Density "Soft-Starter" for Water Pump?

[WaterSoftnerRick]

Hello Community!

pondering: ....... a Density "Soft-Starter" for Water Pump?

I have these four PLC controlled 7.5hp self-priming water pumps for our new basement dewatering system (commercial building residing over an underground spring)

15feet vertical suction(from sump well in basement)
12feet vertical discharge ...... then 90degree out to horizontal gravity to storm sewer.

Both vertical suction and vertical discharge pipes are in "flooded mode" 24/7. via foot check valves at bottom of suction pipes.

In lieu of an electronic "soft-starter" unit for each pump (expensive .... costs as much as one pump $1,000!) I was thinking to
relieve the startup "work load" (27 feet of standing water) being forced to move immediately up the pipe at start-up by injecting micro aeration (bubbles) into the pumps volute. This would drastically change the density of the water resulting in a "soft-start".

..... thoughts on these?

Thanks!

 

[Valveman]

Actually, just the opposite. With zero head against the pump on start up it will have a hard start. Starting the pump against a closed or almost closed valve gives it a mechanical soft start. This has very little effect on the amplitude of inrush currents but will greatly reduce the duration of the start. Both amplitude and duration play a big part on inrush start currents.

Can be done with something like a slow opening check valve.

 

[Sponsor]

 

[WaterSoftnerRick]

..... I'm confused here! : (
(In operation, It's 3- phase 240 vac @18 surge and run @14 amps.)

In repairing this pump I had to remove the pump from the motor. After repairs, I powered up just the motor without the pump attached.

Running the pump only like this, unbolted to anything! ...... it doesn't even show any "start-up" motor torque in its body (the weight of the motor itself quells the rotational torque of the motor case).

When the motor is attached to the pump ..… a slight rotational torque is evident in the motor body.

When piped back into place with 15 feet of water at rest in the suction (foot valve check) and 12 feet in the discharge pipe ..… at start up, there is a dramatic torque evident in the motor (start-up stress under load).


...... from this, I'm assuming that the major "work demand surge" is from the standing water being forced from a standstill to an immediate full flow transition (Newton's first law of motion) ......... is this true?

 

[Valveman]

Lol! Well I don't know about Newton. But I have experienced the same thing many times. With nothing attached to the motor there is almost no inrush current on start up. Even adding a heavy brass impeller doesn't make the starting amps spike at all. But flood that pump case with water and the motor will dim the lights in the house when trying to start.

It is a lot like spinning a boat propeller on a table. Takes very little effort and spins easily. But try spinning that propeller at the bottom of a bathtub full of water and it will take both hands to barely spin it.

Water is very heavy and a pump trying to lift 200 GPM, which is 3 or 4 gallons the first second, will bog down until it gets up to speed.

Funny thing few people understand is water is only heavy to an impeller when there is flow. If you start a pump against a closed valve there is no flow and no weight for the pump to lift. The pump will start almost the same as if there is no water in the pump.

Again, you can get a slow opening, globe style, check valve. A needle valve on top of the valve bonnet can be adjusted to let the control water out slowly. You can simulate this by just closing one of those red handled ball valves on the discharge of the pumps before starting that pump. It takes very little water to keep the pump cool, but you don't want to let the pump run against a completely closed valve for more than a minute or two.