principle

After steam is depressurized by the pressure-reducing valve, it expands and generates a higher flow velocity. The extreme flow velocity when flowing through the valve seat of the pressure-reducing valve can make noise and erosion unacceptable. Therefore, its flow velocity must be limited. In heating systems, it is generally recommended to limit the steam flow velocity to between 20 and 30 m/s. For outdoor environments or already noisy environments, higher flow velocities can be selected. The table below shows the flow rates at different pressures and velocities. 

For example

A steam heating system has an inlet pressure of 7 barg, an outlet pressure of 2 barg, and a flow rate of 440 kg/h. Select appropriate upstream and downstream pipe diameters. As shown in the diagram, the upstream pipe diameter is DN40, and the downstream pipe diameter is DN65. 

Flow rate of saturated steam in a pipe at a given flow rate (flow rate: kg/h)

Incorrect selection is a common cause of pressure reducing valve failure. The following key parameters should be considered:

● Maximum/Minimum Inlet Pressure (P1max/min): The highest and lowest inlet pressures that the valve needs to withstand.

● Required outlet pressure (P2): The stable pressure required by the process.

● Maximum flow rate (Qmax): The maximum flow rate required by downstream equipment, usually expressed in kilograms per hour (kg/h). It needs to be calculated based on the actual load, rather than simply selected according to the pipe diameter.

● Valve body material and diameter: Select the appropriate material and diameter based on the steam temperature and pressure.

● Pressure reduction ratio (P1/P2): If the pressure difference is very large (e.g., more than 10:1), it may be necessary to install two pressure reducing valves in series for staged pressure reduction to avoid noise, erosion and cavitation damage caused by high-speed steam.