13 replies to this topic

[Yogesh Bhatt]

Dear all experts,

 

I'm studying the requirement of PSV on an "Instrument Air Receiver (IAR)". There are 2 more Air Receivers (AR) in the same plot area. Following are the design parameters:

Design pressure: 174 psig

Design temp.: 122 F

Volume of each AR: 1.5 m3

Each air receiver has a PSV of 1/2" x 1/2" which is a non-standard PSV as per API.

 

Now I'm contemplating 2 options:

 

1. Car seal open all the valves (as per the attached P&ID) to provide an open path for pressure relief from IAR to the connected vessels. This way I can avoid installing a PSV on IAR.

Only thing I need to ensure is that the PSVs on other two air receivers can relieve the entire load (of all 3 vessels) in case of fire exposure. As per my calculations, the exposed surface area is 15.2 m2 and calculated orifice area is 1.233 cm2. This gives me E orifice. 

According to above calculations, PSVs on both ARs are under-sized and need to be replaced with higher orifice designation to relieve this load. 

 

2. Since the air receivers have only dry air, fire exposure will result in vapor thermal expansion and a very negligible amount of relief will occur. If this is true, the installed PSVs (1/2" x 1/2") on ARs are adequate and it can even relieve the load from IAR when given an open path.For this, I need to support this with a calculation for relief load and orifice area for vapor thermal expansion. I can not find any calculation on API for this case. HYSYS safety analysis tool  gives me 0.176 cm2 of orifice area. I did not provide any heat flux for this though. 

 

Kindly give your suggestions to a way forward. 

 

Thanks very much in advance!

 

Regards

Yogesh

 

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[Bobby Strain]

Are there any flammable liquids or gas nearby? Could be you don't need a PSV for fire as long as the PSVs on the compressors are set to protect the receivers.

[Yogesh Bhatt]

Yes there is a gas pig receiver nearby at about 40 meters. There is a small PSV at the compressor discharge. But for pool fire case, we need PSVs on receivers right? Because the receiver location is little far from the compressor area.

[Bobby Strain]

I believe you have no case for a pool fire or a gas jet fire. One would have to gather firewood to create a fire.

 

Bobby

[Yogesh Bhatt]

If the fire case is not applicable, on what basis are the relief valves installed on Air receivers? Is it for vapor thermal expansion due to ambient temperature rise or solar radiation? If so, do we have any calculation to find out the relief load or just provide a small PSV (non-API in my case is provided)?

[Art Montemayor]

My comments on your queries are based on what you furnish as the little basic data:

 

Why are you concerned about the fire case on a pressure vessel that is filled with air?  How do you visualize the PSV protecting the receiver against the fire case?  Will it stop the vessel from simply rupturing at the set pressure?  I don't think so.  The only thing that will mitigate a gas-filled vessel rupturing in the fire case would be a total blow down or venting.

 

Are you concentrating on what is furnishing the pressure in the receiver - the air compressor?  As Bobby has stated, the compressor has the mandatory PSV to protect its downstream pressure from going too high.  What about the air receiver?  Isn't it classified as a pressure vessel that must have a PSV mounted on it?   I think it is.  The receiver is directly connected to a source of high pressure air (100-150 psig?) that is being delivered in a constant, fixed displacement mode (I believe) and therefore, it must be rated to relieve that same maximum discharge capacity in the event the compressor instrumentation doesn't stop it from feeding the receiver - whether the receiver needs it or not.  That is the scenario I remember setting as the most credible when I set the maximum credible capacity requirement for my last air receiver.

 

I don't think the fire case is a credible scenario in this application.  Don't forget, the fire case is a pool fire (I believe).  That means you must have a burning, combustible liquid pool at or near the vessel in question.  Not a gas fire.  A gas fire could impinge on the vessel if it were close enough or if the radiation lasted long enough.  I don't think that is the case here.

 

I offer this as my thoughts and my experience in a similar application.  If you are changing the PSV, I believe that calls for at least an informal Hazop since you will have to note the change on your P&ID.  The change would then undergo a Hazop review.

