It's a long story, but I found that my main pump is over sized for my filter which is a Hayward S-244T sand filter rated for 62 gpm. The pump is a Sta-Rie single speed 1-1/2 H.P. with a maximum flow of almost 120 gpm. When the filter is clean the inlet pressure is 24 psig which according to the pump curve corresponds to a flow of 82 gpm. I'm pretty sure I had intermittent episodes of flow channeling because I've seen the inlet pressure drop by 3 or 4 psi when it should have been going up, not down.
I added a valve to the pump discharge and put a pressure gauge on the filter outlet so I could limit the inlet flow from the pump and determine the pressure drop across the filter. What I found was surprising. At full pump flow with an inlet pressure of 24 psig the filter outlet pressure was 6 psig. That's a differential of 18 psi and that is way too much for a sand filter. I varied the inlet pressure and recorded both inlet and outlet pressure and estimated the flow from the manufacturer's pump curve (after back washing). The results are:
Inlet, psi--Outlet, psi---Diff.psi------Flow, gpm
24-----------6--------------18------------ 82
25----------11--------------14------------78
26----------18--------------8--------------73
27----------20 -------------7--------------68
28----------23--------------5--------------63
29----------26--------------3--------------60
30----------27--------------3--------------58
The flow numbers are very rough because it's hard to read the very small graph the Mfg. sent, but the pressures are good numbers. I repeated the test several times and got good repeatability. What is most interesting is the sharp rise in differential pressure between an inlet of 26 to 24 psi. The filter has been running with an inlet pressure of 28 psig for the past week and the differential has only increased 1 psi. I'm not sure at what point on the pressure curve the filter should be back washed but think I'll watch flow from the returns and compare that to the outlet pressure try to make a correlation.
I added a valve to the pump discharge and put a pressure gauge on the filter outlet so I could limit the inlet flow from the pump and determine the pressure drop across the filter. What I found was surprising. At full pump flow with an inlet pressure of 24 psig the filter outlet pressure was 6 psig. That's a differential of 18 psi and that is way too much for a sand filter. I varied the inlet pressure and recorded both inlet and outlet pressure and estimated the flow from the manufacturer's pump curve (after back washing). The results are:
Inlet, psi--Outlet, psi---Diff.psi------Flow, gpm
24-----------6--------------18------------ 82
25----------11--------------14------------78
26----------18--------------8--------------73
27----------20 -------------7--------------68
28----------23--------------5--------------63
29----------26--------------3--------------60
30----------27--------------3--------------58
The flow numbers are very rough because it's hard to read the very small graph the Mfg. sent, but the pressures are good numbers. I repeated the test several times and got good repeatability. What is most interesting is the sharp rise in differential pressure between an inlet of 26 to 24 psi. The filter has been running with an inlet pressure of 28 psig for the past week and the differential has only increased 1 psi. I'm not sure at what point on the pressure curve the filter should be back washed but think I'll watch flow from the returns and compare that to the outlet pressure try to make a correlation.