Close-Coupled Reliability

In actual practice, what is the reliability of close-coupled horizontal pumps vs. properly aligned long mounted pumps?

I will be buying pumps for an application that will require very high reliability. They will be mag drive, where the alignment does not matter as much. Does anyone have experience with mag drive close coupled pumps? Will there be a real difference between close coupled and long mounted?

Fluid: deionized water (radioactive)
Pump HP: 10 to 50

 

[fredb]

Hello Don M

I tend to like close coupled pumps because you don't have the potential alignment problems that come with couplings.
There are so many many variables when doing a proper alignment such as hot vs. cold, soft foot on motor, etc., unless you have someone knowledgeable and experienced it's not likely you will get a good installation.

The downside is if you have a pump seal failure with a very aggressive fluid and it gets into the motor, then it's possible to have a motor bearing or winding failure.

I'm not familiar with a mag connected pump-motor set. But if you can physically separate the motor from the pump and drive it with reasonable efficiency, then I guess you would have the best of all possible worlds.

fredb

 

[electricpete]

It seems to me it's a no-brainer that the close coupled will be more reliable. How much more reliable is a good question.

The downside is that maintenance can be more difficult. Not an option to pull out the pump or motor individually... they gotta come together. If you should have a motor winding failure you'll have to tear into your radioactive system to repair when you wouldn't have had to a coupled machine.

wouldn't have had to? ;-) You know what I mean.

 

[quark]

To All!

In magnetic coupled pumps, actually pump and motor are not hydraulically connected. They are connected by two parts of magnetic material. There is magnetic coupling half inside surrounded by casing and on top again second half of coupling. As the drive depends purely upon magnetic force, the efficiency is lower.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

 

[oth88]

Long coupled pumps were really designed to have a spacer coupling so that the pump rotating element (impeller, shaft, seal, backover) could be removed without disturbing the motor thereby keeping your alignment when the pump was reassembled. In reality people do not want to spend the extra money on the spacer and that advanatge is lost.

You should bear in mind that close coupled pumps come in two basic designs. One is the extended motor shaft (ie special one piece shaft through the motor to the pump impeller) and the other is the standard motor with a coupling which clamps and joins the pump and motor shafts. Sometimes the extended shaft is of two materials (MS in the motor and SS in the pump) and they are joined by friction welding. If you have any shaft failures these are not so easy to replace.

Another issue to consider is hydraulic thrust. End suction pumps have quite a high axial thrust caused by hydraulic imbalance around the impeller. This has to be taken up by a bearing. Close coupled pumps usually have a special bearing fitted at the drive end of the motor to handle this thrust so the motor is not a standard.

I hope this helps.

 

[vanstoja]

You appear to have overlooked or deliberately ruled out zero leakage (canned integral motor) pumps which are perhaps (though arguably) the most reliable for non-or minimal maintence applications like in the chemical processing industry ,some boiler plants and small nuclear powerplants. Which was it? Small canned motor pumps can be either horizontally or vertically mounted. Most of the higher motor power designs are vertically mounted. They are relatively expensive, though, and may need an external cooling water supply system for the higher power ranges. The need for a heat barrier between pump and motor also prevents close coupling of the impeller so the unsupported impeller overhang length may be an appreciable fraction of the radial bearing span thus leading to rotordynamics problems. Process fluid-cooled radial and thrust bearings of the fluid film type have, theoretically, infinite life compared to rolling element bearings in conventional, air-cooled pumpmotors. vanstoja

 

[electricpete]

Maybe someone can help me sort out the terms.

close-coupled
long-coupled
direct-coupled
And what do you call a motor and pump on the same shaft with no coupling?

 

[marktb]

A close coupled pump with a mag coupling has to be the best of both worlds. If the motor fails you remove the motor and leave the pump where it is and the motor is always going to be aligned properly.

I have used mag couplings before but not at this shaft power so I am unable to comment on their reliability. In the smaller sizes they are very reliable.

As the two shafts will be seperated by using a mag coupling there will should be no thrust transmitted from the pump to the motor.

 

[DiaphragmPumps]

If your having mag drive, it is basically an impeller attached to a metal block inside a sealed cup, the magnetic drive is a larger cup, with attached magnets,surrounding the inner cup therefore giving a barrier protection to your product. As the impeller is not physically attached to the motor, i prefer closecoupled, because theyre isn't a worry over shock loads which would normally be absorbed by the flexible coupling on a fixed long coupled pump. Here is a picture showing this

http://www.innomag.com/breakdown.html

Hope this helps