7 replies to this topic

[chem55]

Dear Experts,

 

My Vessel MAWP is 100 Psig and operating pressure is 40 Psig. I have PSV installed on Vessel and Rupture Disc is also there as a secondary safety device. For Blocked outlet we consider 10% accumulation (that means if the PSV is set at 100 psig, it will Fully open at 110 Psig), and For Fire Case we consider 21% Accumulation (that means PSV will Fully open at 121 Psig).

 

If I set my Rupture Disc (of 95% Operating Ratio) at 105 Psig (Code allows 5% above MAWP), does it mean the Rupture Disc will burst before the PSV opens completely (for blocked outlet PSV will Fully open  at 110 Psig.).

 

Please advise.

[latexman]

Yes.  There is nothing wrong with having a PSV set pressure lower than MAWP to give more cushion between primary and secondary devices.

 

My advice is for PSV and RD in parallel, not series.

Edited by latexman, 24 April 2013 - 11:10 AM.

[Raj Mehta]

No, since the rupture disc is placed before the PSV, it will not burst before its set pressure is reached i.e. 105 Psig. Once the rupture disc bursts, PSV will now do its job (since the pressure now is already above the set pressure) and open fully at 110 Psig. 

 

For your reference, API 520, 6th edition, fig - 26, tells that the maximum allowable burst pressure for single rupture disk = design pressure or MAWP = 100 Psig. I don't know why you are selecting 105 Psig. ?

 

Thanks.

[chemsac2]

Chem55,

 

Since you have specified marked burst pressure of RD as 105 psig, I am assuming you have RD and PSV in parallel.

 

In such a case, RD may burst before PSV completely lifts.

 

If RD marked burst pressure is 105 psig, minimum burst pressure considering 5% negative tolerance is 99.75 psig.  

 

PSV overpressure is 10%, but API states that full relief capacity is achieved at 5-6% overpressure itself. For PSV set at 100 psig, if you do not want RD to open, you need to limit overpressure to below 5% and no negative tolerance for RD. Little difficult I would say unless vendor confirms both.

 

You can lower PSV set pressure a bit as you have significant difference in operating and design pressures.

 

Regards,

 

Sachin

[fallah]

Dear Experts,

 

My Vessel MAWP is 100 Psig and operating pressure is 40 Psig.

 

If I set my Rupture Disc (of 95% Operating Ratio) at 105 Psig (Code allows 5% above MAWP)

 

chem55,

 

Is 40 psig considered as normal maximum operating pressure? If so, the operating ratio of the rupture disc cannot be 95%. Please clarify...

[chem55]

Raj, My Rupture Disc is not installed in series with PSV. Rupture Disc and PSV are installed in  Parallel with higher set pressure of Rupture disc so that PSV opens first and if it fails then Rupture Disc is there as a secondary device. Code allows to set the secondary safety device 105% above MAWP. I can provide the reference if you want.

 

Fallah, I am considering the Operating Ratio based on PSV set pressure and Rupture Disc Burst Pressure – considering PSV set Pressure as a reference pressure. As my Rupture Disc is Set @ 105 Psig, if I select the Rupture Disc with 95% Operating Ratio and 0 Manufacturing Range, it will burst at any pressure between 100 Psig (95% of set Pressure) and 105 Psig.

 

I guess it is better that I reduce the PSV set pressure in order to increase the difference between PSV and Rupture Disc SET Pressure in order to accommodate 10% accumulation for PSV.

[Bobby Strain]

Well, if the PSV will provide 100% capacity, then you can specify the rupture disc for 110% or whatever. But you must keep the PSV in service at all times. However, I think maybe you misled us with your statement that the RD is to protect the vessel in the event of PSV failure (meaning failure to open). You need to be very explicit about the function of the rupture disk. If it is intended to protect the vessel when the PSV is out of service for repair or testing, then you need to set the burst pressure at the MAWP of the vessel. In this case, probably both devices will have isolation valves with one closed and the other open and both set at the vessel MAWP or lower. Or some such arrangement. Your description makes us all look like we are tap dancing, each to a different tune.

 

Bobby

[fallah]

Fallah, I am considering the Operating Ratio based on PSV set pressure and Rupture Disc Burst Pressure – considering PSV set Pressure as a reference pressure. As my Rupture Disc is Set @ 105 Psig, if I select the Rupture Disc with 95% Operating Ratio and 0 Manufacturing Range, it will burst at any pressure between 100 Psig (95% of set Pressure) and 105 Psig.

 

I guess it is better that I reduce the PSV set pressure in order to increase the difference between PSV and Rupture Disc SET Pressure in order to accommodate 10% accumulation for PSV.

 

chem55,

 

The operating ratio for rupture disk application is determined by dividing the maximum operating pressure by the burst pressure of the rupture disk. Then it makes no sense using PSV set pressure instead maximum operating pressure for this determination.

 

And yes, you can reduce the PSV set pressure such that you would be able to set the RD at MAWP without having a simultaneous opening of PSV and RD.