2 replies to this topic

For example -- 6" suction pipe from tank going to 3" suction flange on pump.
When eccentric 3" is on top of 6" suction, there is a greater tendency to pull entrained gas.
The arrangement also leaves significant liquid level which can not be pumped
When eccentric 3" is on bottom of 6" suction, there is greater tendency to pull sludge or solids.

Most of the standard details I've seen from large Engineering Firms show the eccentric reducer on top. My perspective is that the eccentric should be on the bottom of larger pipe.

Any ideas which is better??

[Art Montemayor]

orrjd:

At the risk of coming across as pedantic, I'm forced by 43 years of experience in the "business" to advise you the following with regards to centrifugal pump suction piping:

• Placing an eccentric reducer at the pump's suction flange should be done with the smaller diameter (pump flange) AT THE TOP. This is a time and experience-tested method of avoiding the formation of voids or air pockets in the suction line. However, in the special case of slurries, the opposite orientation (small diameter at the bottom) is what should be done. The latter orientation aids in solids flushing action and discourages suction line plugging. Your comment that there is a greater tendency to pull entrained gas when the smaller diameter is on top is simply not true. Trust me.
• Always allow a minimum of 10 diameters of straight suction pipe between the source and the centrifugal pump. This is an old DuPont technique that avoids a lot of suction turbulance and NPSH problems. Trust me again.
• Always be conservative in your suction velocity; keep it as low as possible (except for slurries) by using a liberal suction diameter. The cost in a large diameter suction line (usually one size larger) is minimal when compared with future suction problems.
• Your statement that having the smaller diameter at the top of the suction line also leaves significant liquid level which can not be pumped is not an accurate statement of what is happening in the suction line to a centrifugal pump. I can assure you that there is no centrifugal pump on this earth that will pump it's suction line down to dryness. This is just simply not possible and has never, to my knowledge, ever been expected of a centrifugal pump. Once the pump evacuates all the liquid in the source tank, it will shortly loose its prime and a residual liquid will be established within the pump casing and the suction line. That is why all suction lines should have a nominal 3/4" drain nozzle and valve on the bottom side. This happens whether the reducer is concentric or eccentric - or whether the smaller diameter is at the top or the bottom. You will always have a suction line liquid residual when the pump looses prime due to pumping the source "dry".

I hope these comments and experience serve to put a correct light on the subject of centrifugal pump suction piping practices.

Art Montemayor

[Steve Zeigler]

Art is correct. The orientation of eccentric reducers should be "Flat on Top". You will see the abbreviation "F.O.T." on pump suction lines shown on some P&IDs. Some clients take this a step further and add a section of straight pipe 4 to 6 diameters long between the reducer and the suction flange of the pump. The purpose of this is to reduce turbulence as the liquid enters the pump. If you have a situation where the NPSHa is very close to the NPSHr this can make the difference between a pump that cavitates and one that runs smooth.

In general the higher the vapor pressure of the fluid being pumped the more important the suction piping becomes. Pumping water at ambient temperature does require the same degree of care in the design of suction piping as does water at 180 degrees, or a high vapor pressure fluid like hexane.