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]

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

 

[1gibson]

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.)

 

[LittleInch]

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

 

[bimr]

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

 

[Artisi]

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.)

 

[Artisi]

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.)

 

[DubMac]

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.

 

[moiz19670309]

Dear friends. Thank you very much for the response. For better understanding i have attached my pump sitting sketch. kindly respond.

 

[moiz19670309]

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?

 

[DubMac]

Clean out your sump instead of shortening the pump.

 

[Artisi]

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.)

 

[moiz19670309]

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

 

[Artisi]

Yes, get a hacksaw and start cutting

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.)

 

[bimr]

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.

 

[moiz19670309]

Thanks to all.

 

[LittleInch]

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

 

[Artisi]

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.)

 

[LittleInch]

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

 

[Artisi]

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.)