Recip Compressor Capacity Control Methods

7/26/2019 Recip Compressor Capacity Control Methods

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Engineering Practice

CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2012 51

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here are two main reasons whycompressor capacity regulationis used. The most prevalent rea-

son is to adjust the suction flowto match the process demand. The sec-ond reason is to save energy. As a rule,capacity control is determined by thecompressor discharge pressure. Com-pressor capacity-control methods areutilized to maintain a required deliveryunder variable process conditions [1].

In this article, methods for control-ling the capacity of reciprocating com-pressors are presented in detail. Thesemethods are bypass recycling, un-loader valves, clearance pockets, step-

less valves and variable-speed drives.The advantages and disadvantages ofeach method, as well as their applica-tions, are presented.

In addition, auxiliary devices andpackages used under transient condi-tions (namely, during startup, shut-down and maintenance) and duringnormal operation are described. Bar-ring devices (gear), hydro-jackingsystems, frame lubrication systemsand cylinder lube oil systems are ex-plained. Finally, a recommended pro-cedure for starting up and stopping areciprocating compressor in single andparallel operation is presented, along

with some major points that shouldbe taken into consideration by plantoperators.

Capacity control methodsBypass control.As the name implies,this control method uses an externalbypass around the compressor to recy-cle gas from the compressor discharge

to the inlet, or to the atmosphere inthe case of an air compressor. Thetake-off point for the bypass must bedownstream of a heat exchanger sothat cooled gas will be spilled back tothe suction. If there is no exchangerin the discharge, the bypass mustbranch into the suction line upstreamof an exchanger.

Alternatively, a cooler may beplaced in the bypass line. In any case,the bypass should tie-in upstreamof a suction knockout drum so that

any condensate resulting from theexpansion cannot enter the compres-sor. Bypass control is preferred overother control methods because of itssmoothness, simplicity and low ini-tial costs. It is, however, inefficientbecause excess compressor capacity isexpanded across the control valve inthe recycle line. For this reason, thismethod is commonly accompaniedby the use of inlet valve unloaders orclearance pockets, which reduce com-pressor capacity in discrete predeter-mined steps [15].

In multistage compressors, a bypassaround the first stage or a partial by-

pass can be used. Because of decreas-ing discharge pressure, the absolutepower input would be reduced in thisway, although the mass flowrate is

increased as shown in Equations (1)and (2).

(1)

(2)

Where:W = Compressor power

m

= Rated mass flowMw = Gas molecular weightp2/p1= Compressor ratioK = Gas specific-heat ratioTin = Gas inlet temperaturePD = Compressor piston

displacementQ = Compressor flowC = Compressor clearance

volumeR = Ideal gas constantIt must be remembered that a reduc-tion in the flow to the second stage

causes a drop of all inter-stage pres-sures, and consequently it can lead toexcessively high-pressure ratios anddischarge temperatures in subsequentstages. Moreover, this pressure shift-ing may cause an overload in the laststage. Thus, there should be a limita-tion on the maximum by-passed flowin multistage equipment. The mini-mum capacity that can be obtaineddepends on the number of compres-sion stages.

The more stages used for a givenoverall compression ratio, the widerthe achievable control range.Inlet valve unloaders.Valve unload-

Feature ReportEngineering Practice

Ali Ghanbariannaeeni

Nargan Engineersand ConstructorsGhazalehsadat Ghazanfarihashemi

Sazeh Consultants

Reciprocating Compressors

Startup and CapacityControl Methods

Procedures and guidelines that will help operators

minimize energy consumption and maintenance

requirements of compressor systems


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52 CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2012

ers are mechanisms that are held openor bypass one or more cylinders inlet

valves at each end of double-actingcylinders. This provides complete un-

loading of one or both ends of the cyl-inder. For a single-cylinder compres-sor stage, valve unloaders can achievethree-step unloading that providesnominal cylinder capacities of 100, 50and 0%; and two-step unloading thatprovides cylinder capacities of 100and 0%. Thus, considering two cylin-ders per compressor, the capacity canbe controlled in five stages 100, 75, 50,25 and 0%.

Three-step (100, 50 and 0%) capacitycontrol is used on some general pur-

pose machines, such as air compres-sors. This method of control requiresmore cyclic actuation of the unload-ers than five-step control. Therefore,three-step control is more detrimentalto machine components especiallyto valves.

Standard API 618 [6] introducesthree unloader types: finger, plug andport. The finger type unloader is notrecommended when other types are

viable. Plug type unloaders shouldbe installed on extra suction ports. A

minimum number of valve unloadersteps should be considered in order tomaximize compressor reliability.

Unloaders should be pneumati-cally operated by instrument air andequipped with positioner indication.The unloader actuator should be sizedto operate on minimum air pressureas well. Nitrogen purge ports shouldbe provided for all types. The unloaderstem packing should be provided witha lantern ring and a nitrogen connec-tion for the purging of leak gas. All

purge and vent connections should bepiped up to a single purge and ventconnection. All lines to and from un-loaders are to be AISI 316 L stainlesssteel [6, 7].

