Rated coefficient of Discharge for PSV 2

[Sawsan311]

Dear All,

I would like to enquire about the following case:
- existing PSV was originally designed for fire case and using the required relieving capacity, the calculated ASME area was estimated by vendor using the rated coefficient of discharge. the rated capacity corresponding to that PSV and that FIRE CASE was subsequently calculated using the selected actual area of the PSV.
- In case later on , an overpressure scenario like gas blow by arises with requirement to be checked at 10%, will there be any requirement to recheck with vendor about the rated coefficient of discharge or that would be independent since capacity certification is always carried out at 10% accumulation and using air or steam even for fire case scenarios?

Thanks regards,

 

[The Obturator]

The simple answer is No. ASME VIII requires certification only at 10%. No testing is done at higher overpressures such as 21 % on fire.

If the PRV is to be 'UV/NB' stamped, the PRV will also be stamped with the certified capacity of the equivalent flow of air, saturated steam or water at 10 % overpressure regardless of whether the PRV will see 10% or 21% overpressure in service.

The valve size will be determined using the overpressure for the selected relieving case. The certified ASME coefficient of discharge does not change.

A PRV designed to ASME VIII must attain full lift at 10 %. Full lift is D/4 (D = Diameter of nozzle bore/4 = maximum possible flow area/curtain area) ie., whether 10% or 21% overpressure, this is the largest flow area available.

Understand also the different capacities applicable to a PRV;-

Required Capacity - the user supplied capacity, usually the worst case scenario. This the the capacity used to size a PRV. The minimum it should pass.
Rated Capacity - The maximum capacity the selected size/type of PRV can pass of the given fluid at the stated overpressure relative to the relieving case..
Certified* (or stamped) Capacity - The nameplate stamped capacity of either air, sat steam or water, the PRV has been certified to pass at set pressure with 10 % overpressure

(* - Certified as by the National Board and published in the 'Red Book' NB-18).



*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Sawsan311]

Thank you very much The Obturator , I would also like to enquire on the following:

in one of the vendor data sheets, I saw that vendor calculated first the effective ASME calculated area using the rated coefficient of discharge followed by calculating the rated capacity at full lift based on the selected PSV area ratio to the calculated ASME area and the required relieving rate.

Is this a correct approach?

Regards,

 

[The Obturator]

That calculation explanation doesn't sound right

There are two types of calculation for PRV''s;-

1. Preliminary/API Effective
Using API-520 listed effective coefficient and effective areas. These are done to predetermine a size of PRV. All vendors equal at this stage.

2. Actual or ASME sizing using the certified coefficient and actual area listed in NB-18. This is usually the calculation the vendor provides, though the data sheet may also list the effective area for reference. ASME certified coefficient and actual area is unique to a vendors particular PRV design type.

Never mix effective and actual data.

Please look up my FAQ in this forum. Click above.

*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Sawsan311]

thank you very much Mr.Obturator