Odd Pressure Fluctuation -- Help Please

[Mark Holmes]

I'm a first time poster, but I've benefited from advice in this forum many times. Thank you all for that.

I recently replaced my pressure booster pump with a much larger unit, and I'm seeing a high-frequency (4-5 times a second) fluctuation on the pressure gauge while the pump is running. The average pressure rises as I'd expect, but the needle vibrates between 3-5 psi above/below very quickly. When the pressure switch shuts off at 50psi, the needle spikes downward, causing the switch to turn on and off again very quickly—firing the motor for a split second. The pressure system seems to be working... but that rapid short cycle at shutoff seems really bad for the motor.

Some details on my current setup: see pic below
- 3000g storage tank, about 20 vertical feet above wellhouse; 1-1/4" PVC gravity line to pump suction
- New Goulds 25GBC20 booster pump
- 120g pressure tank, only 3 years old
- Square D adj. pressure switch, set to 30/50 -- also 3 years old, and was working fine with old pump
- New brass 1" swing check valve between pump outlet and tank T

I replaced the pump for two reasons: 1st because it's a requirement for my new house sprinkler system; and 2nd because while away the 3000g tank ran dry, causing the old pump to run and overheat.

I drained the pressure tank completely, reset pressure to 28psi, and it seemed to hold pressure fine. All the tests I can think of show that the tank is ok, and the diaphragm isn't ruptured. Some sediment definitely came down the line when the big tank ran dry, but I don't see how that could be affecting both the gauge and the switch, which are mounted on opposite sides of the T. Manufacturer indicated pump would prime automatically with positive pressure on the suction side, and that seemed fine. I pulled the prime plug to test, hence the puddle below the pump in the pic.

Help! Any advice would be appreciated. Air trapped somewhere? Pump not fully primed? Could sediment cause this? Something with the check valve? Seems like a simple setup, but I'm stumped.

Many thanks,

Mark

 

[Reach4]

I recently replaced my pressure booster pump with a much larger unit, and I'm seeing a high-frequency (4-5 times a second) fluctuation on the pressure gauge while the pump is running. The average pressure rises as I'd expect, but the needle vibrates between 3-5 psi above/below very quickly. When the pressure switch shuts off at 50psi, the needle spikes downward, causing the switch to turn on and off again very quickly—firing the motor for a split second.

Is that 3/4 inch PVC to the pressure tank? If the pressure tank has a 1-1/4 fitting, you could increase the size of the path to 1-1/4. That might need a new tank tee.

 

[Sponsor]

 

[Mark Holmes]

The T is 1", as is the path from the pump to the T (pump->checkvalve->flex connector->T all 1"). Pump outlet is 1". From the T to the house goes through a 3/4" PVC nipple (left over from a temporary install that never got changed). I plan to replace that 3/4" nipple, but it shouldn't be causing this problem, should it?

 

[Reach4]

New brass 1" swing check valve between pump outlet and tank T

I wonder if that could cause an oscillation. Usually people would use a spring-loaded check-valve. I am experienced with this, but I don't think anybody recommends a swing check valve in this kind of situation.

Is that a Prosource pressure tank by chance? https://terrylove.com/forums/index.php?threads/dual-csv.66498/#post-494725

You might want to consider a pressure (plus vacuum?) gauge on the input too.

 

[Reach4]

delete

 

[Mark Holmes]

I wonder if that could cause an oscillation. Usually people would use a spring-loaded check-valve.

I have a 1" PVC spring valve that I can try.

Is that a Prosource pressure tank by chance? https://terrylove.com/forums/index.php?threads/dual-csv.66498/#post-494725

https://terrylove.com/forums/index.php?threads/dual-csv.66498/#post-494725

Not Prosource, but a cheapie from Tractor Supply -- CountryLine I think they call it. But they sell crappy stuff, so I wouldn't be surprised if it was poorly designed inside. Thx for the link, that's a good consideration.

You might want to consider a pressure (plus vacuum?) gauge on the input too.

Maybe the pump is overdrawing on the supply? It's a BIG pump, 25gpm @ 2hp. I'd assumed 1-1/4" straight from an uphill tank would be fine. But I'll consider upsizing the entire connection from the pump to the tank to 1-1/4" also.

 

[Valveman]

Yeah your check valve is not working. If you are not using any water, the pump should stay off. If it drops any pressure, water is going somewhere, like back through the check valve. Those swing checks don't work well on pump systems and they won't work at all when on their side like that. They have to be in the upright position with the flapper hanging down to do any work. Kind of like me.

