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ONTROLSY

STEMS

GAS COMPRESSION

CONTROL SYSTEMS

Oil & Gas

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Contents

1 Control System Overview .................................................................................................................... 1

1.1 PRODUCT DESCRIPTION .......................................................................................................... 11.2 ADVANTAGES OF INTEGRATED CONTROL ............................................................................ 21.3 OPEN ENVIRONMENT ................................................................................................................ 2

1.4 DISPLAY / OPERATOR INTERFACE..........................................................................................21.5 COMPONENT DEVELOPMENT ..................................................................................................21.6 VARIED COMPRESSOR CONFIGURATIONS............................................................................3

2 Control System - Old versus New....................................................................................................... 4

2.1 OVERVIEW................................................................................................................................... 42.2 SURGE LIMIT MODEL................................................................................................................. 42.3 UNITIZATION FOR TUNING AND DISPLAY...............................................................................52.4 CURVE FIT ................................................................................................................................... 52.5 COMPRESSOR OPERATION......................................................................................................52.6 TURNDOWN CALCULATOR ....................................................................................................... 62.7 SURGE MARGIN.......................................................................................................................... 72.8 TURNDOWN (RANGEABILITY)................................................................................................... 7

2.9 PROPORTIONAL AND INTEGRAL CONTROL ALGORITHM ....................................................72.10 POSITIONED VALVES................................................................................................................. 8

3 Piping and Instrumentation ................................................................................................................. 9

3.1 TIME CONSTANT OF THE SYSTEM ..........................................................................................93.2 COMPRESSOR DECELERATION.............................................................................................103.3 HEAT BUILDUP IN UNCOOLED RECYCLE SYSTEMS........................................................... 103.4 RECYCLE LINE PIPING............................................................................................................. 113.5 FLOW-MEASURING ELEMENTS..............................................................................................113.6 COMPRESSOR INSTRUMENTATION...................................................................................... 113.7 CHECK VALVES ........................................................................................................................ 123.8 RECYCLE VALVES.................................................................................................................... 123.9 RECYCLE VALVE TYPES ......................................................................................................... 133.10 MULTIPLE RECYCLE VALVE ARRANGEMENTS.................................................................... 14

3.11 RECYCLE VALVE CONTROL.................................................................................................... 143.12 COMPRESSORS IN SERIES..................................................................................................... 153.13 GAS COMPOSITION CONSIDERATIONS................................................................................15

4 Valves and Associated Components................................................................................................ 16

4.1 SIZE AND CHARACTERISTIC................................................................................................... 164.2 TYPICAL VALVE ARRANGEMENTS.........................................................................................174.3 SURGE CONTROL VALVE ACCESSORIES............................................................................. 174.4 POSITIONED VERSUS DIRECT CONTROLLED VALVES.......................................................214.5 RECYCLING FOR PROCESS CONTROL................................................................................. 224.6 INTERACTION BETWEEN SURGE AND PROCESS CONTROL............................................. 22

5 Flow-Measuring Elements and Transmitters...................................................................................23

5.1 GENERAL SELECTION CRITERIA ...........................................................................................235.2 COMPARISON OF COMMONLY USED FLOW-MEASURING ELEMENTS............................. 23

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Contents, Contd

6 Process Control.................................................................................................................................. 25

6.1 PRODUCT DESCRIPTION ........................................................................................................ 256.2 SUCTION PRESSURE...............................................................................................................256.3 DISCHARGE PRESSURE.......................................................................................................... 25

6.4 FLOW CALCULATOR ................................................................................................................ 256.5 FLOW CONTROL.......................................................................................................................266.6 SPEED CONTROL..................................................................................................................... 266.7 PROCESS CONTROL USING ANTI-SURGE CONTROL VALVE ............................................276.8 SUCTION PRESSURE...............................................................................................................276.9 DISCHARGE PRESSURE.......................................................................................................... 286.10 COMMAND TO ANTI-SURGE VALVE....................................................................................... 286.11 MULTIPLE UNIT LOAD SHARE CONTROLLER AND SURGE MARGIN EQUALIZER ........... 28

Appendix A Yard Valve Sequencing .................................................................................................... 30

A-1 MILESTONES............................................................................................................................. 30A-2 SEQUENCING MATRIX ............................................................................................................. 31A-3 VALVE OUT OF POSITION ....................................................................................................... 31

Appendix B Recycle System Design Check List ................................................................................ 34

B-1 RECYCLE SYSTEM DYNAMICS...............................................................................................34B-2 RECYCLE LINE PIPING............................................................................................................. 34B-3 FLOW-MEASURING ELEMENTS..............................................................................................34B-4 COMPRESSOR INSTRUMENTATION...................................................................................... 34B-5 CHECK VALVES ........................................................................................................................ 35B-6 RECYCLE VALVES.................................................................................................................... 35B-7 PROCESS CONTROL VALVES................................................................................................. 35B-8 COMPRESSORS IN SERIES..................................................................................................... 35B-9 GAS COMPOSITION CONSIDERATIONS................................................................................35

Appendix C K-Value Definitions ........................................................................................................... 36

Appendix D Compressor Data Requirements..................................................................................... 37

D-1 EXAMPLE OF ACCEPTABLE COMPRESSOR DATA ..............................................................37

Appendix E - Surge Control Valve Accessories .................................................................................... 39

Appendix F Glossary of Definitions ..................................................................................................... 40

Caterpillar is a registered trademark of Caterpillar Inc.Solar, Titan, Mars, Taurus, Mercury, Centaur,Saturn, SoLoNOx, and Turbotronicare trademarks of Solar Turbines Incorporated.Specifications subject to change without notice. Printed in U.S.A. 2003 Solar Turbines Incorporated. All rights reserved.SPGCCS/203

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Anti-Surge and Process Controlfor Gas Compressor Applications

1. Control System Overview

1.1 PRODUCT DESCRIPTION

Solar Turbines Incorporated offers a control sys-tem for the avoidance of surge in turbine drivencentrifugal compressors. The system creates amathematical model of the compressor perform-ance and monitors the actual operating point ofthe compressor against the model on a real-timebasis. It establishes a surge line that indicateswhere surge is expected to occur and a control

line that is at a fixed margin, typically 10%, fromthe surge line. If the compressors operating pointreaches the control line, the system initiates cor-rective action.

The hardware required includes a recyclepiping loop with an anti-surge valve, the neces-sary valve accessories, and instrumentation tomeasure the flow through the compressor andthe suction and discharge pressures and tem-peratures. Proper valve and instrumentation se-lection, coupled with a suitable piping layout, iscritical to the successful operation of the system.

The corrective action taken by the system isto open the anti-surge valve. This causes recy-cling of some of the process gas to increase theflow on the suction side of the compressor. Thismoves the operating point away from the surgeline. Once the operating point is to the right of a"deadband" line (typically 12% from the surgeline), the system instructs the anti-surge valve toclose. Operation of the valve is asymmetrical inthat it opens rapidly, but closes slowly.

Solars system scope typically includes thefollowing:

Engineering to determine the optimumcontrol algorithms for the specific applica-tion

Software programmed and tested for theselected compressor staging

Engineering to specify the anti-surge con-trol valve and accessories, including valveperformance evaluation over the compres-sor performance map at varying valve po-sitions

Engineering to specify the flow-meter typeand size

Evaluation of purchaser's piping and in-strumentation diagram and physical layout

Documentation, including all surge controlcalculations and program constants

Modified head-versus-flow control

Automatic override of manual control mode

Speed set point decoupling

Surge detection with step valve opening

On-screen, real-time graphic display

On-screen, real-time control parametersetting

Availability of all surge control parametersvia serial communications link for remotemonitoring

Suction flow transmitter *

Suction pressure transmitter *

Discharge gas temperature RTD(100-ohm platinum) *

Discharge pressure transmitter

* Shipped separately for installationby purchaser

The following components and informationare typically required from the purchaser in orderto facilitate the surge control system design andonsite operation:

Expected compressor operating conditionranges for suction pressure (P1), suctiontemperature (T1), discharge pressure (P2),flow, and gas specific gravity

Flow-meter specification sheet

Purchaser piping and instrumentation dia-gram (P&ID) and physical layout drawing,including suction and recycle pipe sizesand schedule

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Anti-surge control valve and its specifica-tion sheet, unless included in Solar's scope

Suction gas temperature signal (100-ohmplatinum RTD preferred)

1.2 ADVANTAGES OF INTEGRATED

CONTROLSolars anti-surge control system is fully inte-grated in the turbine unit control panel (UCP).This integration allows for better interface with thecapacity and/or fuel control loops for optimumperformance and compressor safety.

