pump shuts off too quickly - new system

Grundfos uses this design for their solar pumps as it is positive displacement and will pump when it is cloudy and the motor runs slow off of the solar panels. Aermotor use to make one years ago. I also saw some on a Chinese manufacturer website a while back. While not common they are around.

Why 2 psi lower ?

valveman said:

Need to let air out of tank now. Will make a lot of difference. Always 2 PSI lower than the pump starts. Good catch! I missed it.

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I am adding a 2 gal. pressured bladder accumulator tank and a 20/40 pressure switch to the 12VDC pump in my travel trailer.
The pump currently has a built in pressure switch that closes at 38 psi and opens at 42 psi. This, of course causes the pump to constantly cycle on/off when using water.

Adding the accumulator and pressure switch should greatly reduce the cycling, however, one thing that I want to take place is for the accumulator tank to be completely emptied of water before the pump kicks on. It seems to me that in order for this to happen, the pre-charge on the tank should be slightly ABOVE the kick on pressure.

Example with 20/40 control switch and 22 psi charge on accumulator tank:
>Supply power to pump
>Pump starts
>Pump fills accumulator tank
>Pump cuts off when pressure increases to 40 psi
>Run water
>Accumulator tank empties
>Pump starts again when pressure decreases to 20 psi

Would this not be the case, or is there something I'm missing here ?

Thanks,
Arky

Arky217 said:

however, one thing that I want to take place is for the accumulator tank to be completely emptied of water before the pump kicks on.

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Why, if it completely empties befor the pump restarts...how is the pump going to know to restart, the tank is empty, there is no pressure in it.

If this is just a pressure tank and not a "bladder pressure tank" then it doesn't matter.

Rancher

Rancher said:

Why, if it completely empties befor the pump restarts...how is the pump going to know to restart, the tank is empty, there is no pressure in it.

If this is just a pressure tank and not a "bladder pressure tank" then it doesn't matter.

Rancher

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This is a bladder pressure tank.
When the tank is empty of water, the bladder will be up tight against the inside of the tank (water connection end) just as it would be before even connecting the tank to the water line.

When the line pressure decreases to the same pressure as the pre-charge pressure on the bladder, the bladder will be pushed against the water end of the tank and the tank will be empty of water, then when the line pressure continues to decrease to the pump kick in pressure, the pump will start.

If you have your switch at 20/40 and set your air charge at 22(instead of 18) then when you get to 22 psi in the tank it will be empty and your pump will kick on BUT during this time your water will stop running for a brief moment till the pump can get started whereas if it was set at 18 psi the tank has very little water in it but enough to keep the water flowing while the pump is getting started--this way no lull in water. Who wants to be taking a shower and the water keep stopping/starting?

prd823 said:

If you have your switch at 20/40 and set your air charge at 22(instead of 18) then when you get to 22 psi in the tank it will be empty and your pump will kick on BUT during this time your water will stop running for a brief moment till the pump can get started whereas if it was set at 18 psi the tank has very little water in it but enough to keep the water flowing while the pump is getting started--this way no lull in water. Who wants to be taking a shower and the water keep stopping/starting?

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I understand what you're saying, but in actual use it doesn't act like that.

In other words, even with the current setup (no tank, and using the 38/42 pressure switch on the pump), there is no lull in the flowing of the water as the pressure decreases and the pump kicks on.

Likewise, with a 22 psi pre-charge tank and 20/40 switch, the water would still flow when the tank is empty due to the line still having pressure and that pressure would again start to increase when the pump started. (perhaps the pressure might drop 1 or 2 psi during the fraction of a second when the pump is starting, depending on how open the faucet was, but the end user at the faucet would not be aware of it)

Likewise, with a 22 psi pre-charge tank and 20/40 switch, the water would still flow when the tank is empty due to the line still having pressure and that pressure would again start to increase when the pump started.

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Water does not compress, so when the tank is empty, you have NO pressure. There are lots of posts here asking why the water stops with a bladder tank and the answers are always the same. "You have too much air in your tank"

bob...

SQF helical model pump

As Pumpman says, this type design was used back in the 60's in what we called the Peerless Wiggle tale pump. One was manufactured by AERO and used a specially tapped Franklin Submersible motor that ran at 1500 rpm. I installed several of them in wells. however they were a positive displacement pump. If the water demand was plugged, the motor either stopped and burnt up or something blew up somewhere. They were manufactured and discontinued shortly after by several manufactures. I still have one as a reminder to never buy another one.

The Grundfos SQF helical model pump may be OK for solar applications? ? ?