It should be noted that there aresome restrictions on actuator sizing atsuction pressures higher than 70 barg.Clearance pockets. Cylinder de-livered capacity can be reduced byincreasing the cylinder clearance

volume; this is done by a clearancepocket. There are two types of pockets:

fixed and variable. Opening the pocketreduces the cylinders inlet volumetricflow by trapping additional gas in the

larger clearance volume at the end ofthe piston stroke. Consequently, clear-ance-pocket flow adjustment is fre-quently applied to the head end only,

leading to a maximum of 5060% re-duction in the head end capacity and a2530% overall capacity reduction ina double acting cylinder [8].

Valve unloaders combined withclearance pockets can provide five-step unloading leading to nominal cyl-inder capacities of 100, 75, 50, 25 and0% in a compressor with only one cyl-inder. This means that 75% capacityis achieved by opening the head-endclearance pocket, 50% load makes useof the crank-end valve unloader, 25%

load makes use of the crank-end valveunloader and head-end clearancepocket simultaneously and 0% load isachieved by activating the valve un-loaders on both ends of the cylinder.

At compression ratios below 1.7, thepocket volume becomes very large rel-ative to cylinder size, thus, it shouldnot be used in this case [2].

Clearance pockets can be arrangedfor local manual operation, manual/pneumatic operation, or automatic/pneumatic operation. However, the au-

tomatic/pneumatic type is preferred.Using clearance pockets is usually

restricted to services with cylinderinlet pressures less than 70 barg.Stepless capacity control. A step-less capacity control system, known asreverse flow control, can provide a ca-pacity range of approximately 100 to40% of rated capacity; however, moreturndown ratios have become feasiblerecently. In this system, an unloadingdevice is fitted to each suction valve.

At partial loads, the unloading device

does not allow the inlet valve to closewhen the piston is at its bottom deadcenter position, but rather delays theclosing in a controlled way. Conse-quently, an amount of gas, which canbe adjusted, is allowed to return tothe compressor inlet manifold beforethe compression starts. As the piston

velocity increases in the compressionstroke, it pushes gas in reverse flowback out the inlet valve faster andfaster. This creates a larger and largergas drag force on the inlet valve plate

that eventually overcomes the un-loader force and closes the inlet valve.

Applying this type of control requires

economic justification. In general it isnot recommended because of limita-tions in performance accuracy and fa-tigue consequences on the valve plate

and seat [7].Inlet throttling. This method of ca-pacity control is not recommendedbecause pressures lower than normalinlet pressures cause higher piston-rod loads and discharge temperatures.Consequently, the mass flow rangethat can be safely achieved in this re-gard is normally too low to suit pro-cess needs.Variable speed drive. Reciprocatingcompressors should normally be speci-fied for constant-speed operation in

order to avoid excitation of torsional,acoustic and mechanical resonances.When variable-speed drivers are used,all equipment should be designed torun safely throughout the operat-ing speed range, up to and includingthe trip speed [6]. Gas engine drives,which could operate in a 75 to 100%speed range, make use of variablespeed method for flow control. Opera-tion below 75% is normally done bybypass control and inlet valve unload-ers or clearance pockets [912].

Startup unloading. Some meansof unloading for startup is usuallyrequired because drivers lack suf-ficient torque to accelerate the trainrotating parts under positive displace-ment compression loads. Inlet valveunloaders and external bypasses arethe two most commonly used methodsfor startup unloading. The machineshould be capable of starting with op-erating suction pressure on each stage.

An unloading system may be designedto operate automatically on a voltage

dip for service continuity where auto-matic reacceleration of motor drivencompressors is required.

Compressor operation modesAuto mode. In the automatic (auto)mode of operation, the compressormeets the demand of all the main pro-cesses, including all cases of operationrequested by the equipment datasheet.In some cases, operation with nitrogenis also requested for pre-commission-ing, cleaning purposes, testing of theautomatic functions and leak testingof other equipment. In this mode, themachine is controlled automatically.


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When the compression gas is changed,set point values are only adjusted forthe new condition with no change incontrol procedure.

Maintenance mode. In this mode,during which the compressor is atstandstill, all auxiliary drivers (suchas auxilliary oil pump, water pumps,and bypass valves) can be manuallystarted by operators.Off mode. In this mode, all driversare out of operation, and standby func-tions are disabled. This mode will beactivated only when the compressor isnot running; otherwise, selecting theoff mode will have no effect on theoperation of the compressor.

Mode selection between auto, offand maintenance is done within thedistributed control system (DCS). Acompressor is provided with severallocal instrument boards (LIBs), withgages for indicating pressure, tem-perature, flowrate and level of processgas, lubrication oil and other utilities(such as water, air and nitrogen), dur-ing compressor operation in all modes.

