Recommended pressure for pool circulation?

I'm wondering if there are any good guidelines for how much pressure there should be in the filter for pool circulation.

I have just replaced the motor on our pool circulation pump (bearings finally got too noisy) and the replacement went faily well and I have the new motor working. I noticed, however, that the filter pressure reading was different than I expected with the new motor. It is rated to be a 1:1 match for HP and RPM (3/4 and 3450 respectively). I have previously replaced the seal only a while back and had a change in the pressure reading as well. At that time, I could not seem to adjust the impeller clearance to increase the pressure which I thought I had to do even though the flow was good so I left it alone. This time, I changed the impeller clearance to drop the psi to about 15 psi since that seemed like less strain on the equipment and the pool return flow seems strong. It got me wondering though about what the pressure should be for a balance of good flow and safe operation. I have some leyway with the impeller to make it higher or lower if there is an advantage.

Any tips?

Thanks.

Background:
When we took over the pool: 17 psi baseline
When I changed just the seal: 14 psi baseline
When I changed motor and seal: 20 psi baseline

Pool: 20,000 gal. inground, indoor
Pump: 3/4HP Pentair HydroPump
Filter: Sand - Tagelus TA 60D (does that indicate the size?)

There have been no other changes to the pool piping or equipment.

Pressure is an indication of two things in a plumbing system, flow rate and head/friction loss. The higher the flow rate, the higher the pressure will be for a given plumbing system. The larger the pump HP, the higher the flow rate and thus the higher the pressure will be.

17-20 PSI is not unusual by any means for a pool. I run at about 19 PSI with a 1 HP Northstar pump (SF = 1.85), 2 1/2" suction plumbing and 2" return plumbing. With the solar system on, the pressure rises to about 27 PSI.

The equipment can usually handle more than 40 PSI but most pumps can't deliver more than about 35 PSI before dead heading so pressure is usually not an issue but a just symptom of the plumbing design.

Also, when you increase the impeller clearance it reduces the pumping the HP of the wet end. Unless you reduced the HP of the motor as well, this reduces the overall efficiency of the pump. You will get less GPM per watt of energy doing that.

However, after you changed the motor, the PSI went back up which is unusual. The pressure is determined by the impeller design and the motor needs to be matched to that so it doesn't over heat. It could be that there was some blockage in the impeller between the second and third steps which cleared after the work was done.

What exactly did you do to the impeller between steps 1 & 2?

Thanks for the info.

All I did between baseline 1 and baseline 2 was replace the pump seal.

I can elaborate a bit though. When I changed the seal with the old motor it was because it was leaking. The motor was a bit noisy but not bad enough to replace. When trying to set the impeller clearance, I was not able to set the clearance as low as I would like to have since it seemed to rub without a fair bit of clearance. In hindsight, that probably should have told me there was a little something wrong. Perhaps the motor bearings were worse than I thought and the amount of play in the shaft was causing the rubbing if the clearance was too low. In any event, I set the clearance as best I could and got about 14psi.

With the new motor and seal installed, I seem to be able to set the impleller clearance quite small, which is how I understand it should be. This gave me about 20psi when running. I was a little worried that this might be too high for my system since when I turned the pump off there seemed to be a bit of a "hickup" in the pressure just before it dropped off. This made me think it might be building a bit too much back pressure at that setting. It is a bit of a smoother shutoff if the pressure is lower. I don't know if it is important but maybe I should have mentioned that the pump is located in a basement, well below water level (That fact is going to cause me a big problem when I decide to try to fix a leaking joint at a ball valve for the pool shut off but I'm trying to avoid thinking of that right now).

I do understand that the larger impeller clearance won't flow quite as much water but I thought that too much pressure might be bad for the life of the motor. I'm not sure about that though. I'm still just figuring things out as I go with our pool system (and these forums have been invaluable). I know everyone's system is different but I wondered if any ranges of pressure are considered "good". It is nice to know that 17-20 psi isn't unusual.

Thanks.

One other thing to note is that the filter's pressure gauge reading is dependent on the elevation of the gauge with respect to the pool's water level. So if it is in the basement with the pump, it will read about 4 PSI higher (9 ft down) than at ground level.

Also, a pump & motor combo is designed for best efficiency when operating in the middle of the head curve. This is somewhat counterintuitive but a motor's load actually goes up with lower pressure and higher flow rates (right side of head curve). A pump will draw the minimum amount of current when it is dead headed at zero flow. It will draw the maximum amount of current at the other end of the head curve near the maximum flow rate. So as long as your operating point is somewhere around the middle of the head curve, you are not overdriving the pump.

By changing the clearance in the impeller, you are changing the dynamics of the pump/motor interaction which will also change the load on the motor. It is probably like stepping down the impeller size on an existing motor. Pressure and flow rates both will drop (i.e. the whole head curve changes) and the load in the motor will drop but this also reduces the efficiency of the pump. I don't think it would damage anything but it really isn't necessary.

I would never have thought about higher pressure reading with guage below water level. Great information to know now.

I see what you about setting the impeller clearance larger. I like to avoid future problems whenever I can but sometimes go too far the other way and end up causing other problems. I will probably set the impeller back to how I had it which was closer to instructions on how to set it.

Now that I know that 20 psi isn't out of a normal range, I can be more corfortable with the higher pressure.

Thanks for all the info.