notsocommonsense
Petroleum
- Jul 2, 2008
- 10
We added a PSV on our high pressure amine (DEA)pump discharge to satisfy code overpressure requirements for high pressure piping. To meet API 521 requirement for no liquid in PSV discharge line, we raised the PSV and are discharging through an adapred manwaybcovee into the top of the Lean amine surge tank. I didn't add a dip leg because A)I thought you were not allowed any liquid in any part of the PSV discharge piping and B) it would be difficult to support without welding in the tank (we are going in through a manway flange to preclude having to weld on the tank) The tank is expected to have an oily RAG layer comprised mainly of naphtha and light gas oil components. The DEA itself is very conductive (approx 10 Ms). The tank is nitrogen blanketed and is equipped with a PVSV I assume that since we are operating pump discharge on spillback pressure control at 1550 to 1600 psig and that the conventional hard seated PSV is set at 1797 psig that we will have the PSV occasionally lifting. It will discharge out of a 6" pipe at roughly 33 feet second. Tank is 21 feet high, normal liquid level is 12 feet and discharge nozzle will come through the top manway and be approximately flush with the roof. Per API RP 2003, I am wondering about a charged mist forming and settling on the rag layer creating a sparking potential. If N2 inerting fails and air is present, a potentially explosive situation could ensue. Other details are Tank diameter 40 feet, amine circulation roughly 4000 USGPM, tank contents 85 deg C and PSV discharge temp is roughly 55 C. Three weeks left in shutdown. Am I allowed per API 521 to add a dip leg and should I or is the N2 inerting adequate to prevent a static induced explosion/fire? Any ideas about design and how to support a dip leg without welding? I was thinking an inverted spring can umder the dip leg too put some pressure against the floor sandwiching the dip leg pipe between the bolted manway cover and the floor. Also thought about a muffler style submerged outlet with lots of holes to reduce exit velocity. Would also add a vac breaker hole at top of dip leg.
