25 replies to this topic

[kkala]

Following technical points on the matter discussed can be useful, of course reflecting my opinion.
1. Original query concerned a propane depressurization line from a refrigeration cycle.
Minimum design metal temperature (MDMT) was generally looked into.
2. Example by kkala concerns a delivered process design, where LPG from a deethanizer or splitter (liquid stock about 12 ton) is depressurized to flare header through a 14" line. Minimum design metal temperature has been taken as -42 oC (normal boiling point of propane).
3. Example by Ankur2061 concerns a report of natural gas depressurization through an orifice. Gas bulk temperature -70 oC downstream of orifice. Minimum internal pipe wall temperature is estimated at -46 oC, "due to heat transfer effects of the pipe metal with the outside surroundings". This according to dynamic simulator "OLGA". Ambient temperature was taken as 0 oC, average wind velocity 5 m/s (max 37.5 m/s).
3.1 MDMT of -46 oC in previous example is judged to need supporting evidence beyond the data disclosed (bulk fluid temperature = -70 oC). Clarifications asked by kkala had not been responded (posts No 11, 16, 21), but these would have been enough only for a general picture.
4. At any case the engineer can adopt the most proper design and take responsibility of it. (2) may be conservative and applicable to small diameter pipes, (3) would need run of specialized simulators and convincing evidence about the results. Necessary study for the latter can start from searching 'depressurization' - "author JoeWong" in ChereSources Engine, one example is http://www.cheresources.com/invision/topic/3504-isentropic-efficiency-and-its-implications. Steel mass of pipe, heat transfer from ambient air, non steady state can result in increase of MDMT.

Edited by kkala, 20 July 2012 - 05:15 PM.