Additionally, the LIB shows whetherlocal motor-control stations are alsoavailable for local starting and stop-

ping of auxiliary equipment in main-tenance mode. Auxiliary motors alsohave local motor-control stations lo-cated within a few meters from the as-sociated auxiliary-drive motor. Visual-ization and control of the compressoroperation is normally done from a cen-tral control room (CCR) station.

Compressor auxiliary systemsHydro jacking device.For large re-ciprocating compressors and motors,manufacturers consider adding an ad-

ditional system to the equipment inorder to lessen the breakaway torquein the bearings in transient conditions(startup, shutdown or maintenance).For this purpose, one oil pump is usu-ally added in series after the lubrica-tion pump. A jacking oil pump assureslasting oil film existence betweenbearing and journal (shaft) surfaces atthe instant a rotation commences.Barring device. The compressorshould be fitted with a barring device(barring gear), which can be manual,electrical or pneumatic. This device isused only for maintenance, while thecompressor is stopped. For compres-

sors with a rated power equal to orgreater than 750 kW (1,000 hp), andtorque requirement equal to or greaterthan 1,600 Nm (1,200 ft-lb), either the

electrical or pneumatic type should beconsidered. Manual barring may beachieved by turning the flywheel. Thebarring device should be designed sothat gas forces acting on the pistonscannot cause the compressor either toaccelerate or to rotate in the reversedirection. With the barring deviceengaged, it should not be possible tostart the compressor [6].

Manual barring devices should havea locking feature, whereas pneumaticand electrical devices should incorpo-

rate appropriate safety switches ormeasures for interlocking with themain driver. Moreover, a pneumaticsystem should be equipped with anair-supply inlet filter and oiler, and afour-way control valve for operation inforward or reverse directions.

The barring device should be de-signed to enable barring over thepressurized machine at suction pres-sure. Before the barring device is en-abled to start, the hydro jacking ofcompressor and main motor have to

be started, and the minimum pres-sure must be obtained.

The barring device operates with alocal push button (forward and reverse)on the local panel near the motor. Thebarring device motor works only whileone of the push buttons is pressed; re-lease of the button stops the barringdevice and locks the crank gear. Whenmaintenance is finished, the stoppedbarring gear must be moved back inthe turn wheel free position, which isdisplayed on the DCS system.

Compressor frame lubricationsystem. A frame lubrication systemis pressurized through two electri-cally driven oil pumps. One is usedfor normal operation (main pump)and the second is in standby modeas an auxiliary pump for necessaryconditions. A crankshaft-driven mainpump is allowed per API 618, but forlarge machines, each oil pump shouldbe individually equipped with anelectrical motor.

In auto mode, the main oil pump

has to be started before starting thecompressor main motor. Generally,pre-lubrication time is set for approxi-

mately one minute. When pre-lubrica-tion time has passed, the startup con-dition Pre-lubrication time passed isactivated and displayed on the DCS.

When neither the low-lube-oil pres-sure alarm nor the low low-lube-oilpressure alarm is activated, the hydro

jacking pump and subsequently themain motor are permitted to start.During normal operation of the com-pressor, the standby oil pump shouldbe in auto mode and start automati-cally when a low-oil-pressure alarm istriggered on the DCS.

If the low low-oil-pressure signallasts more than a specified time (usu-ally 2 s), the compressor main motor

should be stopped immediately by ashutdown (trip).

Since the compressor is stopped nor-mally, post lubrication is started; thismeans that the lubrication oil pumpwill continue to run for a minimum ofapproximately 2 min. When post lubri-cation time has passed and the normalstop sequence is accomplished, the oilpump can either be stopped by the op-erator or continues running. Note thatin maintenance mode, the frame oilpump can be started individually.

Moreover, the operator can enablethe oil heater by pushing a button inthe DCS. If the heater is enabled, thetemperature of the frame lubricationoil is controlled by the switch control-ler inside the heater. Note that thecrankcase oil temperature should notexceed 70C (160F).

For safety reasons, the heater isinterlocked and must be switched offwhen the low low oil level is acti-

vated. It should be noted that duringcompressor operation, the heater shall

always be stopped.Compressor cylinder lubricationsystem. The cylinder lubricationsystem is pressurized by electricallydriven multiple-plunger-type oilpumps (plunger per point) or by thedivider block method. However, usinga divider block system is not recom-mended because of complexity andlow reliability.

The cylinder lubrication system willbe started locally if the cylinder oil-tank level is fulfilled (above low level).

When the pre-lubrication time (ap-proximately 2 min) has elapsed, thecompressor start condition cylinder


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pre lubrication time passed is ful-filled and displayed in the DCS, andthe over lubrication time starts. Thetotal time of the cylinder lubrication

pumps running without the mainmotor running is cumulative via cyl-inder lubrication time.

