PSV on instrument air compressor

[ivegotgas]

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

 

[mvancleave]

The only case I see that would warrant a relief on the shell side of your aftercooler is a blocked-in situation in which the inlet and outlet cooling water valves are closed and the compressor is allowed to run. The heat from the tube side would cause expansion on the shell side. In this case, a 1 x 3/4 relief will do the trick. Just anything to burp a tiny bit of water out. I think RP-520 says something to this affect.

 

[quark]

I am not knowledgeable about this nor thought of it ever, for all the inter and after coolers of compressors I dealt so far have PSVs. There may be a condition when tube cracks and air may go to the shell side. This may be a possibility but I can't say whether it is a case or not.

 

[pleckner]

One clarifcation to Mvancleave's response. That should be a 3/4" x 1" PSV. The stated order is relevant because the first number is the inlet and the second is the outlet.

 

[Ashereng]

For thermal expansion, it is also common to use a 1C1 PSV - 1" x 1", C orifice.

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?

 

[djack77494]

A PSV is not required just because you have a pressure vessel. If there are no relief scenarios, then you do not need a relief valve. For example, if you have several pieces of equipment that are connected without any intervening valves, then a single PSV might serve to protect the whole group.

 

[pleckner]

djack77494: I must take exception to your statement, "A PSV is not required just because you have a pressure vessel. If there are no relief scenarios, then you do not need a relief valve."

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.

 

[jte]

I've gotta second pleckner here. If we are talking about a U stamped vessel, then it seems that ASME VIII Div. 1 would have some jurisdiction. From that code:
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

 

[jte]

'Course, djack also makes a valid point in that the vessel may have a relief path through other vessels. I've seen that done many times.

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

 

[pleckner]

The statment was made that a pressure vessel didn't need a PSV if there were no credible scenarios. This is obviously dangerously wrong and is the reason I put my 2-cent worth in.

I do agree with the second point as I stated in my post but this is unrelated to the first point.

 

[JimCasey]

I don't have the code in front of me, but if memory serves, section VIII limitation does not apply to vessels smaller than 15 cubic feet. A small tube and shell HX may well be small enough so that it is not code-regulated. Or it may just be cheaper to put on a 3/4 x 1 PRV than to argue with the code inspector.

 

[jte]

Jim-

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

 

[pleckner]

Concerning the exclusion in question:

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.

 

[Ashereng]

Then again, a 3/4 x 1 or even a 1x1 is definitely cheaper than arguing with the inspector.



"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?

 

[djack77494]

Allow me to readdress the apparently controversial statement that I made earlier; namely, "A PSV is not required just because you have a pressure vessel. If there are no relief scenarios, then you do not need a relief valve." This statement, which I admit to as being (very) poorly stated, drew a variety of comments, but I believe that it has some validity.

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.

 

[pleckner]

Doug:

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.

 

[djack77494]

Thank you for your reply, Phil. Again I can see we are in agreement.