December 14, 1999
There have been some questions regarding suction pipe velocities etc. that the following may shed some light upon.
The general theory of suction entrapment forces can more easily be understood if I relate the factors to a moving train.
The question is - How hard is it to stop the train by pulling on the caboose? This depends on:
The length of the train (Length of Pool Piping)
The speed of the train (Suction Velocity in feet/second)
size of the train cars
Table top or real (Small pipe size or large)
And of course the Engine (Pump)
To stop the train from the caboose (Main drain), which is what happens when drain blockage occurs:
First - we must either stop the engine (turn off the Pump) or disconnect the engine from the train at some point (break the suction).
Second - we must consider keeping the train as short as possible. The shortest distance between the caboose (main drain) and the engine (pump) or disconnect point (break in suction) is desirable.
Third - anything we can do to keep the train speed low (limit pipe velocity) will further reduce the effort required to "stop the train from the caboose."
Certainly there can be circumstances of higher velocities with a short, small train that will not exceed safe limits.
But as you can see, all factors are inter-related.
Just food for thought (without formulas).