The compressor main motor mustbe started within the over lubricationtime (approximately 5 min) to preventaccumulation of oil in the cylinder.

If the total over-lubrication time haselapsed and the main motor is still notrunning, a compressor main-motor-start inhibit will be activated and analarm remove oil collected in cylindersbefore re-start appears on the DCS.

Resetting the over lubricationtime alarm will be done by slowly ro-tating the compressor for a number ofrevolutions (usually two or three) by abarring device to distribute the accu-mulated lube oil among the compres-sor cylinders. At this time, the alarmcan be reset in the DCS manually.

During cylinder barring-over ormaintenance operations, the cylinderlubrication should not be activated.

At compressor normal stop, postlubrication time starts for approxi-

mately 2 min, and the cylinder oilpump switches off automatically af-terwards. By pressing the emergencystop, the cylinder oil pump stops im-mediately and no post lubrication iscarried out. The cylinder lubrication-oil pumps should run while the com-pressor main motor is running. In theevent of low flow, an alarm will be trig-gered in the DCS and in the case oflow low-flow, compressor trip actionwill be activated.

Moreover, the operator can enable

the oil heater by pushing a button inthe DCS. If the heater is enabled, thetemperature of the cylinder lubrica-tion oil is controlled by a switch con-troller inside the heater body. The heatdensity of the heater should be limitedto 2.3 W/cm2 (15 W/in.2). A tempera-ture switch prevents overheating. Anover temperature switch activatesan alarm and switches off the heater.For safety reasons, the heater is inter-locked and must be switched off whenthe low low-oil level is activated.Compressor cooling water system.Minimum cooling water flow to thecylinders and packings is one of the al-

lowed conditions for starting the mainmotor. As long as low flow is detectedduring normal running, an alarm willbe triggered in the DCS. In a closed

cooling cycle, the system is pressurizedthrough two electrically driven waterpumps. One pump is used for normaloperation (main pump) and the secondis in standby mode as an auxiliary fornecessary conditions. For the compres-sor to get a permissive start, it is nec-essary to run the main water pump inorder to deliver a minimum requiredflow of cooling water. Auxiliary waterpumps can be started, as long as thewater-pressure or low-flow alarm isactivated in the DCS.

Main motor purging system. Themain-motor purge system is an inde-pendent working system. In EExpdrivers, the motor becomes explosion-proof by pressurizing its cage. Themain drive motor should be startedonly if it has been purged with air fora period of time specified by the motormanufacturer. (Purge time is approxi-mately between 30 to 60 min).

Preparation before startingBefore the compressor is allowed to be

started,the following checks and con-ditions have to be fulfilled in the field:

Check if the cooling-water supplyvalves are open (for inter-stage gas-and oil-system coolers). Moreover, thecooling water specifications, such asflowrate, temperature and pressure,should be verified with design con-ditions by local instrument devices.Check the oil level in the tanks; it hasto be above the minimum level on theoil sight glass.

Check if the lube oil systems are

ready for operation and the manualvalves are in the correct position.

Drain valves of all process pipes andvessels (including pulsation dampen-ers, inter-stage coolers and separa-tor drums) have to be checked for thepresence of liquid. If liquid is present,it should be drained. Otherwise, liquidwould be carried over to the cylinderand damage the compressor whenstarting the main motor.

Check if all utilities including in-strument air, hydraulic oil (for high-pressure valves actuation) and nitro-gen for purging of pressure packingsare available.

The main drive motor should bestarted only if it has been purged andthe pre-lubrication time of the crankgear and cylinder have passed. This

should be considered at the time ofstarting the compressor.

Check the operability of all voltagecircuits and the shut down system.

The oil pumps can be started at min-imum ambient temperature, whereasthe compressor only when the lube oiltemperature is above the minimumtemperature recommended by themanufacturer (about 15C). Hence,frame and cylinder lubrication heaterand tracing system should be on (tem-perature start permissive of motors).

The compressor should be barredover by a barring device (for two tothree revolutions) to ensure that liq-uid is not present in cylinders.Start inter-lock system.Before com-pressor startup, several parameters(such as level, pressure, temperatureand flowrates) are checked and com-pared with set point values, specifiedby the compressor manufacturer in theset point list (set point lists include allinstrument devices set values specify-ing alarm or trip signals). Afterwards,

compressor start permission can bepassed by the DCS and the next stepwill be executed. Note that interlockswill only prevent the start sequencefrom continuing, or will interrupt thestart sequence unless all specifiedconditions are fulfilled; interlocks arenot trip signals.

Depending on the manufacturerand user concerns, an interlock sys-tem could be varied, but interlocks(alarm conditions) below are consid-ered in most applications. An alarm is

a signal generated automatically froman irregular state, which does not leadto a compressor shutdown.

