BKU Type Chiller - Propane Temperature Control Loop[

[Arash Fakharnezhad]

thread124-402307

Hello everybody

I have read in the earlier thread (thread124-402307) about BKU Type Chiller Propane control loops that there are two basic process control strategies for the propane side of this exchanger. (vary the liquid level and maintain a fixed pressure or vary the pressure and maintain a fixed liquid level)
Is there anyone who can describe these control loops in detail?
I wanna know, how it is possible to control the stream outlet temperature at specific temperature via these propane control loops?

Also, I have attached the typical of BKU control loop! I wanna know that, is it correct or not?
Thanks
Arash

 

[georgeverghese]

Yes, you've got the second process control scheme with TC varying kettle pressure. Your post is sufficient description of the scheme. Assuming the control valve on the BKU shellside outlet is air fail close, you would set this up as a direct acting TIC acting on the TCV.
Depending on how smooth the flow / temp of the feedstream is, some refinement of the scheme may be required to maintain a constant 25degC into the downstream separator. If feedgas flow is erratic, switch out the RTD / thick walled thermowell on the TE feeding the TIC to a thermocouple with thin wall thermowell to reduce response lag. Another basic refinement to address fluctuations in propane vapor pressure fluctuations downstream of the TCV (vapor leading to intermediate compressor suction) would be to install a slave PC upstream of the CV, whose SP would be reset by the TC acting in a typical master-slave control arrangement.
Your scheme shows feed propane is at 55degC at 18.5barg, check that it cannot drop below 25degC under any circumstances in normal operation (winter operation at air or water cooled C3 condensor?).