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Pressure Setting Of Spare Standby Psv


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#1 SawsanAli311

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Posted 05 June 2019 - 07:28 AM

Dear All, 

 

In-line with API 520 and ASME SEC VIII UG-134 item a,multiple relief valve device configuration mandates setting the first PSV at the equipment's MAWP/design pressure while the subsequent multiple device(s) at 105% of the MAWP/design pressure.

 

I have a case of 3 working multiple PSVs and one common standby PSV among them all. These PSVs are installed on a separator vessel having a design pressure of 19 barg. Governing over pressure scenario is two phase blocked outlet. 

 

- Set pressure of 1st PSV=19 barg

- Set Pressure of 2nd & 3rd PSV = 19.95 barg

- Set pressure of the isolated inlet spare PSV(number 4) is 19 barg.

 

Now, in case one of the 2nd or 3rd PSV is taken for replacement, how to ensure that the spare PSV would have its PSV set point also staggrered with  respect to the 1st PSV. 

 

If one selects the spare PSV number 4 to have a set pressure of 19.95 barg  instead of 19 barg and PSV number 1 is taken for maintenance, then staggering would be defeated since all available (2,3,4) PSVs would pop at the same higher set pressure of 19.95 barg and allowable accumulation of 16% as per ASME would not be achieved but reduced to around 10%. 

 

Appreciate your views on the following:

- Optimum setting of the spare PSV. 

- Whether planned scheduled PSV replacement shall consider the above setting sequence. What if one of the PSVs urgently required replacement on an emergency basis and hence it can't be planned.

- The safest design is to have one spare PSV for each installed working PSV, however this will lead to having 6 PSVs in total instead of 4 optimized PSVs in addition to structural challenges and complexities. 

 

Thank You,

 

Regards, 



#2 fallah

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Posted 05 June 2019 - 11:25 AM

Hi,

 

In general, if you are going to consider one spare for multiple PSV's you have to consider same rated capacity for such multiple PSV's as well. Hence, the spare PSV should be set at the pressure same as the first operating PSV because if it being set at higher value, in the case of using spare PSV relieving can be started at a value higher than MAWP and this may not be acceptable in code limitations standpoints...



#3 Pilesar

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Posted 06 June 2019 - 10:33 AM

I believe the first sentence of the originator's initial post is incorrect. There is no mandate that the set pressure of the first PSV be at the MAWP. The setpoint of your primary relief device should be set NO HIGHER THAN the MAWP of the vessel. If your normal operating pressure allows, consider setting the spare PSV at 18 barg. That way you are assured of keeping the staggered setting.



#4 SawsanAli311

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Posted 08 June 2019 - 12:49 PM

Thanks Fallah and Pilesar, 

 

It is absolutely incorrect to set the PSV at a value higher than MAWP, for this purpose setting the spare PSV to be equal to the setting of the primary PSV. Yet, if it is set at 18 barg then I believe the size of this spare PSV would be non-identical to the remaining PSVs and accordingly rated capacity would be different.If this spare PSV becomes the primary one then with 18 barg, the area will have to be increased to limit the code allowable accumulation to 16% otherwise accumulation would be higher than 16% which is not acceptable by code.

 

Thanks

 

Regards, 



#5 Bobby Strain

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Posted 08 June 2019 - 03:09 PM

Accumulation has to do with the vessel, not the PSV. The maximum relieving pressure has nothing to do with the PSV set pressure.

 

Bobby



#6 SawsanAli311

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Posted 09 June 2019 - 01:03 PM

Accumulation has to do with the vessel, not the PSV. The maximum relieving pressure has nothing to do with the PSV set pressure.

 

Bobby

Thank you Mr.Bobby, I agree with you,I may have confused overpressure with accumulation.

 

Can you please advise whether the staggered arrangement common spare PSV should be set as the 1st main PSV. For 2nd & 3rd PSV, as per ASME  code, they shall not be set higher than 5% of MAWP do you think with such close setting, the set pressure tolerances defined in UG-134 as +/- 3% would be affected?



#7 Bobby Strain

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Posted 09 June 2019 - 03:55 PM

Pilesar's advice has merit. And, what you choose will depend on the jurisdiction responsible for enforcing pressure vessel code adherence. And your insurance company requirements. Is this a new facility? Or are you just buying a spare valve as a hedge against downtime? Either option will provide protection for the vessel.

 

Bobby






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