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Psv Sizing For Centrifual Pump Discharge


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#1 fanof46

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Posted 10 April 2017 - 04:28 AM

Dear All,

 

I am working on a adequacy check for existing PSVs in a refinery. I have a case where a centrifugal pump discharges to a series of filers. The case here is the downstream filters all have a design pressure of 150 psig and the pump's blocked in pressure turns out to be 156 psig. Now technically I have to provide a PSV as there is 6 psig more than the design  pressure, but even If I provide a PSV, it wont be able to have a full lift as I believe these PSVs would have a 10% overpressure and would not lift completely. So I cant predict the area available for a lift of 156 psig and check adequacy. I want to makes sure if this case is really exist. My thought was if API can allow 10% overpressure can I take credit for that and say that PSV is not required for a blocked outlet? please share your thoughts

 

Thanks,

G

 

P.S: The vessels are also stamped for 150 psig for MAWP



#2 ankur2061

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Posted 10 April 2017 - 05:16 AM

Hi,

 

The maximum pressure that the filters can see due to blocked in condition is:

 

Pump Differential Head (Pressure) due to blocked-in condition from pump curve (zero flow for given impeller and highest liquid sp. gravity)  - Static Head (if filters elevation is above pump discharge nozzle)

 

If the pressure calculated as above exceeds the MAWP of the filters then PSV is required. In case I have a continuous process system which is not fully rated for pressure, which means that one or more elements of the system has a lower design pressure, and, requirement of PSV is established, then I would provide the PSV set pressure as per the weakest element in the process system. If the filter happens to be the weakest element in your system, in terms of lower pressure rating then I would set the PSV set pressure as 150 psig and not the shut-off or blocked in condition.

 

Hope this helps.

 

Regards,

Ankur.



#3 Zauberberg

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Posted 10 April 2017 - 06:15 AM

You need PSV because the equipment MAWP will be exceeded in case of blocked pump discharge. Yes, the overpressure will be in the range of only 4-5% above the MAWP and not sufficient to cause a major failure (i.e. vessel rupture), however it will likely lead to a leaking flange and/or possible damage of fittings, instruments/connections, valve components etc. Those incidents will require money and, even more important, (down)time for repairs. You don't want to risk that because of a few PSV's. Read more on overpressure % in the attached file.

 

Secondly, an external plant audit will always identify such and similar items as non-conformity to the Code(s). You also want to avoid that kind of problems.

 

With regards to the piping itself, B31.3 allows you to expose piping (piping only) to pressures 20% above design if exposure is shorter than 50 hours per event and less than 500 hours/year, for which your case would very likely qualify. This does not apply for vessels, fittings, valves, instruments etc.

 

Attached File  CCPS Overpressure Considerations.pdf   639.73KB   132 downloads

 



#4 Bobby Strain

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Posted 10 April 2017 - 11:20 AM

If you look at the code, your relief valve should provide the required relieving quantity at 10% overpressure. That quantity, in your case, is zero. If the filters are too small to require a PSV for overpressure protection from fire, then you need a PSV for zero flow capacity at 165 psig to meet code requirements. As Dejan says, if this is the case you can consider the filters as piping components.

 

Bobby



#5 fanof46

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Posted 11 April 2017 - 12:04 AM

Thanks for all your replies.

 

To reply for Bobby, the filters i am working are on Amine package and are quite big enough and cant be considered as piping and also as they re stamped, i have to consider them as pressure vessel and do the analysis. 

 

TO reply for Ankur, the fire relief rate is lower than that of the blocked in discharge and so the governing case for the PSV is a blocked outlet and i cant set the PSV for fire

 

Now i am stuck with this, a PSV is required which will not fully lift in the event of blocked outlet which by the way is governing case and i wont be calculate area required for relief rate at the blocked in head. 

 

I am sorry if i havent absorbed all your technical details, i am just learning from this unique case.

 

Thanks,

G



#6 Bobby Strain

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Posted 11 April 2017 - 09:03 AM

Like I said before; the PSV for blocked outlet requires 0 flow at 110 % of PSV set pressure. Is that so difficult?

 

Bobby



#7 mirandomka

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Posted 12 April 2017 - 11:31 AM

Base on your description, the PSV should be set at 150 psig to protect against fire case.

As you have already identified, blocked outlet will not drive the pressure above 10% accumulation over the filter DP,150 psig. Therefore, blocked outlet is not the sizing governing case.

In reality, you will have PSV partially lifted when you have blocked condition ( loss of contents /leakage may be a concern if your discharge is not returned to your auction vessel), however, it's not a design case for the PSV as it's not protecting against anything.

Hopefully, it's clear to you.

#8 mirandomka

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Posted 12 April 2017 - 11:44 AM

I suggest you to go to API520 for set pressure guidance and definitions.

#9 Zubair Exclaim

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Posted 27 April 2017 - 01:00 AM

My piece of advice is .. look at the Minimum allowable flow point of pump ... set PSVs to discharge pressure at that point set the flow rate of PSVs equivalent to pump minimum flow ....  this will land you slightly at or below 156 psig depending on pump curve and wil protect your pumps too



#10 ChemEng01

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Posted 30 April 2017 - 02:00 AM

If Blocked outlet cannot exceed max allowable accumulation of 10% an RV of any size would be OK to protect against blocked outlet. 

 

If thermal expansion / fire is the only other applicable case, then this RV should be sized for this. 

 

Have you considered pump overspeed as a relief case? 


Edited by ChemEng01, 30 April 2017 - 02:11 AM.


#11 balatpl

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Posted 01 May 2017 - 01:34 PM

It is not always necessary to set the relief valve set pressure same as design pressure, as is usually considered by many designers.

 

1. You may set a slightly lower pressure so that mechanical integrity is always ensured. 

 

2. This pressure could be the calculated from the head equivalent to min flow rate of the pump. 

 

3. Use the as-built capacity curves submitted by pump vendor.

 

4. However, add a margin above this, but less than the shut off pressure.






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