two sump pumps

[j9DuBois]

So there are two NEW sump pumps for sewage. One lead, one lag. a total of 4 circuits. each pump is 240 volts, 2 circuits each. when either is running the amperage is correct. However, when both run at the same time the amperage doubles, to 39-40amps.
When we got there the 30 amp fuses blew.
Any ideas as to why the amperage would double?

The well was not purged when the new pumps were installed. The old pumps were rated for only 15 amps.

 

[Dennis Alwon]

Does the amperage stay at 39 or is it the initial surge? I am trying to understand why this would happen and I come up blank unless the shut off is not working but I would think that would just make the pumps run freer but I have never seen a sewer lift station with 2 pump.

 

[augie47]

You might discuss the mechanics with someone, It's just a shot, but I picture them pumping against each other when both try to run especially if the main discharge pipe is small.

 

[Dennis Alwon]

Why did they just put a pump that can do the job? Gus has a good thought.

 

[dicklaxt]

If I recall correctly these type systems have several features,,,,,,,,the lead/lag order is selectable and can be programmed so that which ever pump is selected as lead the lag pump will come on and augment the lift capabilities,that being the basic funnction tho there are more.If you say they do fine individually under test conditions minimum load but then when both are running the amperage increases,it sounds to me like the discharge piping may be undersized and the increased head pressure is putting the motors under a strain they were not designed for,,,,,,,,,,,just a thought.Did a manufacturer rep commission the system or was it two separate contractors Electrical and Mechanical.

dick

 

[JDB3]

I have just been to one site similar to this. The reason for 2 pumps (at that location) was that if one went out (quit working) the other one would come on. That system was not designed for both pumps to run at the same time.

 

[automate0042]

If the sytem has a pump labeled lead and a pump labeled lag, then the system is most likely designed to run two pumps at the same time. FYI when a centrifugal pump is blocked in, the amps usaully drop, as the load on a pump is a function of head and flow.

 

[templdl]

Would there be a chance that there was an attempt to start both pumps at once. By doubling the start current that may have initiated a voltage drop that resulted inthe pumps stalling draw current for long enough duration to tke out the fuses.
It may be assumed that this is not posible with the starters configured as described that would not allow both motors to start at once but the posibility that they wold be allowed to start at the same time under an unusual situation shouldn't be overlooked.

 

[A/A Fuel GTX]

So there are two NEW sump pumps for sewage. One lead, one lag. a total of 4 circuits. each pump is 240 volts, 2 circuits each.

Two circuits feeding one pump:?

 

[renosteinke]

Your amperage goes up when something - especially something stringy- gets wrapped around the impeller.

When the pump draws more amps than it should, your first step -if they're 3-phase pumps- is to revers the direction and see if that clears it. Otherwise, you need to reach under the pump and try to spin the impeller by hand. It ought to spin freely. If you can't do that, then you need to pull the pump out. Wear gloves, be wary of needles, have the appropriate HIV vaccinations.

Stuff tends to get wedged between the back of the impeller and the pump case.

How many are more amps than they should draw? Well, that's why I make a note in the panel when the pumps are new, something like 'Left Pump 7 amps 6/6/12." Now I have a benchmark for future visits.

 

[GoldDigger]

You might discuss the mechanics with someone, It's just a shot, but I picture them pumping against each other when both try to run especially if the main discharge pipe is small.

Since these are sewage pumps, I expect that they are impeller type rather than positive displacement type. In that case a restriction in the discharge line should actually reduce the power requirement of each pump by reducing the volume moved. An centrifugal impeller type pump will not consume appreciably more power to move the liquid against a greater head pressure, and will theoretically consume no power when pumping against a closed valve.
Now the back pressure from a restricted discharge might possibly cause one pump to try to start with a full output pipe system when the other is running and so have an immediate load during startup, while when running by itself the pump could be initially sucking air until it self-primes. If they are submersible pumps, then that whole idea does not apply though.

 

[ptonsparky]

The pumps in our local village seal themselves into place on the discharge pipes via gravity. The weight of the pump and motor holds it into place unless the discharge becomes blocked, then it will raise up somewhat and recycle what it is pumping. They may increase amperage at this time but I do not recall them ever approaching double amperage. Ours are in a holding tank. Each pump has a check valve before they merge into a common pipe on the way to the settling pond.

 

[tom baker]

when the discharge head of a centrifugal pump increases, the motor load decreases (this is due to the pump or infinity laws) Example if a motor is overloaded, then often the discharge valve is throttled. When the two pumps run at the same type, the increased flow increases the head loss, and the amps should go down.

Almost always when a motor is overloaded its a mechanical problem, but the electrician gets the blame. Typically with sewer pumps they get "ragged up".
You may just have two motors starting at the same time, creating excessive inrush.

recommendations
Starting time delays on each motor
Time delay fuses for motor starting
Look at a lead lag controller this one is very simple and easy to wire http://www.marshbellofram.com/diver...ternating-triplex-and-quadraplex-controllers/

 

[j9DuBois]

Selectable and programmed

Selectable and programmed

If I recall correctly these type systems have several features,,,,,,,,the lead/lag order is selectable and can be programmed so that which ever pump is selected as lead the lag pump will come on and augment the lift capabilities,that being the basic funnction tho there are more.If you say they do fine individually under test conditions minimum load but then when both are running the amperage increases,it sounds to me like the discharge piping may be undersized and the increased head pressure is putting the motors under a strain they were not designed for,,,,,,,,,,,just a thought.Did a manufacturer rep commission the system or was it two separate contractors Electrical and Mechanical.


