3 replies to this topic

[achilles09]

Hello everyone,

While going through API RP 520 PART- I got this term " SUPPLEMENTAL PSV " for additional hazard.

In case of multiple PSVs we used to set first valve @ 100 % of MAWP and remaining valves @ 105 % MAWP.

API recommends to set supplemental valve @ 110 % of MAWP apart from the other valves . It also mentions that supplemental valve is used for additional hazard such as " Fire".

To make my point clear,

Let us consider Set pressure Accumulated pressure
Valve A -First valve 100 121
Valve B -Second valve 105 121
Valve C -Supplemental valve 110 121

All values are % of MAWP.

My question,

* While in multiple valve installation fire case scenario to be handled by separate valve such as " Supplemental valve ".Why it cannot be taken care by valve A & B ???

* If the valve C is used for fire case alone it defeats the purpose of accumulated pressure of 21 %. Since this valve is set @ 110 % of MAWP. Actual accumulation is 10 % only ???

Request to correct me if am wrong in my opinion. Waiting for ur feedback.

regards,

Achilles

[fallah]

Hello everyone,

While going through API RP 520 PART- I got this term " SUPPLEMENTAL PSV " for additional hazard.

In case of multiple PSVs we used to set first valve @ 100 % of MAWP and remaining valves @ 105 % MAWP.

API recommends to set supplemental valve @ 110 % of MAWP apart from the other valves . It also mentions that supplemental valve is used for additional hazard such as " Fire".

To make my point clear,

Let us consider Set pressure Accumulated pressure
Valve A -First valve 100 121
Valve B -Second valve 105 121
Valve C -Supplemental valve 110 121

All values are % of MAWP.

My question,

* While in multiple valve installation fire case scenario to be handled by separate valve such as " Supplemental valve ".Why it cannot be taken care by valve A & B ???

* If the valve C is used for fire case alone it defeats the purpose of accumulated pressure of 21 %. Since this valve is set @ 110 % of MAWP. Actual accumulation is 10 % only ???

Request to correct me if am wrong in my opinion. Waiting for ur feedback.

regards,

Achilles

To answer your first question:

I think the supplimental valve is used in the cases with the following conditions:

a-The fire scenario is govern case

b-The difference between fire relief load and maximum relief load among other scenarios is considerable such that considering fire relif load in integrated PSV system sizing may lead to chattering of the first PSV in nonfire contingencies (in this logic fire case would be assumed as a relatively rare case).

Therefore it is logical to consider separate PSV for the additional load produced in fire case with respect to maximum relief load among other nonfire cases.

To answer your second question:

As Accumulation is the pressure increase over the MAWP allowed during discharge through the PSV,then 10% overpressure on the set point (110% of MAWP) lead to relieving pressure of 121% as maximum accumulated pressure and would be fully acceptable.

[sheiko]

* If the valve C is used for fire case alone it defeats the purpose of accumulated pressure of 21 %. Since this valve is set @ 110 % of MAWP. Actual accumulation is 10 % only ???

Remember: Maximum accumulated pressure = MAWP + accumulation = set pressure + overpressure
Usually, set pressure is taken equal to the design pressure of the protected equipement or piping. Design pressure is lower than or equal to MAWP.

Edited by sheiko, 08 October 2010 - 04:08 PM.

[Qalander (Chem)]

* If the valve C is used for fire case alone it defeats the purpose of accumulated pressure of 21 %. Since this valve is set @ 110 % of MAWP. Actual accumulation is 10 % only ???

Remember: Maximum accumulated pressure = MAWP + accumulation = set pressure + overpressure
Usually, set pressure is taken equal to the design pressure of the protected equipement or piping. Design pressure is lower than or equal to MAWP.

Dears, Although quiet a lot is said/ discussed but for safety reasons

Please consider it to be mandatorily ensured that

• In no case maximum pressure to which any vessel or containment (is internally subjected to) may exceed its stress withstanding limit for a particular temperature i,.e design pressure
• As such the reference made to MAWP is not independent of the stress value considered for design thus the figures referred above will have to be trimmed/ modified accordingly;kindly appreciate.
• In order not to subject vessels,containments in question to higher stress values pressures.

Please be extra careful in doing so i.e. all values during pressure reliving/ withholding must stay below the design value

Thanks and Hopes for Best/safest operations.(Please do correct me if wrong?)