5 replies to this topic

[US_ChemE]

Hello,

 

Normally spring setting for conventional PSV is adjusted to take account of the superimposed backpressure. However, if we refer to API 520 Part 1 Clause 5.3.2.3, it allows to use conventional PSV without adjusting spring setting provided the allowable accumulation is not exceeded during relief. In doing so, do we have to set PSV lower than MAWP or we set the PSV at MAWP but in service it will open at pressure above MAWP (due to superimposed backpressure) and will be open fully at pressure upto maximum accumulation?

 

Although we do not cross the accumulation beyond that allowed by code but we start opening the PSV late than required by code in second approach. Are we allowed to do this?

 

Thanks in advance for your valuable advice.

 

Regards,

US_ChemE 

[fallah]

US_ChemE,

 

In conventional PSV always "set pressure+(overpressure percent multiplied by set pressure)*100" should be equal to or lower than "MAWP+Maximum accumulation"...

 

Then suppose you have a vessel with MAWP=100 barg and the superimposed back pressure is 10 barg. You cannot set the PSV higher than 90 barg without adjusting spring because at set pressure equal to 90 barg the PSV will be started to be opened at 100 barg due to superimposed back pressure. In this case the allowable overpressure would be around 22% of the set pressure in nonfire scenarios.

[US_ChemE]

Thank you Fallah for your response.

 

When we say that

 

In conventional PSV always "set pressure+(overpressure percent multiplied by set pressure)*100" should be equal to or lower than "MAWP+Maximum accumulation"...

 

Are we allowed to have set pressure same as MAWP and reduce the overpressure? When set pressure is set as MAWP, but due to superimposed backpressure it will open at pressure higher than set pressure. However, due to reduction in allowable overpressure, it will not exceed the allowable accumulation of the protected equipment?

 

For example-

 

MAWP= 100 barg

Allowable accumulation for protected system= 10% of MAWP 

Superimposed backpressure= 1 barg

Set pressure= 100 barg (without adjusting spring setting)

Actual PSV opening pressure in service= 101 barg

Overpressure for which PSV is specified= 9 bar (9% of PSV set pressure)

Relieving condition pressure= 110 barg which is same as maximum accumulated pressure

 

Is this allowed?

 

Thanks & Regards,

US_ChemE

[fallah]

US_ChemE,

 

For such low superimposed back pressure because the allowable accumulation isn't exceeded during relief, it is allowed.

But suppose the superimposed back pressure in the example you submitted to be 8 barg (same relieving pressure of 110 barg then overpressure would still be 10% from which 8% will be in charge of superimpoed back pressure and remained 2% free of that charge); means during an overpressure the vessel might be in operation at pressure close to 108 barg for a long time while the PSV wouldn't pop up and this couldn't be acceptable in code's requirement standpoint.

Edited by fallah, 22 August 2013 - 01:17 AM.

[US_ChemE]

Thanks for the feedback. I was wondering if opening of single PSV above MAWP is allowed per ASME Section VIII Division 1, UG-134 clauses (a) and ©. It clearly says that effect of constant backpressure shall be accounted for in setting the set pressure. If allowing single PSV to open in service above MAWP, then is there any limit for the maximum opening pressure above MAWP (I understand it should be as low as possible, but do we have any specific value allowed by code)?

[fallah]

US_ChemE,

 

There is no specific value by code...It  is better to avoid such setting but if we have to do so, as you mentioned, the value above MAWP should be as low as possible...