Pump when Dry-running 1

[macmet]

Hello everyone,

I hope you are all doing well.

I have a situation where I am trying to transfer cleaning water between two tanks. Currently I manually turn the pump on and monitoring the level in the tank being emptied. When the tank is completely empty I turn off the pump and isolate the tanks. The tanks are both less than 500L.

I would like to automate this process and as usual, it has to be as cheap as possible. The pump is a small, 1HP, centrifugal pump. For reliability I would prefer to use a flowswitch in the drain line of the tank being emptied, but I'm curious if I could use the power draw on the pump to trigger shutting off the pump. I think that may be a cheaper option.

I am concerned that the power load will not change significantly when the flow goes to zero. I have come across a few web pages that indicate the same, but I would like to get additional feedback from people more knowledgeable then I about pump operations.

Does anyone have any experience using the power draw to indicate dry running?

I'm also curious if I able to use the pump curves at the zero flow condition to determine the expected power flow at this condition?

Please let me know if any additional information would be helpful.

Cheers,
Macmet

 

[Artisi]

Why re-invent the wheel, use simple, well proven float switch/s to stop and start the pumps.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[itsmoked]

I would expect you could see a pretty significant power drop when the water runs out. Best bet would be to use something like a WattsUP to monitor the power to see. I'd expect a small pump to have its power drop by about 50%.

The BIG question is what are you using to monitor the power and open the contacts? I don't believe those are very common.

The problem is if you mean CURRENT. Lots of things can measure the current and open contacts but the current unlike the power will NOT change much on a small motor that runs dry...

Keith Cress
kcress -

http://www.flaminsystems.com

 

[macmet]

Keith,

You are right, I should have been referring to current. What would you estimate the current drop to be on a small pump such as this?

I looked into the Wattsup like you suggested and it looks like a good device, but too expensive for this use. I appreciate the suggestion though.

Artisi,

I also thought of using a float switch, but got some push back. Maybe I should reconsider that option.

Thanks all,

Macmet

 

[georgeverghese]

As suggested, one way is to pick this up on the ammeter if the ammeter resolution for this 1HP motor is good enough for this.

Else, if the tank is atmospheric, and the liquid density is more or less constant, another alternative to the FSL would be a PSL reading liquid static head.

 

[Compositepro]

Current is not a good run-dry indicator. These guys have good literature explaining it:

http://loadcontrols.com/

But those are not very cheap. A pressure switch on the output of the pump is usually the simplest and least expensive option. Use a momentary contact push button switch to start the pump. When the pressure rises enough to indicate flow (close the pressure switch), a relay contact latches across the push button contacts so that you can release the switch and the pump remains on. As soon as the pump run dry, the press sure switch opens and drops out the latching relay so the pump stops automatically. This is the same wiring diagram as the standard two button start/stop arrangement for electric motors except the pressure switch is the stop button.
A float switch in the tank or a paddle flow-switch will also work. Pressure switches tend to be cheaper and more reliable.

 

[itsmoked]

That sounds like a great solution Comp. I like it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

macmet,

Some questions to ask yourself.

1) Is the volume you transfer more or less constant - if so why not just use a timer? If not can you adjust the operation based on the volume in the tank?
2) For a pump of this size, does running fry for a while actually do it any harm?
3) what is the required / actual operation when the pump runs dry? does the feed get a mixture of air / water for a while ebfroe you stop it? Does the discharge sit at a constant pressure (pushing in at the bottom of the tank) or does the discharge line empty?
4) wrt float switches and pressure switches, how empty does your tank need to be when transferring?
5) Does the pressure stay steady now until you run dry? You may need a timer on the pressure switch to avoid accidental tripping if you really want the pump to completetly empty the tank

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ziptron]

I'm not sure why a float switch is out of the question, perhaps maintenance or size of tank opening is the problem? If so, you can perhaps try an ultrasonic level sensor. Mount this above one of your tanks and it knows how much water is within and thus knows when to cut power to the pump.

http://www.flowline.com/level-switches.php

Just a thought, though the other options presented would likely work as well.

 

[itsmoked]

What would you estimate the current drop to be on a small pump such as this?

I see I didn't respond to this.

I'd guess there'd be only about a 10% change in current. The reason is because the power factor takes a huge dump on the motor when it goes unloaded. The work and actual power does change substantially. However, when the power factor drops, the current component that does no actual work, increases dramatically while the current component that does actual work, drops, also dramatically. The end result is that the current sensor, (being completely ignorant of the power factor), continues to see the sum of the two components, hence the small change in measured current. This is what renders current monitoring to be unreliable as a run-dry curative.

Keith Cress
kcress -

http://www.flaminsystems.com