The start sequence will be abortedif an alarm occurs. In other words,an alarm will inhibit the start of themachine as long as it is active. Alarmsand warnings do not stop the compres-sor. Causes of alarms must be inves-tigated and rectified immediately byoperators, or else they could cause themachine to trip (shutdowns). The com-pressor is ready to start if none of the

following alarms are active: Low lube-oil tank level Low cylinder oil-tank level


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Low suction pressure High discharge pressure Low packing-purge pressure Low cylinder lube-oil flow

Low crank-gear oil pressure Low crank-gear oil temperature High oil-filter differential

pressure Bypass valves over stages are

opened Barring gear is in safe position

(barring gear disengaged) High level in separators Low compressor cooling-system

flow Compressor has been barred over

for a minimum of 1 min (mandatory

if cylinder over-lubrication time haselapsed before)

Crank-gear lube-oil pump pre-lubri-cation time has passed

Cylinder pre-lubrication time haspassed

A trip is an irregular state that re-quires an immediate and automaticshutdown of the compressor in orderto avoid damage to the equipment andpersonnel. Note that prior to shutdownan alarm will be indicated in the DCSto warn the operators. The following

trips commence the normal stop pro-cedure at any time of operation.

The compressor is ready to start ifnone of the following shutdown condi-tions are active. The start sequencewill be aborted if a trip occurs. High high-suction-gas temperature High high-discharge-gas tempera-

ture Low low-gas pressure on suction

side High high-level in separators High high-main-bearing tempera-

ture High high-vibration on compressor

casing and rod drop low low-up-stream-bearings oil pressure

Low low-cylinder lube-oil flowStarting sequence of the compres-sor in auto mode. Regarding com-pressor-package control philosophy, itcould be started from the DCS or LCS(local control station). After startup, allfunctions will be carried out throughthe DCS automatically. Bypass valvecontrollers should be activated by the

operator when the compressor startsequence has been completed success-fully. For the compressor starting pro-

cedure to be deemed successful, theposition of the manual valves must beas follows: Gas suction isolating valves are

open Gas discharge isolating valves are

open Blow-off valve and vent valve to safe

location, flare, and atmosphere areclosed

Control valves and isolation valveon separator drain are open

Shut-off valves for pressure instru-ments are open

Bypass of separator drain is closed Separator drain valves have been

put in auto mode, and their relevant

interlocks are activatedThe preconditions for startup sequenceare as follows:1. All heaters, including oil heaters

and the motor space heater, must beswitched on.

2. Purge air must be available. If thepurge time of the main motor haselapsed, the message purge end isdisplayed in the DCS.

3. The compressor has been barredover. To do this, first the compressorsoperating mode should be changed

to maintenance mode in the DCS,and the operator in the field shouldengage the barring device manually.Then, the frame oil pump is startedand compressor/motor jacking oilpumps will be started (with delay)afterwards because barring deviceoperation is only possible as long asthe hydro-jacking inlet-oil pressurealarm is not activated. At this time,barring over should be enabled fromthe local panel by pressing two but-tons for operation of barring gear in

clockwise and reverse direction. Itshould be noted that, none of pre-lubrication time, low-oil-tempera-ture, or the lube-oil-pressure alarmsshould be activated. When barringover time (approximately 5 min) haselapsed, the operator should de-en-gage the barring device and removeany oil.

4. No low cooling-water-flow alarm.5. No low-level alarm on interstate

separators.6. No low-level alarm on oil tanks.

7. No high-temperature alarm on themain motor.

Permission to start. At this stage,

the frame oil pump is started, andthe jacking pumps of the main motorand compressor will be started if pre-lube time, low oil temperature and

low-pressure alarms are not active.Additionally, the following conditionsshould be satisfied: No low-suction-pressure alarm No high-discharge-pressure alarm Barring device in safe position (dis-

engaged)Next, the cylinder lubrication pumpwill be started if all preconditions arefulfilled. This will also start the cylin-der pre-lubrication timer. If the cylin-der pre-lubrication time has elapsed,the compressor start condition cyl-

inder pre-lubrication time passed isfulfilled and will be displayed in theDCS and over-lubrication time startsafterwards. The cylinder lubricationtimer should be programmed as a cu-mulative timer. If it is not reset, it willaccumulate the total time of the cyl-inder lubrication pump running whilethe main motor is not running up tothe over-lubrication time limit. Dur-ing this time range, it allows multiplestartups as long as they are within theover-lubrication time.

If the over-lubrication time haselapsed and the main motor is not run-ning, the start-up sequence will be can-celled and an alarm to remove collectedoil appears in DCS. Accumulated oil inthe cylinders has to be removed by turn-ing the barring device before anotherstarting sequence commencement.

Since cylinder lube-pump start is thelast step in the start sequence prior tomotor starting, impact on plant avail-ability is minimized.

