Engineering Practice: Discharge Coefficients and Flow Resistance Factors
By Dilip Das Bayer CropScience |
This article presents the fundamental concepts of the discharge coefficient, KD, of a relief valve or a safety relief valve, and flow resistance factor, KR, of a rupture disk, and their proper use in sizing an emergency relief system. When these devices are used to protect a pressure vessel, they must be certified according to ASME code in the U.S. Both the discharge coefficient and the flow resistance factor are dimensionless numbers determined by measurement and theoretical computation. These factors are prescribed in Performance Test Code (PTC) 25 of ASME VIII, Division 1.
The discharge coefficient of a relief valve or safety relief valve is the ratio of the measured relieving capacity of the valve to its theoretical relieving capacity computed by a prescriptive theoretical equation matching the test fluid, test pressure, temperature and flow area of the valve. In the determination of the discharge coefficient, there is a choice of the test fluid among steam, air, gas or water. The deviation of the discharge coefficient from the value of unity comes from the deficiency of either the flow model (mass flux through the valve as a function of density and pressure) or the fluid model (density as a function of pressure),…