vertical pump problem

[rateb]

pls advise me we had sea water pump used for plat form washing and at this time the operation time is part of aday not continuas operation and after many years we need it for anther service to feed heat exchanger for generator which high jacket temperature problem is repeatedly occur at summer condition so we put anew pump with new flow rate and TDH secan hand one this pump at its Initial condition with 10 stages and its rpm is 1750 we didnot find in our work shop motor with this rpm so we use motor with 3600 rpm and make destaging (remove 7 stage from this pump) and we trim impeller .5 inchto match recommended condition after installing it .there is afailure found . the failure as the follow we found pump shaft broken and clearance on sleeve bearing and many line shaft broken we made amodification on material of pump shaft we increase its grade we found life time increased but the pump failure happened again what is ur imagination about this failure pls help me to solve this problem i want to know where is the failure come?

 

[DubMac]

That's the longest sentence I've ever seen.

You cannot just assume a pump that runs at 1800 rpm can run at 3600 rpm; especially at 10 stages. Your original pump may have had a maximum speed of 1800 rpm. If so, the bearings and clearances are not designed to run at 3600 rpm.

Can you supply any other technical information on the pump other than what you have given? If you don't have any more information than what you have given, it terrifies me that you are working on a platform.

 

[LittleInch]

Changing (doubling) the rotational speed will increase some forces by a factor of 4 and some by a factor of 8.

Even trimming the impellors (not clear what the original size was), will reduce these a little bit, but you have completely changed the design of your pump and I am surprised it works at all. motors are fairly cheap - as are pumps so changing it to the extent you have seems a lot of trouble for little benefit.

Taking out 1 or 2 stages in 10 might be ok, but 7 is far too many.

You supply no details, but I suspect your revised pump has a higher duty, hence a different motor but still the same size shaft. Instead of spreading the work over 10 stages you now have more work over 3 stages.

You need a new pump.

My motto: Learn something new every day

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

 

[TenPenny]

If it's a vertical pump and you've increased the speed, I would do a critical speed analysis. Usually, when a vertical pump like this is designed, the bearing spacing is selected to avoid running near a critical speed. When you change the speed, it's possible that you are suddenly running near critical.

That's just my thoughts before I have my morning coffee, so take it for what it's worth.

 

[Artisi]

Just sounds like a complete basket case of on-going disaster/s.


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.)

 

[rateb]

the first issue what is the relationship between pump shaft diamter and RPM?the second one is ,if there is aviberation where is come froem is it may be come from clerance on bearing or clearance on wear ring ?

 

[DubMac]

Sounds like your pump needs what we used to call a float test. Lay it out on the deck, tie onto it with the crane, raise it up over the handrails, boom out over the water and drop it. If it floats, keep it. Otherwise, buy a new pump.

 

[JJPellin]

If you removed impellers but did not remove the corresponding bowls, you may have eliminated too much shaft support, resulting in shaft deflection. With 10 stages, they may have had different designs to limit down-thrust. Some impellers are designed to thrust upward in order to counter the high down thrust of the others. If you eliminated the impellers that thrust downward, but kept the impellers that thrust upward, you could buckle the shaft. You may have stumbled upon a torsional resonance. You may be exciting a lateral rotor resonance. Where did the shaft break?

The others above are correct. It is possible that the pump is not capable of running at 3600 rpm. But most vertical pumps would be capable. The shaft is probably not breaking from simple overload. You can calculate the torque in the shaft if you know the speed and horsepower. Crack open a text book and check to see if your shaft can take that much torque.

Examine the failed shaft. Did it fail in bending (square break) or torsion (45 degree angle)? Examine the fracture surface under the magnifying glass. Does it have beach-marks suggesting fatigue?

If you really want us to solve your problem, you will need to give us more details including a cross-sectional drawing, a pump curve, details on the operating point, etc.

Johnny Pellin

 

[rateb]

hi JJPellin thanks for your answer can i ask u if there is abook for vertical pump problem and type of failure? and i want to ask about how to calculate pump shaft size?

 

[JJPellin]

You would not calculate the shaft size. You would measure the shaft size. You would calculate the torque being transmitted. You would calculate the stress in the shaft based on the shaft size and the transmitted torque. You would compare the stress in the shaft to the allowable stress for the given shaft material.

I am not familiar with a book specifically devoted to problems with vertical pumps. There is a lot of good information in “Pump User’s Handbook” by Bloch and Budris.

Johnny Pellin

 

[rateb]

thanks jjpellin i want abook to calculate stress based onp[um shaft size pls advise me

 

[rateb]

how can i calculate thrust force on impeeller on vertical pump

 

[Artisi]

You hire an engineer capable of undertaking all the "little" simple requests that you have.


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.)

 

[TenPenny]

If you want to calculate the thrust factor for a vertical pump, many of the curves will list the 'K' factor, or the manufacturer can supply it, and work it out from there.

What is the shaft diameter and bearing spacing? What is the tpl (pump length)? what is the shaft material?

Your best bet is to contact the manufacturer, and see if there are obvious issues such as shaft strength or critical speeds. Based on the speed, the manufacturer picks the shaft diameter and bearing spacing to avoid critical speeds, so this should be one of your first checks when you change the speed (or when you apply variable speeds to an existing vertical pump).

 

[DubMac]

As others above stated; you will NOT be able to find a book to help calculate the values you're looking for without help from the manufacturer.

You cannot calculate the thrust without the K value for your specific bowl and impeller combination. It is proprietary, based upon test data, and comes from the manufacturer. That being said, if you can locate the manufacturer, and get the K factor, you can easily calculate thrust with conditions of service.

They will also be able to give you HP limitation for each lineshaft based upon rpm, material, diameter.

Also as others have stated, removing impellers and/or bowls changes the natural frequency of the entire rotor string (motor included). You may well have created a machine that is running at or very near its critical speed. This could easily be causing the catastrophic failures you are seeing.

 

[rateb]

what is the test used for vertical pump commisioning ? pls advise me with help of doucmentation