Two Lead and Lag Reciprocating compressors control 1 stage each
Two Lead and Lag Reciprocating compressors control 1 stage each
(OP)
Hi,
I am not so experienced in this field and want to share some thoughts regarding some standard skids of 2 reciprocating compressors that are suppose to work as lead and lag: First, the lead is getting ramped up till getting 100% load and if the inlet pressure still raises then the second (lag) compressor is starting and keeping ramping till getting a stable pressure on suction. If the Lag goes under 50% load when pressure getting stabilized, them the lead is ramping down so both compressors are reaching the same load, let say around 60-70%, that actually fits the economic regime of the gas driven engine and energy saving. Attached is the sketch of the skid/system. From I&C point of view I am wandering if such a skid can be controlled by a single control panel, single PLC based, that also will be able to handle some of the utilities feedbacks. Compressors speed (actually capacity control) is controlled based on suction pressure PT by SC.
Thanks,
Alex
The Drawing
I am not so experienced in this field and want to share some thoughts regarding some standard skids of 2 reciprocating compressors that are suppose to work as lead and lag: First, the lead is getting ramped up till getting 100% load and if the inlet pressure still raises then the second (lag) compressor is starting and keeping ramping till getting a stable pressure on suction. If the Lag goes under 50% load when pressure getting stabilized, them the lead is ramping down so both compressors are reaching the same load, let say around 60-70%, that actually fits the economic regime of the gas driven engine and energy saving. Attached is the sketch of the skid/system. From I&C point of view I am wandering if such a skid can be controlled by a single control panel, single PLC based, that also will be able to handle some of the utilities feedbacks. Compressors speed (actually capacity control) is controlled based on suction pressure PT by SC.
Thanks,
Alex
The Drawing





RE: Two Lead and Lag Reciprocating compressors control 1 stage each
PT to PIC to master SIC to master load signal splitter then to individual engine SIC.
Feedback signal from each engine ST to individual SIC
Feedback from each engine ST to master load signal splitter also.
Master load signal splitter to contain these additional logics you've described - sounds workable.
RE: Two Lead and Lag Reciprocating compressors control 1 stage each
I have been about in the same wave of having the control done. Actually was thinking to have the PIC as master for both SIC, but more dedicated approach will just make more reliable the control as you have proposed. Have attached some sketches. On the bottom side are the 2 versions of controls in two different layout. The feedback though can be picked by master via bus from individual controllers of speed. On the uppers side of the sketch there are 2 options of the PLC architecture. 1st with one single PLC along with a Panel View (of course) and the 2nd with 3 PLCs with a Panel View or Working Station as well. I think the 1st option could be considered moneywise at the most convenient than the option 2. Taking in counts that for option 2 all 3 PLC shall be SIL rated.
Thanks for your input and time.
Alex
RE: Two Lead and Lag Reciprocating compressors control 1 stage each
In case the two units are different, you need to work out the load-efficiency curves and see what is the best load sharing concept for this particular application.
Your sketch does not show everything, so I guess you have covered other important subjects, i.e.: how the lead/lag operation is configured (set points for pressure switches), overspeed protection etc.
Get a copy of Bela Liptak Instrument Engineers' Handbook, Volume 2, Paragraph 8.15 "Compressor control and optimization". It contains some great deal of information.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Two Lead and Lag Reciprocating compressors control 1 stage each
Thanks,
Alex
RE: Two Lead and Lag Reciprocating compressors control 1 stage each
Error in my previous post on first line
PT to suction PIC
to master SICto master load signal splitter then to individual engine SIC.Also, you also need a low suction pressure controller for the case when both compressors are at min speed and suction pressure continues to drop - this should open the overall recycle valve to keep suction pressure at the low suction PC setpoint.
Further, there should be a discharge PIC which overrides the compressor load controls through an auto sginal select which selects either the suction PC output signal or the discharge PC output signal. So revised first line will be
PT to suction PIC to signal select to master load signal splitter then to individual engine SIC.
Signal select to autoselect between suction PIC or discharge PIC.
Hope you know how to set up these 2 PICs in order to enable the auto select
When discharge PIC is running the compressors, and compressors are at min speed by signal from this PIC, then the discharge pressure continues to rise, then the recycle valve should be opened. Hence the recycle PCV receives its command signal from either low suction pressure PIC or high discharge pressure PIC via an auto signal select.
Also this means the discharge PIC has to be a split range controller - one range to operate the load signal for the compressors, and the other range for operating the recycle PCV.
Also, a high suction PIC may be required to cut back the source feeds automatically for the case when source supply flow is higher than permitted compressor capacity at that time.
Process controls never fail to bring up suprises, so keep working on this scheme and watch out for complications during programming due to misinterpretation and/or poor definition of the process control narratives or code language limitations.
RE: Two Lead and Lag Reciprocating compressors control 1 stage each
a) The master SIC shown on your sketch is the master load signal splitter in my posting
b) Dont have working knowledge on actual wiring of these signals - whether to use serial links or FFB etc. etc - this is the working responsibility of the plant control systems instrumentation engineer - there may be others in this forum who can help.
As Dejan says, add on for other controls functions and keep building up a controls and safeguarding sketch that gives you a birds eye view of everything you need to know from a controls / safeguarding perpective.
RE: Two Lead and Lag Reciprocating compressors control 1 stage each
RE: Two Lead and Lag Reciprocating compressors control 1 stage each
To trip this compressor skid when pressure reaches 40barg at pipeline inlet may be one way to run this unit. Another way would be to use a discharge PIC which could be used to reduce the speed of the upstream compression unit. Then we wont need to trip this new compression train / existing compression unit. If this is of interest, you could use this discharge PIC low and high range signals to operate the speed and recycle valve of the upstream compressor.
RE: Two Lead and Lag Reciprocating compressors control 1 stage each