All unit parameters are available to the sta-tion process control system for station optimiza-tion based on compressor performance and op-timum envelopes. Because the anti-surge controlis part of the control processor program, all pa-rameters and status indications are available viaa serial link, e.g., transmitters, control parame-

ters, PV, SP, OUTPUT, SM and status variables.Integration of the process control, anti-surgecontrol, and the unit control minimizes interactioncomplications. A seamless process providescontrol from a reactive mode to a coordinatedmode of surge and process control.

With an integrated control system, the anti-surge, process and station control valves can allbe used to avoid surge. This results in superiorperformance through better resolution, faster re-sponse (reduced risk of surge), and significantlyless noise. Also, redundant components areeliminated, reducing cost.

1.3 OPEN ENVIRONMENT

Solar's anti-surge control system is easy to mod-ify by changing the source code (changing as-signed K-values). This feature is important whengas conditions are expected to change or if thecompressor is restaged.

1.4 DISPLAY / OPERATOR INTERFACE

Solars system provides real-time indication of allvariables associated with anti-surge control onone screen. The anti-surge control screen alsoprovides indication of the compressor operating

point relative to the recycle and surge lines. Thisfeature allows the operator to see how close thecompressor is operating to the unstable region.The system has a manual mode, enabling theoperator to operate the recycle valve manually.However, the system will automatically overridethe manual mode if the operating point reachesthe control line.

1.5 COMPONENT DEVELOPMENT

Solar has made and continues to make a signifi-cant investment in the development of surgecontrol components.

1.5.1 Control Processor

Solar uses control processor hardware and soft-ware produced by Allen-Bradley. Solar worksclosely with Allen-Bradley on the development ofhardware and software specifically for turbo-machinery control. This has enabled the surgecontrol algorithms to become faster, more accu-rate, and more sophisticated.

1.5.2 Instrumentation

Solar works continuously with instrumentationmanufacturers to improve the performance of theinstrumentation components. In flow measure-ment, Solars primary focus is on the low delta-P

transmitters used with orifice plates and venturis,since the speed of response of these devicestends to be inversely proportional to their range.

1.5.3 Control Valves

Solar works continuously with valve manufactur-ers to improve the performance of anti-surgecontrol valves. Solar-specified anti-surge controlvalves employ asymmetrical stroking operation.This enables the valve opening response to betuned beyond critical damping without producinginstability. Solar currently supplies valves withopening speeds of less than 100 milliseconds per

inch of port size. Future plans call for doublingthat performance; i.e., reducing the time to 50ms.

1.5.4 Flow Measurement Elements

Solar has worked with a wide variety of flow-measuring elements, such as orifice plates, flowtubes, and venturis. Solar has also worked withcompressor impeller eye flow measurement withboth Solar's compressors and other manufactur-ers' compressors. Solar is currently working onerror correction schemes based on compressor

speed and pressure ratio when using impellereye flow measurement.

1.5.5 Tailored Algorithm

Solar provides anti-surge control in many differ-ent forms. The anti-surge control algorithm is of-ten tailored to the application. This provides uni-formity with other compressor anti-surge control

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2. Control System - Old versus New

Solar's enhanced anti-surge control system pro-vides fast system response without sacrificingcontrollability. Brief descriptions of several en-hancements follow.

2.1 OVERVIEW

Solar's anti-surge control system determines thecompressor operating point using pressure andtemperature monitored at the suction and dis-charge sides of the compressor, along with flowmonitored by a flow-measuring element andtransmitter. In the earlier algorithm, only flow (Q)through the compressor and compressor head(P2 - P1) defined the compressor operating point.

The new system compares the compressoroperating point to a third-order polynomial modelof the compressor's surge limit (theoretical surgeline). The difference between the operating pointand the surge limit model, minus the protectionmargin, is the control error. A proportional andintegral (P+I) algorithm adjusts this difference, orerror, to provide a control signal to the recyclevalve. In the earlier system, the compressorsurge limit was defined by a straight line (y = mx +b), as opposed to a polynomial equation.

2.2 SURGE LIMIT MODEL

The surge limit of the compressor can be ex-pressed in terms of mathematically reduced

polytropic head and volumetric flow. The full ex-pressions for polytropic head (Hp) and flow (Q)are as follows:

Since a number of terms are common to boththe head and flow equations, they can be re-duced by:

where:

T = Temperature

Z = Gas compressibility

SG = Gas specific gravity

The resulting terms are as follows:

Reduced Head

and

Reduced Flow

where:

hW = Flow element pressure differential

and, for ideal gases

Since the reduced terms have an equal im-pact on both head and flow, a model of the surgelimit, in terms of reduced head and reduced flow,

is insensitive to changes in these parameters.Since measured P1is used in both terms, chang-ing compressor suction pressure is accountedfor.

Prior to 1999, the surge limit model wasbased onDP versus hW, whereDPis the pressuredifferential across the compressor and hW is thepressure differential across the flow-measuringdevice. While simpler and usually suitable for

11

2

P

P

HR

1P

hQ WR

1

2

1

2

ln

ln

P

P

T

T

SG

ZTP

P

kHp

11

2

1

SGP

ZThkQ W

1

2

2

SG

ZT

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pipeline applications, this was inadequate in anenvironment of changing gas conditions. Pres-sure, temperature, or specific gravity compensa-tion was difficult and made the algorithm com-plex.

2.3 UNITIZATION FOR TUNING

AND DISPLAYThe data ranges for reduced head and reducedflow are unitized; that is, they are expressed aspercentages (0 to 100%) instead of dimensionedvalues for both the X and Y axes. This approachproduces similar surge lines for a wide range ofcompressors. The gains for anti-surge control aresimilar for different installations and onsite tuningis simpler. With a dimensioned system, however,the gains for different installations can vary byorders of magnitude.

2.4 CURVE FIT

For a single-stage compressor moving light natu-ral gas (pipeline, sales quality gas), a straight-linesurge model was usually sufficient. However,multi-stage compressors running heavy gas pro-duce a more complex curve. When surge limitsfor these complex curves are modeled with astraight line, the protection margin can be twotimes the actual requirement near the center ofthe curve. This causes unnecessary recycling.The new, third-order polynomial-based systemmodels compressor surge limits accurately anddoes not recycle gas unnecessarily.

From the compressor manufacturers surge

limit data, coefficients for a third-order polynomialare developed for the operating range of thecompressor using the equation listed below:

x = Ay3 + By2 + Cy+ D

where:

x = Reduced flow (Q) elementy = Reduced head (H) element of the

surge control algorithm

The constants, A, B, C, and D are the con-stants or K-values defining the polynomial (seeAppendix C).

For display purposes (Figure 1), the surgeline and the accompanying control and deadbandlines are shown as straight lines. The surge lineis a tangent to the actual curve defined by thepolynomial, corresponding to the value of re-duced head at the compressors operating point.

2.5 COMPRESSOR OPERATION

The operation of a compressor can be describedin terms of three parameters: head, flow, andspeed. The operating point is often defined interms of its relationship to surge, typically as theratio of any of these parameters to that parameterat surge, holding any of the other parameters

constant. The three most commonly used valuesare turndown, surge margin, and head rise tosurge.

Turndown is the ratio of flow greater thansurgeto flow at the operating point, at a constanthead (Figure 2.) It is often used to describe themargin of safety of operation of a compressorespecially where only head and flow are moni-tored. It is typically expressed as a percentage.

Another definition of turndown is used to de-scribe the rangeability of a compressor; that is,the distance between surge and choke. In thiscase, turndown is often expressed as a ratio; i.e.,

2:1.Surge marginis the ratio of flow greater than

surgeto flow at the operating point, at a constantspeed (Figure 3.) This is the most common de-scription of the margin of safety of operation of acompressor. It is usually expressed as a percent-age.

Head rise to surge is the ratio of operatinghead to head at surge, at a constant speed.Again, this describes the margin of safety of op-eration of a compressor. It is usually expressedas a percentage.

Speed loss to surge, at constant head, de-

fines how far speed can be reduced, at constanthead, before the compressor surges. This rela-tionship is useful in explaining the problem ofavoiding surge during a shutdown. If the volumein either side of the compressor is maintaining thehead across the compressor, reducing speed willinduce surge.

The remaining two relationships are headrise to surge at constant flow and speed fall tosurge at constant flow. Neither of these relation-ships is very useful. Head across an operatingcompressor is set by the upstream and down-stream system. Head changes slowly in direct

proportion to the flow. Flow is only constant whenspeed, upstream and downstream resistancesare held constant and the system has reachedequilibrium. Relating a change in any parameterto a constant flow does not reflect an operatingscenario.

Solars control system uses the calculatedvalue of turndown to protect the compressoragainst surge.

