Need Guidance on Well Pump

[munzelj]

I am installing a well on a property where I run a retail tree nursery. Someday down the road I would also like to build a home on the property and serve both the tree nursery and home from the same well.

The well drilled is 6” casing, static water level at 67’, drilled to 280’ to ensure adequate flow is available. The driller was pumping 45 GPM at 140’ for an hour.

My water needs could be all over the place in the future. Right now I get by with a 5GPM 12V pump drawing water from tanks but it isn’t great and I have to break it down into four separate zones. My current irrigation system only needs 25 PSI but that could change in the future, up or down. I can vary the number of zones watering at a given time to increase flow if necessary.

My main concern is to make sure I have enough flow to meet future needs (which are unknown) but also be able to run just a house safely while not wrecking my pump.

With all of that said, if I go with a larger pump, say a 30 GPM 3HP Starite with a variable speed drive, is it bad if I end up running at the lower end of the curve a lot? It seems even dropping to a 20 GPM pump doesn’t change things much and the minimum recommended flow on both is the same. I am trying to factor in wear and tear on the pump and also efficiency.

Put another way, how big can I go without harming the pump if i end up only using a small amount of water most of the time?

 

[Valveman]

I am installing a well on a property where I run a retail tree nursery. Someday down the road I would also like to build a home on the property and serve both the tree nursery and home from the same well.

The well drilled is 6” casing, static water level at 67’, drilled to 280’ to ensure adequate flow is available. The driller was pumping 45 GPM at 140’ for an hour.

My water needs could be all over the place in the future. Right now I get by with a 5GPM 12V pump drawing water from tanks but it isn’t great and I have to break it down into four separate zones. My current irrigation system only needs 25 PSI but that could change in the future, up or down. I can vary the number of zones watering at a given time to increase flow if necessary.

My main concern is to make sure I have enough flow to meet future needs (which are unknown) but also be able to run just a house safely while not wrecking my pump.

With all of that said, if I go with a larger pump, say a 30 GPM 3HP Starite with a variable speed drive, is it bad if I end up running at the lower end of the curve a lot? It seems even dropping to a 20 GPM pump doesn’t change things much and the minimum recommended flow on both is the same. I am trying to factor in wear and tear on the pump and also efficiency.

Put another way, how big can I go without harming the pump if i end up only using a small amount of water most of the time?

Trying to factor in wear and tear on the pump then controlling it with a variable speed derive (VFD), which is designed to shorten the life of pumps/motors doesn't make sense. Minimum flow with a VFD is another problem. A VFD is also not going to make a 30 GPM pump efficient at 5-15 GPM. No matter how you control it, any pump is only efficient close to its BEP or Best Efficiency Point.

Pumping from 140' at 50 PSI at 30 GPM, 3HP will do 33 GPM and a 20 GPM, 2HP will pump 20 GPM. You need to size the pump to the max possible flow needed. The 2 HP pump is not going to work if you need more than 20 GPM, but will be more efficient at 1 GPM than the 3HP.

With the right pump control the 2HP can safely operate down to 1 GPM and the 3HP down to 3 GPM. The minimum flow can be much lower when the pump is controlled by a Cycle Stop Valve than a VFD. While the Sta-Rite pump will work fine with a CSV, a Goulds or Grundfos will be more efficient at lower flows. When restricted with a Cycle Stop Valve to say 3 GPM, the Grundfos will drop 50%, the Goulds will drop 30%, but the Sta-Rite will only drop 10% in amperage. Still not as efficient as running at BEP, but the better the amp drop the less the electric bill.

 

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[munzelj]

Thank you for the insight. I realized I probably should have added a couple more pieces of information:

1) I feel the minimum flow I can get away with is 20 GPM @ ~60 PSI to have enough water for a residence plus one large overhead sprinkler.

2) Right now my tree nursery is mostly watered via drip irrigation (20-25 PSI). That will be run daily for as much as six months a year so I need to make sure the pump is efficient for that use more so than for household water use. But, because I have it broken down into zones, I could use anywhere from 5 GPM up to 40 or 50 GPM. A 3 HP pump is as big as I'm considering, but my point here is that the challenge is that there is what I NEED (20 GPM @ 60 PSI) and then what I could use if I had it. I could water one zone at a time at 5-10 GPM or all six ones at once at 30-?? GPM.

Not sure if this changes anything?

 

[Valveman]

Again a 2HP will only be efficient when using close to 20 GPM. A 3HP will only be efficient when using close to 30 GPM. A 3HP will be much more expensive to purchase and maintain than a 2HP. I would use the 2HP and set up 2 drip zones for about 15 GPM. That will leave about 5 GPM for the house to use while still being fairly efficient on the drip. If you use all 20 GPM the 2HP can make, the house pressure will be low while the drip is on.