Yes this correct. These systems were replaced by 'others', prob. two contactors, one electrical one plumbing. The only way that I know that the old pumps were 15 amp motors is that they were left to rust outside. I do not know what size the pumps are that were installed and the amperage does not decrease as the two run. When they are reun individually, the amperage does decrease.
Okay, I am going to use this as a start, the discharge undersized. I already asked that the well be purged, and sumps looked at. ICKY!

Thanks everyone for your input

 

[ActionDave]

Yes this correct. These systems were replaced by 'others', prob. two contactors, one electrical one plumbing. The only way that I know that the old pumps were 15 amp motors is that they were left to rust outside. I do not know what size the pumps are that were installed and the amperage does not decrease as the two run. When they are run individually, the amperage does decrease.
Okay, I am going to use this as a start, the discharge undersized. I already asked that the well be purged, and sumps looked at. ICKY!

I don't know what is wrong. I suspect the problem is with the pumps.

It is not anything to do with an undersized discharge. It doesn't work that way. The smaller discharge line would make the pump motor draw fewer amp because it moves less waste.

How many buckets of water can you bail out of a sinking boat if you used a one gallon bucket? How much harder is it if you use a five gallon bucket?

 

[renosteinke]

I have to challenge the assertions made that additional resistanse to flow (head, restriction, whatever) causes a drop in amp draw. Bull Pucky.

Every pump has a discharge curve, with the horsepower curve superimposed on it. Please produce one - just one - that shows the horsepower going down as the head increases.

Complete blockage? That's different. In that instance, you have cavitation - the head drops because, in effect, the impeller is trying to push vapor. Or, you simply have the same water racing around inside the casing.

The two pumps pushing against each other? Only if the plumber really, really undersized the pipe and used the wrong fittings.

Typical pump controls will have the pumps taking turns as the 'lead' pump. The second pump will only come on when the tank level continues to rise. Ironically, a fuller tank has the individual pumps draw less current, as the 'suction head' is greater, and offsets more of the required discharge head. Deeper water = greater pressure into the pumps.

Whatever the FLA of the pumps, the pumps generally do not draw anything near it. A pump will draw the FLA only when being pushed to it's limits. If the pump is capable of producing 30-ft of head, yet the plumbing only requires 15-ft. of head .... the pumps will draw only a fraction of the FLA. Look at the pump curve and you can make a pretty good estimate of the actual current drawn.

If, for any reason, you're drawing more than the FLA, it's a safe bet you have a pump problem. You either have something in the pump casing keeping the impeller from spinning, or you have a bad bearing in the pump keeping the shaft from turning. When it's a bad bearing, you can usually hear the pump scream as the tank is emptied.

 

[ActionDave]

I have to challenge the assertions made that additional resistanse to flow (head, restriction, whatever) causes a drop in amp draw. Bull Pucky.

Whatever the FLA of the pumps, the pumps generally do not draw anything near it. A pump will draw the FLA only when being pushed to it's limits. If the pump is capable of producing 30-ft of head, yet the plumbing only requires 15-ft. of head .... the pumps will draw only a fraction of the FLA.

Which side are you on.

Look at the pump curve and you can make a pretty good estimate of the actual current drawn.

I have worked on lots of pumps; never had to look at a pump curve. It makes little difference to me what they say. I know that if you restrict the flow the amps will go down.

 

[tom baker]

I have to challenge the assertions made that additional resistanse to flow (head, restriction, whatever) causes a drop in amp draw. Bull Pucky.

Every pump has a discharge curve, with the horsepower curve superimposed on it. Please produce one - just one - that shows the horsepower going down as the head increases.

With a variable torque load, the amperage decreases as the flow decreases, due to the affinity laws or pump laws
1. Flow is proportional to shaft speed
2. Pressure or Head is proportional to the square of shaft speed
3. Power is proportional to the cube of shaft speed

What the law means is if you reduce pump speed by 50% you cut the power by 1/8. Its the basis for VFDs on fans and pumps.
And I know if I have a pump that is overloaded, I can throttle the discharge and reduce the amperage. I have a 50 hp pump that pulls 75 amps and the discharge is throttled, as the FLA should be 62. The real solution is to trim the impeller.

A motor is dumb and will just keep pulling power if overloaded, it needs to run at full speed. Often the motor is overloaded due the pump being sized wrong

Mechanics will say its an electrical problem when the OL trips when most of the time its a pump problem.

Pump Curve? Don't have at home. But the pump laws are all we need for centrifugal pumps


I wo

 

[jumper]

I have worked on lots of pumps; never had to look at a pump curve. It makes little difference to me what they say. I know that if you restrict the flow the amps will go down.

That is what I have been taught. Works for air flow also.

 

[renosteinke]

So easy to say "I know ..." and not produce any data.

Two rebuttals, and neither bother to post anything like this: http://www.pumpstroubleshooting.com/wp-content/uploads/2009/09/centrifugal-pump-performance.JPG

There, I did your work for you ... and ....

By gosh, by golly, I couldn't have been more wrong had I predicted a Super-Bowl win for the Chicago Cubs.

I stand corrected.