High Vibration at Low RPM on high capacity Vertical Centrifugal Pump

We have a vertical type muti-stage centrifugal pump which operates at 1500 RPM and has smooth operation with vibration limit max of 0.8mm/sec RMS. During shutdown or start-up when the said pump reaches 80 to 90 RPM, its vibration increases significantly i.e. above 80 mm/sec RMS and causes tripping of Steam Turbine and ultimately complex shutdown. Its a classic example of structural resonance but for now we are looking to ensure plant reliability by increasing the delay factor in vibration sensors to allow pump to surpass that RPM. Following are the questions

1. Is there any recommended time delay factor for Vibration probes so that we can add that limit since the issue is limited to a very sort time period
2. Is there any standard that permits time delay setting for different sensors.

Thanks
 
We have a vertical type muti-stage centrifugal pump which operates at 1500 RPM and has smooth operation with vibration limit max of 0.8mm/sec RMS. During shutdown or start-up when the said pump reaches 80 to 90 RPM, its vibration increases significantly i.e. above 80 mm/sec RMS and causes tripping of Steam Turbine and ultimately complex shutdown. Its a classic example of structural resonance but for now we are looking to ensure plant reliability by increasing the delay factor in vibration sensors to allow pump to surpass that RPM. Following are the questions

1. Is there any recommended time delay factor for Vibration probes so that we can add that limit since the issue is limited to a very sort time period
2. Is there any standard that permits time delay setting for different sensors.

Thanks
Hello ALL,

My First question would be where did this vibration spike located ( bearing , surface?? )

What is the vibration monitoring rack system Model/type & Instrumentation OEM associated installed?

Steam Turbine controls systems..,,?

Then we can try to support you as best as we can on this great forum.

I strongly advise you to take in consideration some satandards like ISO 10816-1, and other API standards , like API610 For centrifugal pumps, API 670 .....

Hope this can help,
Controls Guy25.
 
Thanks for the response

Following are the remarks
My First question would be where did this vibration spike located ( bearing , surface?? )
Vibration spikes are located on Pump DE and Motor DE side bearings.

What is the vibration monitoring rack system Model/type & Instrumentation OEM associated installed?
OEM installed instrumentation.

Steam Turbine controls systems..,,?
Alstom Steam Turbine with DCS for controlling.

I have already checked the up stated standard but it not much helpful for the subject issue.

Regards
Ahmad
 
Thanks for the response

Following are the remarks
My First question would be where did this vibration spike located ( bearing , surface?? )
Vibration spikes are located on Pump DE and Motor DE side bearings.

What is the vibration monitoring rack system Model/type & Instrumentation OEM associated installed?
OEM installed instrumentation.

Steam Turbine controls systems..,,?
Alstom Steam Turbine with DCS for controlling.

I have already checked the up stated standard but it not much helpful for the subject issue.

Regards
Ahmad
Thanks for your response,



1. Is there any recommended time delay factor for Vibration probes so that we can add that limit since the issue is limited to a very sort time period
2. Is there any standard that permits time delay setting for different sensors.

I cannot advise you on adjusting /increasing time delay factor on such instrumentation equipments.
OEM is best position to advise you on that case.

Also did you performed Orbital , Polar Graphics, amplitude phase diagram FFT??

That's what I can comment for the moment,
Controls Guy25.
 
AAkhan,

When did this vibration problem start? In other words, how long has the turbine been commissioned and running without this vibration problem, and how long ago did this vibration problem start?

What happened, operationally or mechanically or maintenance-wise (maintenance outage; pump repair; repair requiring removal and replacement of this pump; etc.) before this vibration problem started?

Has there been any operational change to this pump recently? Some valve position change? A relief valve change? Does this pump normally operate, or is it a standby pump which has recently had to be put into service?

Are you just trying to get through to a scheduled maintenance outage to repair or replace the pump by temporarily changing the trip time delay?

Is the vibration "real"--meaning, has someone stood near the pump and actually observed the high vibration the sensor(s) are reporting?

How many sensors are reporting this high vibration? (Presuming there is more than one vibration sensor on this pump.)
 
Thanks for your response,



1. Is there any recommended time delay factor for Vibration probes so that we can add that limit since the issue is limited to a very sort time period
2. Is there any standard that permits time delay setting for different sensors.

I cannot advise you on adjusting /increasing time delay factor on such instrumentation equipments.
OEM is best position to advise you on that case.

Also did you performed Orbital , Polar Graphics, amplitude phase diagram FFT??

That's what I can comment for the moment,
Controls Guy25.
Yes we have performed the FFT and the cause is structural resonance because of piping and we intend to eliminate by installing the flexible bellows. But for that we need outage and for now our reliability is being compromised hence we need to add a specific time delay to overcome that issue for time being.

Also we have raised query to OEM and awaiting their response also.
 
AAkhan,

When did this vibration problem start? In other words, how long has the turbine been commissioned and running without this vibration problem, and how long ago did this vibration problem start?

It was reported after less than a year of operations and plant was commissioned in July 2018. Please note vibration is only on low rpm i.e. from 80 to 90RPM and on rated speed i.e. 1500 RPM, pump operation is very smooth.


What happened, operationally or mechanically or maintenance-wise (maintenance outage; pump repair; repair requiring removal and replacement of this pump; etc.) before this vibration problem started?