 

[Mark Holmes]

valveman -- there was no leak, so the valve was technically working ok: it passed water, but stopped it when necessary. But I still think it was part of the problem. Based on Reach4's suggestion, I swapped the 1" swing check for a PVC spring check and that improved the situation -- still fluctuating, but not quite as badly. And it improved enough that the pressure switch cutoff is clean now -- not flipflopping.

I think the swing valve was causing turbulence (very asymmetrical, partially obstructed flow path through the valve body--just the way it's designed). Add that to the restrictions in/out of the pressure tank and it is probably creating a lot of turbulent pressure spikes. To test this, I opened the tank drain hose bib off the tee wide open while the pump was running. It filled more slowly of course, but the pressure rise was much more steady.

I've got a 1-1/4" tee on order and will install next week, and I'll update the post. Might just swap out a 1-1/4" spring check too while I'm in there... can only help I think.

Thanks for the prompt responses guys, it definitely solved the worst of the problem today, and I feel comfortable running my motor now.

 

[Valveman]

That is a very small tank tee for a 25 GPM pump. But it still should not be causing the pressure switch to "flip-flop". That is usually because the actual opening in the tank is not accepting water as fast as the tee can supply it. In other words sometimes adding a 1 1/4" tank cross doesn't help if the tank opening you screw the tank cross into has only a 3/4" hole. It will bounce the pressure switch because the pressure is backing up from not being able to get into the tank fast enough. Restricting the outflow of the pump to less than the inflow of the tank is the only way to solve this problem without getting a tank with a larger opening.

Sometimes a snubber on the pressure switch line will help, sometimes not. And of course a CSV1A screwed right next to that check valve would solve all of those problems.

 

[Mark Holmes]

Tank opening is 1-1/4", so the new tee coming this week will hopefully help. This system wasn't intended for a 25gpm pump -- that's a big requirement from our county to meet pressure minimums for the fire sprinkler system designed for our (in progress) new house. Seems I'm having to upgrade everything else to match, which is fine.

Thanks for the great suggestions -- looks like I've got some options to continue improving the situation. I'll update the thread when the new tee is installed. I'll have to read more about the CSV1A you mentioned -- the initial system involved running a small pump on solar power and large pressure tank with a custom controller integrated with my battery/charging system. It's smart enough to bias pressurizing the tank during peak hours of excess PV when possible. The larger tank effectively became a 'water battery', and I'm not sure practically how the CSV would impact that. I'm thinking of upgrading the PV system to handle the larger 2hp pump, but I have to study it further, as this could be integrated. Alternately I can add a second big tank, keeping the same charge-during-excess-sunlight-hours model.

If anyone is interested in this solar/plumbing integration, I posted some details and pix here: http://forum.solar-electric.com/dis...l-well-and-pressure-pumping-during-peak-hours

 

[Valveman]

Even though the pressure tank has a 1 1/4" elbow to connect to, many times the elbow will be screwed onto a little 3/4" hole coming out of the tank. At 25+ GPM the pressure in the 1 1/4" tee will backup and shut off the switch before the tank has accepted the water it should. You have to look back through the elbow into the tank to see what I am talking about. Restricting the flow from the pump to an amount the 3/4" hole can accept will keep the switch from bouncing.

That is a pretty cool solar system you have. And you don't want to restrict the pump with a CSV on any valve unless you have to. As using a pressure tank for a "water battery" is a good idea the way you are doing it, and you want the most gallons per watt you can get.

If you don't mind me asking, how much have you spent on the solar system?

 

[Mark Holmes]

Ah, gotcha. I'll peek up the elbow when I replace the tee. It's sounding like a 2nd big pressure tank is likely in my future, but I'll see how the tee helps.

Thanks re: solar -- it was (and continues to be) a fun, unusual project. Lots of odd skills and techniques to explore to make the whole thing work. As for price, I don't mind: the Grundfos pump and controllers were a bit over $2k w/o install, the inverter and main panel combined was about $5k; the charge controllers are about $650/each; I have 8 6-v golfcart batteries, 220ah wired @ 48vdc to match the inverter input. It's a very small bank for my current plans... but this is a system that has evolved in fits and starts, not something planned in advance (long story). Every other part of PV systems are easily upgradeable, but batteries are more fickle about changes. Arizona Wind and Solar (not sure of their current name) who host the forum I linked to above are great, and that forum is very vibrant—like this one. Tremendous resource for the DIY-on-steriods crowd ;-)

As for panels, I've got an older legacy set, plus a newer set (purchased just for the Grundfos) that I combined for a total of 2.2kw PV input. Prices on panels and batteries seem more like commodities, so check current prices. All the other gear prices are likely similar today, or a bit more.