dead head associted with low current?

[jor1492]

Good day everyone,

Yesterday I was a issue with one pump operated by motor, where it wasnt deliver enougth flow rate and it was running with low current. at the end I discovered that the problem was a dead head operation

So my question is, a symptom of a pump working in dead head is low current in the operation of the motor?

If this is true what is the law or mathematical/physical point of view to demonstrate it.

also is this associated with low velocity of the impeller ?

Regards

 

[RVAmeche]

See Rusika's response in this thread:

https://www.eng-tips.com/viewthread.cfm?qid=152951

Take a look at your pump curve. Power increases as flow increases.

 

[LittleInch]

Yes.

A qick look at your pump curve will show the power required at no flow is often lower than any other flowrate.

This is because the pump is doing very little work (no flow) other than internal re-circualtion flows within the pump at low efficiency.

Lets say this is like lifting a bucket of water up a set of stairs.

If you stop you still have to expend energy holding the bucket, but this is a lot less than climbing the stairs holding the bucket.

The impellor speed is also important but for most motors this is a fixed speed, unless you have a VFD.

Then the pump affinity laws come into play.



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

 

[Valvecrazy]

Throttling a full speed centrifugal pump can reduce the amp draw almost as much as varying the pump speed. This is counter intuitive and very few people understand it. As a matter of fact I have been chastised nearly thirty years for saying so. I am glad to see it explained so well in these two threads as it is a hard concept for many people to understand. However, both ways still cause less efficiency. The pump will be using more energy per gallon produced, even though the amp draw is reduced by throttling or varying the pump speed.

 

[TenPenny]

For a typical centrifugal pump, the power load drops as you approach shutoff. But, like everything in life, there are centrifugal pumps where the power does not reach its lowest point at shutoff, your best bet is to look at the pump power curve.

Somewhat related, I was on a webinar put on by a manufacturer yesterday, where the point was made that a 1 hp motor always draws 746 Watts. I didn't have an opportunity to ask, doesn't that depend on how much the motor is loaded? I dislike people who make sweeping generalized statements without clarifying what they actually mean.

 

[Valvecrazy]

I completely agree! Pump curve, pump curve, pump curve! That is where you get the answer to this question. The drop in amps is just proof.

 

[Artisi]

TenPenny, from memory these very small motors don't have much change in power input across their operating range.

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.)

 

[Valvecrazy]

The pump is what has changes in power required across the operating range. Some brands do better than others, but there are many small pumps that will drop 50% to 60% in amperage at deadhead or close to deadhead conditions.

 

[TenPenny]

Thanks for that; I don't have much experience with such small motors, and it's possible that he was saying more. However, later on, he referred to 'break horsepower' on a power point slide, and then converted hp to kWh, and multiplied that by hours and cost per kWh to get operating cost, so I don't think I'll take his comments too seriously, LOL.