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Flow
Velocity in Plastic Piping Systems
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| 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. |
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Shock
Pressure |
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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. |
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Safe
Flow Velocity |
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| 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. |
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How
can you determine the shock pressure potential in a system? |
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| 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. Just
click here to open the calculator. |
| 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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to Hayward Back to PEP
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Copyright
HAYWARD Industrial Products, Inc. 1999
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Flow
Velocity in Plastic Piping Systems
