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Reducing the length of a DWT Pump

Reducing the length of a DWT Pump

Reducing the length of a DWT Pump

(OP)
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

RE: Reducing the length of a DWT Pump

There should be no negative effect on the discharge head as the pump is being moved vertically, hence the decrease in inlet head is exactly matched by the increase in height of the pump. The differential head remains the same, but inlet head is 4 feet less, but height of pump is 4 feet higher, therefore any d/s pipe will see the same head / pressure.

Issues to think about are to check the NPSHr to see if you're still within it by a reasonable margin (3-4 ft min above NPSHr is recommended) and 4 feet coverage could start to induce a vortex if the inlet is vertically upwards and there are no anti vortex plates. Hence over time if a vortex does appear you could suck air in. 4 feet is probably just on the limit, but it depends on your flowrate, inlet size and shape and inlet velocity into the pump.

My motto: Learn something new every day

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

RE: Reducing the length of a DWT Pump

He says 4 ft above the min submergence, not "4 ft submergence" so that should be fine. Usually submergence is the limiting factor before NPSH, but if NPSHR is more than 32 ft (atmospheric @ sea level) plus submergence, then it needs to be checked.

The only other thing to consider, hydraulic institute has guidelines for distance from sump floor to suction bell (1/2 to 1/2 bell diameter.) There can be flow disturbances with open spacer under the suction of the pump. Probably only a concern if flow is more than 10,000 gpm (arbitrary guess.)

RE: Reducing the length of a DWT Pump

Good point, so long as the minimum level is set at a water temperature above what yu have and you're not at 7,000 ft ASL you should be ok, but NPSH needs to be checked, don't rely on minimum submergnece. Given the normal type of these pumps the only other item I can think of is vibration analysis and whether the section you remove has a bearing in it or not.

My motto: Learn something new every day

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

RE: Reducing the length of a DWT Pump

Have you run this question past the pump manufacturer? That should be the first step.

RE: Reducing the length of a DWT Pump

It is very unlikely that NPSHa/r will be a problem. If at sea level or thereabouts NPSHa is 32ft + submergence to the inlet - doubt if the NSPHr will be anywhere near to NPSHa.
OP also has indicted that submergence is 4ft above minimum submergence.

However if the silt load is high what advantage do you see in raising the pump? if the silt has time to settle before being pumped then you are only delaying the time before it reaches a level to become a problem once again.
Probably better installing a small hard impeller pump in the sump to handle the silt as it builds up.

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.)

RE: Reducing the length of a DWT Pump

Edit last post -- hard metal impeller submersible pump.

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.)

RE: Reducing the length of a DWT Pump

Another thing to consider with sand intake is whether or not your lineshaft is open or enclosed. Would be much better for lifespan if enclosed. If open, make sure your elastomers (bowl & Lineshaft bearings) are natural rubber and not nitrile or buna; natural rubber is best against sand erosion.

When shortening a vertical pump from its original length MAKE SURE you check with manufacturer to see if there could be a conflict with your new natural frequency moving into the neighborhood of running frequency. Its a small chance of actually happening, but if it does you will go through a lot of failures and fingerpointing before you figure out what it is.

Just to be clear (I think everyone is) but minimum submergence value and NPSHR value are not the same and should not be confused as such. Min submergence is to prevent vortexing, NPSH is to prevent cavitation. You can easily have plenty of Min submergence margin and still cavitate.

RE: Reducing the length of a DWT Pump

(OP)
Point wise reply;
1. As shown in the sketch NPSHr is 17.5 feet.
2. As you see in this case currently NPSH available is 32 + 8 = 40 feet
3. With respect to submergence the sketch shows the clear picture.
4. As shown the discharge flow is 7600 IGPM at 4.55 psig discharge pressure
5. Water temperature is 26 to 28 C
6. The pump is a closed shaft pump with rubber lined bearing with demineralized water supply
7. The silt settling rate is 4 inch per year at average.
Hence, as advice I have to do vibration analysis and check that the running frequency of modified pump is not closed to the natural frequency of modified pump.
Does the discharge flow and pressure remains the same?


RE: Reducing the length of a DWT Pump

Clean out your sump instead of shortening the pump.

RE: Reducing the length of a DWT Pump

I agree with DubMac, clean the sump every 6 months or so.

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.)

RE: Reducing the length of a DWT Pump

(OP)
Thanks for the suggestion. However, we have decided to shorten the length. Any advice in this direction please.

RE: Reducing the length of a DWT Pump

Yes, get a hacksaw and start cutting smile

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.)

RE: Reducing the length of a DWT Pump

There will be an insignificant change in the discharge flow and pressure.

One area to be considered about is creating a vortex and air entrainment with the lower submergence.

You should have a quality pump repair shop make this adjustment for you.

RE: Reducing the length of a DWT Pump

(OP)
Thanks to all.

RE: Reducing the length of a DWT Pump

Of course your other option is simply to raise the pump 4 feet ( or maybe a bit less) by means of a steel frame under the pump frame mounting and re-align your pipework and power cables as required.

The big advantage of this is that you avoid touching the pump and its shaft / column. If your lift was actually a single stage then it would be easier, but I would be wary of making these changes and expecting any vendor to guarantee it unless they do it themselves.

doing it this way should also be a bit faster

My motto: Learn something new every day

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

RE: Reducing the length of a DWT Pump

Raising the pump 4 ft cause result in performance problems at the low level, a 4 ft increase in head on an axial flow pump which I assume this unit is could cause hydraulic problems. You will note that the performance detail given on the drawing gives 4 something psi, I assume this is its rated head.
changing the pump length, although not my solution to the problem, is easy, and straightforward.

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.)

RE: Reducing the length of a DWT Pump

Artisi, unless I've missed something here where do you get "a 4ft increase in head"? The differential head of the pump surely remains the same, but the pump has "lost" 4 feet of inlet head but is now 4 feet higher. Therefore surely the out put head/pressure measured at the pump outlet flange on the surface remains the same? Dropping a pump to the bottom of the ocean doesn't increase it's static head compared to just below the surface.

There might not be physical room to lift it 4 feet in the air, but it all depends who is changing the pump length - the pump vendor or some other contractor...

My motto: Learn something new every day

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

RE: Reducing the length of a DWT Pump

My last message (don't you just love predicted text) typed cause instead of could. (First line)

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.)

RE: Reducing the length of a DWT Pump

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

RE: Reducing the length of a DWT Pump

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

RE: Reducing the length of a DWT Pump

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.

RE: Reducing the length of a DWT Pump

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.)

RE: Reducing the length of a DWT Pump

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

RE: Reducing the length of a DWT Pump

(OP)
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

RE: Reducing the length of a DWT Pump

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.)

RE: Reducing the length of a DWT Pump

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

RE: Reducing the length of a DWT Pump

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.

RE: Reducing the length of a DWT Pump

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".

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