Flexible or Rigid Coupling 1

[Eabrar]

I am going to select a pump which is of following characteristics;

TYPE - Vertical
Fluid - Steam condensate
Capacity - 50 m3/hr
Temp - 98 deg C (max)
Seal - Mechanical type

Now my question is "Which type of coupling whould be better for this vertical condensate pump".
My experience is that, there are certain manufacturers which offer rigid coupling in case the pump thrust is faced by motor. But for that arrangement we have to align the pump using dowels on motor foots and its checking is tricky and quite labourous. Ofcourse there is no problem during operation.
Can a flexible coupling be a better choice for this arrangement or it can effect the performance of mechanical seal during long run.
Thanks in advance.

 

[Artisi]

A lot will depend on the pump type / motor configuration and how much room /space is available to fit a coupling. You have given very limited detail so it not feasable to give you any difinitive answer.

 

[Eabrar]

Thanks for the reply Artisi.
At the time pump type and motor configurations are not final. The Situation is in requisition stage. If the configuration is known then it will be quite straight to finalize the coupling, for e.g if the vendor is giving the thrust bearing in motor then the whole of the pump load will be bear by the motor and in this case he will use rigid coupling. On the other hand, if the vendor is giving thrust bearing in pump and will not allow the motor to take up the load then he will use flexible coupling.
For now it is up to me to select the type of the configuration to be used.
Please guide me which configuration will be more effective and efficient from the maintenance and performance point of view.
Thanks.

 

[Artisi]

For me I would go with seperate bearings in pump and motor with a flexible coupling between. But up to you, if your specifying the unit at this stage you make the decisions on what you want.

 

[Heckler]

For me I would go with seperate bearings in pump and motor with a flexible coupling between. But up to you, if your specifying the unit at this stage you make the decisions on what you want.

I second this statement. An engineer before me did a gear pump that was driven by a very powerful kollmorgen brushless DC motor with a splined rigid couple. I was his replacement and got to fix the design.......we tried a crown splined rigid couple that allowed for some misalignment but the ultimate solution was a flexible couple.

Heckler
Sr. Mechanical Engineer
SWx 2007 SP 3.0 & Pro/E 2001
XP Pro SP2.0 P4 3.6 GHz, 1GB RAM
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

(In reference to David Beckham) "He can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. Apart from that, he's all right." -- George Best

 

[JJPellin]

I would almost always prefer a flexible coupling. The only exception might be if I did not have enough head room for the extra height required with the flexible coupling option. The reason I prefer flexible is the distance from the impeller to the first true bearing. With a rigid coupling, the first true bearing is in the motor, quite a long distance from the impeller. Even with a support bushing, this long overhung shaft allows for more shaft deflection. This makes it very difficult to maintain tight wear ring clearances and plays havoc with the mechanical seal. Plus, those rigid couplings are very touchy. The tiniest little burr on the shaft can result in very high run-out out at the end of the shaft.

Johnny Pellin

 

[Eabrar]

Thanks for the replies guys.
I have also considered the same but a senior machinery engineer told me that you also have to see the fluid which you are pumping. Mechanical seals have very fine clearances between the seal faces and donot encourage the axial movement to be acted on them in vertical position. In my case i am going to pump steam condensate collected from a condenser and the temperature is almost 95 deg C. The axial play which the flexible coupling allows to encounter slight misalignment can cause the seal faces to open with time and can allow leakage.
Is the theory correct and flexible coupling can act in this way....
Although the designs with flexible coupling in vertical pumps are there and are proving successful, i mostly worked with rigid couplings at the fertilizer plant in condensate service and they are trouble free.
Please guide me as i have to select a trouble free design for the new installation.

 

[Artisi]

I can not believe that you could ever want the coupling to maintain the seal faces together, surely that is a function of the pump bearings. The coupling whether rigid or flexible should not be allowed to influence the seal.

 

[JJPellin]

There should be no more axial movement allowed with a flexible coupling than there would be with a rigid coupling. In either case the shaft is restrained axially by ball bearings (motor bearings or pump bearings). In my experience motor manufacturers tend to leave more axial float (axial clearance) in their ball bearing arrangements than do pump manufacturers. If this is the case, then you will get less axial movement with the flexible coupling with the pump shaft supported on its own bearings. You describe this as a steam condensate pump. Most of the ones we have are vertical turbine pumps rather than vertical in-line. This affects some of my earlier comments. A vertical turbine pump will have a line shaft down to the impellers in the bottom of the "can" and thus will have support bushings even if it has the flexible coupling configuration. My concerns about radial movement (shaft deflection) are not valid in this case. The shaft will have the same opportunity for radial displacement regardless of the choice of coupling arrangements.

Johnny Pellin

 

[Eabrar]

Thanks guys.
The information provided is very helpful and surely help me out in my decision.
I would go for flexible coupling design. I have also come across with a design in which there is an intermediate bearing housing between pump and the motor. This design involves two set of coupling, one between motor and bearing housing which is flexible and the other between housing and pump which is rigid. In this arrangement we have a dual advantages of using flexible coupling for better support and rigid coupling for ease of maintenance i-e removing and installing mechanical seal.

 

[rotaryguy]

i agree with JJ. i have just gone through with vertical turbine pump requistion for wastewater application. we opted for flexible couplings and about bearings issue raised by ARTISI, in this case all bowl shaft, column section shaft and head shaft are supported by bushings (journal bearings)and in motor they provide thrust bearings which can handle all the down trust and up thrust(occasional). Make sure that impellers are hydraulically balance inorder to redice axial thrust on shaft which is critical in vertical application due to Shaft run out.

 

[Slickstir]

One thing I'd like to add. With vertical turbine pumps that have mechanical seals I prefer a spacer coupling. Otherwise, the user probably will be required to remove the motor every time they service the mechanical seal. In some applications this can be quite often. Instead of the job being handled by a lone mechanic with a wrench or two they must also have an electician and sometimes a crane. I've seen installations that required a $1500/day crane to change out a $500 mechanical seal.

 

[rotaryguy]

yep agree with Slickstir, we have dry spacer type flexible coupling.

 

[Tmoose]

Ahhhh.
Design For Maintainability.

When talking with folks who have had to fix things it takes about one comment in a design review.

On the other hand, I witnessed a several day "discussion" with a stubborn apartment dwelling Russian engineer about providing a decent sized access hole for assembly and maintenance.