Pump sound

[moiz19670309]

We have a simple Centrifugal pump with 150 m3/hr flow vs 185 feet head. It has a simple 5 inch vertical suction pipe with a vertical length of 12 feet. Than a long radius elbow than horizontal 5 inch pipe 8 feet long. It then has a concentric reducer from 5 inch to 2 1/2 inch pump suction port. By the way the pump also has a 5 inch foot valve. The submergence above foot valve is 3 feet. We are facing problems with start up. When we start this pump after the priming. Their appears to have sound as if small blasts are being taken place in the pump. This happen for 3 to 4 minutes after which the pump runs quietly.
I think the problem is with the concentric reducer air traps in the area above concentric reducer. The air takes time to get out of this area after which the pump works smoothly. I am right. Should i change the concentric reducer with eccentric reduce?
Guidance required ....

 

[LittleInch]

Can you describe how the "priming" takes place, especially how you bleed air from the inlet piping / fill it from the tank / liquid entry. It certainly sounds to me like you have air in the inlet piping which takes its time to work through the pump system.

The key is your priming system and where it is removing the air. If this is on the discharge side then yes a concentric top flat reducer and sloping up (1: 100) horizontal section would help a lot.

There are other self priming pumps and systems available.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[bimr]

Yes. Use eccentric reducers rather than concentric reducers at the pump suction. Concentric reducers will trap air. Be sure the eccentric reducer is not installed up side down. The top of the reducer should go straight into the suction flange.

Rules of thumb for pumps

http://www.mcnallyinstitute.com/02-html/2-7.html

http://www.pipingstudy.com/reducer.html

 

[Artisi]

Question, if you have a foot valve, why do you need to reprime for start-up?
Sounds like cavitation talking place on startup -but why - more information is required.

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.)

 

[1gibson]

What are the discharge conditions? It is likely the pump is running at high flows during those initial few minutes, and that can cause cavitation.

 

[bimr]

Use and install an eccentric reducer to avoid air pockets as per the drawing below: