Pump switching

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frediefreon

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United States
In several of my buildings for the heating system we have 2 pumps, one runs all the time the other is for stand by, I would like to have the pumps alternate run times, switching say once a week, and also in case the running pump fails, to have the other pump start .... I would also like to use the existing starters..... Any ideas would be welcomed... Fred
 
In several of my buildings for the heating system we have 2 pumps, one runs all the time the other is for stand by, I would like to have the pumps alternate run times, switching say once a week, and also in case the running pump fails, to have the other pump start .... I would also like to use the existing starters..... Any ideas would be welcomed... Fred

You could use a digital timer that would control a no/nc 2p contactor

Example: You would set the timer to turn on at 12:00a.m which would energize the contactor, opening 1 pole on the contactor and energizing that motor while at the same time it closes the other pole, de-energizing the other motor and at 11:59 p.m it would switch over again. As far as load sensing part of it I'll have to think about that one! Lol
 

JFletcher

Senior Member
Location
Williamsburg, VA
You're talking about a lead/lag system. Not sure of the electronics you need however you're going to need to make sure you have the power to run both pumps simultaneously (as they werent designed to do so initially). Are these small boiler/recirc pumps? To have the lag pump start on failure, you will need some kind of sensing, like a pressure switch or flowmeter. I take it there are also check valves in place to keep one pump from spinning the other backward, or are they mechanically isolated by ball/globe/gate valves only?
 

iwire

Moderator
Staff member
Location
Massachusetts
In several of my buildings for the heating system we have 2 pumps, one runs all the time the other is for stand by, I would like to have the pumps alternate run times, switching say once a week, and also in case the running pump fails, to have the other pump start .... I would also like to use the existing starters..... Any ideas would be welcomed... Fred

I would just use a simple alternating relay. They are made for this purpose.

http://ab.rockwellautomation.com/Relays-and-Timers/Alternating-Relays
 

JFletcher

Senior Member
Location
Williamsburg, VA
Lead lag?

To me a lead lag system is when you set up say multiple compressors and if the lead unit cannot keep up the lag unit kicks in.

Yes. I've seen it more with pumps and wetwells than compressors but same principles. Lead and lag often alternate duty cycles as well. OP wanted not only switching, but the ability of the back up to kick on if the primary fails, tho most l/l systems Ive seen are if the primary pump can't keep up, not if it fails for whatever reason.

If he wants to equalize run times for getting the most of the equipment, what you linked would work fine. If he wants back-up redundancy, it's gonna be a bit more complicated. iirc, small boiler recirc pumps tend to shear/break the springs in the motor/impeller coupler, or just lock up altogether. I'm failing to see how such a relay would be able to sense a motor/pump fault and switch to lag (other pump) w/o at least timer and pressure switches, and the pressure switch would have to override the timer. It's doable, but Im not sure in this application it's worth it, unless I'm grossly underestimating the pump sizes and costs.
 

iwire

Moderator
Staff member
Location
Massachusetts
OP wanted not only switching, but the ability of the back up to kick on if the primary fails,

I had missed that part so I agree, he needs more than the alternator relay.

Seems needs a more complicated controller of some type.

Perhaps current sensing of the pump motors? Flow switches?
 

J.P.

Senior Member
Location
United States
Some geothermal units circulate by alternating two pumps. I have installed the controllers before.

They do exactly what your after....of course I don't remember who made it.

It was a PLC, relays and two motor starters. It did all come in a handy enclosure.

There weren't any electrical pressure sensors. Maybe there were some mechanical ones installed by the plumbers?

It wasn't a very complicated set up. I don't think it mattered if the pumps overlapped or not. Just that they circulated the coolant %99.9 of the time.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
It doesn't need to be terribly complicated. A "duplex controller" does exactly this, using an alternating relay. But in a typical duplex controller for pumps or compressors, there is something that calls for the first machine, such as a float or pressure switch, then a second sensor that adds the 2nd one if the first one can't keep up. So if pump 1 fails, the level gets higher and automatically adds pump 2.

If this system only has one sensor, it's a little trickier, but not that bad. I used to do it with a n On-Delay timer and NC contacts of the starters feeding a relay that acts like the second sensor. The timer coil is in parallel with the alternator coil being controlled by the sensor. Then a NO delayed contact of the timer is in series with the NC aux contacts of the starters, then to the relay coil. A NO contact of the relay then goes around the alternator contact to each starter.

So when a pump is called for, whichever one is selected by the alternator comes on. At the same time the timer starts timing and a second later, the delayed contact closes. If the starter is running, the NC contact is open so the relay doesn't come on and only the one pump runs. If it fails and the sensor is still calling for a pump, its NC contact closes and the relay closes, which turns on the other pump.

Now I would do all of this in a Smart Relay or Micro PLC because it would cost less than the relay and timer and all that wiring, but not everyone is comfortable with that technology.
 
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