7 replies to this topic

[splendid.ye]

Dear Forum members,

 

I have got one question regarding PSV allowable overpressure.

As far as I understand 10 % overpressure can be applied to non fire case and 21% to fire case.

This is obvious to me.

However, the thing is when there are several cases for one PSV including fire case and non-fire case,

let's say, a PSV should cover blocked outlet case, CW failure case and fire case and the fire case is governing case for the PSV then which allowable overpressure can I apply 10% or 21%?

I assume 21% will be applied if a PSV only has one case which is fire case not the other credible cases which are non-fire for a PSV. So 10% overpressure should be applied.

Please let me know if my understanding is correct.

 

Thanks in advance.

Edited by splendid.ye, 08 November 2016 - 09:08 PM.

[fallah]

 

I have got one question regarding PSV allowable overpressure.

As far as I understand 10 % overpressure can be applied to non fire case and 21% to fire case.

This is obvious to me.

However, the thing is when there are several cases for one PSV including fire case and non-fire case,

let's say, a PSV should cover blocked outlet case, CW failure case and fire case and the fire case is governing case for the PSV then which allowable overpressure can I apply 10% or 21%?

I assume 21% will be applied if a PSV only has one case which is fire case not the other credible cases which are non-fire for a PSV. So 10% overpressure should be applied.

Please let me know if my understanding is correct.

 

 

Hi,

 

As you mentioned the overpressure for fire case is to be considered 21% and for other cases, if any, 10% of the PSV set pressure...

 

At the stage of specifying sizing/governing case of a PSV you will specify required relieving area for all cases based on relevant overpressure and will select the case with highest required relieving area as PSV sizing case...

 

If the PSV would be sized based on the sizing case which can be fire case or another case, it will handle other credible cases might or might not be included fire case...
 

[mirandomka]

Definition of PSV overpressure is based on your relieving pressure over your PSV set pressure.
Therefore, for non fire case usually is 10% if your set pressure = MAWP and for fire case is 21%.
Your reported relief load and relieving case and relieving pressure should be consistent.
If you are reporting fire case, then it should be 21%.

[splendid.ye]

I appreciate for all your valuable replies.

I know the sizing procedure of PSVs as fallah mentioned and I have always followed that procedure.

But there's still one thing that I am confused.

 

If a PSV is sized based on the fire case among the other cases, it will be designed to be fully opened at 21% overpressure.

When it is designed to be fully opened at 21%, it would not be fully opened for the other non fire case and then how the other cases can be handled by this not fully opend PSV?

Is it possible to handle other credible cases even though it is not fully opend just because its size is decided based on the highest required relieving area?

And is my understanding that the PSV wouldn't be fully opened for non-fire case correct?

If it is wrong, please correct me.

 

Thanks in advance.

[fallah]

Hi,

If the PSV has been sized to handle the fire case as sizing case; yes it will be full open at fire case relieving conditions with the OP of 21% but it will not be in full open position at relieving conditions in other non fire cases although it might will open fully at the first moments of non fire cases relieving...

[splendid.ye]

Thank you very much fallah.

 

Now it is now really clear to me.

Though, can I ask one more thing?

 

As you mentioned, PSV might will open fully at the very first moments of non fire cases relieving.

Then the equpipment protected by this PSV will undergo 21% overpressure of design pressure or MAWP for very short moment.

Would it be acceptable when code allows 10% overpressure for non fire case?

I assume it would be OK becasuse, even though, code allows 10% overpressure for non fire cases, the duration of 21% is very short.

And most importanly the equipment's allowable stress would be much higer so the equpipment will well be protected.

Is my understanding is correct?

 

Sorry for the too much question.

[mirandomka]

PSV starts opening at the set pressure not relieving pressure.

For compressible fluid, expansive force will full lift the valve at first as Falla explained before reach to maximum overpressure allowed. For liquid or incompressible fluid, valve opens on the reactive force, (%lift becomes a more of a function of flow rate than only pressure) your valve may not be able to achieve full lift, but it will be certified for the rated flow by Vendor at the allowed overpressure.

In another word, as long as you set your PSV correctly, sized bigger than required for all cases and piping size is adequate (i.e. overall pressure drop below overpressure for conventional PSV), the equipment will not experience over pressure more than what is allowed by code.

Another point I want to emphasize is, the CODE is the CODE. For non fire case, equipment shall not experience 10% overpressure as mandated by ASME CODE.

And this is verified as your selected PSV orifice capacity is greater than your required load at 10% over pressure for the credible cases.

21% over pressure is ALLOWED by ASME code ONLY for fire case.

Edited by mirandomka, 11 November 2016 - 12:13 AM.

[fallah]

Hi,

If the fire case is sizing case for a PSV means the other cases will need much lower relieving area than the fire case hence in non fire cases neither the PSV will fully open nor the OP will reach to 21% then the full open position in the first moments of relieving may be just due to PSV pop action.

Obviously if in a non fire case the PSV will fully open; the OP will certainly be lower than 10%....