Limiting Suction Outlet Plumbing Velocities

Standards Mandate the MAXIMUM SPEED LIMIT
 
Standards dictate that the MAXIMUM velocity limit in the plumbing at the suction outlets is 6 feet-per-second (ANSI/APSP-7).  I recommend that the maximum designed velocity on the suction side of the pump be limited to 4 feet-per-second.

Why lower that the maximum?  In suction lines speed is not a good thing.  In fact, excessive speed causes equipment wear, friction, lost energy and pump noise.

Since pump selection is the last task performed when designing a suction system, oftentimes there is not a pump curve that EXACTLY fits the system requirements.  By designing below the maximum allowed velocity, the designer has the ability to select the closest pump curve, even if it slightly exceeds the 4 FPS design - safely remaining below the maximum.

Isolation Valves 

When isolation valves are placed in suction lines between a pump and the suction outlets, any singular suction line supplying water to the pump MUST be capable of providing 100% of the systems’ designed flow rate at a maximum line velocity of 4.5 FPS.  

This is required, because if valves are closed and lines are improperly isolated, and if the pipes are not of sufficient diameter, the line velocity would quickly exceed the 6 FPS standard.

Energy Efficiency

Energy efficiency is greatly enhanced by lowering the line velocity.  Cavitation, operating noises & suction entrapment hazards are also reduced by lowering the line velocity.

Line velocities cannot be reduced much lower than 4 FPS, otherwise pipe scour would not occur.  Debris and sediment could eventually collect in the pipes.

Paolo Benedetti, SWD Aquatic Artist, Watershape Consultant, Expert Witness, International Construction Management 
Contact the author at: info@aquatictechnology.com or 408-776-8220 
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