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Figure 1. Anti-Surge Control Display

2.6 TURNDOWN CALCULATOR

The turndown (TD) calculator takes inputs ofpressure, temperature, and flow to calculate theturndown. Turndown is defined as the horizontaldistance (i.e., at constant reduced head factor)between the reduced flow factor at the operatingpoint and the reduced flow factor at the surgelimit line, expressed as a percentage, showngraphically in Figure 2 and mathematically as:

where:

and: Figure 2. Turndown

SRGRQ

OPRQ

REDUCEDFLOWFACTOR

REDUCEDHEADFACTOR

SRG

SRG P

hQ WR

1

OP

OP P

hQ WR

1

SRG

SRGOP

R

RR

Q

QQTD

100

SRG

SRG P

hQ WR

1

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OP

OP P

hQ WR

1

2.7 SURGE MARGIN

The surge margin is calculated in the same wayexcept that the value of QRsrg is derived from theintersection of the constant speed line and thesurge line, as shown in Figure 3:

Figure 3. Surge Margin

2.8 TURNDOWN (RANGEABILITY)

When describing rangeability, turndown isdefined as:

where:

QMAXSPEED = Flow at maximum speed(See Figure 4.)

Figure 4. Turndown (Rangeability)

2.9 PROPORTIONAL AND INTEGRALCONTROL ALGORITHM

In any control system, the maximum control gainis limited by the time constant or system resonantfrequency. If gains are continually increased toimprove response, the system oscillates. Sincemany system components respond non-linearly,the system time constant changes. Even if the

system is initially optimally tuned, process condi-tions and/or the equipment change over time, andthe system can oscillate.

An anti-surge control system must 1) avoidsurge with severe process changes and 2) con-trol continuous recycle without oscillation orhunting. Off-the-shelf valves and transmittersmust be used in a piping system where flow-meter runs are not ideal and control volumes canbe large.

Often, performance requirements cannot bemet with a conventional single gain system. Toensure surge avoidance, valve opening speedsfar in excess of the system time constant can berequired. To return to the normal processsmoothly, a much slower closure rate is required.To achieve these conflicting objectives, Solaruses two gains: high gains for opening the recy-cle valve and low gains for closing the valve.

Solar's anti-surge controller uses conven-tional proportional plus integral control with gains

OP

SRGOP

R

RR

Q

QQSM

100

SRGQ MAXSPEEDQ

HEAD

FLOWREDUCEDHEADFACTOR

REDUCED

FLOWFACTOR

SRGRQ OPRQ

SRG

MAXSPEED

QQTD

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modified, depending on the location of the com-pressor operating point relative to the control line.Variable gains allow the recycle valve to open orclose appropriately, depending upon process re-quirements.

2.10 POSITIONED VALVES

Until recently, Solar used only direct-actingvalves; i.e., valves controlled via a current / pres-sure (I/P) transmitter, for anti-surge control. Ear-lier valve assemblies that included positionerswere not judged acceptable due to their poor re-

sponse to commands from the control processor.However, improvements in valve technology andthe use of ancillary components in the system,including for example, a one-way volume boosterand a needle valve, now permit the successfulapplication ofpositionedvalves. Therefore, Solarnow recommends the use of positioned valves.

Solars anti-surge system responds fasterthan most, if not all, competing systems and pro-vides precise control that allows continuous op-eration at the surge control line.

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3. Piping and Instrumentation

Design of the piping and the selection andplacement of instruments will significantly affectthe performance of an anti-surge control system.The benefits of a good piping and instrument de-sign cannot be overemphasized. The cost of cor-recting a poor design once the equipment is inoperation can be extremely high. The followingguidelines are provided to help ensure properoperation of the anti-surge control system.

The typical simple recycle system is shown inFigure 5. The system includes a flow-measuringelement in the compressor suction, a compres-sor, an aftercooler, a discharge check valve, anda recycle line and valve connected upstream ofthe discharge check valve and upstream of thecompressor flow-measuring element. The controlmonitors the compressors operating parametersand compares them to the surge limit and opensthe recycle valve as necessary to ensure the de-sired surge margin is maintained.

3.1 TIME CONSTANT OF THE SYSTEM

For surge avoidance, the system time constantcan be defined as a volume and a valve. In the

simplest system, the volume is bounded by thecompressor, discharge check valve, and recyclevalve. The suction volume is typically several or-ders of magnitude larger than the discharge vol-ume and, therefore, is ignored (considered infi-

nite, constant pressure). The time constant of a

volume / valve system can be estimated by theformula:

where: = Time constant in seconds

(63.2% decay in pressure)

Figure 5. Typical Piping and Instrumentation Outline

ZTPC

SGVP

V 2

135

FT PT TT

COMPRESSORENGINE

PT TT

ANTI-SURGE

CONTROLLER

SOLENOID

ENABLE

24VDC

4 - 20mA

LIMIT

SWITCH

POSITION

TRANSMITTER

4 - 20mA

ANTI-SURGE

CONTROL VALVE

FAIL OPEN

SV = SUCTION VALVE

LV = LOADING VALVE

VV = VENT VALVE

DV = DISCHARGE VALVE

TT = TEMPERATURE

TRANSMITTER

FT = FLOW TRANSMITTER

PT = PRESSURE

TRANSMITTER

AFTERCOOLERSV

LV

VV

DV

SCRUBBER

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CV = ISA flow coefficient for the recycle valve

P1 = Inlet pressure of the recycle valve

P2 = Outlet pressure of the recycle valve

SG = Specific gravity of the gas

T = Absolute temperature of the gasupstream of the recycle valve

V = Volume bounded by the compressor,

the check valve, and the recycle valve

Z = Compressibility of the gas

In a surge avoidance system, half of thevalves capacity will be consumed recycling thecompressor. Only the remainder can be used fordepressurizing the discharge volume. Therefore,in the equation above, only that portion (CV) ofthe valve not consumed in recycling the com-

pressor can be considered for depressurizing thedischarge. Ultimately, this time constant will de-termine the fastest rate at which the surge avoid-ance system can operate.

3.2 COMPRESSOR DECELERATION

The worst-case scenario is when the compressoris operating near surge, without any recycle, andan engine shutdown occurs. With the initiation ofa shutdown, the compressor can be expected todecelerate approximately 30% in the first second.With a 30% loss in speed, the compressor's headcapability at its surge limit will drop by approxi-

mately 50% (Fan Law). The surge control valvemust, therefore, reduce the pressure across thecompressor by one-half in that first second. To dothis, the surge control valve must move a propor-tional amount of gas out of the discharge into thesuction. This is in addition to the flow requiredthrough the compressor to avoid surge.

The larger the volumes in the system, thelonger it will take to equalize the pressures, themore sluggish will be the response of the surgecontrol system, and the more likely the compres-sor will surge. The larger the valve, the better thepotential of avoiding surge. However, the largerthe valve, the poorer the controllability will be atpartial recycle. The faster the valve, the quicker itcan get to the position where the required flowcan be achieved and the more likely surge will beavoided. However, the speed of a valve cannotsimply be turned up infinitely. Increasing the gainof the valve positioning system will at some pointproduce instability. This can be overcome by in-creasing the power of the actuator (improving therelationship between the command and the valve

position). At some point, this becomes impracticalin both size and cost. An alternative is a valveboosted only to open. This provides high openingspeeds for surge avoidance, while avoiding os-cillation by very slow closing.

If the discharge volume / recycle valve can-not be designed to ensure surge is avoided, a

short recycle loop (hot recycle valve) may beconsidered. If only a single recycle valve can beused, a rotary valve typically has 50% more turn-down than a globe valve. Subsequently, use of aball valve can cut the depressurization time by50% over a globe valve.

3.3 HEAT BUILDUP IN UNCOOLEDRECYCLE SYSTEMS

Virtually all of the energy put into the compressoris reflected as heat in the discharged gas. In anuncooled recycle system, this heat is recycledinto the compressor suction and then more en-

ergy added to it. At 100% recycle, eventually thiswill lead to overheating at the compressor dis-charge. Low pressure ratio compressors often donot require aftercoolers. Compressors with onlyhot recycle systems are not intended to recycle atall during normal operation. The problem usuallyoccurs when there is a long period between theinitial rotation of the compressor and overcomingthe pressure downstream of the check valve.

A cubic foot of natural gas at 600 psi weighsabout 2 lb (depending on composition). The spe-cific heat of natural gas is about 0.5 Btu/lb (againdepending on composition). 1 Btu/sec equals

1.416 hp. If the recycle system contains 1000cubic feet, there is a ton of gas in it. 1416 hp willraise the temperature of the gas about 1 degreeper second. This approximates what happenswith 100% recycle.

The analysis of the partial recycle scenario ismore complicated. The compressor dischargetemperature will rise asymptotically until the en-ergy of the gas leaving the system equals theenergy input to the compressor.

Extending the length of the recycle linedownstream of the recycle valve increases thetotal volume of gas in the recycle system, thusreducing the heat buildup rate. Some heat will belost through the pipe walls. If the outlet is far up-stream into a flowing suction header, dilution willoccur.

