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Posted 28 February 2013 - 03:23 AM
I have to do PSV sizing for a HP condensate flash Tank. The condensate from all sources (steam traps) gets collected in this flash tank @ 100psig. The tank PSV is set at 50 Psig. The tanks normal operating is at 16 psig. There is level control valve for condensate and also a control valve on the outlet vapor. I have flashed the condensate at reliving pressure to find out amount of vapor produced and its nearly 4-5% only. my question is wheather i need to size it was liquid closed outlet also?, which means liquid comes out of PSV. I have calculated relief loads for vapor valve closed outlet and fire case. I m quite confused about cases to consider.
Posted 28 February 2013 - 08:16 AM
Just size PSV for vapor relief.
If liquid outlet is blocked, the flash tank liquid level controller will alarm or shut down the inlet.
If the level controller fails, the rising liquid level will squeeze the vapor space to raise vapor pressure to no more than16 psig+, because the condensate inlet is controlled by upstream letdown valve at 16+ psig. As soon as the flash tank pressure reaches 16 psig+, there will be no flow into the flash tank anymore because there is no pressure difference anymore.
Posted 28 February 2013 - 11:33 AM
If it were up to me, I would size the valve for blocked liquid and vapor outlets. If the level valve fails closed for some reason, it's likely that the operator will close the vapor outlet valve to prevent liquid from flowing out with the vapor.
Posted 28 February 2013 - 02:18 PM
We usually calculate all possible scenario's without safeguards (like level control, ...) Gives you a good idea of what might happen if ... So calculate for blocked outlets for liquid overfilling ... usually a liquid scenario results in smaller oriifce sizes
Posted 28 February 2013 - 02:21 PM
Do you consider you are presenting a double jeopardy scenario? Bobby
Posted 01 March 2013 - 02:41 AM
Dear Shan /Bobby / gemhag , Thanks for timely help. At present i m going ahead with Shan's reply. Will get back if i find anymore difficuty. Hope u will be helpful as u are now.
Posted 01 March 2013 - 03:19 AM
1. When sizing PSv, we need to analyze several emergency scenarios and use the worst case scenario as the design basis.
2. Don't forget the case of steam break through to the flash drum instead of condensate. This is probably the worst.
Edited by S.AHMAD, 01 March 2013 - 03:22 AM.