Pump Eccentric reducers - bottom flat / top flat

[mfelzien]

Fellow chemical engineers:

I have a question regarding how to best explain the process rationalization between TOP-FLAT and BOTTOM-FLAT eccentric reducers used on pumps. From my understanding I wrote the following paragraphs:

"Two process conditions are issues to pump performance. Gas coming out of the process fluid and solids coming out of the process fluid. Both of these are issues before the pump and can harm the pump impellers."

"Top flat is used when there are gas issues in the process fluid. This arrangement of the eccentric reducer allows the gas to have less residence time to form or coaless to larger gas bubbles from smaller bubbles. Larger bubbles mean more harm to the pump when going through the impeller. The bubbles rise and are pulled through the top-flat eccentric reducer quickly."

"Bottom flat is used when there are solid issues in the process fluid. This arrangment of the eccentric reducer allows the solid to have less residence time and to be quickly moved throught the impeller thereby causing less coalessence and issues for harm in the pump. Solids are an issue best to have quick flowrate through the pump before too many solids accumulate. The solids come-out of solution and are drawn quickly through the bottom-flat eccentric reducer to the discharge side of the pump."

This is my current understanding on the subject (which I am positive is wrong). And I'm still not happy with the explination too much vaguary. Is there testing for the choice of these different arrangements?

If anyone could provide the correct answer, it would be appreciated. I am not exactly satisfied with this hand-waving answer. It lacks the intuition I would like. Any suggestions?

Thx

mfelzien
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[25362]

Try the following threads:

thread407-181888 Its heading has been "expanded" to cover suction piping
thread407-173003
thread407-157254
thread407-149171
thread407-133376
thread378-140479

 

[ApC2Kp]

mfelzien,

Your understanding of eccentric reducers flat on top or on bottom is same as my view point, and would be correct for horizontal flow piping. There is no concern in a vertical flow piping, so a concentric reduceer could be used in the vertical flow situations.

The eccentric reducer situation also occurs at many control valve stations wherre the control valve has one or two size smaller body than the piping. There the inlet eccentric reducer could be Flat on Top for gas bubbles, or Flat on Bottom for solids. The eccentric reducer on control valve outlet would usually have Flat on Bottom for free draining of the piping.

The Flat on Bottom eccentric reducer is also used in pipe racks to keep the pipe supports at same elevation, after a pipe size change. If a concentric or a Flat on top reducer was used in pipe rack, then more than one support detail would be required.

 

[11echo]

I have never used an ECC. RED. “Bottom Flat” on a pump installation! Because as you initial point out it's a collection point for gases, and ingestion of these gas bubbles is very harmful to the pump impellers! Now "IF" solids were an issue, I'd be using some type of inlet strainer, and not "hoping" the solids would settle on the bottom of the inlet piping! ...My $0.02 anyway!

 

[itdepends]

Unfornately strainers don't work when you actually want to pump the solids...........

 

[11echo]

..Then "IF" you want to pump solids, why use an eccentric reducer at all? I've seen applications like this and we just used concentric reducers.

 

[itdepends]

Because the heavy/coarse solids tend to flow along the bottom of the pipe- it's better to give them a smooth flow path into the pump- less chance of soilds build up and/or wear.

 

[11echo]

itdepends ...That's interesting, THX! ...SO along that line would it not be a better piping configuration to have a "long leg" of the same diameter pipe to the suction connection on the pump, as apposed to changing diameters at the pump flange?

 

[itdepends]

Not really- because generally the velocity at the inlet to the pump is considerably higher than the main line velocity. Dropping the pipe size further back can result in excessive pressure drop in the suction line and associated high wear rates.

We generally try and keep our slurr velocities around 2-3m/s in discharge lines (high enought to avoid settling out, low enough to prevent excessive wear). You can run higher average velocities in larger pipes because the velocity at the pipe wall is lower (longer velocity gradient from central core to pipe wall).

Suction lines are normally one pipe size bigger than the discharge to reduce the pressure losses.

Cheers.

 

[rocketscientist]

Here's something useful.

I think this nicely encapsulates the discussion and my experience.

Dirk Willard

 

[twr]

We use ecc bottom with liquids that have a solid component to avoid the settling issue. If anything does have the tendency to settle to the bottom of the pipe, it will still be in line with the suction and will not build up a residue. For our application it is also important to have self-draining lines.

 

[KimBellingrath]

Why do many system specs call for flange bolts to straddle the center - so as to not have one on the top?