For start-up, a relatively small control valvecan be placed across the discharge check valve.As compressor discharge temperature increases,this valve can be opened, pushing some of thehot recycled gas into the suction header tempo-rarily.

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3.4 RECYCLE LINE PIPING

3.4.1 Compressor Suction Side

The suction side of the recycle system isbounded by the compressor suction, the suctionblock valve or inter-stage check valve, and theoutlet of the recycle control valve. The connection

of the recycle line outlet should be as far awayfrom the compressor as possible. Conversely, theflow-measuring element should be as close to thecompressor suction as possible. With this ar-rangement, the introduction of recycle flows willnot overly adversely affect flow measurement.

3.4.2 Compressor Discharge Side

The discharge side of the recycle system isbounded by the compressor discharge, the dis-charge check valve, and the inlet of the recyclecontrol valve. This volume is critical to the per-formance of the surge control system and should

be kept to a minimum.

3.4.3 Recycle Line Sizing

The recycle valve should have greater than 90%of the total pressure drop across the recyclesystem under maximum throttling conditions(maximum head and speed).

For short recycle systems (

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ENGINE

ASC

AFTERCOOLER

SCRUBBER

ENGINE

ASC

AFTERCOOLER

SCRUBBER

COMPRESSOR

COMPRESSOR

HBP

HBP

SV

LV

DV

VV

DV

VV

SV

LV

SRV

FM

FM

HBP = Hot Bypass Valve

ASC = Anti-Surge Control Valve

SRV = Station Recycle Valve

FM = Flow Measuring Device

Figure 6. Typical Recycle Valve Configurations

3.7 CHECK VALVES

A surge control system must contain at least onecheck valve. Typically, it is located in the dis-charge immediately downstream of the entranceto the recycle line. When hot and cooled recyclecontrol valves are used, a second check valvemay be added immediately downstream of theinlet to the hot recycle line. This second checkvalve serves to reduce the effective dischargevolume for the hot recycle valve. Reducing thevolume to be depressurized improves the re-sponsiveness of the surge avoidance system.

Check valves at the discharge of the com-pressor do not lessen the impact of surge on thecompressor. A check valve at the discharge ofthe compressor may actually increase thedamaging effects of surge by decreasing therecovery time, subsequently increasing therepetition rate. A check valve at the discharge ofthe compressor will prevent backspin of thecompressor. However, a properly designed surge

control system, with a minimized dischargevolume, cannot maintain enough energy tobackspin the compressor.

Premature compressor bearing failures havebeen attributed to a check valve being placed tooclose to the compressors discharge. If a checkvalve is to be placed at the discharge of the com-pressor, it should be at least 10 pipe diametersdownstream of the compressor.

3.8 RECYCLE VALVES

Recycle valves are used to allow the operation ofa compressor at delivered flows lower than thosewithin the operational boundaries of the com-pressor.

A compressor driven by a two-shaft gas tur-bines can decelerate very quickly with a loss ofpower. The compressors ability to develop headvaries with the square of speed (Fan Law). Lossof compressor speed will cause a very rapidapproach to the compressors surge limit. To

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avoid surge, the valve(s) must be able to respondaccordingly. Conversely, when there is a gradualapproach to the surge limit (line packing), thevalve must be able to smoothly and preciselythrottle recycle to maintain the desired margin.These demands push the valve size and posi-tioning system in opposite directions.

The valve(s) responsible for surge avoidancemust meet two sizing requirements:

1. Capacity to recycle the compressor

2. Capacity to depressurize the discharge inless than one second

If the capacity required to depressurize thedischarge exceeds three times the size requiredto recycle the compressor, two valves should beused. If only one valve is to be used, an equalpercentage characteristic will expand the throt-tling range. For equal percentage, characteristic

globe valve capacity (Cv) is approximately equalto travel squared. Noise-attenuating ball valveshave a characteristic where capacity (Cv) is ap-proximately equal to travel cubed. As such,noise-attenuating ball valves are a superior singlevalve choice.

Valves specified or supplied by Solar will beable to open in less than 50 ms per inch of portsize. If the enable solenoid is de-energized, thevalve should reach 63.2% open (first-time con-stant) in less than 50 ms per inch of port size.With a 50% step change in the input to the posi-tioner (20 to 12 mA), the valve will reach 50%

open in less than 50 ms per inch of port size.Throttling with large pressure drops across a

valve can result in significant temperature drops.This can cause freezing both inside and outsidethe valve, if sufficient water vapor is present.Freezing can render the recycle valve inoperable.These conditions most often occur with capacityor station control valves around multiple com-pressors in series with aftercoolers.

In most cases, the throttling process takesplace so rapidly and in such a small space thatthere is neither sufficient time nor a large enougharea for much heat to transfer. Therefore, we as-

sume the process to be adiabatic. Since h = f(T)for an ideal gas, we could expect no temperaturechange during this constant hprocess. However,even for nearly ideal gases, T2will differ from T1.This is known as the Joule-Thomson effect. TheJoule-Thomson coefficient (Uj) is defined as:

The Joule-Thomson coefficient can be de-rived from the gas composition and characteris-tics. For typical natural gas compositions, the

temperature can be expected to drop 1F forevery 20 psi.

3.9 RECYCLE VALVE TYPES

3.9.1 Start Bypass Valves (on / off)

The start bypass valve is an on / off valve con-nected across the compressor inside the blockvalves and upstream of the check valve. It isopen during start-up and shutdown. It is fail open.It is configured for fast opening so that it canbleed down the discharge to prevent surges inthe event of unscheduled shutdowns. A start by-pass valve is seldom used alone. It is usuallyused in conjunction with a surge control valve.

3.9.2 Surge Control Valves Modulating

The surge control valve is a modulating valveconnected across the compressor inside theblock valves and upstream of the check valve. Itis open during start-up and shutdown. It is failopen. It is configured for fast opening so that itcan bleed down the discharge to prevent surgesin the event of unscheduled shutdowns. A surgecontrol valve is often used alone. When the surgecontrol valve is used alone, its sizing becomes acompromise between an appropriate size forthrottling recycle around the compressor and de-pressurizing the head across the compressor. Assuch, the noise-attenuating ball valve should be

the first choice. With its exaggerated equal per-centage characteristic (70% travel is only 1/3 ofthe fully open flow), it can best address both ap-proaches to surge problems.

3.9.3 Capacity Control Valves

Capacity control valves are in parallel with thesurge control valves. They are fail open. The ca-pacity control valve typically utilizes noise-attenuating trim. If they are to be used to aid thesurge control valve and vice versa, the capacitycontrol valve must also have fast open capability.This approach will reduce the size of both valves,

improve controllability, and reduce valve re-sponse time in the event of a shutdown.

3.9.4 Station Recycle Control Valves

Station control valves are connected outside theunit check valves and block valves. They are failclosed. The station control valve typically utilizesnoise-attenuating trim. The flow capacity of thestation control valve cannot be considered forsurge protection.dP

dTUj

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3.10 MULTIPLE RECYCLE VALVEARRANGEMENTS

3.10.1 Parallel Valves, One Control Valve,One On / Off

This combination (valves properly sized) providesgood protection for the compressor along with

good controllability at partial recycle. This is typi-cally a rotary valve for the on / off and a globe asthe control valve. The control valve is sized toflow at least the surge conditions. The on / offvalve is sized for discharge volume depressuriza-tion. The on / off valve is often grossly oversized,since it is not used for throttling. Both valves mustbe configured for fast opening.

3.10.2 Parallel Control Valves (Globe)

This arrangement strives to achieve the turn-down and noise control of a noise-attenuatingrotary valve. This combination (valves properly

sized) provides good protection for the compres-sor along with good controllability at partial recy-cle. The two valves are sized to flow at least thesurge conditions. A piping analysis may showthat the capacity needs to be larger. Optimally,the valves should be cascaded by the control,one having a slightly higher surge protectionmargin (set point) than the other. This enablesboth valves to open with large movements of theprocess.

3.10.3 Station Control Valve

The station control valve is placed across multiple

compressors in parallel. It enables the operator tocontinue operation of the current number of com-pressors on line during periods of reduced de-mand or supply. The arrangement of multipleunits with on / off surge avoidance valves (seeSection 6) at the unit level and a station controlvalve will provide maximum protection for thecompressors, along with excellent controllabilityat partial recycle with minimum piping and in-struments.

3.11 RECYCLE VALVE CONTROL

3.11.1 Anti-Surge Control Valve (ASCV)

The valve will begin opening at less than 10%surge margin (control line) and begin closing withgreater than 12% surge margin. High gains wouldbe employed if the operating point is to the left ofthe control line. Low gains would be employed ifthe operating point is to the right of the controlline.

3.11.2 Cooled Recycle Control Valve (CRCV)

The valve will begin opening at less than 11%surge margin (control line) and begin closing withgreater than 11% surge margin.

