Cooling Medium Expansion Drum Lower than Consumer

[DoraeS]

Hi,
I am working on one offshore brownfield modification where a new module has to be placed on existing platform topsides. It requires cooling medium on the new module but that means the new compressor aftercooler will be located higher than the existing Cooling Medium Expansion Drum.

I understand that normally CM expansion drum needs to be located on the highest location with respect to all its consumers. However, can anyone tell me what would be the problem if the consumer is located above the drum?

What I know is that HC may leak from process side into the CM side and the HC vapour can migrate to the expansion drum if it is at highest location and therefore vented safely. When expansion drum is not located on the highest location the HC leaked vapour may get trapped at the highest location and affecting the cooling effect. Is my understand correct?

Also is there any other concern?


Thanks everyone.

 

[georgeverghese]

Yes, the entire retuen line back to the expansion drum must self vent back in to the drum. The aftercooler exit nozzle must also be at the top of the HX to allow the cooling water side to salf vent and prevent the CW side from getting vapor locked, resulting in HX thermal failure at low leak rates. Obviously, the PSV on the HX or the CW expansion drum should cater for tube rupture flow. A TSLL on this return line will also help to trip this system if the CW side approaches freeze point as a result of the tube rupture, if this freeze point approach is possible. Also check that the control vent line ( not the PSV) on the expansion drum can deal with say up to 5-10% of the tube rupture flow that may occur for small operational leaks.

 

[DoraeS]

Hi georgeverghese,
Thanks for your reply.
In this brownfield project, it seems not feasible to relocate the CM drum to highest point.
Do you have any opinion if installing an automatic venting for the new aftercooler would help solving the issue?

Thanks.

 

[georgeverghese]

An auto vent at a local high point may help somewhat, but I'm not sure if this is a good solution. A local high point degassing drum would be better.

 

[sshep]

If the drum is pressurized, it is not neccessary to be at the highest point. I have seen many tempered water systems with expansion drum at grade, and serving elevated exchangers.

Best wishes,
Sean