1:Determine the size  of the valve needed based on the system demands. The term fall-off refers to the difference between the pressures on the valve outlet and the inlet pressure. If the demand is more, there will be a greater fall-off. Study the performance curve provided by the valve manufacturer. These charts, also known as valve capacity charts, show a plot of the flow rate against the pressure fall-off for valves of various sizes. Choose the chart for the maximum flow rate of your system and locate the fall-off for this flow rate. Select the valve size that gives a fall-off in the range of 10 to 15 psi for your system flow rate.

2:Determine theparameters by measuring the inlet pressure. Choose the outlet pressure you require, which should typically be in the range of 50 to 70 psi for domestic applications. Decide on the minimum and maximum flow rates you need to maintain in gallons per minute or liters per minute.

3: Consider the inlet water pressure. Pressure regulating valves can be direct acting, piston operated or externally piloted models. Choose direct acting valves for flows ranging from low to moderate. For moderate to high flow rates, use piston operated or externally piloted pressure reducing valves.

4: Choose the right valve size needed. Choosing the right size is critical to the longevity of the pressure reducing valve. Over-sized valves will lead to noise, valve failure and wire draw during low flow conditions. If more than one size of valve meets your required capacity, choose the valve that is less expensive and allows you to use smaller diameter pipes.