3.11.3 Station Recycle Control Valve (SRCV)

The valve will begin opening at less than 12%surge margin (control line) and begin closing withgreater than 14% surge margin. Between thecontrol line and the deadband line, the valvecontrol signal will not change.

3.11.4 Deadband

Valves, as with all mechanical devices, have fric-tion, sticktion, and inertia. As such, there is aminimum control signal change that will causeany movement at all (resolution). In any controlsystem, there is always some error. With an inte-grating control system, this will be reflected as a

continuous change in the control output. This willresult in hunting of the valve. If rather than thecontrol responding to the sign and magnitude ofthe error it responds to 12%, there will be a 2% region or deadbandwhere the control system does nothing at all.With cascaded recycle valves, it is desirable tohave only one valve throttling at steady state. Toensure this, a deadband is only incorporated intothe outermost loop (the valve that will be in con-trol at steady state). This allows the differencebetween the set points of the cascaded valves tobe infinitely close together and ensures that two

valves will not be open in steady state.

3.11.5 Ramp for Initial Closure

Oftentimes, valves are ramped closed initiallyrather than simply operating on the piping andinstrumentation diagram (P&ID). This provides formuch smoother onloading of the equipment. Withcascaded valves, the valves should be rampedclosed individually rather than simultaneously.This can be accomplished as simply as ensuringthe next innermost valve is completely closedbefore the ramp for a valve is released. In thecase of a system with hot and cooled recycle

valves, it may be desirable to close the hot recy-cle valve much earlier in the start sequence toavoid unnecessarily heating the compressor.

3.11.6 Cascaded Recycle Control Valves

Compressor recycle valves are operated in cas-cade to optimize various aspects of the compres-

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sor control that would be a compromise using asingle valve. Additionally, properly cascading thecontrol of the valves can significantly reduce thesizes of the individual valves.

Hot recycle valves (directly around the com-pressor) will provide the control system with veryfast response, since the controlled volume can be

very small. (A check valve must be installed im-mediately downstream of the hot recycle loop).However, continuous operation at significant per-centages of recycle will overheat the compressor.Cooled recycle valves allow continuous operationat 100% recycle, but the impact to the processvariable lags behind the valve position due to thesize of the control volume (discharge piping andcooler). Station control valves typically providerecycle for several compressors (either series orparallel). Station control valves are used to avoidinteraction between the unit recycle control sys-tems. Again, the compressors can be recycled

continuously and the impact to the process vari-able lags behind the valve position due to thesize of the control volume.

Cascading is accomplished by slightly in-creasing the set points for the control of eachsucceeding outer loop. The gains for each suc-ceeding outer loop should be significantly lowerthan the preceding inner loop. This occurs natu-rally because the time constant of each suc-ceeding outer loop is greater since the volume ofits system is larger.

At steady state, the outer loop will integrateout its entire error, causing the error of the innerloop to become positive and, subsequently, clos-

ing its valve completely.

3.11.7 Cooled Recycle Valves with a HotBypass (on / off)

In this arrangement, the cooled recycle valveprovides all the modulating control and the by-pass valve is used only during start-up and shut-down.

This combination (valves properly sized) pro-vides the maximum protection for the compressoralong with good controllability at partial recycle.This approach does not require a control as com-plex as the cascaded valve arrangement; how-ever, the size of the valves cannot be reducedsince the valves operate independently (one doesnot aid the other).

3.12 COMPRESSORS IN SERIES

Compressors in series can be treated just asthough they were individual compressors. How-ever, due to the close coupling and slow instru-

ments, recycling one compressor drives the oth-ers into surge before their controls can react.

A single control valve can be used aroundthe entire series. By cascading this valve with theunit surge control valves, interaction betweenunits is avoided as long as the unit valves arekept from opening. This approach works well, as

long as there are no side streams. The control ofthe single valve is much simpler than controllingthe unit valves. The valve will be smaller than anyof the unit control valves due to the increasedpressure drop. The increased pressure dropleads to an increased temperature drop at theoutlet of the valve. If the inlet to the recycle valveis downstream of an aftercooler, freezing mayoccur downstream of the recycle valve. To en-sure this does not happen, a heater line may beadded between the inlet of the valve and the dis-charge of the compressor.

The other approach is some form of control

anticipation. If any unit in the series begins recy-cling, the outputs to the other recycle valves canbe forced open in some proportion to the amountand speed at which the initial valve was opened.This is not a form of feed-forward control sincethe processes are interactive. An improperly de-signed anticipation system will drive all the valvesfully open due to wrap around.

3.13 GAS COMPOSITIONCONSIDERATIONS

3.13.1 Corrosives

The presence of various chemicals in the gasmay attack the piping and components. Gener-ally, the recycle system is designed and fabri-cated to the same requirements as the main pip-ing. However, since the recycle system is onlyused intermittently, it may not need to meet thesame requirements as the piping for continuouslyflowing gas.

3.13.2 Hydrate Formation and TemperatureDrop at the Outlet of the Surge Valve

In cases where the pressure differential across arecycle valve is sufficiently high, hydrates (forma-

tion of water vapor) may drop out of the gas. Ifthis occurs and the outlet temperature of the gasis too low, ice may form at the outlet of the valve,restricting recycle flow back to the compressor.For these cases (usually occurring in multi-bodycompressor sets), a temperature control valvemay be necessary to heat the recycle valves inletgas to levels high enough to prevent ice forma-tion at the outlet of the valve.

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4. Valves and Associated Components

Surge control valves are primarily sized to fit thecompressor. During steady-state recycling, therequired capacity of the recycle valve can be di-rectly derived from the compressor map. To han-dle transient conditions, the required capacitymust be greater to allow for the volumes on eitherside of the compressor. With the initiation of ashutdown, the compressor can be expected todecelerate approximately 30% in the first second.With a 30% loss in speed, the head the compres-sor can develop at its surge limit will drop by ap-proximately 50%. The recycle control valve must,therefore, reduce the pressure ratio across thecompressor by one-half in that first second toavoid surge.

The following guidelines pertain to a typicalone valve, one compressor arrangement. More

complex systems of cascaded valves or valvesaround multiple compressors require a more de-tailed analysis.

To facilitate both precise throttling at partialrecycle and the need to reduce the DP across thecompressor quickly during a shutdown, Solarrecommends surge control valves with an equalpercentage characteristic (Figure 7). The equal

percentage characteristic spreads the first of

the valve's fully open capacity over the first ofthe valves travel. This greatly improves control-

ability at partial recycle throttling. In order to avoidsurge during a shutdown, the valve must open tothe required capacity in significantly less than onesecond.

Solar recommends surge control valves thatmeet the following:

4.1 SIZE AND CHARACTERISTIC

Surge control valves are sized to meet two di-verse objectives. During steady-state recycling,the required capacity of the recycle valve can bedirectly derived from the compressor map: thesmaller the valve, the smoother the control. Dur-ing transient conditions, the required capacityincreases due to the volumes on either side ofthe compressor. Therefore, to avoid surge duringa shutdown, the bigger the valve, the better.

To facilitate both smooth throttling at partialrecycle and the need to reduce the pressure dif-ferential (DP) across the compressor quicklyduring a shutdown, control valves with an equalpercentage characteristic are recommended.With an equal percentage characteristic, themore the valve is opened, the greater the in-crease in flow for the same travel. Solar recom-mends two types of valves for surge control:globe valves and noise-attenuating ball valves.The globe valves capacity (Cv) varies with the

Figure 7. Typical Globe Valve Flow Characteristics (% Open) versus Compressor Map

500 1000 1500 2000 25000

1.0

1.5

2.0

2.5

3.5

3.0

FLOW, acfm

GLOBE VALVE EVALUATION

EQUAL PERCENTAGE CHARACTERISTIC

(Percentage Open)

60% 70%

100%

RA

TIO,

P2/P1

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square of the percentage travel. The noise-attenuating ball valves capacity (Cv) varies withthe cube of the percentage travel. Both valvesare sized to be throttling at about two-thirds openat surge conditions. As such, the noise-attenuating ball valve will have 50% more capac-

ity to depressurize the discharge volume than theglobe valve. This additional capacity makes thenoise-attenuating ball valve the better choice ininstallations where there is a single surge controlvalve, which means there is no hot bypass andthe discharge volumes are large; e.g., the dis-charge system includes an aftercooler.

4.1.1 Operation

The surge control valve assembly should be in-creasing signal to close. The surge control valveassembly should be fail open. The valve shouldopen with loss of either electrical signal or controlair supply.

4.1.2 Interface Definition

The surge control valve closure should be en-abled by supplying 24 volts to a three-way sole-noid. Removal of the 24 volts should cause thevalve to open. Solenoid valves with openingspeeds of less than 20 ms and a Cv of greaterthan 0.6 are recommended.

