Actual orifice area vs effective area?

[tr6]

Why is a valve's actual (certified) area less than the effective area, which is used in flow formulas to determine the capacity of a relief valve?

For example, for an API "L" orifice (effective area = 2.853 sq in) the certified orifice areas (ref National Board NB 18) for Crosby and Farris are:
Crosby: 2.025 sq in
Farris: 2.009 sq in

I would have thought that the "effective area" used for flow capacity calcs would have been less than the "actual" to provide a bit of unpublished flow capacity.

This came up as I was performing a Caesar relief valve dynamic analysis that required the "minimum diameter of the relief valve throat". At first I used the "API" orifice diameter and the calculated capacity was almost twice the published capacity for the valve. Once I used the "actual - certified" value for the particular valve, the calculated capacity was pretty close to the published value for an "L" orifice.

Anybody have any thoughts?

 

[CMA010]

Actual area is larger than the API effective area. This is due to the 0.9 ASME de-rating factor of the discharge coefficient. In order to use the API discharge coefficients and standard sizes manufacturers have chosen to increase the area. Calculating the certified capacity using the certified de-rated discharge coefficient and the actual area gives approximately the same capacity (or required area) as using the API discharge coefficients and the standard API effective areas. Check the Consolidated catalogue.

 

[tr6]

CMA010,
My mistake.

Thanks, you made me look closer at the two values.

I was comparing API effective area to actual Orifice Diameter. I hate it when that happens.

Regards
Bill