It seems these days that having a relief valve header is the way to go; but why is that? Why are so many businesses deciding an “RV header” is the way to go with their RV Design Basis? In the process safety world, a facility is required to have a “relief design basis” and the use of a header should be documented as to why a header is needed, not just wanted. So where did the use of an RV header come from and why?
RV headers became a necessity actually out of some environmental efforts, although their use can be traced back long before EPA came to be. But headers came widely accepted and used when facilities began to have the need to use environmental control/treatment devices such as flares and scrubbers. In an effort to cut fugitive emissions to zero, facilities began to use these environmental control/treatment devices more and more and so the headers grew larger and more complex. This “zero fugitive emissions” goal also began the concept of tying relief devices into these environmental control/treatment systems.
But we also see headers used in RV systems at facilities that have no zero emission goals. When asked to see their RV design basis we find no sound engineering rationale for the use of a header (e.g. no use of environmental control/treatment device(s). Yes, in the RV Design basis the rationale for the use of a “header” is necessary. Just saying that you want to vent the HHC/EHS to a “safe location” is not enough, as this can be achieved by venting directly to the atmosphere with a simple/single vent line. Venting all the RV’s to the same location does NOT increase process safety or personnel safety – in fact, in many of our assessments, we find increased risks due to improperly designed headers. One item that you will never see mentioned in 1910.119 or Part 68 is a drawing called “isometric drawing”, also called “ISO’s” for short. This drawing is VERY detailed and will show run, pitch, diameter, valve locations, flange locations, etc. of the pipe(s). In an RV header design, these drawings are a MUST HAVE to ensure that RV’s discharge into the header at the proper location with the proper size. We get a lot of pushback when we ask for this drawing, but without a doubt, an RV Header design basis MUST have an ISO drawing in order to document its design is to an RAGAGEP.
Another serious concern is the “design scenario” for the RV’s dumping into the header. In almost all cases, other than a runaway exotherm, the fire scenario will be the scenario at which most RVs will be sized to. When each vessel has its own vent line, sizing is rather simple; however, when the RVs tie into a header the sizing can become extremely complex, as well as making the header excessive in size. The trick comes into play when the engineer(s) have to defend their rationale as to why they did not assume that a single fire event would impact multiple vessels within the same fire area. This consideration can sometimes cause the header size to be unworkable and thus require the RVs of the vessels discharge directly to the atmosphere, which for some reason, as I said in the beginning, is a bad thing these days. There is just NO LOGIC in the mindset that two (2) vessels sitting within the same secondary containment (or room) would not be exposed to the same fire, which means that the header would HAVE TO BE SIZED for BOTH RVs on the two vessels in the shared containment/room to lift in the same fire scenario. How any engineer can argue this is not the proper sizing scenario defies logic. It is one thing to say “the code allows it” and it means something entirely different to make every effort to ensure your final layer of protection is designed PROPERLY.
So if your facility uses an environmental control/treatment device such as a flare, scrubber, diffuser tank, etc. then a header is absolutely appropriate; however, if your header merely discharges to the atmosphere and your reason for the header is “fewer roof penetrations” you may want to reconsider this “engineering concept” a bit further!
PLEASE NOTE that some international/state/local codes PROHIBIT using headers or as some codes call them “manifolds”. For example, in the state of Ohio flammable liquid tanks are NOT permitted to be tied into a header system, unless the header goes to a vapor recovery, vapor conservation or air pollution control device…
(e) 3404.2.7.3.5 Manifolding. Tank vent piping shall not be manifolded unless required for special purposes such as vapor recovery, vapor conservation or air pollution control.