The surge control valve assembly shouldtransmit a 4-to-20 mA signal corresponding to theposition of the valve, ranging from fully open to

fully closed.The surge control valve assembly should

provide isolated contact closures, representingfully open (contact closure >2% travel, contactopen

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Figure 8. Instrumentation for Globe Anti-Surge Valve

4-20 mA

LIMIT SWITCH CLOSED

LIMIT SWITCH OPEN

24 VDC

EXHAUST

BOOSTER

NEEDLE VALVE ANDCHECK VALVE

THREE-WAY 24- DC

SOLENOID VALVE

POSITIONTRANSMITTER

4-20 mA

PRESSURE REGULATOR

(AIRSET 35-50 psig)

INSTRUMENT

AIR SUPPLY

ELECTRO-PNEUMATICPOSITIONER

OKE-MOUNTED

4-20 mA, 6-30 psig4-20 mA

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Figure 9. Instrumentation for Ball Anti-Surge Valve with Single Spring and Piston Actuator

EP5

F5

4-20 mA

CLOSED LIMIT SWITCH

OPEN LIMIT SWITCH

4-20 mA

80-100 psig

24 VDC

ELECTRO-PNEUMATIC POSITIONER

OKE-MOUNTED 4-20 mA, 6-30 psig

POSITIONTRANSMITTER

4-20 mA

THREE-WAY 24- DC

SOLENOID VALVE

EXHAUST

BOOSTER

NEEDLE VALVE ANDCHECK VALVE

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Figure 10. Instrumentation for Ball Anti-Surge Valve with Double Spring and Piston Actuator

EP5

F5

4-20 mACLOSED LIMIT SWITCH

OPEN LIMIT SWITCH

4-20 mA

80-100 psig

24 VDC

ELECTRO-PNEUMATICPOSITIONERYOKE-MOUNTED

4-20 mA, 6-30 psigPOSITION TRANSMITTER

4-20 MA

THREE-WAY 24-VDC

SOLENOID VALVE

EXHAUST

BOOSTER

NEEDLE VALVE AND

CHECK VALVE

VOLUME

BOOSTER

80-100 psig

EXHAUST

BOOSTER

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Solar recommends positioned valves for allrecycle applications.

4.5 RECYCLING FOR PROCESSCONTROL

The majority of process control is managed by

manipulating the power from the engine. Whenthe process conditions change more rapidly thanthe engine can respond, recycling can be em-ployed to ensure the minimum suction pressure ismaintained, maximum discharge pressure is notexceeded, or the flow set point is not exceeded. Ifthe process demands conditions below the mini-mum speed for the compressor, recycling canalso be employed. This is achieved in the sameway as cascading the valves. When the differ-ence between the set point and the process vari-able exceeds some value, the recycle valve will

be opened to ensure this distance from the setpoint is not further exceeded. If the engine powerreaches the point where the compressor canmeet the process conditions, the recycle valvewill close.

4.6 INTERACTION BETWEEN SURGE

AND PROCESS CONTROLSurge and process controls operate continuouslywithin the turbo-compressor control processor.The control demanding the recycle valve(s) to bethe least closed will be in control. Each processcontrol has gains best suited for that controlmode. Surge control has different gains foropening and closing the recycle control valve(s)to ensure maximum protection for the compres-sor without the risk of control oscillation. Hand-offbetween any of the controls requiring recycle isbumpless.

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5. Flow-Measuring Elements and Transmitters

5.1 GENERAL SELECTION CRITERIA

5.1.1 Cost

The total life cost of the instrument should always

be the deciding factor. Low acquisition costs mayresult in higher operating cost and low reliability.In a control system, slower response or pooreraccuracy may increase costs due to their impacton the overall process controlled.

5.1.2 Permanent Pressure Loss(operating cost)

Ideally, the differential developed across a flowmeter would be totally recovered downstream.With the best flow tubes, the pressure recovery isgreater than 90%; with orifice plates, the pressurerecovery is more likely to be about 50%. Forcomparison purposes, permanent pressure lossshould always be expressed as a ratio.

5.1.3 Signal Level

High signal levels typically produce higher signal-to-noise ratios. High signal levels are read byhigher range transmitters, which typically haveshorter time constants.

5.1.4 Reliability

Devices installed perpendicular to the flow pathmay resonate at certain flow conditions, eventu-

ally leading to failure. Devices with ports facinginto the flow path rather than perpendicular to itmay be susceptible to clogging. Devices withfragile parts or moving parts in the flow streamare more likely to fail compared to those withoutsuch parts.

5.1.5 Characteristic

Typically, flow is proportional to the square root ofthe differential pressure. Devices with more com-plex characteristics will produce a more inaccu-rate flow prediction or require complex compen-sation when used over a wide range of flows.

5.2 COMPARISON OF COMMONLY USEDFLOW-MEASURING ELEMENTS

5.2.1 Orifice Plates

An orifice plate is a plate mounted perpendicularto the flow path with a hole bored in it (typically inthe center). The bore is sometimes chamferedfrom the downstream side, producing a sharpedge at the upstream face. An orifice produces

an abrupt reduction in the flow area. The fluidvelocity increases to pass through its bore; sub-sequently, the pressure at the orifice is reduced.The orifice is the most commonly used device formeasuring compressor flow. It is probably morecommonly used than all other types combined.

Advantages

Well documented, standardized, well understood,low initial cost, low cost to change.

Disadvantages

High permanent pressure loss. Deviation fromtypical Q is proportional to the square root ofpressure differential.

5.2.2 Nozzles

A nozzle is a horn-shaped device that produces agradual acceleration of the fluid. Flow nozzleshave gradual reductions in the flow area. As thefluid is accelerated, it remains attached to thewalls of the flow-measurement device.

Advantages

Well documented, standardized, understood.More accurate than an orifice when characterizedby a single coefficient,

when used over a wide range of flows.

Disadvantages

High permanent pressure loss. Higher initial costthan an orifice.

5.2.3 Compressor Suction to Impeller Eye

The issue of "compressor suction-to-eye flow

measurement" has been confused by its names,including "impeller eye" and "eye of the volute." Itis only a nozzle. In single overhung wheel com-pressors where the shaft does not extend throughthe impeller, this is easier to visualize. The pres-sures obey the same laws as those for a nozzle.The differential pressure is directly proportional tothe increase in velocity resulting from the de-creased flow area between the compressor suc-tion flange and the pressure tap in the inlet

5.0

1

'

P

hCQ w

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shroud near the impeller inlet: V2 = 2GH. Pre-dicting the differential pressure across an orificeis much more complicated, although there aremany inexpensive computer programs availableto do it.

Advantages

"Suction-to-eye" is lower in initial cost. It is pro-vided as standard on all of Solars compressorsthat have it available. It is not an option; there isno additional cost. The cost of providing an addi-tional compressor flow-measuring device isavoided along with the cost of the additional per-manent pressure loss (horsepower).

"Suction-to-eye" provides a much higher sig-nal level, typically four times that of an orifice.With the higher signal level, the signal responsetime is reduced. This is especially important insurge avoidance. With the higher signal level, thesignal-to-noise ratio is often increased, reducing

the need for filtering the flow signal.Solar's Surge Control Engineering has

worked with "suction-to-eye" flow measurementon both Solar's compressors and compressorsfrom other manufacturers. Solar's Surge ControlEngineering recommends "suction-to-eye" flowwhenever it is available.

Disadvantages

The compressor must be properly calibrated. Thismay require special testing. Deviation from typicalQ is proportional to the square root of pressuredifferential. Often more difficult to calibrate ini-

tially.

5.2.4 Nozzles with Divergence Cones(Flow Tubes)

In these devices, the fluid is gradually acceler-ated to its maximum velocity in the throat andgradually decelerated back to its original velocity.The fluid flow remains attached to the walls of theflow-measurement device. Subsequently, perma-nent pressure losses are often less than 10%.

Advantages

Low permanent pressure loss. More accuratethan an orifice when characterized by a single

coefficient, {Q = C(hw/P1)0.5}, when used over a

wide range of flows.

Disadvantages

High initial cost.

5.2.5 Venturi, ISO 5167 (Classical)

As with nozzles with divergence cones (flowtubes), the fluid is gradually accelerated to itsmaximum velocity in the throat and gradually de-celerated back to its original velocity. The fluidflow remains attached to the walls of the flow-measurement device. Subsequently, permanentpressure losses are often less than 20%.

Advantages

Low permanent pressure loss. More accuratethan an orifice when characterized by a singlecoefficient, {Q = C(hw/P1)

0.5}, when used over a

wide range of flows.

Disadvantages

High initial cost.

5.2.6 Averaging Pitot Devices(Annubars and Verabars)

In the pitot flow-measuring system, the static andvelocity heads are measured. The pressure dif-ferential between the pitot and static is propor-tional to the square of the flow. The same form ofequation is used for pitot devices as is used for

orifices and venturis. A flow coefficient (K) re-places the discharge coefficient and a pipe

blockage factor replaces the d/D ratio ().

