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Allowable Overpressure For Psvs

Started by Guest_Karteek80_*, Aug 15 2013 06:28 AM
overpressure percentage overpressure set pressure

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#1 Guest_Karteek80_*

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Posted 15 August 2013 - 06:28 AM

Dear Forum members,

I am facing an interesting problem with regards to accpetance of PSV supplied for an ongoing project.

 

For 3 of the pressure vessels with design pressure 145 psig contaning oil & water, fire case PSVs are provided in design (governing case). The PSV set pressure for these vessels is 145 psig and the % overpressure specified in the datasheet is 21% which is typical for fire case and as specified in API 520, Part 1 (sizing) and 521. 

 

All the calculations and datasheets from the supplier also show the above details.

 

However, when the PSV is delivered the nameplate and test certificates show that the %overpressure is 10% and not 21% as specified in the datasheets. Remember these valves are to be used for fire case and hence original requirement is for 21% overpressure. I am posed with the question of whether these PSVs are acceptable with 10% overpressure.

 

The supplier also states that as per ASME Section 8 UG-125 through 137 the capacity certificate shall be with 10% overpressure hence this valve meets the requirement and is acceptable. 

 

In this regard i have the following questions:

1 - What is the criteria that sets % overpressure allowable is 21% for fire case?

Is it that during fire case (which is an emergency case) the main aim is to protect the vessel such that personnel is provided with adequate time to move away from the vessel safely and that in the meanwhile even though there is a higher pressure in the vessel due to fire it is okay. This is unlike other cases (i.e process upsets like cooling water failure, gas blowby etc) in which case the vessel mechanical integrity is still required to be manitained hence allowing only 10% overpressure (more conservative).

I am lead to this approach because in the area calculations for the releif valve (liquid service) as per API 520, Part 1 the releiving pressure = set pr + % overpressure + atm pr. is considered in the denominator. This will result in smaller area for 10 % overpressure as compared to 21%. The flowrate is dictated by vessel dimensions and heat absorption anyway. 

 

2 - In the current scenario, can the valves be accepted as delivered?

Since the valve is tested that it will relieve at full flow at 10% overpressure then the %overpressure of 21% will never be experienced and this is conservative.

 

3 - If 2 above is true, then can 10% overpressure for relief valve sizing for liquid case be considered as a general design rule since it is conservative?

 

4 - If so how to document this, is it required to change the datasheet stating 10% overpressure for fire case?


#2 fallah

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Posted 15 August 2013 - 11:36 AM

Karteek80,

 

The point is appears the PSVs are to be sized such that can handle the required relief loads in fire case at 10% overpressure, means they are sized a little bit higher than required because they could be sized smaller to handle the same relief load with 21% overpressure.

 

Anyway, you can accept these larger PSVs if the vendor would want to receive th initial amount of the contract (that amount was agreed for the PSVs with 21% overpressure) otherwise you can reject the PSVs and ask to receive the PSVs as per your initial requirement in overpressure value standpoint...


#3 gegio1960

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Posted 15 August 2013 - 11:59 AM

In my knowledge, only European codes limit at 10% the fire case overpressure.


#4 Lowflo

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Posted 15 August 2013 - 12:16 PM

gegio1960 -  Refer to PED Guideline 5/2. Europe (PED) doesn't limit the accumulation to 10% for fire exposure. Interestingly, they say that 10% isn't the limit for fire, but they don't say what the limit is. They're simply silent on this question. Consequently, I use 21% in Europe and that hasn't been challenged by any NoBo's.

 

http://ec.europa.eu/...ped/guidelines/


Edited by Lowflo, 15 August 2013 - 12:17 PM.

#5 gegio1960

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Posted 15 August 2013 - 03:23 PM

Lowflo,

thank you for your comment. I purposely wrote my post in that way. Have you ever worked for Italian plants?

Does anyone know where the 10% rule is applied?

Thanks


#6 Lowflo

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Posted 15 August 2013 - 03:50 PM

Lowflo,

thank you for your comment. I purposely wrote my post in that way. Have you ever worked for Italian plants?

Does anyone know where the 10% rule is applied?

Thanks

 

The PED guidelines apply in all CEN member countries which includes Italy. If your question is whether this guideline applies in Italy, the answer is yes.


#7 Guest_Karteek80_*

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Posted 20 August 2013 - 11:03 PM

Thank you for your inputs Mr. Fallah...

