Flow Velocity in Plastic Piping Systems

Flow Velocity

Flow velocity is the speed at which the process media is flowing through the pipe. It is expressed in feet per second. High flow velocities in themselves may not be dangerous, however, the shock pressure (sometimes called water hammer) they can generate is.


Shock Pressure

This shock pressure, which can be hundreds of pounds per square inch, is often more than enough to cause severe damage to plastic piping systems that are often rated for 150 psi. This shock pressure can be created in a piping system when a valve in the system is opened or closed quickly, a pump is started or stopped, or when a pump is started in a empty system or section of a system.


Safe Flow Velocity

In plastic piping systems, Hayward defines a generally accepted safe flow velocity as no more than 8 feet per second for piping systems that do not have fast opening or closing valves. For systems that do have these types of valves (such as solenoid valves or pneumatically actuated valves without speed controls) the maximum velocity is usually 5 feet per second. Metal piping systems are often designed with flow rates of up to 15 feet per second, much higher than would be acceptable for a plastic system. This is an important consideration if you plan on installing a plastic valve, strainer, filter or other component in a metal system.