Advantages

Low initial cost.

Disadvantages

Low differential pressure. Potential for failure es-pecially if misapplied. Ports facing flow pathrather than perpendicular to it may be susceptibleto clogging.

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6. Process Control

6.1 PRODUCT DESCRIPTION

Solar offers a control system to control the proc-ess for gas compressors. The system maintains a

desired process variable by automatically adjust-ing the speed of the turbine. For single turbinecompressor packages, depending on the optionschosen, control is available based on:

Suction pressure

Discharge pressure

Suction or discharge flow

A combination of any two of theabove variables

All three of the above variables

For multiple identical turbine compressorpackages operating in parallel, the same types ofcontrol are available, including equal load sharingbetween compressor trains.

Set points for the process variables may beentered at the unit control panel (UCP) or pro-vided to the UCP from a remote location via seriallink or hard wire connection.

6.2 SUCTION PRESSURE

A pressure-indicating controller (PIC) controls thesuction pressure of the compressor train. Under

this control mode, the gas turbine speed (NGP) ismodulated to control the suction pressure (PV) ofthe compressor train to the desired set point. ThePIC increases the NGP when the pressure isabove the set point. The PIC decreases the NGPwhen the pressure is below the set point.

The PIC uses a proportional-integral (PI) al-gorithm to modulate NGP. The gains (KP and KI)for the PI algorithm can be adjusted at the op-erator interface. The pressure set point can beadjusted locally at the operator interface. Alterna-tively, a remote pressure set point can be usedby the PIC. The local set point (LSP) or remote

set point (RSP) can be selected at the operatorinterface.Figure 13 shows the specific input / output

(I/O) used with the PIC.

6.3 DISCHARGE PRESSURE

A PIC controls the discharge pressure of thecompressor train. Under this control mode, the

Figure 13. Suction Pressure Control I/O

NGP is modulated to control the PV of the com-pressor train to the desired set point. The PIC de-creases the NGP when the pressure is above theset point. The PIC increases the NGP when thepressure is below the set point.

PIC uses a PI algorithm to modulate NGP.KP and KI can be adjusted at the operator inter-face. The pressure set point can be adjusted lo-cally at the operator interface. Alternatively, aremote pressure set point can be used by thePIC. The LSP or RSP can be selected at the op-erator interface.

Figure 14 shows the specific I/O used withthe PIC.

Figure 14. Discharge Pressure Control I/O

6.4 FLOW CALCULATORA flow calculator (FC) generates a volumetric flow(Q) based on inputs of pressure (P), temperature(T), pressure differential across the flow meter(hW), gas specific gravity (SG), gas compressibility(Z), and design pressure (PD), design tempera-ture (TD), design gas specific gravity (SGD), anddesign gas compressibility (ZD). The followingequation is based on AGA Report 3:

SPNGP

SP

R

E

M

-

L

O

C

PI

PV

PIC-6104 KP

PIC-6104 KI

PIC-6104 RSP

PIC-6104 LSP

PT-6104

GAINS

PIC-6104 OP

SPNGP

SP

RE

M

-

L

O

C

PI

PV

PIC-6102KP

PIC-6102KI

PIC-6102RSP

PIC-6102LSP

PT-6102

GAINS

PIC-6102OP

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Figure 15 shows the specific I/O used with

the FC. The units for Qare MMSCFD.

Figure 15. Flow Calculator I/O

6.5 FLOW CONTROL

A flow-indicating controller (FIC) controls the f lowof the compressor train. Under this control mode,the NGP is modulated to control the flow PV ofthe compressor train to the desired set point. TheFIC decreases the NGP when the flow is abovethe set point. The FIC increases the NGP whenthe flow is below the set point.

FIC uses a PI algorithm to modulate NGP.KP and KI can be adjusted at the operator inter-face. The flow set point can be adjusted locally atthe operator interface. Alternatively, a remoteflow set point can be used by the FIC. The LSPor RSP can be selected at the operator interface.

Figure 16 shows the specific I/O used withthe FIC.

Figure 16. Flow Control I/O

6.6 SPEED CONTROL

The speed-indicating controller (SIC) selects theNGP set point that will be used to control the fuelactuator. The SIC has two operating modes:auto and manual. The output of these two modesare inputs to the SIC fuel control.

6.6.1 Auto Mode

In auto mode, the NGP set point output from theprocess controller described above is input to arate limiter. If the compressor surge margin dropsbelow an operator adjustable set point (typically12 to 15%), then the rate limiter reduces the NGPacceleration and deceleration. The rate limitingprevents any interaction between the NGP con-trol and recycle valve that will be modulating tokeep the compressor away from the surge limitline. If the surge margin is greater than the setpoint, then NGP is allowed to accelerate or de-celerate at its maximum rate of 0.5% per second.

The NGP set point output of the rate limiter isan input to the load share controller (LSC) andsurge margin equalizer (SME). Refer to Sections6.11.1 and 6.11.2 for descriptions of these func-tions. If the LSC is off, then the NGP set point isinput to the fuel control algorithm. If the LSC ison, then the NGP set point is compared withother units in the same group and the maximumvalue is selected. This base NGP set point is in-put to the SME. If the SME is off, then the baseNGP set point is input to the fuel control algo-rithm. If the SME is on, then the surge marginequalization speed bias is added to the base

NGP set point and is then input to the fuel controlalgorithm.

6.6.2 Manual Mode

In manual mode, the NGP set point is adjusteddirectly by the operator (LSP) or by an RSP intothe control system. If the compressor surge mar-gin drops below an operator adjustable set point(typically 12 to 15%), then a rate limiter reducesthe NGP RSP acceleration and deceleration. Therate limiting prevents any interaction between theNGP control and recycle valve that will be modu-lating to keep the compressor away from the

surge limit line. If the surge margin is greater thanthe set point, then NGP RSP is allowed to accel-erate or decelerate at its maximum rate of 0.5%per second.

The manual mode NGP set point output is aninput to the LSC. If the LSC is off, then the NGP

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set point is input to the fuel control algorithm. Ifthe LSC is on, the other units in the group followthe unit's NGP set point. Note that in NGP Mastermode, the SME is automatically disabled. TheNGP set point is then input to the fuel control al-gorithm.

6.6.3 Fuel ControlIn the fuel control algorithm, the NGP set point iscompared with the maximum NGP limit based onT5, the minimum power turbine speed (NPT), andthe minimum NGP limit. The SIC selects the low-est set point from this group and generates theappropriate output signal to the fuel actuatorcontroller.

Figure 17 shows the specific I/O used withthe SIC.

6.7 PROCESS CONTROL USINGANTI-SURGE CONTROL VALVE

In addition to anti-surge control duty, the anti-surge control (ASC) can control the compressortrain suction and discharge pressures as well.

The ASC can be coupled with up to twoPICs: one for suction pressure and one for dis-charge pressure. The set point for the ASC suc-tion pressure controller is the set point of thestandard suction pressure controller subtractedby an operator-adjustable pressure offset. In turn,the set point for the ASC discharge pressure

controller is the set point of the standard dis-charge pressure controller added to an operatoradjustable pressure offset. Staggering the setpoints eliminates interaction between the speedand valve control loops.

6.8 SUCTION PRESSURE

The ASC suction PIC (designated here as PIC-6102A) will modulate the anti-surge valve (ASV) ifthe suction pressure drops below its set point.When PIC-6102A is in control of the suction pres-sure, standard suction pressure control (PIC-6102) will continue to reduce speed because itsset point has not been achieved. As PIC-6102reduces speed, PIC-6102A will detect a rise insuction pressure and begin to close the ASV.

Figure 17. Speed Control I/O

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PIC-6102 will continue to reduce speed until itsatisfies its set point, which will cause PIC-6102Ato close the ASV because the pressure will beabove its own set point. At this point, the controlsystem has recovered from the process transient.However, if PIC-6102 reaches the minimum NGPlimit before achieving its set point, then PIC-

6102A will continue to recycle gas to maintain itspressure set point.

Figure 18 shows the specific I/O used withthe suction PIC.

Figure 18. Suction Pressure Control I/O

6.9 DISCHARGE PRESSURE

The ASC discharge PIC (designated here as PIC-6104A) will modulate the ASV if the dischargepressure exceeds its set point. When PIC-6104Ais in control of the discharge pressure, standarddischarge pressure control (PIC-6104) will stillreduce speed because its set point has not beenachieved. As PIC-6104 reduces speed, PIC-

6104A will detect a fall in discharge pressure andbegin to close the ASV. PIC-6104 will continue toreduce speed until it satisfies its set point, whichwill cause PIC-6104A to close the ASV becausethe pressure will be below its own set point. Atthis point, the control system has recovered fromthe process transient. However, if PIC-6104reaches the minimum NGP limit before achievingits set point, then PIC-6104A will continue to re-cycle gas to maintain its pressure set point.