If the main motor starts while over-

lubrication time has not elapsed, motorspace heaters and lube oil heaters willbe stopped at the same time. More-over, for approximately 2 min, the lowlow-pressure trip on the compressorsuction will be overridden to allow astable condition without tripping themachine. After that, the overriddenfunction will be deactivated.

The jacking oil pump will be stopped1 min after a successful start of thecompressor. In this position, the com-pressor is now running at no load con-

dition. In order to obtain the desireddischarge pressure for each stage,the bypass valve setting must be per-


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formed manually. For this purpose, thefirst stage bypass valve shall be en-abled to control suction pressure au-tomatically. Note that automatic con-

trol capability of all bypass valves willhave been deactivated during startup.

At this time, close the second-stagedischarge to first stages dischargebypass valve gradually until the de-sired second-stage discharge pressureis obtained. For the last stage, the de-sired pressure is obtained by closingthe discharge isolating valve. In thisway, the stroke position of the previ-ous bypass valves are slowly adjustedto control pressure between eachstage. Thus, the compressor runs in

0% capacity continuously and bypassvalve positions are set for 0% capac-ity (or 100% turndown). It is of highimportance to close the bypass valvesslowly and gradually to stabilize theconditions and prevent overshootingof discharge pressure.

Note that in compressors, whichare only started by bypass method,the starting procedure is performedwith bypass valves fully open. If anunloader valve also exists, the bypass

valves are fully closed and unloader

devices open the inlet valves, untilcompressor reaches the rated speedat 0% capacity. Due to increasing suc-tion-valve temperature in this condi-tion, the compressor is usually runbetween 5 to 10 min as maximum.

After that, the performance step is in-creased by loading each cylinder cham-ber in the sequence shown in Table 1for a compressor with two double-act-ing cylinders (HE = cylinder head end,CE = crank case cylinder end).

After a period of time (adjustable

between 5 to 15 min), in each perfor-mance step, solenoid valve activationon unloaders will be automaticallyswitched over. This switching pre-

vents increasing the temperature ofthe suction valve. Moreover, if pur-chase requisition states that the com-pressor shall run at 0% capacity for along time, the compressor should runat 25% step capacity, and total flow isrecycled by bypass valve thereafter.Due to unbalance matters, minimumload on each stage could be changed to

50%. Note that compressor mechani-cal stability must have priority overenergy consumption.

Bypass valvesIn this section, we investigatethe capacity control procedure ofa reciprocating compressor with

bypass valves in two stages. As aprinciple rule, each stage is con-trolled with one bypass valve,which is independent of the otherbypass valves, but valve set-points are adjusted together andfinalized during the compressorcommissioning period. Per APIrecommendation, the maximumpredicted discharge temperatureon each stage should not exceed150C (300F). This limit appliesto all specified operating and load

conditions. However, in actualdesignation, this limitation is de-creased to 135C (275F) by com-pressor manufacturers.

To control discharge tempera-ture, pressure ratio or differentialpressure is a parameter that isconsidered as a controlling value.Designers mention relevant set-tings in the instrument set-point-list document.

Figure 1 shows an example of atypical capacity-control diagram.

In this figure, PCV is pressurecontrol valve and PT is pressuretransmitter. The main objective ofthe capacity control is to maintainconstant suction pressure (PT1).The capacity of the compressor iscontrolled by bypass over the firststage (PCV1) and bypass overthe second stage (PCV2). If compres-sor suction pressure PT1 is decreased,the first stage bypass valve (PCV1)could be opened continuously up to apercent of stroke (between 50 to 60%)

until suction pressure (PT2) is abovethe setpoint of the minimum suctionpressure in the second stage.

If suction pressure (PT1) still falls,controller will open the second to firststage bypass valve PCV2. This bypass

valve will open continuously to 100%.Now the first stage bypass (PCV1) willbe able to control the main suction pres-sure (PT1) with PCV2 in parallel. Thus,the desired pressure is obtained by high-pressure gas, which is recycled throughfirst- and second-stage bypass valves.

When both valves are fully open,the compressor will operate in full re-cycle mode, until suction pressure is

increasing again. When suction pres-sure is increasing, the bypasses willclose in reverse.

Rapid valve movement causes rapidload change on the compressor. To pre-

vent this from happening, the bypasscharacteristic is linear and the stroketravel rate is approximately 2 min;however, this rate can be adjustedduring compressor commissioning.

Please note that, in case of highercapacity requirements, increasing thesuction pressure (PT1) could increasethe compressor capacity.

