I see a lot of plumbing installations that mention having straight pipe runs of 3x dia of the pipe minimum before the pump. Whilst I understand why that would be for a normal water pump, almost all pool pumps break this theory by making the water turn through 90 degrees in the pre filter before entering the pump.
Should the pre filter be mounted further in front of the pump intake (3 x the dia, if that were possible) to smooth the water flow entering pump?
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Thanks Terry, it was getting lonely here for a while. It would seem that mainstream pump manufacturers are forgetting this when designing their pumps. Because I have just installed an Eco Pump the unions used to connect the pre filter to the pump (separate items Waterway 2" prefilter) mean I can increase the distance but as I have reduced the speed of the pump there is far less turbulence in the pre filter anyway especially as the suction side is all in 2 1/2" with flowed bends so the water is much smoother entering the pump. I also removed the eyeballs from the returns to see the difference in water flow and pressure reduction. We always used to flow our cylinder heads and match ports to carburettors to ensure smooth flow but pool fittings are poorly designed for this with steps everywhere in fittings etc. The eyeballs I drilled out to their max size in the end as I found directing the flow was very important to acheive a clean pool with no main drain. The eyeballs are terrible and create pressure and turbulence with their internal shape but once enlarged the whole system not only flowed better at reduced pressure but also energy consumption fell a bit lower too. It would be nice to see the back of the eyeballs bellmouthed so when used off axis to direct flow they didn't begin to obscure some of the fittings through put.
I have seen this stated as 4 times the pipe diameter or minimium of 12 to 18 inches for typical 2 to 3 inch diameter residential pool systems. This does reduce the suction resistance which adds to the overall total dynamic head of the system with pressure side friction / restriction "losses". It also helps to prevent caviation and water vaporization (water "boiling" under vacuum) on the suction side of the pump which can lead to loss of prime. It is usually a good idea to use eccentric reducers at the pump suction with the flat horizontal on top to preclude air pockets from from forming on the suction inlet. Use of concentric unions eleswhere rather than reducing bushings help to reduce the total friction losses throughout improving the system efficiency and flow performance.