Allowable Back Pressure per ISO-4216 Part9

[Pavan Kumar]

Hi All,

I am asked to calculate the Back Pressure on a PSV installed on Electric Boiler. The PSV is sized per ISO-4126 Part 7. I understood the back pressure in the discharge line is as per ISO-4216 Part 9. I do not have access to this standard, I wanted to know from you if you know what is allowable back pressure on Conventional, Balanced Bellows and Pilot Operated PSVs per this Code. I know it is 10% per API 521 Part II for Conventional PSVs, up to 50% for Balanced Bellows.

Thanks and Regards,
Pavan Kumar

 

[The Obturator]

There is no back pressure restriction in ISO-4126. Back pressure is limited by the design of the Pressure-relief Valve and is the same as in ASME XIII, API etc.

Note that the limits in API-520 stop at 50%. This does not mean that the valve is limited to this percentage. Each manufacture will have their own limitations by valve type/design that will either be below or above the published limits. High variable back pressures above 50% are generally be catered for by pilot operated designs, but some manufacturers have spring operated designs that can also handle such. Its a case by case issue with further discussion with the vendor.

 

[Pavan Kumar]

Hi Obturator,

Is there a limitation imposed by the Boiler Code of Construction, in this Case EN -12953-1. ASME Sec I for Power boilers just says the back pressure is limited to the extent that will start reducing the PSV capacity. So it looks it is up to the PSV Type and Manufacturer. Is ASME Sec VIII Div 1 any different?.

Thanks and Regards,
Pavan Kumar

 

[The Obturator]

No different. Back pressure at certain values, reduces the flow. API 520 pt. I explains these scenarios, and what back pressure correction factor is applied to the sizing/flow calculation, based on the type of back pressure and valve type.

 

[The Obturator]

Just to be clear,

ISO 4126-9
Paragraph 7.1 states "...The allowable back pressure, which is the sum of the built-up back pressure, and the superimposed back pressure, is typically specified by the valve manufacturer or national code or regulation".

ie., As I stated earlier, back pressure is limited by each design type of Pressure-relief Valve.

ISO 4126- 9 Annex D
Provides calculation method of built up back pressure (for use in ISO 4126-7 sizing calculations).