Good day

recently one boiler feed water pump tripped on high vibration. when physically inspected the coupling was found damaged. Many shims were sheared. a new shim pack coupling was installed and pump was restarted after alignment. the new values were high and close to the trip level as shown in the attached file. vibration was at 1x. phase was steady and stable. the NDE readings were very low as shown in the first page of the attachment. another interesting observation was the bode plot. As the speed was going down the vibration was increasing up to 190 microns. It is surprising to see the vibration increasing instead of decreasing. Also vibration exceeded bearing clearance which was 110 micron during the inspection.

 

 

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Original Post
Ngo Dinh posted:

Have you checked run out of coupling hub yet? the vibration was high on both side of coupling or just on pump side?

What kind of coupling do you use?

No we didn't check the coupling run out. what are the limits for the coupling r/o?? Motor side we don't have sensors so i don't know the level there.

for pump, in normally, run out could be not exceed 0.05mm; the vibration at slow roll is high, it is high possibility due to run out problem.

i suggest you:

1. Use dial indicator to check run out at sensor probe reading, run out of coupling hub?

2. Check alignment condition.

3. Coupling check.what kind of coupling are using?

 

Ngo Dinh posted:

for pump, in normally, run out could be not exceed 0.05mm; the vibration at slow roll is high, it is high possibility due to run out problem.

i suggest you:

1. Use dial indicator to check run out at sensor probe reading, run out of coupling hub?

2. Check alignment condition.

3. Coupling check.what kind of coupling are using?

 

We tested runout at probe area. It was .03 mm. alignment was less than .05 mm . It was tested by laser device from skf. Coupling is ok no apparent damage. Type is shim pack coupling.

How about run out of coupling hub?

I do not see the trend of vibration at NDE side. How about vibration value of NDE side at low speed 600-800rpm?

The new coupling is the same type of previous coupling (weight, type)? How about axial displacement value from normal running to stop? bearing temp?

BONI 2 posted:

If you know the moment of inertia of motor and pump, you can calculate the torsional resonance if it is 600Hz at 640 rpm.

A torsional resonance at 600 Hz would require a huge amount of energy.  

Normally the one-node torsional is down around 10 Hz to about 20 Hz; the two-node torsional in the "mid" 30 Hz region and the three node up in the 60 Hz to 90 Hz region.  These are ballpark ranges.

I would very carefully reinspect the coupling and personally witness the measurements.  

 

i read a case recently published by BN about coupling pre-stretch. is this can be a similar one and how can we measure that to confirm it is ok or not ??

another thing i can suggest is to loose and re tighten the coupling bolt to release any stress at the coupling. do you think this will work ??

As my experience, i not see the pre-stretch applied for coupling of pump with motor driver as thermal growth of shaft is not much. But you can recheck the Flange to flange distance and compare with coupling drawing.

Do you tight equally torque for coupling bolt? 

By the way, pls reply some question above.

Ngo Dinh posted:

As my experience, i not see the pre-stretch applied for coupling of pump with motor driver as thermal growth of shaft is not much. But you can recheck the Flange to flange distance and compare with coupling drawing.

Do you tight equally torque for coupling bolt? 

By the way, pls reply some question above.

Hi Ngo dinh

Sorry if I missed some of your questions. Regarding coupling runout we didn’t check last time. In next time I will check. Bolts torque , yes we did it equally. 

Now assume coupling runout exceeded .05 mm what is the solution.? Is it possible to give it a balance shot to reduce the force ??

For me, i prefer to solve the root cause of problem than just for phenomenon. Actually, you are interesting case that the vibration increase at low speed. So, you should compensate to see how many effect of unbalance force in the vibration value at steady state before you conduct balancing (if not, balancing could be not effect).

I concern that:

- This phenomenon of high vibration at low speed is occurred just after replacing new coupling or in the past, it is appeared.

- When you replace new shim pack, have you remove coupling hub? 

- Have you checked the distance of coupling and flange to flange to compare value yet?

- Have you checked the bolts which used to press the shim pack when the coupling was installed? is it removed or not?

- Run out at probe is normally less than 0.01mm. Have you check the run out at journal bearing?

Regards,

 

 

Ngo Dinh posted:

For me, i prefer to solve the root cause of problem than just for phenomenon. Actually, you are interesting case that the vibration increase at low speed. So, you should compensate to see how many effect of unbalance force in the vibration value at steady state before you conduct balancing (if not, balancing could be not effect).

I concern that:

- This phenomenon of high vibration at low speed is occurred just after replacing new coupling or in the past, it is appeared.

- When you replace new shim pack, have you remove coupling hub? 

- Have you checked the distance of coupling and flange to flange to compare value yet?

- Have you checked the bolts which used to press the shim pack when the coupling was installed? is it removed or not?

- Run out at probe is normally less than 0.01mm. Have you check the run out at journal bearing?

Regards,

 

 

Thanks for the reply i will put your questions in the action plan and will revert back with answers once it is done

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