Turbocompressor Load Sharing Control

[rafcu]

Hello Guys,

I’m working on a new project where there are 6 turbo compressors in parallel, my problem is that the customer wants install a sophisticate Load Sharing Control (LSC) controlling driving operating limits in cascade with the inlet for each compressor via the compressor speed, the driver limits are: Turbine speed, exhaust temperature, air compressor speed, etc. From my opinion as the compressors are similar, for the LSC is enough, control for each compressor, the distance to the surge control line.

Could anybody advise about the subject?

In addition, does has any body experience related with the sequence to start up three turbo compressor in parallel?

Thanks,

Rafcu

 

[vzeos]

I’m not exactly sure what your intent is. I think you’re saying that you want to control each unit under a flow control mode with multiple control mode overrides based on turbine speed, exhaust temperature, air compressor speed, etc. In addition, you would like to use the surge line of the compressor map as the operating guide. I guess your units also will have an automated surge control system.

Assuming you are talking about a typical transmission compressor station, what you describe sounds like what could be the normal operation of a unit control system (UCS). However, this in and of itself is not sufficient. In addition to a UCS, you need to design a plant control system (PCS) that will coordinate the operation of your compressor units with the operating parameters of the plant. Even if you only had one compressor, you would still need a PCS. For example, when your units are on flow control, you need to make sure that your MAOP of the pipeline is not exceeded. You may need an override control mode that controls on station outlet pressure. You will need to design the PCS to communicate with the UCSs so that PCS will be able to regulate the units to maintain control of the outlet pressure. Similarly, you may need an override control mode that controls on station inlet pressure to make sure your units do not under pressure the inlet system. There are many more station parameters to be considered.

Your concern about start-up sequencing is appropriate. You should also be concerned about shut down sequencing as well as unexpected unit trips and station emergencies. The PCS should be designed to take these operating conditions into account as well as other plant safety controls.

 

[pmover]

yes,

i would definitely give load sharing control due and close consideration, including all possible operating scenarios (i.e. start-up, shutdown, 1, 2, 3, etc. unit operation). basically, lsc are for similar centrifugal compressor units operating in parallel mode and it is accomplished by rotor speed or otherwise. as far as the driver limitations, these limitations are controlled by the driver control system and not the surge and/or load sharing control system. the lsc simply sends output signal to driver for making adjustments in compressor operating point. one matter to consider is the piping system. this mainly pertains to the tuning of the control system, but if not planned/designed correctly, there may be operational constraints.

the mfg of lsc do have technical articles that explain the process of lsc and provide justifications. the mfg should be willingly provide this information.

just because the compressors are similar/identical, does not imply nor mean that all operating compressors will operate at the same operating point. there will definitely be subtle differences between the compressors and piping configurations which will have an impact on the operation of the compressors. imagine this scanerio by inspecting the compressor curves; compressor #1 operating at a higher speed and recycling gas and compressor #2 operating at a lower speed and not recycling gas. while this scanerio is rather extreme, it is a possibility should no lsc system be implemented and the operators are not paying attention to the unit operation.

lastly, please post your question in only one forum, that way there is minimal confusion the no redundancy.

hope this helps - please advise.

good luck!
-pmover

 

[jsummerfield]

There are varying levels of load sharing control. The simple versions just bring all machines up and down together. This may be based upon suction pressure, discharge pressure, flow or a combination of setpoints and override limits. The more complex versions adjust for the differing efficiencies, etc.

Often, the same organizations that market surge control for centrifugal compressors offer load sharing controls too.

John

 

[tgmcg]

Station suction pressure control cascaded with individual unit flow controllers would balance flows between units while maintaining constant station suction pressure. You could also provide a station discharge pressure override. Such a system would automatically compensate for different number of units online. Individual units should be sequenced up and down automatically in the usual manner with appropriate automated isolation, pressurization and blowdown valving. Each compressor train or body..depending on number of stages....should have it's own anti-surge control system and recycle loop.