It has grown tiresome, but it remains a teachable moment, to debate “engineers” who want part of ASME/NBIC to be the gospel and other parts not so much. The ammonia refrigeration industry just can’t seem to grasp the critical need in their “Common PRV Header Design” v.s. having more “holes in the roof” (as they like to point out). And today, I got word a technician was performing their 5-year PRV maintenance with the refrigeration system in full operation. A compressor RV lifted, discharged into the shared PRV header, and gassed the employee who needed ALS medical care. He would have been flown to a trauma center, had the weather permitted. No LOTO in place for these “line breaks”, but more importantly, there is NO WAY to isolate each RV from the common header such that LOTO can be applied. This is a “newer” process, built in 2019/2020 and this was the first time the RVs needed to be changed out. In my PHA/Engineering Code Analysis, it was suggested (in 2017) that if the process was going to utilize a common header then ASME/NBIC requirements should be considered in this design. The “contractor” hired to design and build the process stated he had never done it like that before and “in fact, it’s illegal”, so the business took the easy path and built the common header with no means to isolate each PRV. In other words, the PHA recommendation was to shut down the process and perform the maintenance since there was no mention of isolation. The business did neither and now they are worried about my PHA recommendation(s) creating a citation path – how about worrying about the worker you put into ICU?
So I am providing the latest from the NBIC Part 4 regarding when, where, and how to use “intervening valves” in PRV path(s) so that maintenance on these PRVs can be performed safely. And yes, the code calls out the ammonia refrigeration common PRV header design as one such design where an intervening valve is not only allowed; it is called for.
(Emphasis by me)
Part 4 – Pressure Relief Devices
2.5.6 INSTALLATION AND DISCHARGE PIPING REQUIREMENTS
…
e) There shall be no intervening stop valves between the vessel and its pressure relief device(s), or between the pressure relief device(s) and the point of discharge, except under the following conditions:
1) When these stop valves are so constructed or positively controlled that the closing of the maximum number of block valves at one time will not reduce the pressure relieving capacity below the required relieving capacity.
2) Upon specific acceptance of the Jurisdiction, when necessary for the continuous operation of processing equipment of such a complex nature that shutdown of any part is not feasible, a full area stop valve between a pressure vessel and its pressure relief device may be provided for inspection and repair purposes only. This stop valve shall be arranged so that it can be locked or sealed open, and it shall not be closed except by an authorized person who shall remain stationed there during that period of operation while the valve remains closed. The valve shall be locked or sealed in the open position before the authorized person leaves the station.
3) A full area stop valve may also be placed on the discharge side of a pressure relief device when its discharge is connected to a common header for pressure relief devices to prevent discharges from these other devices from flowing back to the first device during inspection and repair. This stop valve shall be arranged so that it can be locked or sealed open, and it shall not be closed except by an authorized person who shall remain stationed there during that period of operation while the valve remains closed. The valve shall be locked and sealed in the open position before the authorized person leaves the station. This valve shall only be used when a stop valve on the inlet side of the pressure relief device is first closed.