Restricting flow on aerator lines make amps go down?

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hockeyoligist2

Senior Member
I think this has been answered, but a search doesn't give me any results. I used to know the answer but forgot. Oldtimers disease.

I have a wastewater aerator blower that is shutting down on low amps. The amps are actually dropping below the set point before it shuts down. It is 100 hp 460v, centrifugal. Two other identical blowers are running and attached to the same header.

I know that restricting the flow at the inlet will cause the amps to drop but will restricting the flow on the outlet also cause it to drop?
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
If a pump or fan is not pushing fluid, it will be doing less work, and its motor will draw lower amps. It does not matter if the reason for the low fluid flow is a "nearly closed" inlet valve or a "nearly closed" discharge valve. The impact is similar.

The fluid dynamics is a bit different, so the amp draw on the motor will not be identical in both cases. With the inlet valve closed down and the discharge path open, the pump's impeller (or fan blades) will tend to draw a vacuum at the center of rotation. With a low pressure within the machine, I believe the amps will be lower. If you switch it around, close down the discharge valve and open the inlet valve, the impeller (fan blade) will be pushing the internal fluid against a static head. That will tend to raise the pressure, and cause the machine to do more work. So I believe the amps will be higher, but not as high as they would be if the machine were pushing fluid in the normal operating mode.
 

Besoeker

Senior Member
Location
UK
I think this has been answered, but a search doesn't give me any results. I used to know the answer but forgot. Oldtimers disease.

I have a wastewater aerator blower that is shutting down on low amps. The amps are actually dropping below the set point before it shuts down. It is 100 hp 460v, centrifugal. Two other identical blowers are running and attached to the same header.

I know that restricting the flow at the inlet will cause the amps to drop but will restricting the flow on the outlet also cause it to drop?
Essentially, the power for a centrifugal fan or pump is head (pressure) times flow. Less flow, less power, lower current.
 
T

T.M.Haja Sahib

Guest
I think the vanes on the impeller whether forward, backward or axially curved may have an effect on the current draw when the discharge port is closed.
 
T

T.M.Haja Sahib

Guest
I don't think, given your parameters, that you are at all correct.
Check with the centrifugal fan characteristics for forward, backward and radial blades. Look at the power consumption for each type at zero flow. You may find that forward blades type fan consumes largest power, backward blades type fan lowest and radial fan in between at shut off also.
 

kwired

Electron manager
Location
NE Nebraska
I think the vanes on the impeller whether forward, backward or axially curved may have an effect on the current draw when the discharge port is closed.
I don't think, given your parameters, that you are at all correct.
Different impeller design with valve in same position may give different loading results but current is still going to be lower with restriction than without restriction. Gallons per minute / cubic feet per minute is what the bulk of the work comes from.

If you were bailing water from a tank the faster you work the faster you become tired - but you did move more volume of water or did more work. Using a larger bail bucket will also increase productivity but will tire you faster because you are doing more work in similar amount of time. Now if you have the larger bucket but have to dump it in a drain that will only take it at a certain rate - you have a restriction similar to the partially closed pump discharge and will need to slow down the work because of the blockage - the net result is you do less work in same amount of time - just like the pump.
 

hockeyoligist2

Senior Member
I found the problem yesterday with the help of our mechanics. They were tied up Monday and I wanted to be sure that we would be working on the right end Tuesday. It is big heavy equipment, so I didn't want to start on the wrong end! That was the reason for my OP.

I didn't have big enough wrenches to take it apart, 1 1/2" nuts and bolts. The biggest I have is 1". I couldn't have gotten it apart by myself anyway, we had to haul in our gas powered compressor and use a 3/4" impact wrench, which almost didn't work.

It took 4 people about 2 hours to get that sucker apart. The bolts were stuck.

The inlet valve had a broken shaft. The indicator on the actuator was showing that it was opening.

Thanks again, I didn't mean to start a large discussion!
 

kwired

Electron manager
Location
NE Nebraska
I found the problem yesterday with the help of our mechanics. They were tied up Monday and I wanted to be sure that we would be working on the right end Tuesday. It is big heavy equipment, so I didn't want to start on the wrong end! That was the reason for my OP.

I didn't have big enough wrenches to take it apart, 1 1/2" nuts and bolts. The biggest I have is 1". I couldn't have gotten it apart by myself anyway, we had to haul in our gas powered compressor and use a 3/4" impact wrench, which almost didn't work.

It took 4 people about 2 hours to get that sucker apart. The bolts were stuck.

The inlet valve had a broken shaft. The indicator on the actuator was showing that it was opening.

Thanks again, I didn't mean to start a large discussion!
No problem, this has come up a few times before, the engineers like to go too far into detail, when the typical service guy in field mostly needs to know the more volume being used the more work it will take to move that volume which equates to more current demanded by the motor. This is true for most any pump or blower, especially centrifigual type. Positive displacement pumps are more complex, volume/time is still a certain amount of the load, but pressure begins to become a factor also, especially with media that will not compress easily, like most liquids.
 

Besoeker

Senior Member
Location
UK
No problem, this has come up a few times before, the engineers like to go too far into detail, when the typical service guy in field mostly needs to know the more volume being used the more work it will take to move that volume which equates to more current demanded by the motor. This is true for most any pump or blower, especially centrifigual type. Positive displacement pumps are more complex, volume/time is still a certain amount of the load, but pressure begins to become a factor also, especially with media that will not compress easily, like most liquids.
You want less detail and post that??
See post #4.
:p
 

hockeyoligist2

Senior Member
Can't be helped. Sometimes we're like fire in a field of dry grass... all we need is a little spark.
Good one Jraef!

kwired, When our Positive displacement pumps give a problem it is usually easy to solve the cause, but not the problem. We usually have a busted pipe from the pressure! LOL I had a mess a few months ago when an actuator valve failed on one, I had to put on hip boots and wade in the "sludge". Not a pleasant thing at 2 AM.
 

kwired

Electron manager
Location
NE Nebraska
Good one Jraef!

kwired, When our Positive displacement pumps give a problem it is usually easy to solve the cause, but not the problem. We usually have a busted pipe from the pressure! LOL I had a mess a few months ago when an actuator valve failed on one, I had to put on hip boots and wade in the "sludge". Not a pleasant thing at 2 AM.
I once was trying to find out why a submersible pump at a swine facility waste water holding area was not working. I had a helper with me that day, the pump had a 2-1/2 or 3 inch discharge hose that was not too far from the pump panel. I don't remember exactly what the problem was anymore but I did something in the pump panel and decided to "try it". It worked, but a fitting broke, came apart or something of that nature in the hose and sprayed all over my helper, I got just a little mist because he pretty much shielded me. He did hose himself off as best as he could but this place was about 30 miles from home and I still had to ride with him. @#&* happens:)
 

hockeyoligist2

Senior Member
Indeed it does.
My boss and another electrician in our shop were checking out a sump pump on a sewer lift. I don't know what happened exactly, but the pump kicked on, a fitting came apart, and they both got a face full.

The joke went on for many weeks about what they would like for lunch.
They probably didn't want any corn. That stuff looks the same after it's been... well you know. When I first started working in sewer I couldn't eat corn for a while.

We have a saying in the sewer business, You get used to the smell, but you never get used to the taste! That is a very true saying, I did get used to the smell, but after 14 years and many unwanted "tastes", I know!
 
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