Effective orifice area 1

[Safety and Control]

Can anyone explain how effective Orifice area is calculated in api-520?
Eg: D- 0.11 in2
T- 26 in2
How these area is calculated?

 

[RVAmeche]

It's based on the API 520 assumed discharge coefficient. The API discharge coefficient and orifice area are used to get you in the ballpark.

Then you go to catalogs and find a manufacturer's ASME orifice area and ASME discharge coefficient; that will be what the valve is certified to have.

If you're asking how do you determine what preliminary size you need; you use the area sizing equations in API 520 for sonic/subsonic/liquid flow.

 

[The Obturator]

Those are not calculated areas but standard orifice areas as listed in API-520

They are labelled EFFECTIVE Areas and are used with EFFECTIVE coefficients of discharge listed in API-520 (0.975 for gas/vapour).

API-520 is a Recommended Practice. The calculations used within are to calculate and select an effective orfice size. The idea here is to determine a PRELIMINARY size in order that to put all manufacturer sizes on the level.

Once that size is established, the manufacturer would normally provide his calculations using his specific products using the ACTUAL (ie. as measured) area and ACTUAL coefficient of discharge Kd - this being the one granted by National Board of Pressure and Vessel Inspectors (USA) and listed in the NB-18 "Red Book". The coefficient of discharge is de-rated by 10 to give Kd to be used in calculation.

You will find actual areas larger than effective and different from each manufacturer due to design. Actual coefficients are smaller than API 0.975. The approximate same size is resulted using final actual calculation Vs API effective. NEVER mix actual and effective areas/coefficients.

Actual area sizing is sometimes called ASME sizing. Which it basically is since the National Board tests are in line with ASME VIII requirements for Unfired Vessels from which we get the "UV" Code Stamp authorisation by the National Board.

Once calculated size is determined, you generally select the next standard API Orifice area larger in size than the calculated area. Based on the selected API area, you can then calculated what the valve can pass "the rated capacity".



Per ISO, only the term Safety Valve is used regardless of application or design.