[breizh]

Hi ,

I believe you receiver is a pressure vessel thus a relief valve is required . check the Code , should be related to P*V .

All receivers I've been operating were equipped with safety valves .

good luck

Breizh

Edited by breizh, 12 February 2019 - 04:57 AM.

[Sharma Varun]

Yogesh, going by the P&ID shared by you since a single PSV is provided without installed spares it is certain that this is a fire case PSV. 

 

Breizh is correct, its a common practice to provide PSV on pressure vessels to meet ASME code requirement. However it may be noted that as per ASME code (ASME Boiler and pressure vessel code, 2010 edition, VIII div. 1, rules for construction of pressure vessels) also, it is not mandatory to provide PSV on all pressure vessels and over pressure protection can be provided by system design. In past we have done away with PSV merely provided for gas filled case.

 

Bobby has raised a valid point in order to have pool fire you must first establish that there are chances of liquid (fuel) spillage. For gas leaks normally safety valves are not considered as jet fires result in vessel impingement & localised heating where vessel will fail before the PSV set pressure is reached.

 

As rightly explained by Art for cases like external fire for gas filled vessels it may happen that vessel wall may fail before the PSV relieving conditions are reached and thus providing a PSV is generally of no use and thus such vessels are protected by making them accessible to fire fighting or by providing them with water deluge, fire proofing or a vapor depressuring system.

 

You have defined the design conditions however operating conditions are also required to calculate relieving temperature. Assuming your vessel operating pressure as 6 kg/cm2g & operating temperature as 40degC, relieving temperature comes to be 430 deg C & with operating pressure as 5kg/cm2g relieving temperature is 547 degC. Under these circumstances your vessel will not fail (Assuming vessel to be of CS) & PSV for fire case will be helpful if there are chances of a pool fire.

[Bobby Strain]

There is no code that I am aware of that requires a PSV on a pressure vessel if there is no source of overpressure. But all engineers I know install one regardless.

 

Bobby

[Yogesh Bhatt]

Thanks Bobby, Art, Breizh, Varun.

Art,
I checked the relief load for max compressor discharge as well and that is 160kg/h for which D orifice is oversized. Probably that's the reason 1/2" × 1/2" PSVs are installed on air receivers.
Now, my question is can I provide a CSO valve between air receivers (AR) and IAR vessel so that all 3 vessels are connected and I can avoid installing a dedicated PSV for IAR. Both air receivers have PSVs on them.

[breizh]

Hi,

 

To add to my previous answer.

 

https://www.national...geID=134&ID=284

 

hope this is helping you.

 

Breizh

[latexman]

There is Code on this in the U.S.

 

https://www.osha.gov...r/1910/1910.169

 

It says, "Every air receiver shall be equipped with an indicating pressure gauge (so located as to be readily visible) and with one or more spring-loaded safety valves" and "No valve of any type shall be placed between the air receiver and its safety valve or valves."

 

The reg applies to "New and existing equipment."  Therefore, retrofit is mandatory.

 

IMHO, air receiver safety valves are usually so small and cheap, it's not worth much debate.

Edited by latexman, 13 February 2019 - 01:35 PM.

[Yogesh Bhatt]

Thanks a lot.

[PRVBoss]

Hi Yogesh,

 

Agree with what has already been posted in reply, in particular that overpressure protection is required per ASME VIII and PSVs are required per OSHA. Ensuring overpressure protection by system design can be a little tedious and devolve into an effort in trusting your documentation management system so as latexman says, it is usually easiest to install a nominal sized PSV and move on. 

 

Wanted to point out a couple of things:

 

1. You might want to ensure that the maintenance isolation valves on the inlet to the existing PSVs have valve controls (locks / carseals) and that the P&ID is updated to reflect them. 

 

2. ASME VIII Appendix M provides guidance on valve control requirements for isolation valves between equipment that are protected by a common relief device(s). I would refer here for what to do with the IAR. 

 

3. API 521 6th Edition (Jan 2014) Section 4.4.13.2.4.3 provides guidance for relief of thermal expansion of vapor. You directly calculate the required PSV area. The heat input is implicit in the dT included in the F' term.

 

Hope this helps.