Flow Velocity in Plastic Piping Systems

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

Shock Pressure

This shock pressure can be hundreds of pounds per square inch, 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 stated 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 valve, such as solenoid valves or pneumatic 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.

How can you determine the shock pressure potential in a system?

If it's a Schedule 80, plastic piping system, 1/4" to 12" and flowing water or a liquid with a viscosity similar to water at ambient temperature Hayward has an automatic calculator to do the work for you. .

If it's a different system you'll need an engineering handbook or the pipe manufactures catalog to figure it out. Charts and formulas are available to convert flow in gallon to minute to feet per second for different size and materials of pipe. Charts are also available that show a factor, called a wave surge constant for each pipe size as well as for factors for liquids at different temperatures and viscosities. The simplest method is to multiply the flow velocity in feet per second by the wave surge constant and then add in the system pressure to determine the shock pressure potential. Although, the instructions furnished with the charts may be different and should be followed.

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