Dear all,

I’m monitoring a 1.2MW submerged cooling water pump in a Powerplant. This pump was being monitored regularly due to suspected bearing defect (~78Hz, ~60Hz peaks and harmonics in envelope spectrum aligns with BPFI and BPFO for 6036M bearing). There are no obvious in increase in vibration trends since Jan.  I have however, started to notice an abnormal sound from the bearing housing since late April (bit like a chain block sound). The guide bearing was inspected during a stoppage and found to be in acceptable condition. Drained lube oil showed no wear particles, debris etc.

Any thoughts on what could be causing the abnormal noise. Please refer to attached data. The bearing change is well overdue as per OEM and ideally we plan to do it during a scheduled outage in October. 

Any help on this is much appreciated!

Best regards,

Thimba

 

 

 

 

 

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Personally I would view it with some degree of urgency.  Yes, I know you don’t see it showing up in the velocity spectrum, but it’s a slow speed machine and you DO see it in the acceleration time waveform (12g’s peak-to-peak).  Also the presence of two different defect patterns (inner and outer) is a caution flag for me (bearing may be progressing to the point that damage from one race is propogating to the other).   The noise you mentioned is another caution flag. 

 

I assume you are getting inside the column onto the bearing housing to get your measurement. If you are measuring from the outside of the column, that suggests even higher severity.

 

> "The guide bearing was inspected during a stoppage and found to be in acceptable condition"

I would not put much faith in that.  There is pretty limited ability to inspect the races and rolling elements on a deep groove bearing without cutting it apart.  By the way, did they rotate the shaft during this inspection? If not, then you can call B.S., the inspection is almost meaningless - the portions of the race accessible for inspection are severely limited without rotating  (and even with rotation very very difficult / limited).   

 

Comment on the design: I see an upper (guide, 6306) and lower (thrust, 29322E) bearing in the reservoir with oil level maintained in middle of lower bearing.  There is a slanted port in the "runner" (runner might also be called "collar" or "bearing carrier") whose apparent purpose is to "pump" oil to the area just above upper (guide) bearing.   However, it's not obvious to me that the bottom of that slant tube will be able to capture any oil at all for pumping ,since the bottom of the slanted port is far above the reservoir oil level maintained by the oiler.  I presume this area inside the runner will have similar oil level as the reservoir since the area between runner/standpipe is vented to atmosphere below via the shaft/standpipe clearance and presumably the oil reservoir is also vented (for that matter, you would have oil leaking through the shaft/standpipe clearance if oil got high enough to reach the bottom of the slant port) . In other words it doesn't look like this design will reliably lubricate the upper bearing.  Am I missing something? 

 

electricpete posted:

Personally I would view it with some degree of urgency.  Yes, I know you don’t see it showing up in the velocity spectrum, but it’s a slow speed machine and you DO see it in the acceleration time waveform (12g’s peak-to-peak).  Also the presence of two different defect patterns (inner and outer) is a caution flag for me (bearing may be progressing to the point that damage from one race is propogating to the other).   The noise you mentioned is another caution flag. 

 

 

Thankx heaps for your input. My initial assesment was to recomend to schedule the job at the next oppertunity. Although I wanted a 2nd openion just to make sure as this is a big job. 

the readings are taken from inside the colomn on the upper bearing cover. 

The inspection was done by rotating the outer race. Once the top bearing cover is lifted (up to the coupling) theres just enough space to reach the outer race of the guide bearing to rotate it. (please refer to attached pic)

RE: comment about the design; The oil level is maintained around the bottom of the slanted port. The item marked 646 is an oil retaining tube and prevents any leakage. 

 We may have an oppertunity in mid July for just long enough the complete the job, so I have recomended to schedule the job then. Do you recon it will be safe till then. Of course I will be monitoring this very closely untile then. 

And once again, thank you verymuch for your input. I will post any findings during the replacement here once done. 

 

 

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Thanks for the feedback. 

I see how they can pull the cover and rotate the outer ring. That's a little better.  I still think visibility is extremely limited.  If you looked up underneath that bearing you'd be looking at a cage. Solid brass cage would extremely limit the view. fabricated steel is a little better. I  would still treat that inspection with healthy skepticism unless you did it yourself.

 

Piece 646 is what I called standpipe.  It is a stationary part with clearance to the shaft. In order to do it's job of preventing oil leaking out of the clearance, oil level has to stay below top of the standpipe.  If oil level is above the top of that standpipe (as it would have to be to reach the slant tube), then it would come leaking down through that clearance.  Regardless of all that, I don't expect the level to be that high because the oiler maintains level near the top of the lower bearing as shown in the drawing.   I really don't see how the upper bearing can get lubricated with anything more than vapors (which may or may not be enough). 

 

 

 

 

"The oil level is maintained around the bottom of the slanted port"

The oil level is maintained by the constant level oiler at the level of the tap (pipe) for the constant level oiler. Wouldn't you agree that the drawing shows the constant level oiler pipe much lower than the bottom of the slanted port? 

electricpete posted:

"The oil level is maintained around the bottom of the slanted port"

The oil level is maintained by the constant level oiler at the level of the tap (pipe) for the constant level oiler. Wouldn't you agree that the drawing shows the constant level oiler pipe much lower than the bottom of the slanted port? 

That was my understanding of it. But when I checked with the maintenance guys and i was told it's kept higher than that. I will double check this and get back to you.

About the inspection, I didnt do it my self im afraid. And as you say its bearly visible. the maintainence guys just rotated the outer race and said it felt pretty smooth. It is not possible to visually inspect the bearing components at all. Where the top cover is on the picture is as far up as it goes. 

Hi everyone, This is just a follow-up on this topic from May.

so we dismantled the pump as planed in July. Attached are the bearing pictures and one of the SEVERELY corroded standpipe/ oil retaining tube. Found some damage (Looks like corrosion and contamination to me) on the bearings, some rub marks on the shaft where the standpipe was.

Looks like there was water leaking in to the bearing carrier/ oil bath through the gland packing down below and through standpipe(there was some leakage through the gland packing  every few months and we would replace it ) There's a clearance of about 2-3 mm from the shaft to standpipe. There is what looks like an o-ring just below the standpipe on the shaft on the design drawing as shown on attached pic part #411, but its not listed on the parts list and apparently wasn't supplied and was overlooked during the last overhaul 3 years back (by supplier TA). I'm thinking this is the root cause of the corrosion of standpipe issue and bearing deterioration due to water and particle contamination of the lube oil. I have recommended to have an o-ring as per the design drawings. 

Any thoughts anybody on this based on the attached pics and previous data? specifically, failure mode of the bearings and root cause of all this?

p.s- @electricpete : True! the level is as shown on pic as you suggested. And I have pointed out this as a possible design issue. However, there's not been any issues due to lack of lubrication of the guide bearings on either of the MCW pumps so far so I'm guessing it is sufficiently lubricated somehow.  

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