Issues with Centifugal Pump under vacuum on the suction side.

[KDUN]

Hello. I've used the forum several times in the past for information, but just recently decided to join.

I have a Gould's 3196 1x1.5-8 that is under vacuum on the suction side of the pump (-15 to -20 in hg). The pump is supposed to pull the water out of the tank and feed some nozzles that spray down on a filter belt. The issue is that we are getting very little flow the through the nozzles and the tank is filling up. As a result, the excess water is being pulled in to a vacuum pump and overloading it. The gauge on the discharge is reading 4 psi and does begin to raise when discharge valve is closed. When the suction line is opened and closed, nothing changes on the discharge gauge, it reads 4 PSI. There is no notable build-up or wear in the pump bowl or impeller. Impeller clearance is correct. With vacuum off the tank and the tank full of water, no leaks were noted on the pump seal or suction line. One small leak was noted on a recirculation line that goes from the pump discharge back to the tank. Any other ideas as to what to look at?

Thanks in advance.

KDUN

 

[KDUN]

....one other thing, the nozzle have been cleaned and are not plugged.

 

[Artisi]

What speed is the pump running.
Where is the vacuum gauge located, you need to measure the vacuum at the pump inlet.
Have you checked the total flowrate thru'the nozzles
Can you post a curve.

How can closing the inlet valve not result in a change of discharge pressure.
Is this a new or existing pump installation.

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.)

 

[KDUN]

I have attached the curve. I am not sure how closing the suction valve would not effect the discharge pressure. The suction side gauge is located approximately 3-1/2' above the centerline of the pump. I have not check the total flow rate from the nozzle, I really have no way to do so. This is an existing application.

 

[Artisi]

Was it operating ok previously or is this a new problem?

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.)

 

[KDUN]

It was ok before. There are 5 other identical setups that are operating fine. The only difference being the others go to a drain instead of the nozzle, but it has operated ok before now.

 

[Artisi]

Has it been out of commission for repairs or any electrical work do to it or close by - - - check for correct motor direction.

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.)

 

[KDUN]

We were down for a plant shutdown for a week. No work was done on the pump though. Motor direction is correct.

 

[Artisi]

If direction is correct, assuming you have personally viewed it, it is starting to become one of those little mysteries in life. At this stage all that can be offered is a few suggestions :

Is the inlet pipework clear - no blockages when the unit is operating.
For a unit that was operating the normal checks would be
Direction
Impeller loose
Inlet blocked
Vacuum higher than design increasing inlet head and reducing discharge head.
I am unable to open the pump curve on my phone, will try later on my comp.

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.)

 

[Compositepro]

First of all it is not really correct to say that a pumps pulls material out of a tank. Fluid head in the tank pushes material into the pump.
It sounds like there is not enough net positive suction head for the pump to work. This could be caused by not enough pressure in the feed tank or by a blockage in the piping.

 

[LittleInch]

Kdun, you need to give us a lot more.

Like a pump curve we can open, a diagram of the set up, some flow data, pressures and head at different locations.

Details of this mysterious recirculation line. How is flow or pressure controlled??

What pressure does the nozzle need?

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

 

[Artisi]

Check for air leakage into the inlet side of the pump or maybe via the shaft seal.

A pump that was operating ok previously and which now doesn't is usually something straightforward, although at times can be difficult to track down.

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.)

 

[LittleInch]

OK, I had to rename your file as a pdf to get it to open.

With nothing really to go on, my only suspsicion is that your flow through the pump is higher than you think and for some reason a lot more flow is going back through the bypass. This would equate well with the small rise in pressure when the discharge valve is shut as then the bypass would have 100% of flow instead of maybe 80%?

Down at the end of the curve you might have only 35ft differential head instead of 55 at your base flow rate.

Try isolating the re-circulation line and see what happens....

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

 

[Artisi]

For flow check, direct the flow from one spray into a 5 gallon drum and time it, then multiple by the number of nozzles.

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.)