Pump couplings and VFD's

Rich_L

I've been noticing lately that circulation pumps that have a polymer/ rubber type coupling and are controlled by a variable frequency drive seem to wear out much faster than without a freq drive. Are others seeing this? I seem to have the perfect test bed, 4 pumps, 2 controlled by freq drives, 2 standard motor starters. All 4 pumps were aligned with a laser alignment tool eighteen months ago. I'm back replacing the couplings on the 2 with the freq drives.

Thanks, Rich L

LarryC

Identical motors?

  

  

"I seem to have the perfect test bed, 4 pumps, 2 controlled by freq drives, 2 standard motor starters. All 4 pumps were aligned with a laser alignment tool eighteen months ago. I'm back replacing the couplings on the 2 with the freq drives."

 

1)  Are all four pumps identical?

 

2)  Are all four motors identical?

 

3)  Are all four hydraulic loads similar in size and operation?

 

What is different between the two VFD pumps and the the two conventional motor starter pumps?

Brad White

Bearings intact?

There was a transient phenomenon regarding electrical arc damage from static discharge on bearings with VFD's. A grounding ring was the solution and there is one that is proprietary, licensed to various manufacturers.



I read that Taco recently added this as an option to their pump line. Just a thought, but if the bearings go, the coupling becomes the whip.

HydroMan

Coupling solution

Rich,

This phenomena is geniune - most coupling manufacturer's like Woods recommend going with a metal grid type coupling (e.g. Falk type coupling) when using VFDs with pumps.  Some manufacturers like Grundfos/PACO require using metal grid couplings on VFD systems in order to promote long term life expectancy, while some other pump mfgrs. still default to polymer/rubber types in an effort to cut costs. Hope this helps!

Rich_L

Thanks!

LarryC: Good points, they are not identical pumps or loads. The two that needed repairs are heating loop pumps. They serve air handlers and perimeter building heat. The two beside them are chilled water pumps serving only air handlers.



Brad: I've heard of that also and recently noticed those rings available aftermarket. The bearings seem fine, though I did not check them with a dial indicator for run-out, end play, etc.



Hydroman: This is what I've heard too, from my PACO pump rep. I was wondering if others have been experiencing this trouble too.



On three large commercial buildings we've done recently with numerous pumps I've experienced this on EVERY pump operated by a VFD that has a polymer coupling. I did building start-up/ pump alignments and have been called back to every one. I've done hundreds of pump alignments over the years with much success and all of a sudden we can't make a coupling last longer than a year or so. I installed a couple of PACO pumps on a large radiant floor project almost a year ago and they came with metal grid (Rexnord) style couplings. Talking with the pump rep he told me that's the only type of coupling they'll supply with a pump that's VFD controlled. Those pumps are still purring along! I was just wondering if any others had experienced this.



Thanks for your input, Rich L

Tom Blackwell_2

Stiffness

Pumps over 5 hp need to have a torsionally stiffer coupling if used variable speed. Woods markets a polyurethane coupling half for this purpose.  On the bearing issue we just had a disaster on 4-250hp chilled water pumps.  Bearings showed significant damage after less than a year.  Before this, we had never seen bearing discharge damage on motors less than 1,000 hp.  The fix was a grounding brush to drain off the charge built up on the rotor, rather than have it discharge through the bearings.  It seems the current crop of Pulse Width drives are causing much more of this than we saw even 3 years ago.  The coupling manufacturer can recommend the proper type for vfd applications.  Last, but not least, the stiffness of the pump base can have an effect.  As bases get flimsier to cut costs, the torque of the motor can cause the base to deflect and cause misalignment under load.  I have done more pump re-alignment in the last year than the previous 10.

ef325

Coupling Selection

The Taco base mounted pumps standardized on the TB Wood's Dura-Flex coupling to solve this problem. 

The previous coupler used was the TB Wood's Sure-Flex.  When  the Sure-Flex was used in VFD appilications, the result was 'black pepper dust' (Fine pieces of worn rubber) falling just below the coupling. Because the VFD is constantly compensating the speed, the backlash in the coupling teeth gradually increases over time as the rubber teeth wear out.

Here are what the couplers listed above look like on the Wood's site:

http://www.tbwoods.com/Products_Mechanical_Couplings_Duraflex.asp

http://www.tbwoods.com/Products_Mechanical_Couplings_Sureflex.asp

Gene Fina

Taco, Inc.

Product Manager-Commercial Pumps

paul_79

vfd brush

this company  has a brush kit for vfd bearing problems and their product has worked well for us.  www.est-aegis.com