API-520/-526 Lists Effective Orifice Areas for Flanged Steel Pressure Relief Valves (PRV). These are represented by Letters (D, E, F…Q, R. T) each having a corresponding cross-sectional area.
Effective orifice areas should only be used when sizing using the API-520 method, together with the stated Effective Coefficient of Discharge (eg.0.975 for vapour).
The purpose of this, as indicated in API-520, is to use an assumed value for the Coefficient of Discharge and Effective Orifice Area, in order to bring all manufacturers onto a level playing field at enquiry stage. ie., They will all calculate and select the same orifice area. API-520 is a user standard.
This is also because all PRV manufacturers have different actual coefficients of discharge and actual (as measured) orifice areas, due to the design differences in their PRV product lines...
(There is history of how this all came about, but it does not apply to this current practice).
Once a manufacturer and/or PRV Type is selected, the Manufacturers Actual Coefficient of Discharge ‘K’ and Actual Orifice Area ‘A’ is used together to calculate the Actual Rated Capacity, (which will be at least equal or greater than the initial API-520 effective method).
This method is often referred to as ASME Sizing and should be used from order stage.
Manufacturer Actual Coefficients of Discharge and Orifice Areas are usually stated in vendor catalogues. They are also published by the USA based National Board of Boiler & Pressure Vessel Inspectors (NBBPVI) in what was the “Red Book” NB-18 now updated weekly online https://www.nationalboard.org/SiteDocuments/NB18/NB18.pdf
Relevant Notes
In 1962, ASME De Rated PRV certified capacities by a safety factor of 10%.
Subsequently, Actual Coefficient of Discharge ‘K’ = KD x 0.90
Where KD = Average Coefficient of Discharge from a series of PRV flow tests conducted by the NBBPVI against the ASME Boiler & Pressure Vessel Code requirements (BPVC ASME VIII).
A manufacturers PRV Type listed in the current NBBPVI NB-18 accreditation, means also that that manufacturer can ASME Code Stamp that PRV for the certified fluid. No other document is used.
Quite simply, use the published K & A figures in NB-18 (K is already de rated).
The API listed Orifice Letters (D, E, F…Q, R, T) are often used when meaning Actual Areas, as in the NB-18. Always be sure which one you are referring to. Actual Area differs by manufacturer but will be cross referenced by the API Letter. (API Effective sizes are the same for everybody).
Example. A "J" Orifice will mean a 1.287in2 API- Effective Area, and also a 1.453in2 Actual Area for one typical manufacturer (It is the same nozzle part).
There is no such thing as “ASME Approved or Certified”. ASME provides the BPVC (specification) and the NBBPVI independently certifies the flow capacity/coefficient
Calculations for ASME actual area and rated capacity performed using the ASME Actual method, will meet or exceed those initially done by effective API-520 sizing (in practice not by much).
Users often (wrongly) believe that ASME Actual sizing gives them added capacity. Unfortunately, and all too often, they have misused and/or mixed their Effective and Actual figures.
Most manufacturers sizing programs provide the option whether to use effective or actual sizing. Some provide results for both. Be sure you know what you are reading and quoting.
Actual area and rated capacity should always be the figures used once the valve manufacturer/PRV Type is established. That’s what the PRV will ACTUALLY pass in terms of capacity. albeit with the 10% Derating Factor already built in (KD x 0.90 Coefficient).
Published Actual Coefficients of discharge (sometimes areas), change according to the certified flow test fluid used (liquid, air, steam) - as can newly listed restricted lift options (different sizing). NB-18 will have different results for the same design tested on alternate fluids.
The ASME BPVC calculation method is the same as the API-520 method except API only states effective areas and coefficients (API also adds other intervening factors such as back pressure correction etc.).
GOLDEN RULE:
NEVER MIX API EFFECTIVE & ASME ACTUAL, AREAS AND COEFFICIENTS.
This document specifically applies to pressure relief valves following API-526 listings, with designs to ASME BPVC VIII (2019), and sizing commentary taken from API Standard 520 Ninth Edition July 2014 (Section 5.2).
