System Pressure Anomaly

[Sailracer]

I have some odd behavior that I can't explain, so I thought I would post here looking for an explanation.

I recently replaced the A.O. Smith 1.5 HP 2 speed motor on my Hayward Superpump with the variable speed Ecotech EVSJ3-NS. The two speed motor was getting really loud and the low speed setting was not getting enough flow through my piping for the SWG to work properly.

When I replaced the motor, I found the impeller had a bit of pine needle crud clogging it in a few places, so I cleaned it out. (That may have been contributing to the low flow on the old motor on low). The new motor is now working great- I can set it at 55% to keep the SWG happy and bump it up to 75% when the Polaris is running. What is confusing me is the pressure readings. With the old 1.5HP pump on high and a freshly backwashed filter I would get about 16 PSI on my gauge. With the new 3HP pump at 100% the pressure on the gauge is only at 12 PSI. How can the pressure be lower with the new pump when it should be turning the impeller as fast as the old pump? Could the crud in the impeller with the old pump have impacted laminar flow and resulted in a higher pressure? Could air have been collecting in the filter with the reduced flow of the old pump/cruddy impeller and increasing the pressure reading?

 

[RonsPlc]

I may be wrong here, but I think you are reasoning that ... higher HP=higher flow ... What you should be looking at rather is the GPH, or GPM that the pumps have. (Gallons Per Hour, Gallons Per Minute)

A 1.5hp with 3600GPH is going to show higher on you gauge than the same HP, or higher that only has 3000GPH.

 

[Sailracer]

Since the impeller and pump stayed the same between the two, I'm assuming that at max speed both motors are turning the impeller at 3450 RPM which should create an equal GPH between the two, provided the 1.5HP motor had enough umph to achieve 3450 RPM. With an equal GPH I would expect equal pressure readings. Further, I would expect the cruddy impeller to decrease GPH and hence decrease the pressure reading, but I am seeing the opposite. I guess one other variable is the sand in the filter. Is it possible it has gotten channels in it over a period of 2 weeks that decrease the pressure?

 

[mas985]

All that is true but are you sure you are running at 3450 RPM?

Also, are all the valve settings the same and did you clean the filter when you changed the motor?

 

[Sailracer]

I don't really know that I am running at 3450 RPM, just that the pump is set to 100% and that is its spec'd RPM. The flow looks to be greater than what the old motor did on high (as measured by eye-balling the flow through the clear part of the SWG). I didn't do a thorough cleaning of the filter when I changed the motor, but I did a backwash a few days after the change. Valve settings are all the same for comparison purposes.

 

[mas985]

So was the pressure reading after the backwash? That could explain it.

 

[Sailracer]

The 12 PSI on the new motor was after the backwash, but with the old motor it would only drop to 16 PSI after a backwash. I wonder if doing the backwash with the impeller cleaned up might have done a better job and resulted in the lower pressure afterwards.

 

[mas985]

Yes, a clean impeller would do a much better job removing debris from the filter.

 

[Sailracer]

Thanks. I can accept that as a plausible explanation