Cycle Stop Valve (CSV) and shroud question

[FredG]

The back pressure is always less than most people would think. A 10 GPM, 1/2HP pump can only make about 100 PSI max. You can scale up the HP and flow without increasing back pressure. A 20 GPM, 1HP or a 40 GPM 2HP will also only make about 100 PSI max. It is the small volume, high horsepower pumps that build the most back pressure. A 10 PM, 1HP can build almost 200 PSI. But a pump like that should be installed in a well where the water level is at least 150'-200' deep, which will eat up a 100 PSI of the 200 that pump can build. Which means you still only see about 100 PSI back pressure before the CSV. Only pumps that are way over sized, like when made for a 500' deep well and the water level is only 100' does back pressure become an issue. But no matter how much back pressure it makes, each pump was designed to make that much. So, as you said, the only issue, even with way oversized pumps, is to make sure the pipe before the CSV is rated for enough pressure. After the CSV, any pipe will work as it will only see 40 to 60 PSI max.

What is the most back pressure a 2hp 25gpm pump can make? Thanks.

 

[MicEd69]

It depends on the specific pump, but the shutoff pressure of a pump I looked at was about 115 PSI.

 

[Valveman]

Here is an example an how back pressure is figured. You can see at zero flow this 25GPM, 2HP makes 340 feet of head. Divide 340 by 2.31 to get a total back pressure of 147 PSI. That is the max pressure this pump can build and also how much would be at the pump. If the water level in the well is at the surface or maybe 1' deep, that is also how much back pressure there would be on all the pipe prior to a CSV. However, for every 2.31' the pump must lift from the water level, there will be 1 PSI less back pressure at the CSV. So, with a water level in the well of 100', there will be 43 PSI less back pressure at the CSV than the 147 PSI the pump can make or 104 PSI. Those are very normal numbers when working with a CSV. But max pressure the CSV can handle is about 200 PSI and minimum it needs to work with a 40/60 switch is 70 PSI.

Also notice the horsepower curve. At 25 GPM it draws 2.5HP and at 4 GPM it drops to only 1HP load. The RPM never varies from the nominal 3450 RPM but the amps drop by more than half. Some pumps do this better than others but nearly all centrifugal type pumps have a reduction in amps that is proportional to the flow rate, regardless of the back pressure.

 

[FredG]

Thank you! I have been doing research and learning about pumps etc. since I bought a house on a well in 2015. Have read a lot on Terry Loves forum where I first learned about the CSV. As soon as my 30+ year old well x trol tank goes, I”ll be buying a psidekick kit. Just wan’t to make sure I won’t have excessive pressure before the CSV.

 

[JEG in Raleigh]

Thanks to all, for the explanations. This discussion has cleared up several questions I have had for a long time. I have an 8" diameter well bore (long story) with a 4" diameter pump with a cooling shroud per Valveman's instructions about 5 years ago and it has worked great. QUESTION: If you have a well pump that is the appropriate size for the diameter of the well bore so it does not potentially get stuck (example: 3" diamter pump for a 4" bore hole), does that eliminate the need for a cooling shroud? The 0.50" clearance between the the outside of the pump and the wall of the bore would not seem to be enough clearnace for a shroud. But the close clearance would not seem to solve the problem of pulling water from above and not properly cooling the pump. It would seem to me that only with a shroud can you direct all water being pumped to enter the pump from below and simultaneously cool the motor. What should you do about a cooling shroud in that situation?

One other question that's probably a dumb one with regard to the back pressure generated by a pump: If you have a 1/2 hp pump that pumps 10 gpm and a 1 hp pump that pumps 10 gpm, why do you use one over the other? Do you need a higher hp pump, when the vertical distance the water is being pumped is greater, even though the gpm production is the same between the 2 pumps? I must have some mental block, and I still don't understand the concept of "head" and how it changes depending on the static water level. Any first-grader level of explanation of this would be appreciated.

Thanks.

 

[Valveman]

Technically, if the well screen or open end of the casing where the water enters the well is below the motor, no shroud is needed. But many times the water above the pump will need to be pumped out before water starts entering the well from below the pump. On deep set pumps with a high static water level the pump could be top feeding for quite some time. This is one of many reasons I like a shroud on any/every submersible pump. The exception being when the well diameter is too small to add a shroud. In those cases all you can do is just HOPE the pump is not being top fed for very long.

The tighter fit the shroud to the motor the higher the velocity of water flow past the motor. Motor cooling is rated in Feet Per Second. 0.5 fps being about the minimum required, so the smaller the shroud the better. There is a lot of area in the circumference even though the pump maybe 3.5"OD and the shroud slightly less than 4.0" ID. This 1/4" of room in the circumference will let up to 60 GPM pass with very little restriction, as I have tested it myself.

Flow, like 10 GPM, is the amount of water being pumped. Head, like feet of lift or pressure is the force needed to move the flow. With a submersible even one 10 GPM impeller will pump 10 GPM. But stacking numerous impellers increases the lift by the number of impellers as well as increasing the motor load. A 1/2 HP only has 8 impellers and can pump 10 GPM but only lift half as high or build half as much pressure as a 1HP that has 16 impellers.