 

I would like to add to my earlier correspondence as follows:

  1. This is a new PSV sized for fire case and not a existing PSV which is being recirtified.
  2. The vendor calculation for required area for fire case releif is based on 21% overpressure.
  3. However, for certification and stamping on the name plate, vendor claims that capacity can be indicated only at 10% overpressure as per ASME code. ASME code certification does not allow to capcity test a PSV at 21%.
  4. The vendor also rejected our request to stamp / certify cpacity as per 21% overpressure. Stating that ASME certification / stamping cannot be done at 21 overpressure.

In this regard, i seek the following clarifications:

  • Has anybody in the past come across this issue, where the PSV is sized for fire case (21% overpressure) but the ASME certification and capacity stamoing on the name plate will only be based on 10% overpressure.
  • If so is it acceptable since there will be no consistency between documents and nameplate capacity. The PSV datasheet and calculations will show fire case sizing with 21% overpressure, where as the nameplate will show capcity at only 10% overpressure.  

Thanks & Warm Regards,

Karteek


#8 fallah

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Posted 21 August 2013 - 03:28 AM

 

  • However, for certification and stamping on the name plate, vendor claims that capacity can be indicated only at 10% overpressure as per ASME code. ASME code certification does not allow to capcity test a PSV at 21%.
  • The vendor also rejected our request to stamp / certify cpacity as per 21% overpressure. Stating that ASME certification / stamping cannot be done at 21 overpressure.

 

Karteek,

 

Please see attached page from ASME... As can be seen Code allows capacity certification tests of PSVs up to 120% of the stamped set pressure... Then appears vendor isn't correct in this approach and you can ask them to do accordingly...

Attached Files


#9 Guest_Karteek80_*

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Posted 21 August 2013 - 03:45 AM

Thank you Mr. Fallah for the reference.

I did read the clause you mentioned UG-131 c(2);  but as you see this is applicable for PSVs provided on vessels under clause UG-125c(3). 

 

UG-125c(3) is applicable to  vessels that are provided with no permanent supply connection and used for storage at ambient temperature conditions of non refrigerated liquefied compressed gases. The vessel in our case is a production separator handling mainly oil & water with very less associated gas, which i feel does not fall under the perview of this clause.

 

Clause UG-131c(1) is more appropriate in our case. Which states capacity certification tests shall be conducted at a pressure which does not exceed the pressure for which the pressure relief valve is set to operate by more than 10% or 3 psi (20 kPa) which ever is greater except as provided in UG-131c(2) (referenced by you earlier).

 

If this interpretation is correct, then vendor seems to be right!

 

Thanks & Warm Regards,

Karteek


#10 fallah

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Posted 21 August 2013 - 07:16 AM

Karteek,

 

If the vessel in your case isn't under the preview of the clause UG-125c(3) you are right and seems vendor should submit the capacity certification based on UG-131c(1).

 

The main point is that: the certified capacity reported by vendor at 10% overpressure should cover the required relief load. It is obvious if it can cover, then the PSV will be a little bit conservative for operating at 21% overpressure and will result in no problem in this regard. Please if you would check this point inform us by coming back to this thread...


#11 Guest_Karteek80_*

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Posted 16 September 2013 - 04:18 AM

Goo day forum members...

Sorry I was unable to respond above query to close out the topic. We were in touch with the PSV vendor for some information.

 

Dear Mr. Fallah, the vendor confirms that the capacity test is done at 10% overpressure and the same is stamped on the nameplate. The sizing however, is based on firecase (i.e 21% overprssure). As you have pointed out, once the PSV is capcity tested for 10% overpressure, it should be suitable for 21% overpressure condition as well.

 

In our case, the vendor will also provide an additional stainless tag plate with the PSV sizing basis (Fire Case) and % overpressure (21% overpressure) mentioned on it apart from the regular nameplate on the valve (which will contain the capacity test figures as per ASME) .

 

Apart from this, we were able to obtain reference of some older projects where similar case was encountered. i.e sizing case for PSV is fire case with 21% overpressure but the capacity certification for teh same PSV was done with 10% overpressure as per ASME.

 

Thanks & Warm Regards,

Karteek


#12 fallah

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Posted 16 September 2013 - 05:29 AM

Karteek,

 

Thanks a lot for informing...

 

Please submit the values of the certified capacity in 10% and 21% overpressures as per the relevant nameplates, if it's possible...


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