API Recommended Practice 520, Part I provides (in Section 4.5.1 of the 6th  edition) the standard sizing equation for liquid releases that are not expected to flash across the relief valve. The flow is a function of the relief valve nozzle area, the liquid density, and the pressure drop across the valve. This equation includes correction factors for the liquid’s viscosity (Kv), the back pressure (Kw) (in the case of a balanced valve), and flow non-ideality (Kd). The viscosity correction factor decreases from a value of one only as the flow becomes highly viscous. Values of Kv as a function of the Reynolds number are given in RP 520. Values of Kw for balanced relief valves should normally be obtained from the valve manufacturer; if the value for the specific valve under consideration is unavailable, the generic curve given in API RP 520 may be used.
Values of the discharge coefficient (Kd) are determined experimentally in the process of the ASME-required certification of an individual pressure relief valve model’s flow capacity. Kd is simply the valve’s measured flow capacity divided by the valve’s theoretical capacity under the same conditions of relief pressure, temperature, and fluid. Values of Kd are given in manufacturer’s catalogs, and are published in NB-18, Pressure Relief Device Certifications. Note that in calculating the theoretical flow capacity, the valve nozzle’s actual discharge area is used. Therefore, this measured value of Kd must be used only with the actual discharge area when calculating the valve’s flow capacity. The measured value of Kd and the actual discharge area are a “matched set,” and should always be used as such. This point is raised because API Standard 526 sets forth “standard orifice areas” for flanged pressure relief devices. In the absence of specific manufacturer data for Kd, API recommends a value of 0.62 for liquid flow calculations. This value should be used only with the standard orifice areas (a matched set different from the model-specific set). Mixing a measured Kd with a standard orifice area or an actual area with API’s default Kd will generally lead to inaccurate calculation of a pressure relief valve’s flow capacity.
The ASME Pressure Vessel Code formerly permitted 25% overpressure for liquid PSVs to reach full capacity; therefore, the Company sized liquid PSVs using 25% overpressure. The ASME Pressure Vessel Code was revised in 1985 to require that liquid PSVs pass their full rated capacity with 10% overpressure. The latest designs of liquid PSVs can meet this requirement. When reusing old liquid PSVs designed for 25% overpressure, it is necessary to recalculate the PSV capacity using 10% overpressure. Routine maintenance of old PSVs does not require recalculation with 10% overpressure unless the relieving case has changed.