Double Water Pumps Question

[SLOBuds]

I have 10 acres that I installed irrigation pipe on a few years back. This is a well system and has a pump with a bladder pressure tank to provide pressurized water through the parcel.

I am thinking about, or rather questioning, about putting a second pump/pressure tank on an opposite side of the property. The idea is to provide a secondary backup system for water pressure, and maybe to even out the pressure during heavy usage.

My well supply guys tell me that I shouldn't install a 2nd pump like that. They tell me that the 2 systems would 'fight' each other. But I don't see how that would happen - the fighting part.

Say one pump is set to go on at 50 lbs/off 70 lbs, and the other is set at 40lbs/60lbs. I think that the 50/70 will cycle most of the time unless there is heavy usage bringing pressure down below 40. Then the 40/60 would kick in. Both would run until pressure exceeds 60. Then the 40/60 turns off. Above 70 the 50/70 turns off.

So basically, the 50/70 does most of the work. If one fails, the other kicks in though.

Am I missing something here? Any other ideas on providing a redundant system?

Thanks

 

[Sackett2]

You are right about the pump action, we ran a similar system on the fire system at my ICBM Missile site. However, both bumps had there own lines into the water tank.
Now, (and I'm not a expert by far-just throwing things around) I wonder about the "draw" of the pumps. Can the well support increased water draw? Is there room for two pumps, I'm assuming they are above ground. How about a increased volume submersible pump.

 

[MikeD74T]

The fighting can occur when the demand pressure is just below the start setpoint of the second pump. With the first pump running & demand keeping the system pressure just below the second start setpoint, the second pump start can boost pressure quick enough to shut it's self off & pressure will immediately drop back below the start point of the second pump. This is short cycling.

You can work this out by balancing the pump sizes, demand rates, & setpoints but that could be a lot of work - especially if the pumps are 1000' mile apart. You might be able to control demand better by putting a couple of check valves somewhere in the system & setting both pumps the same start/stop. This would work best if a check valve was each side of the largest single demand. i.e. Both pumps could feed the largest demand but not back feed to the rest of the other pumps system. With both pumps at the same setpoints the pump closest to the demand should do most of the work.

With distance between the pumps you might also be able to just offset the start setpoints by 5-10 lbs & set shutoff the same. MikeD74T

 

[SLOBuds]

The fighting can occur when the demand pressure is just below the start setpoint of the second pump. With the first pump running & demand keeping the system pressure just below the second start setpoint, the second pump start can boost pressure quick enough to shut it's self off & pressure will immediately drop back below the start point of the second pump. This is short cycling.MikeD74T

I thought that bladder/pressure tanks removed that possibility. So when the first pump got pressure just below start point of the second pump, it would take time to fill the pressure tanks attached to both of the pumps. It would not be possible to boost pressure fast enough for a short cycle.

Maybe I am asking this wrong by providing an solution first. I would like to have a backup pump that would kick in and work in the case where my primary pump stopped. How can I accomplish that?

Thanks.

 

[Kays Supply]

Almost every rural water district has booster pumps along the runs to maintain pressure. The booster has its suction taken off the feed downstream of a main pump. If a system is built in series not parallel, that would work the same as a multistage pump. One boosting the other. I have no input on parallel systems.

 

[Egon]

Maybe draw and post a schematic.

 

[SLOBuds]

Maybe draw and post a schematic.

OK, here's the design.

There is about 1200 feet of 2" PVC between the 2 pumps.
Each pump would have a pressure tank.
Each pump would draw from a separate water storage tank.

 

[Egon]

The chances are very good that you will only have one pump running most of the time. The other pump will only kick in if the demand rate is large enough that the first pump cannot maintain pressure.

 

[SLOBuds]

The chances are very good that you will only have one pump running most of the time. The other pump will only kick in if the demand rate is large enough that the first pump cannot maintain pressure.

You think they will 'fight'?

 

[tommu56]

If you surge is in short duration you would only need more pressure tanks by the flow.

Instead of another pump I would put 4 or 5 more pressure tanks on the system by the houses then your existing pump will run longer but short bursts would come out of the tanks.

tommu

 

[SLOBuds]

If you surge is in short duration you would only need more pressure tanks by the flow.

Instead of another pump I would put 4 or 5 more pressure tanks on the system by the houses then your existing pump will run longer but short bursts would come out of the tanks.

tommu

I am trying to get a backup pump on the system - so if one pump goes down, the system will still have water pressure while the first one gets fixed.

Any idea how to do that?

 

[tommu56]

Then the other pump is the only way ta add redundancy the only other thing would be a separate suction line for the second pump or up size the one to the first to prevent cavitation of the pump with both running.

tommu

 

[MikeD74T]

SLOBuds, Your first post indicated you did not understand how "fighting' could occur. I was trying to explain that. Also you provided little info about pipe size, pump size, demand, etc. I think with 1200 feet between pumps your diagram would work. I'd still stagger the starts & keep the stops the same. Remember that all bladder tanks will drain until the first pump starts, not just the one on that pump. Multiple / larger bladders just allow more time between pump starts and longer pump runs. You can even run with only one bladder tank and staggered settings despite the pumps being separated. If I did that the tank would be toward the middle of the system to absorb water hammer when demand suddenly decreases. With your system that might not be an issue. MikeD74T

 

[Sackett2]

I don't recall the specs, but on the dual pump system at the missile site,
The primary pump supplied water to the building, but if water demand became high, IE during a fire situation where the fire hose was in use, the secondary pump would engage.
the kick-on pressure for the secondary pump was at least about 20 lbs below the primary pump- but the kick off pressure was within 5 lbs of the primary pump- kept them from fighting

 

[Egon]

The system as show should work very well if one pump is intended as a primary backup.

 

[SLOBuds]

Thanks guys for the great feedback and ideas. At the end of the day it sounds like a pretty simple setup.

I have never liked having the large number of mechanicals for my water system on that parcel. So many things can break.

My neighbor has his storage tank on top of a hill. He does not use any pump equipment at all, except to get water from the well to his tank. The pressure is a bit low - 40 lbs - but he uses it OK at that level since he only needs to supply a house with 2 occupants and a bit of landscape immediately around the house.

I don't have that same elevation rise to my benefit. And also I need to supply multiple houses as well as field irrigation, so the low psi would not work with that much demand.

My problem in the past has been systems going down without backups. Another problem is that it isn't always easy to tell if the system is down.

If the pressure system is down, that's pretty easy because we can detect it inside the house. If the well pump is down, we have to notice that the storage tanks are lowering - which could also be from heavy consumption. So then I have to go over and see if water is flowing into the tank, from the well.

Thanks again.