Boil off pressure control valve

[JJ10]

Okay, so I am looking at a cryogenic storage tank that uses boil off control to maintain pressure.
When the vapour pressure reaches the defined level the PIC open the boil off control valve, and it flows to the vapour compressor and then around a re-compression loop.
So my question is: is their any reason why the system needs the pressure control/boil off control valve. From my little understanding, it feels like the system would work the same without it...i.e the vapour pressure would build up in the pipeline to the vapour compressor, and tank room, and enter the compressor. So I am guessing there's probably a good reason why the boil off control valve is required but I cannot figure out why - does anyone have any thoughts?

 

[SNORGY]

If the valve is modulating, as opposed to "on / off", then I think you actually need it for control, do you not? Or is there a modulating valve downstream of the tank on the suction to the vapor compressor?

 

[JJ10]

Hi, Thanks for your response. No this is the only valve between the tank and the suction to the vapour compressor. I guess my question is whats the propose of the valve, or why do we need the valve for control? i.e. why don't we treat the pipe as an extension of the tank and let it feed directly into the suction of the vapour compressor

 

[SNORGY]

What is the boil off rate relative to vapor compressor capacity? Presumably, it is less, otherwise things would not work with or without the valve. Is operation of the vapor compressor such that it is only running when its suction is open to the boil off tank, or is there more to the system than that? Without the valve, assuming the vapor compressor capacity potentially exceeds the boil-off rate, what prevents continuous drawdown of the tank pressure? Presumably the suction pressure of the machine is tied into its capacity control, in which case you could indeed be right in that you might not need the valve. It's the kind of question that I would need a P&ID to answer more correctly.

 

[itdepends]

If you ran the boil off pressure control valve open continuously- more than required by the current boil off rate then, assuming the compressor can handle the flow you will boil off excess liquid that will be recompressed, cooled and returned to the tank. The net effect would be a cooling of the tank contents and a lowering of its operating pressure.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"