Bypass and unloading valvesIn most process applications, compres-sor capacity control is established bya suction pressure controller actingon the bypass valve and step capacitycontrol (0, 25, 50, 75 and 100%). The

TABLE 1. LOADING SEQUENCEFOR A COMPRESSOR WITH TWO

DOUBLE-ACTING CYLINDERS

For 0% load (X = Loaded; 0 = not loaded)

Cylinder A Cylinder B

Time (min) HE CE CE HE

Maximum(5-10 min)

0 0 0 0

For 25% load (X = Loaded):


HE CE CE HE

0 X 0 0 0

5 0 X 0 0

10 0 0 X 0

15 0 0 0 X

20 Return to 0 Min.




0 X 0 X 0

5 0 X 0 X

10 Return to 0 Min.




0 X X X 0

5 0 X X X

10 X X 0 X

15 X 0 X X

20 Return to 0 Min.For 0% load (X = Loaded):


HE CE CE HE

NoLimitation

X X X X


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choice of the step depends on the set-points of the bypass valves.

Up-stepping is one-step increasingin capacity, for example, 25 to 50% anddown-stepping is one step decreasing,for instance, 75 to 50%.

The suction pressure controllerreaches a balance in opening or clos-ing the bypass valve depending onactually adjusted step load (02550

75100%) and actual flow through thecompressor. If the actual bypass flowis smaller than 30% and higher than5%, no up- or down-stepping action isrequired in the step capacity control.This means that suction pressure isonly controlled by the stage bypass

valve in this case.If bypass flow is lower than 1% or

for a time duration (approximately3 min) lower than 5%, bypass flow is

very small, and therefore, the suctionpressure controller is not able to main-tain suction pressure with fully closedbypass. The step capacity control hasto make the up-step, increasing the

compressor performance over 25% byadjusting the next higher step.

Increasing compressor performancewill cause a momentary suction pres-sure drop. Hence, time delay (1 min)is considered to prevent activationof low or low low-suction-pressurealarms or trips. After a certain time,suction pressure will reach stabilityagain and restore the suction pres-

sure to its normal value.If bypass flow is higher than 35%

or for a time duration (approximately3 min) higher than 30%, bypass flowis very high and the compressor iswasting energy. To avoid unnecessarybypass flow, the step capacity controlshould perform down-stepping, de-creasing the compressor performanceby 25%. Decreasing compressor per-formance will cause a pressure peakin suction. Consequently, a time delay(approximately 1 min) should be con-

sidered to prevent activation of highor high high-suction-pressure alarmsor trips. After passing a certain time

PT1

PCV1

Cooler 1

Stage 1

PT2

PCV2

Cooler 2

Stage 2

FIGURE 1. Shown here is a typical setup for capacity control in a two-stage re-ciprocating compressor using bypass valves (PCV = pressure control valve; PT =pressure transmitter)

TABLE 2. CONDITIONS FOR UP- AND DOWN-STEPPINGPARALLEL COMPRESSORS

Up-stepping:

Compressorload B (Slave)

Compressor load A (Master)

25% 50% 75% 100%

25% Upstep A Upstep B Upstep B Upstep B

50% Upstep A Upstep A Upstep B Upstep B

75% Upstep A Upstep A Upstep A Upstep B

100% Upstep A Upstep A Upstep A No Action

Down-stepping:

Compress orload B (Slave)

Compressor load A (Master)

25% 50% 75% 100%

25% No Action Downstep A Downstep A Downstep A

50% Downstep B Downstep B Downstep A Downstep A

75% Downstep B Downstep B Downstep B Downstep A

100% Downstep B Downstep B Downstep B Downstep A

for example:

Sludge drying

Glycol recovery

Used oil recovery

Lubricant recycling

Environmental Applications

Waste

toma

s

e

om

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Circle 15 on p. 72 or go to adlinks.che.com/40271-15


8/8



duration, the suction pressure willreach stability again and restore thesuction pressure to its normal value.

The lowest automatic down-step

will be to 25%. The 0% performancestep is only used in compressor start-up and shutdown sequences.Compressor parallel working.Gen-erally, 10% increasing flowrate is per-formed by increasing suction pressure,but in many applications, two compres-sors are needed to send out enoughflowrate downstream of the system.For this purpose, one of the compressorcontrollers will be master and the otherthe slave. The master takes the controldecisions about suction pressure con-

trol and up- and down-stepping of thecapacity. A slave compressor alwaysfollows the master decisions.

To start two compressors, when themaster machine is stable at 100% ca-pacity, the slave machine should bestarted in 0% load. Then, during severalsteps, performance of both are equaledas much as possible. In this regard, ineach stage, 25% of master compressorcapacity (one step) will be decreasedand 25% capacity of slave machine willbe increased subsequently. Meanwhile,

enough time delay (approximately1 min) between each step should beconsidered so that flow stability is ob-tained at suction of compressors.

The conditions for up- and down-step criteria are the same as for singlecompressor operation and are shownin Table 2.

If the operator decides to take outone of the two running compres-sors, the selected compressor will bestopped with the following automaticsteps with a time delay (approxi-

mately 1 min):The load of the selected compres-

sor will be decreased by 25%. At thesame moment, the load of the com-pressor that is intended to remain inoperation will be increased by 25%.This method will be repeated until theselected compressor is completely un-loaded (at 0% capacity).

