PSV on instrument air compressor
PSV on instrument air compressor
(OP)
I work in a rather old sour gas plant, it has been brought to my attention that we do not have any shell side protection on the air after cooler on our instrument air compressor.
The shell side is Cooling water @40psig while the air going into the tube side is @55oC and 125psig.
The design pressure of the shell is 149psig.
The tubes are protected with a PSV.
I cannot come up with any failure case. The cooler is located in an unclassified building that has little to no chance of ever becomming engulfed in flames.
The inspector argues that it is considered a pressure vessel therefore requires a PSV on the shell side. I am at a loss of how to calculate the flow through the PSV when I can't even think of the failure case. Can anyone point me towards some reference information on how to size this particular PSV. Or better yet, a paragraph in API 520/521 that doesn't require there to be protection on this side :)
Thanks in advance
The shell side is Cooling water @40psig while the air going into the tube side is @55oC and 125psig.
The design pressure of the shell is 149psig.
The tubes are protected with a PSV.
I cannot come up with any failure case. The cooler is located in an unclassified building that has little to no chance of ever becomming engulfed in flames.
The inspector argues that it is considered a pressure vessel therefore requires a PSV on the shell side. I am at a loss of how to calculate the flow through the PSV when I can't even think of the failure case. Can anyone point me towards some reference information on how to size this particular PSV. Or better yet, a paragraph in API 520/521 that doesn't require there to be protection on this side :)
Thanks in advance





RE: PSV on instrument air compressor
RE: PSV on instrument air compressor
RE: PSV on instrument air compressor
RE: PSV on instrument air compressor
The inspector seems to be correct to me. Anything over 15 psi is a pressure vessel under ASME VIII. Your shell side is 40 psig.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: PSV on instrument air compressor
RE: PSV on instrument air compressor
A PSV is not required just because you have a pressure vessel ONLY if it meets the exlusions of the Codes (usually size constraints or for very specific applications, and this isn't one of them)or you can invoke CODE CASE 2211 or you can protect with an approved safety instrumented system. BUT, not having a relief scenario is not going to get you off the hook, although it seems to make sense that it should. If one can't meet one of the criteria I listed above, then one MUST provide a PSV on a pressure vessel. That PSV can be nothing more than a minimum 3/4" x 1" that we would typically use for thermal relief of a liquid full vessel. The controlling scenario listed would then be, "thermal relief of a liquid filled vessel".
In addition, your example does not really address this statement. I do agree that with the PROPER DESIGN, one can have a single relief valve protecting multiple equipment. This is not the same as not needing a relief valve when there is no obvious credible scenario.
I hope this clarifies things for you.
RE: PSV on instrument air compressor
UG-125 GENERAL
(a) All pressure vessels within the Scope of this Division, irrespective of size or pressure, shall be provided with pressure relief devices in accordance with the requirements of UG-125 through UG-137. It is the responsibility of the user to ensure that the required pressure relief devices are properly installed prior to initial operation. These pressure relief devices need not be supplied by the vessel Manufacturer.
Its really pretty clear. Unless the vessel happens to be in a non-code state. So put a 3/4 x 1 on and call it a day.
jt
RE: PSV on instrument air compressor
The pressure relief devices required in (a) above need not be installed directly on a pressure vessel when either of the following conditions apply:
(1) the source of pressure is external to the vessel and is under such positive control that the pressure in the vessel cannot exceed the maximum allowable working ressure at the operating temperature except as permitted in (c) above (see UG-98), or under the conditions set forth in Appendix M.
(2) there are no intervening stop valves between the vessel and the pressure relief device or devices except as permitted under UG-135(d).
jt
RE: PSV on instrument air compressor
I do agree with the second point as I stated in my post but this is unrelated to the first point.
RE: PSV on instrument air compressor
RE: PSV on instrument air compressor
I believe you are remembering an excemption to the scope of the code from U-1. Lots of people seem to remember that number, but no one can seem to quote chapter and verse. In any case, if the vessel is out of the scope of the code per U-1, then it should not have a U stamp on it. If it may be out of the scope but if someone chose to U stamp it then the U-1(c)(2) paragraph needs to be fully complied with: … however, any pressure vessel which meets all of the applicable requirements of this Division may be stamped with the Code U symbol… So if the vessel has a U stamp, there is no excemption to the UG-125 requirements that I'm aware of. I'd be happy to have someone quote chapter and verse of the code if I need some education…
jt
RE: PSV on instrument air compressor
ASME, Section VIII, Division 1, Introduction, U-1 Scope, Paragraph U-1(i):
"vessels having an inside diameter, width,
height, or cross section diagonal not exceeding 6 in.
(152 mm), with no limitation on length of vessel.."
Nothing explicitly says if you stamp the vessel even if it doesn't need to be stamped, you buy all the requirements of meeting code from that point on, but nothing explicitly excludes this either so in my humble interpretation, you stamp it, you buy it lock-stock and barrel. The previous paragraph also stipulates it is the local government that determines applicability of all of the code requirements and they can change or require anything they want.
RE: PSV on instrument air compressor
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: PSV on instrument air compressor
The point that I intended to make was that a mechanical "relief valve" was not necessarily required for each and every "pressure vessel". I was not trying to avoid any requirements based on size, lower than 15 psig pressures, safety instrumented systems, etc. Let me present a concrete example of what I'm talking about.
Picture a system of multiple Air Receivers on the discharge side of an air compression system. Let's say there are four Receivers (for whatever reason). If the four are fed from a common header and/or discharge into a common header AND there are no intervening block valves, then I would contend that a single PSV could be employed to protect the group of Receivers. I don't read the code as specifically mandating a one to one correspondance between pressure vessels and PSV's. Even in the cases mentioned for thermal relief, I think there's an implicit assumption that the vessel has been isolated. I am refering to cases where it cannot be isolated, and I know of many such cases.
Please let me know if you think I'm mistaken.
Thanks,
Doug
P.S. I'd make a similar argument for API atmospheric storage tanks not needing mechanical vent valves if they employ open vent pipes.
RE: PSV on instrument air compressor
Your example is fine and valid. However, and please don't take this wrong, your initial statement is not valid. Having a common PSV for several vessels is NOT the same as saying you don't NEED a PSV just because it is a pressure vessel. If it is a stamped vessel, then (baring the exemptions I pointed out previously) you do NEED a PSV. If you want to put a common PSV in for several vessels then OK but this is still providing a PSV for a pressure vessel. I don't want anyone to get the wrong idea from your original statement.
RE: PSV on instrument air compressor