Reducing the length of a DWT Pump

[moiz19670309]

We have a deep well Turbine (DWT) type pump installed at our pump house. The minimum water level remains 8 feet above minimum submergence of pump. Due to high silt moving into the pump house we are thinking about reducing the length of pump in the pump house. We are planning to reduce the length by 4 feets (by reducing one of the middle shaft length and reducing one column length). By doing so we will still have a 4 feet water above the minimum submergence level of this pump. I need advice before doing this. What things we should keep in our mind. Does it is going to have any negative effect on its discharge head.
Help needed. Moiz Khan

 

[LittleInch]

My tablet won't work anymore on this site but I agree it's not perfect. However even with "could" - the question remains - where do you think 4 more feet of head come from when the pump is still submerged?

My motto: Learn something new every day

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

 

[LittleInch]

My issue is that I don't believe your statement "therefore raising the pump 4 feet increases the head from water level to the discharge flange" is correct. Raising a submerged pump 4 feet higher ( but still below the surface) should have no effect on the discharge head relative to water level as the extra height of the pump is exactly matched by the reduction in the inlet head into the pump, hence no change. Lowering a pump well below the surface increase NPSH and avoid vortexing, but unless the increase in piping length is significant, there will be no difference discharge head. If my basic physics is wrong then I would like to know why.

In the OPs case the tidal change in water level will affect the output head / pressure but that will be the same regardless if the pump was at the bottom or the top of the sump providing it is under water.

My motto: Learn something new every day

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

 

[1gibson]

Raising the pump 4ft would actually decrease the head 4 ft *at the discharge flange* because now that point of reference is 4 ft higher elevation. Once you elbow back down to the existing portion of the discharge pipe, there would be no change in head (aside from minor losses.)

The head created by the pump between water level and existing discharge pipe, will not change no matter where you put the pump, within reason.

 

[Artisi]

LittleInch: Clarifying static head imposed on a submersible pump.

When calculating static head on a submersible pump the measurement is from the standing water level in the sump to the reference point under consideration, irrespective of the pumps location below water level.

1gibson: Quote. Raising the pump 4ft would actually decrease the head 4 ft *at the discharge flange* because now that point of reference is 4 ft higher elevation. Unquote.

Did you intend to say increase the head rather than lower the head?

N.B. I have requested my post of 22 Sep 14 7:45 to be removed as I included contradictory info (late at night in my part of the world)





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]

moiz,

just to summarise here I believe we all now are of the same view, i.e. raising your pump 4 feet, but still under water, will not affect the discharge head seen by your surface pipework from current conditions, noting that this head will vary by tide level as I'm sure you are aware.

If you take up my suggestion of simply raising the whole pump as it is 4 feet by building a structure between pump mounting and the surface, then the head at the discharge flange will be 4 feet less, but if the pipework is then elbowed back down to the current pipe, this difference will disappear.

My motto: Learn something new every day

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

 

[moiz19670309]

Thanks to all for their valuable comments.
So, now i would lift the pump inlet by 4 feet. By removing the middle shaft and middle column in such a way that the pump length would reduce by 4 feet. The discharge remains at its current point and i am not going to disturb the outlet piping network. I think we have adequate skill to reduce this assembly perfectly. It is a single impeller pump. i would take care of the rubber lined bearing and its placement. But i would have to check for vibration analysis and natural frequency discrepancy. By the way this is an old pump manufactured in 1963. The manufacturer is out of market.
Moiz Khan

 

[Artisi]

Do you know what the bearing centres of the line shaft are?

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]

Why don't you just remove the "suction bell" as shown on your drawing and fabricate a new piece with two elbows, a reducer to double the size and reduce inlet velocity, put some anti vortex plates in it and point it upright.

Messing around with anything 50 years old(!) would seem to be not a good thing. As soon as you take this thing apart to modify my guess is that you'll end up replacing most of the pump. what material is this made of that has lasted 50 years in sweater??

My motto: Learn something new every day

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

 

[1gibson]

The manufacturer may be out of market, but there is a good chance that the product line is carried by an existing manufacturer, who can provide parts and service. Reworking a shaft and spool piece isn't exactly rocket science though.

 

[DubMac]

Somewhere down the line someone will ask: "Why didn't they just clean the damn sump out instead of reinventing the pump every 6 months?....Sure glad those knuckleheads are gone".