Stopping the compressorGenerally, three compressor-stopmethods are considered regarding thepermitted overhaul time of compres-sion units and the safety level of themachine for plant protection.

Normal compressor stop.A normalshutdown is manually initiated bythe operator from the DCS or LCS, ifthe compressor needs to be stopped

for reasons such as overhauling themachine or unit maintenance. Thefollowing actions will be taken auto-matically afterwards: All bypass valves will be opened by a

slow ramp up to fully open position.Note that ramp shall be set slowlyenough to avoid over pressurizationof lower stages downstream of thebypass valves

The hydro jacking pump of the mainmotor and the compressor will bestarted

The main motor will be stopped, andall control valves will be de-ener-gized and return to their fail posi-tion, especially bypass valves, whichwill be closed

The isolating suction valve will beforced closed, and the frame lube-oil pump and cylinder-oil pump willcontinue to run after post-lubrica-tion time

Oil tank heaters and the motorspace heater will be enabled

At last, the discharge isolating valve

will be closed and the compressorwill be manually depressurized byrelief valves (vent valves) installedin each stage

Automatic stop based on trip.Theautomatic shutdown of the compressoris used to avoid damage of equipmentand to ensure personnel safety. Thisstop sequence is exactly the same asa normal stop, except that the bypass

valves are not opened at the first step.Emergency stop. In case of danger,manual actuation of the emergency

push buttons, located around the ma-chine or on the emergency shut down(ESD) panel, shall switch off all theelectrical consumers (main motor, oilpumps, heaters, solenoid valves on by-pass valves and so on). This stop se-quence is similar to automatic trip stopexcept that no post lubrication is neededby frame and cylinder oil pumps.

Final remarksMost reciprocating compressors arespecified for constant speed operationto avoid excitation of torsional criti-cal speeds. For all constant speed ap-plications, it is recommended that an

automatic bypass control be provided.For more flexibility of the system, anunloader valve or pocket may be fur-nished to decrease power loss during

turndown capacity. Moreover, if thestepless method is employed, it shouldbe supplemented with a bypass con-trol arrangement.

Edited by Gerald Ondrey

References1. Bloch, Heinz and Soares, Claire, Process

Plant Machinery, 2nd ed., Elsevier Science& Technology Books, November 1998.

2. Bloch, Heinz P. and John J. Hoefner, Recip-rocating Compressors Operation & Mainte-nance, Gulf Publishing Co., 1996.

3. Bloch, Heinz P., A Practical Guide To Com-pressor Technology, 2nd ed., John Wiley andSons, 2006.

4. Hanlon, Paul C., Compressor Handbook,McGraw-Hill, N.Y., 2001.

5. Chlumsky, Vladamir, Reciprocating and Ro-tary Compressors, SNTL- Publisher of tech-nical Literature, 1965.

6. Reciprocating Compressor for Petroleum,Chemical and Gas Service Industries, API618 5th ed., December 2007.

7. Southwest Research Institute, AdvancedReciprocating Compressor Technology, De-cember 2005.

8. Leonard, Stephen M., Fugitive EmissionsControl Technology For Reciprocating Com-pressor Cylinders, Dresser-Rand, PaintedPost, N.Y.

9. Giampaolo, Tony, Compressor Handbook:Principles and Practice, the Fairmont Press,

2010.10. Forsthoffer, W.E., Forsthoffers Best Practice

Handbook for Rotating Machinery, ElsevierScience & Technology Books, 2011.

11. Forsthoffer, W.E., Forsthoffers RotatingEquipment Handbooks, Vol. 3, Elsevier Sci-ence & Technology Books, 2005.

12. Pichot, Pierre, Compressor Application En-gineering, Gulf Publishing Co. 1986.

AuthorsAli Ghanbariannaeeni is arotating equipment engineerat Nargan Engineers andConstructors Co. (Tehran CO15 98 98 3116, Iran; Phone:+98-21-88-908-104-8; Fax

+98-21-88-91-0173; Email:[email protected] [email protected]). He is specializedin reciprocating, centrifugaland screw compressors, gas

and steam turbines, process pumps, engines andelectric machines. He obtained a B.S.M.E. degreefrom Iran University of Science and Technology(Tehran, Iran).

Ghazalehsadat Ghazan-

farihashemi is a rotatingequipment engineer at SazehConsultants Co. (Tehran CO1587657413, Iran; Phone:+98-21-88-532-156-7; Fax+98-21-88-731-503; Email:[email protected] [email protected]). She is specialized in

reciprocating and centrifugalcompressors, process pumps,engines and electric machines. She obtainedB.S.ME. and M.S.M.E. degrees from Sharif Uni-

versity of Technology (Tehran, Iran).

Documents

Recip Compressor Capacity Control Methods