Pump Protection and VFDs

benhargreaves

LawnSite Member
Location
South Dakota
I've seen some good threads on pump protection lately but was looking for some advice about a specific situation. I would like to find a solution to protect some "booster" pumps from dry-run and deadheading. I also need to regulate pressure coming out of some of these pumps.

I have a site with multiple independent sprinkler systems, each with their own pump acting as a pressure booster. The pumps are 2 HP or less. All of the pumps are activated with a pump start relay. The water source is city water and there are no pressure tanks. All of the pumps also have thermally protected motors but there are no other protection devices on the pumps. All of the pumps are also indoors in maintenance rooms, making a pressure release valve an unlikely option.

Recently I had an issue with one of the pumps. The impeller destroyed itself from what appears to be overheating. We checked the system and all zone valves activated. It's unlikely the water supply was disrupted. Whatever the cause, clearly the motor thermal protection didn't activate or didn't activate fast enough to protect the impeller.

After checking the system, we also found that the boosted pressure is a bit high, 75 - 80 psi. Static pressure is 45 psi. Without the pump most zones run 25 -30 psi.

So my questions are thus:
  1. Is it possible to retrofit an existing pump with a VFD? It seems a good VFD could solve my pump protection issue and get the dynamic pressure into a better range.
  2. Is a pressure switch my next best option? Would it be better to spec a pressure switch to maintain the 80 psi or spec down to a lower psi and end up with pump cycling?
  3. Would some of the other pump protection devices (e.g. SmartBox) be a good solution for this situation? I have no experience with these devices and the fact that the motor protection failed to protect the impeller has me uncertain that these other devices would do any better.
  4. Is there a better option than the ones I listed?
I don't like that the only protection on these pumps is the motor thermal protection. I'd like to come up with a solution that could be applied to all the pumps present onsite. I still need to check the pressures on the other sprinkler systems to see if they all have a similarly high dynamic pressure when boosted. I appreciate any guidance y'all can provide.
 

Sprinkus

LawnSite Silver Member
Location
San Antonio, TX
So my questions are thus:
  1. Is it possible to retrofit an existing pump with a VFD? It seems a good VFD could solve my pump protection issue and get the dynamic pressure into a better range. (Pump motor would need to be a three-phase motor. Single phase can be used to supply the VFD but the VFD would need to be derated.)
  2. Is a pressure switch my next best option? Would it be better to spec a pressure switch to maintain the 80 psi or spec down to a lower psi and end up with pump cycling? (Low pressure cut off pressure switch could be used for dry-run and cyclestop valve, which will need a small pressure tank, to control pressure).
  3. Would some of the other pump protection devices (e.g. SmartBox) be a good solution for this situation? I have no experience with these devices and the fact that the motor protection failed to protect the impeller has me uncertain that these other devices would do any better. (SmartBox MPLC24T shows to provide protection from low pressure and "run-dry", as well as high temperature due to dead-head. What about a combination of that and a cyclestop valve?).
  4. Is there a better option than the ones I listed? (Possibly, but I don't have one right off the top of my head).
 
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ArTurf

LawnSite Fanatic
Location
Ark
Sounds like the pump is over sized for the job

A flow meter with a controller that can be set to shutdown and notify would do the trick. High and low flow
 

Wet_Boots

LawnSite Fanatic
Location
metro NYC
Could also install a pressure switch with a cutoff of 80 psi or so
This should be how a booster pump is controlled in the first place. 4 gallon expansion tank can complete the control setup. This setup assumes the sprinkler guy can make zone flows match up with the supply, so no rapid cycling. If there is a mismatch with zones too small in flow for the supply, the recirculating valve can come into play, in combination with pressure switch tweaking.
 

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