Pump Inlet Conditions?

[jasonreber]

I am retrofitting an old wastewater station that is feeding a centrifugal pump. The station provides an average of 4' of head at pump inlet. The existing pump is configured with a 4" inlet. I want to install a new centrifugal pump that has a 6" inlet. If I install a 6" pipe several diameters upstream should I be concerned? This would allow me to avoid major plumbing construction. I was told this was a "big no-no", but I can't prove it.

Thanks for any help you provide,
Jason Reber
Honolulu, Hawaii

 

[PUMPDESIGNER]

Insufficient information has been given.
Although pump connections sizes are important, they are minor compared to pump data such as curve geometry, NPSHr, the system curve, and operating conditions such as flow range and duration of the different flows that will be seen.

PUMPDESIGNER

 

[Scipio]

Agreed with pump designer, nowhere near enough info for a complete answer. Even with the limited information, however, I'd also agree with whoever told you it was a big no-no, if for no other reason than the suction piping for a 6" suction nozzle should be at least 8", especially with only 4' of NPSHA (or is that 4' of static head?) Unless you're 110% confident of your NPSH calculations anything less will probably cause excessive line losses and result in cavitation, which, in waste water, can be pretty agressive.

 

[jasonreber]

Ok, let's see if I can explain this better. I have a 4" line coming into an old pump. The line is providing 4' of static head and the line runs 8 ft from the wet pit to a pump placed in a dry pit. A gate valve is 5 ft from the inlet of the pump, and this is where I would like to expand the pipe from 4" to 6". The pump has duty conditions of 6' of static head, 12' of TDH, and capacity of 700 gpm, and 22.4 ft of NPSHre. My question is, "Will the pump inlet flow be steady enough to allow the pipe change?"