There is no any maintenance performed on these pumps neither any operation upset reported. Just OEM recommended PMs are being performed which includes Lube oil change and parameters monitoring.

Has there been any operational change to this pump recently? Some valve position change? A relief valve change? Does this pump normally operate, or is it a standby pump which has recently had to be put into service?

No any operational change since on rated 1500 RPM, pump operation is very smooth. Both pumps has same issue and it usually happens when OPS performs unit changer as per their routine.

Are you just trying to get through to a scheduled maintenance outage to repair or replace the pump by temporarily changing the trip time delay?
Yes we intend to add a flexible bellow to piping to reduce the impact of piping on natural frequency of motor and pump.

Is the vibration "real"--meaning, has someone stood near the pump and actually observed the high vibration the sensor(s) are reporting?
Yes it is real.
How many sensors are reporting this high vibration? (Presuming there is more than one vibration sensor on this pump.)

Sensors are equipped on MNDE, MDE, PDE.
 
If the unit is not under manufacturer's warranty, you are free to make whatever changes you are comfortable making--as long as you fully understand the risks and accept them.

Speaking of the manufacturer/supplier of the pumps, have you discussed this issue with them to get their input and insight on the potential causes of the problems?

Have you considered NOT having the operators do these regular, scheduled pump changes until the repairs/modifications can be made?

It's obvious, either something has changed--or something was missed or overlooked during commissioning. We aren't there. We don't know the whole situation.

You mentioned other than PMs, nothing has changed. Could it be that the "lube oil change" (I presume that means the lube oil for the pump(s)--not the turbine lube oil) has changed? Could it be that the wrong lube oil, or one not recommended by the pump manufacturer is not now being used?

It is understood--you or someone at site is/are just trying to get to a scheduled outage so that some changes can be made, changes that someone has identified are prudent and necessary. And, you are looking for some kind of "back up" or "justification" so that just in case you make these vibration time delay changes and something catastrophic occurs you want to feel justified (protected) in having made the changes.

Life, and work, is not always this way. You are on site and experiencing the problem. It seems the operations and mechanical departments--and possibly the instrumentation and controls departments--are in some kind of agreement (or perhaps not???) about the proper decision to either make these changes and hope for the best or to shut down the unit and make the repairs/modifications now.

I'm not aware of any international standard that "permits" or "justifies" or "excuses" making this kind of change that could lead to a possible catastrophic failure that you could use to protect yourself or the company in the event of a disastrous problem.

Perhaps you are an operator or other employee that is trying to dissuade the powers (managers; supervisors) that are wanting to make this decision. BUT, that's what managers and supervisors get paid the big bucks to do: Assess risks and make decisions. If you feel you are being forced to do this and want some kind of "protection:, ask for a letter stating that you have been instructed to make this change for this purpose. (Be prepared to be dismissed, though.)

Good luck. This is a sticky wicket. Hopefully, this is a decision by committee, or by someone who has taken the advice and counsel of a sufficient number of experienced individuals to make a considered decision. Sometimes, you just have to sit back and watch, listen and learn.

And, sometimes, if you are in the position of having to make this decision--you just have to man up and recognize you are the person in charge and you have done your best to assess the risks and live with the consequences of the decision (presuming the unit just can't be shut down to fix the problem, or the materials are not available at this time to do so).

There is a saying: "Good judgement comes from experience. Experience comes from poor judgement." It's very true. Sometimes we see it happen to others; sometimes it happens to us (poor judgement and the experience that comes from it). But, as long as we learn something (and good judgment can be just as good a lesson as bad judgement), that's what we are supposed to do. Live and learn.
 
Thanks CSA for the enlightenment.
We have taken up the issue with OEM and have raised warranty claims also and after sharing the Vibration Analysis report, we had same response from them as its because of piping frequency interference.

And we have previously started these pump by bypassing the sensor for the time being as its only appear between 80 to 90RPM and on rated RPM 1500, pump operation is smooth. But in case of sudden tripping of any pump, the standby cuts in but gets trip because of the stated issue hence we are looking to increase the time delay to protect the complex from tripping. We do understand there is a risk but definitely we will take all necessary precautions before implementing it hence was discussing here to have experts opinion.

Its quite clear now that there is no stated rule or standard and we need to take decision under calculated risk.

Thanks for the feedback.
 
AAkhan,

Many thanks for the feedback! We are very happy to hear you have involved the OEM in the troubleshooting and problem resolution, and that you have a work-around for the time until the repairs/modifications can be completed.

We wish you continued good luck with safe operations until the repairs/modifications are complete--and afterwards as well.

Again, "management" is really just a shortcut word for 'risk management.' Usually, people chosen to be supervisors or managers are chosen for their experience and demonstrated ability to assess situations and make reasoned decisions to minimize risk, particularly when unusual operating circumstances are required or being considered. And, that's what experience does for us: makes us better risk managers, better at assessing risks and mitigating risk to the extent possible. But, sometimes, there is no choice but to step up and take the reins and do what is most prudent for the given situation--and hope you understand all of the knock-on effects and consequences and have done everything to minimize the danger to personnel, and to equipment (to the extent possible).

Best of luck!
 
AAKhan,
If the problem hasn't already been resolved, I suggest you look to the Affinity-Law for Pumps... especially at what happens at such a low RPM (less than 6%) in a vertical application !
Regards, Phil Corso
 
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