Figure 19 shows the specific I/O used withthe discharge PIC.

Figure 19. Discharge Pressure I/O

6.10 COMMAND TO ANTI-SURGE VALVE

The outputs of ASC suction PIC, ASC dischargePIC, and the ASC are, in turn, inputs to a mini-mum function that selects the most open valvecommand of the three controllers. This ensuresthat the ASV will open as necessary to satisfyany pressure or ASC requirements. The ASV will

close if the pressure and ASC requirements aremet. The final valve command is used as a resetfor the three controllers to prevent wind-up.

Figure 20 shows the specific I/O used withthe min. function.

Figure 20. Minimum Function I/O

6.11 MULTIPLE UNIT LOAD SHARECONTROLLER AND SURGEMARGIN EQUALIZER

A load share controller (LSC) and a surge marginequalizer (SME) manage the operation of multipleunits to ensure stable load sharing of the proc-ess. One LSC and one SME reside in each of theUCPs. A dedicated link is used for communica-tion of selected control variables.

Units can be organized into load share

groups. Each unit is assigned a load share groupnumber. This number identifies which group theunit is currently configured to operate in a load-sharing scheme. For example, in a five-unit sta-tion, two units share the same suction and dis-charge headers. These two units are defined tobe Group 1 and they load share. The remainingthree units share the same suction and dischargeheaders, different from the first two. These threeunits are defined to be Group 2 and they loadshare as a group. Later on, the two units areswitched over to operate in parallel with the otherthree. In this case, all the units' group numbers

are set to "2" and they load share as a group.The group numbers are defined by the operator.Up to four groups can be used.

6.11.1 Load Share Controller

The LSC has three modes of operation: off, on,and NGP master.

Off. When the LSC is off, the UCP ignores theactions of the other units. Likewise, the other

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units ignore the actions of the UCP. There is noload sharing.

On. When the unit is under automatic control(under the control of a PIC or FIC) and the LSC isturned on, then the unit load shares with otherunits in the group. The units communicate their

respective speed set point to the rest of thegroup. The highest of these is selected as thegroup's base speed set point. All the units in thegroup control on this base speed set point. Notethat as process conditions vary, the base speedset point will change to keep the process variablein line with its process set point.

NGP Master. When the unit is under manualspeed control and the LSC is turned on, then theunit becomes an NGP master. The other units inthe group will follow the speed of the NGP masterunit.

6.11.2 Surge Margin Equalizer

The SME has two modes of operation: off andon.

Off. When the LSC is off, the SME is automati-cally turned off. When the LSC is on, the SMEcan be turned on or off. When the SME is off, thebias added to the LSC base speed set point iszero.

On. When both the LSC and SME are on, then

individual biases are added to the base speed setpoints of each unit to equalize the group's com-pressor surge margins. The units communicatetheir respective surge margin to the rest of thegroup. In the case of multiple compressors, thelowest surge margin is used. An average of allthe surge margins is calculated and used as thegroup's surge margin set point. Units operatingbelow the surge margin set point automaticallyhave their speed set point biased up to 2%, whichincreases the surge margin. Note that the speedset point communicated to the LSC above doesnot include the speed bias generated by the

SME. Units above the surge margin set pointhave a bias of zero. A PI algorithm is used togenerate the speed bias of each unit. KP and KIfor the PI algorithm can be adjusted at the op-erator interface.

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Appendix A Yard Valve Sequencing

The control system will sequence the yard valvesduring start-up, normal operation, and shut-downs.

A-1 MILESTONES

Sequencing of the yard valves is associated withcertain milestones, which are defined below andused in Table A-1.

A-1.1 Manual Yard Valve Sequence

The operator has manual control of the yardvalves from the UCP. Manual operation of theyard valves is permitted only during pre-start.Manual operation of the suction and dischargevalves is permitted only when the compressor ispressurized.

A-1.2 Compressor Pressurized

The process piping and compressor are pressur-ized and the yard valves are positioned as shownin Table A-1.

A-1.3 Compressor Depressurized

The process piping and compressor are notpressurized and the yard valves are positioned asshown in Table A-1.

A-1.4 Start

A unit start is initiated when the following condi-tions have been met:

Start Command. The command to start the unitis given at the UCP or from a remote input.

Permissives OK. Interlocks from other systems,such as emergency shutdown (ESD) and fire andgas, must be OK to allow a UCP start.

Automatic Yard Valve Sequence. The UCPhas automatic control of the yard valves. Selec-tion of automatic and manual yard valve se-

quence modes is performed at the operator in-terface. When a start is initiated, the UCP placesthe yard valves under automatic control.

Compressor Seal System OK. The UCP acti-vates and checks the compressor seal system. Asuccessful check allows the start sequence tocontinue.

Engine Lube Oil System OK. The UCP acti-vates and checks the engine lube oil system. Asuccessful check allows the start sequence tocontinue.

A-1.5 Compressor Purge

The process piping and compressor are purgedthrough their respective loading valve and vent orblowdown valve for a period of five minutes (ad-justable).

A-1.6 Pressurize Compressor

Process gas upstream of the suction valve isused to pressurize the process piping and com-pressor through their respective loading valveupstream of the compressor. If during the com-

pressor pressurization sequence, the pressurizedstate is not reached within five minutes (adjust-able), then the start is aborted and the unit is faststopped pressurized.

A-1.7 Depressurize Compressor

Process gas is vented through the vent or blow-down valve(s) with the suction and dischargevalves closed.

A-1.8 Idle

The unit is operating at the NPT idle set point (50

to 60% typical) or the NGP idle set point (72 to78% typical), whichever is greater.

NPT Idle Set Point. The lowest operating speedrequired to avoid power turbine and drivenequipment critical speeds.

NGP Idle Set Point. The operating speed foridling the gas producer.

A-1.9 On Load Speed

The unit is operating above both the NPT load setpoint and the NGP load set point.

NPT Load Set Point. The lowest normal oper-ating speed for the driven equipment. For com-pressors, the set point is the last speed line of theperformance map.

NGP Load Set Point. The lowest speed re-quired for normal operation of the gas producer.

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A-1.10 Cooldown Lockout / Non-LockoutShutdown

A shutdown that causes the UCP to ramp the unitdown to idle speed. The unit remains at idle foran operator-defined time period (typically 10 min-utes) before the UCP automatically fast stops theunit. If during the cooldown idle time the operator

acknowledges and resets the shutdown, then thecooldown timer is reset to its full period. If duringthe cooldown idle time the shutdown conditiongoes away and the operator acknowledges andresets the shutdown, then the unit may be re-loaded. A lockout type shutdown does not allowremote acknowledge or reset of the shutdown. Anon-lockout type shutdown allows remote ac-knowledge and reset of the shutdown.

A-1.11 Fast Stop Lockout / Non-LockoutShutdown

A shutdown that causes the UCP to close the fuelvalve to stop the unit. A lockout type shutdowndoes not allow remote acknowledge or reset ofthe shutdown. A non-lockout type shutdown al-lows remote acknowledge and reset of the shut-down.

A-1.12 Fast Stop Pressurized Shutdown

The unit is stopped with the compressor pressur-ized (the vent and/or blowdown valves remainclosed). The unit remains pressurized during thepressurization hold time period (field adjustable).When the timer expires, the unit is depressurized.

A-1.13 Fast Stop Depressurized Shutdown

After the suction and discharge valves haveclosed, the vent and/or blowdown valves open.To minimize the flaring of gas, only the followingfast stop shutdowns will cause depressurization:

Compressor seal system failure

Engine lube oil system failure

Fire detected

Manual fast stop

A-2 SEQUENCING MATRIX

The yard valves are sequenced as in Table A-1.

A-3 VALVE OUT OF POSITION

The control system verifies the correct position ofall the valves described above with their respec-tive commands. If a valve fails to transfer or holdits position, then the valve is considered out ofposition (OOP). Valve out of position logic isgiven in Table A-2.

A-3.1 Valve Out of Position Checks

There are three types of possible valve checksthat are performed:

Both Limit Switches On (ZSC & ZSO On)

If the valve's open and closed limit switches areboth on, then the specified action is initiated.

Command versus Limit Switches(Cmd vs ZSC/ZSO)

If the valve's open or closed limit switch is incon-sistent with the fully open or fully closed com-mand for the specified time delay, then the speci-fied action is initiated. After the specified timedelay, if a valve falls out of position when its re-spective command is static at fully open or fully

closed, then the specified action is executed im-mediately (without the time delay).

Command versus Position (Cmd vs Pos)

This criterion applies to valves with positiontransmitters. If the valve position feedback is in-consistent with the command ( the hysteresis)for the specified time delay, then the specifiedaction is initiated.

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Table A-1. Yard Valve Sequencing

LV

SuctionLoadingValve

SV

Sucti