Sizing of an additional PSV on a brownfield system 3

[Sam_89]

Hi All,

I have an existing PSV installed at site sized for blocked outlet (required flow 364,536 kg/h / rated flow 432,371 kg/h).

The flow rate through my system is now increasing to 561,815 kg/h and hence we will add an additional smaller PSV to cover the extra flow.

When sizing the new PSV do I take credit of the required or rated flow of the existing PSV ?

i.e. am I sizing the new PSV for 197,279 kg/h or 129,444 kg/h ?

 

[The Obturator]

If you do sizing by the method suggested by you adding to the existing rated capacity of the existing valve, you run a real risk of oversizing. Size and select against the total required condition only. You should ideally run sizing on a the new total flow required, then select the size required, being vary on the size you already have. Since there will be 2 PRV's protecting the system, you will need to set one of valves at 5% higher than MAWP/Set Pressure. This is called staggering the set pressure so as not to result in valve chattering due to a sudden large area at set point. This is allowed by ASME VIII and API-520. If you use a vendors sizing programme, they should have options for multiple PRV calculations and allow you the option of 16% overpressure for the valve at normal set pressure.

*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Sam_89]

Thanks for the quick reply.

I am familiar with staggering and the 16% overpressure.

I agree that you should size for the "total required condition". Based on your answer the new PSV will be sized as follows:

561,815 kg/h (Total required condition)- 432,371 kg/h(existing PSV rated capacity) = 129,444 kg/h (new PSV required capacity).

 

[The Obturator]

Hi again. It'll work, just bear in mind calculated capacities will be influenced by the overpressure used (10 or 16% in your case). Hence I suggested using a programme that allows you to do this. Presumably, the 2nd 'new' PRV will be the one set at 105 % MAWP in which case the 1st (original single) PRV will need to be recalculated using 16 % overpressure (and increasing the stated rated capacity)

*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Sam_89]

You are right about the need to check and re-confirm against the 16% overpressure.

Thanks for your help. Have a great day!

 

[MortenA]

If this is en EU remember that PED does NOT allow for stacking to 16% overpressure - but you CAN adjust the SP of both valves so that the max pressure is below 110% of MAWP.

say your MAWP is 99 barg - and your max relief pressure then 109 barg.

In eu you are not allow to have max pressure=115 barg

But if the first valve has a SP of say 93 barg, then theres a 6% margin to your next PSV with a SP of 109 barg and a stacked pressure of 109 barg (or 10% on actual pressure)

However, remember than you are now "eating" of your max operating pressure since margins for blowdown etc starts at 93 barg instead of 99 barg!

--- Best regards, Morten Andersen

 

[don1980]

Adding to Morten's comments, this PED restriction (limiting the relieving pressure to 110% of MAWP even when multiple PSVs are used) effectively undermines the benefit of setting the second PSV at 105% MAWP. That's because the second valve isn't fully open - there's not enough overpressure for this PSV to reach its rated capacity. Thus, one is incapable of sizing that partially open PSV. I think this was a detail that was mistakenly overlooked by the authors of PED.

 

[georgeverghese]

This is a very large load. Reaction forces on downstream piping and nozzles can be onerous. In such situations, it is better to set a smaller PSV at 100% of set pressure and the larger PSV at 105% of set pressure. With this setup, the load from many relief incidents may fall within the envelope of the smaller PSV which will lift first, thus preventing the larger one from activation. That is much safer.

 

[MortenA]

@Don1980, i would say that its against regulation to have a SP at 105% unless you can demonstrate that max capacity will be met at around 104% for this valve...

I think my suggestion to set the first valve SP lower than 100% MAWP if you need stacked PSV's

--- Best regards, Morten Andersen

 

[MortenA]

@georgevergese, IMO in EU this will be against code (PED) unless you can prove that 110% will not be exceeded (expect for fire case)

--- Best regards, Morten Andersen

 

[don1980]

@Don1980, i would say that its against regulation to have a SP at 105% unless you can demonstrate that max capacity will be met at around 104% for this valve...

Morten, I agree that the only practical option is to lower the set pressure of the first PRD.

My point was that PED actively states that one can set the second device at 105% MAWP. So it's not against regulations to do that, but by saying this, the PED authors apparently didn't recognize the trap they set for the users who attempt to implement this option.

They didn't recognize the "Catch-22". It makes me think of Cap. Yossarian's dilemma in trying to set up an appointment with Major Major. Sure, he's free to do that, but he can only do so when Major Major is not in!!

 

[The Obturator]

There are of course European based manufacturers that have obtained certification for 5% overpressure for 'full lift' PRV's on compressible fluids. This certification, for example, follows German AD 2000 Merkblatt A2 and satisfies current PED and ISO-4126-1.

Not forgetting that there is also the PED Guideline 5/2 which basically allows overpressure higher than 10% in case of fire relief.

I agree that the PED is in need of some edits.





*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[georgeverghese]

Morten,
Yes, for European applications, max relieving pressure for this case will need to stay within 110% of set pressure.
So permissible overpressure for the larger PSV set at 105% will have to be 5% - the OP will have to ask the PSV manufacturer what the corresponding value for the coeff of discharge for 5% overpressure.
If I remember revised Shell standards correctly, even for firecase, the max relieving pressure for multiple PSVs' with staggered setpoints is still 110% of MAWP.

 

[MortenA]

@Obturator, ok great to know star for you. With regards to the firecase - yes but here i think its logical. After a fire you cant just put he vessel back in service.

--- Best regards, Morten Andersen

 

[The Obturator]

Thanks for the acknowledgement MortenA.

*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***