutine for plotting it. He also plotted the conventional curves used forFigures 3 & 8. Bob discovered the errors caused by the effects of "k"variations when the flow element is used at the compressor discharge, anddeveloped the Ts/Td correction factor for eliminating these errors. Finally, Iam most grateful for our many useful discussions during which he sharedhis practical expertise and in depth knowledge of compressor performance.

Gary has been contributing to my knowledge of compressor performancefor many years. I am grateful for that, and especially for his careful review ofthis paper to verify the accuracy of statements and data related tocompressor performance.

15) REFERENCES

1) White, M. H., "Surge Control for Centrifugal Compressors", ChemicalEngineering, ( Dec. 25, 1972)

2) Gaston, J. R., "Antisurge Control Schemes for Turbocompressors",Chemical Engineering, (April 19, 1982, pp 139 - 147)

3) Boyce, M. P. et al., "Tutorial Session on Practical Approach to Surge andSurge Control Systems", Proceedings of The Twelfth TurbomachinerySymposium, Texas A & M University Turbomachinery Laboratories, (pp145-173, November 1983)

4) Staroselsky, N. and Ladin, L., "Improved Surge Control for CentrifugalCompressors", Chemical Engineering, (May 21, 1979, pp175 -184)

5) Gaston, J. R., "Centrifugal Compressor Operation and Control", ISAInternational Conference, Houston, Texas, (Oct. 1976)

6) "Instrumenting and Controlling Centrifugal Compressors", Proceedingsof The Fifteenth Annual ISA Chemical and Petroleum InstrumentationSymposium, San Francisco, California, (February, 1974)

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turbocompressor antisurge control, new solution for an old problem

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