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Tuesday, January 15, 2008

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A question concerning requirement of overpressure protection device on "Final Vessel" was raised.
"API 14C SAC A.4.c.5 seems to indicate that a PSV is required on a pressure vessel (max pressure >5 psig) unless it is the final vessel on a flare header (i.e. Flare KO Drum). However, this implies that any other vessel venting into the flare header (e.g. Closed Drain Sump) needs a PSV to protect against blockage of the final vessel."
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My personal opinion is if the connection between Closed Drain Sump and Flare KO Drum has NO internal that potentially create blockage which lead to overpressure scenario and affecting relief path, a Pressure relief device is not required on the Closed Drain Sump even though it is not the "final vessel". This is inline with the API14C SAC A.4.C.4.

The following examples show an overpressure protection device is required even though the pressure vessel is "final vessel"

a) Degassing drum with vent line and flame arrester discharge to atmosphere
This drum is a "final vessel" in the relieving path. However there is a potential of flame arrester blockage and may lead to overpressure of this drum. Thus, an overpressure protection device is required. Normally a Pressure relief valve or rupture disk is installed across the flame arrester. In this case, it is inline with the API14C SAC A.4.C.1.

b) Flare KO Drum with Flare Recovery system
In certain environment sensitive area, "zero emission" and As-Low-As-Reasonable-Practical (ALARP) principle are adopted. Those in most cases, a Flare Recovery System (FRS) will be implemented in Flare system. It could be a recovery compressor, ejector, etc. In most cases, the Flare KO Drum downstream flare line which connected to flare stack may have an on-off valve to facilitate plant maintenance. Under this case, eventhough the Flare KO Drum is the "final vessel" but an overpressure protection device (i.e. rupture disk) would be installed across the on-off valve to ensure a clear relief path. In this case, it is inline with the API14C SAC A.4.C.1.

The following examples show an overpressure protection device may NOT required even though the pressure vessel is NOT "final vessel"

a) Two drums connected together with no internal in the interconnecting pipe
As there is NO possibility existent of potential blockage on the interconnecting line and the overpressure protection device located at downstream has been designed for worst case (of two drums), an overpressure protection device is NOT required for the upstream drum. In this case, it is inline with the API14C SAC A.4.C.4.

b) Flare KO Drum with Water Seal Drum
As large relief will destroyed the water seal in the seal drum, thus there is NO possibility existent of potential blockage results overpressure scenario. Overpressure protection device is NOT required for the Flare KO drum. In this case, it is inline with the API14C SAC A.4.C.5.

If you can think of any typical example, let share it in this post.

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posted by Webworm, 1:11 PM

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