Pressure Relief Valve Calculator
Design Philosophy
(Source: CPHEEO Manual on Water Supply and Treatment, 2013; supplemented by IS 2906)
A Pressure Relief Valve (PRV) is a safety device installed on a pressurized pipeline or vessel to protect it from overpressure by allowing a controlled discharge of fluid when the internal pressure exceeds the set pressure. In water supply systems, PRVs are installed at the delivery side of pumps, at the base of long rising mains, and at the inlet of service reservoirs to limit transient surge pressures.
The sizing of a PRV orifice is based on the principle of orifice flow under pressure. The required orifice area is determined such that the valve can pass the full design flow at the set pressure without the upstream pressure rising above the permissible limit. The governing orifice flow equation is:
Rearranging for the required orifice area:
A = Q / (Cd × √(2 × g × H))
where,
Q : Design flow rate (m³/s)
Cd : Coefficient of discharge (dimensionless); typically, 0.62–0.70 for sharp-edged orifice
A : Required orifice area (m²)
g : Acceleration due to gravity = 9.81 m/s²
H : Differential pressure head across the valve (m)
Discharge Coefficient (Cd)
The discharge coefficient accounts for the contraction of the flow jet at the vena contracta and frictional losses through the valve body. For a standard sharp-edged circular orifice, Cd = 0.62 is the theoretical value. Valve manufacturers typically specify Cd between 0.65 and 0.80 depending on the valve type and internals. The default value of 0.65 is recommended as a conservative design assumption unless the manufacturer’s certified Cd is available.
Multiple Headers
Where multiple PRV headers are provided in parallel (for redundancy or flow splitting), the total required orifice area is divided equally among the active headers. Each header is then sized independently for the proportional flow.
Calculator
(Note – all Yellow are user defined value, all white are fixed values, all green are calculated value, all orange is calculated not to be shown)
Instructions
Enter values highlighted in the Yellow
Green highlights are the results
In case of any invalid result of non-functioning of the calculator, please write to us at design@swaenviro.com. We will try and help you with your sizing and calculation
INPUT
| Parameter | Values | Units | Remarks |
|---|---|---|---|
| Enter flowrate and choose the unit | Select unit from dropdown | ||
| Flowrate | 5.100 | cum/s | |
| Pressure Rating | 6.3 | kg/cm2 | |
| Discharge Coefficient | 0.65 | ||
| No. of headers | 4 | ||
| Output values | |||
| Flow | 18360 | m3/hr | |
| Area Required | 3537.17965116913 | sq-cm | |
| Area required, per header | 884.29 | sq-cm | |
| Diameter, Required | 33.56 | cm | |
| Diameter, Required | 335.63 | mm | |
| Diameter, Provided | mm | ||