This might be more Lub than vib but we have a bigger audiance on the vib board and I'm desparate for some quick info:

We purchased and just installed a new 3000 HP 1200 RPM vertical motor for a centrifugal pump application.

The upper bearing is a Renk EVEKU-011. The thrust element of this bearing is tilted pad and the raidal element of this bearing is sleeve. Bearing is immersed in a 55 gallon oil reservoir. Oil reservoir has cooling coils specified by motor OEM to supply 28gpm cooling water. We are supplying on-site 40gpm of cooling water at around 73F (expect that the cooling water supply temperature may increase up to 95F in the summer).

Here are our temperature results with readdings identified as
"UG" = Upper Guide (radial sleeve bearing), "TH" = Thrust tilted pad bearing

In the factory, Motor had run w/o thrust load with 28gpm water in the neighborhood 60F-70F and after several hours they recorded stabilized temperatures
UG=183F, TH = 156F
All vibration readings < 0.08 ips (rms pk/0).

On our site, Ran motor uncoupled approx 1 hour and then read temperatures:
UG = 187F, TH = 155F (seems reasonably close to factory results).
All vibration readings < 0.03 ips (rms pk/0).
Verified oil viscosity (VG68) and level per OEM specs. Electrical checks on thermocouple circuits ok.

Secured motor., Coupled motor (took several hours).

Restarted motor coupled.
~ 1 hr after start UG=189F, TH = 164F
~ 3.5 hr after start UG = 193F, TH = 162F appears stable.

We don't have a communication channel to Renk and very sparse documentation.
Motor OEM tells us some info:

The UG is expected to run higher than the TH because the oil flow in this bearing flow through thrust first where it is heated and then through UG where it is heated further. We don't have experience with this type and I'm just curious if anyone has seen this same pattern UG higher temp than thrust on Renk bearings in vertical motor?

Motor OEM had provided the following limits:
194F (90C) = Alarm temperature per motor OEM
203F (95C) = Shutdown temperature per motor OEM

Obviously when our water temp goes up by 20F or more we will have some problems meeting these alarm limits.. The motor OEM is re-evaluating the limits. I would like to be able to understand and evaluate them somewhat independently.


*********My questions: ***********

Similar limits (90C alarm and 100C shutdown) seem pretty common in the literature but I don't really understand the basis..can someone explain the basis for these lmits?.

What kind of dangers are faced if we were to operate the motor continuously for months or years with UG at 90C, 95C, 100C, or even 105C? Melting point of modern tin babbit grades is much higher. It seems bearing will be relatively uniform temperature (considering there is not much load zone in well-aligned a vertical machine.)

Is the reason for the margin between trip limit and melting point to allow for uncertainty of measurement (we're not seeing the hottest spot). Or are there some considerations other than melting which limit the temperature?

Does anyone know how close to the bearing working surface is temperature sensed in these bearings? How much hotter would we expect hottest spot compared to our sensed temperature in this UG configuration?

Do these readings sound typical to you for Renk bearings? UG 25-30F more than TH?

Considering in most cases hot bearings are heavily loaded and in this case the hot bearings are almost unloaded (UG), are the same temperature restrictions applicable to lightly loaded bearings as heavily loaded bearings?

Does anyone have any resource for Renk bearings other than www.renk.biz?
Does anyone have contact info for Renk technical support accessible to end users?

I did some reading on failure mechanism and it seems to me some temperature-accelerated bearing degradation mechanism would be:

corrosion - always faster when hotter

thermal fatigue - more extreme oif higher temperatures. However not a concern if not a frequent start/stop application.

Warmer metal is slightly softer. So might be slightly more sensitive to things like vibration-induced fatigue. Note even though we have low static loading we still have possible dynamic loading from vibration. Warmer material will be softer and undergo more plastic deformation/fatigue.

Also warmer softer metal might be more susceptible to denting in presence of particles.

I read there is some creep-induced rippling that can occur which would be more at higher temperature but I don't think it is a concern for low static load.

** I am starting to believe a limit such as 100C is more of a time-tested rule of thumb (which is good). However it would probably be complicted to identify specific failure mechanisms.

Pete,

I'm curious. Does the higher temperatures have anything to do with putting 40 GPM through rather than 28?? Has anyone done a thermal heat transfer calc??
In other words, is there enough time (or large enough temp differential) for the extra flow of water to pick up the heat and transfer it??

Dave

Pete,

I've had similar experience, but with a Kingsbury tilting pad/radial guide thrust bearing in a vertical pump. The reservoir size was about 90 gallons, and we only measured the bearing temperatures with an RTD on the back of one of the tilting pad shoes. There was a reservoir thermocouple for lube oil temperature as well. Most curious was the infrared image of this bearing, which showed hotspots ABOVE the oil level in the bearing. The radial guide bearing sat partially below and partially above the oil level, and had 4 return holes above oil level that shot the oil out and to the reservoir wall where we saw the hotspots. The hotspots could be as high as 193 deg F in the summertime. On one occasion, the cooler was inadvertently valved out, and the oil temp ran as high as 370 deg F for a period of time. Interestingly enough, only the oil suffered irreparable damage. There was varnishing and deposits that needed to be scraped from the bearings and cleaned from the cooler, but the bearings went back into service without damage.

The oil doesn't last very long at these temperatures, unfortunately, even when using a PAO synthetic. Varnishing is a common and recurring problem. Synthetic oil will resist oxidation longer, but frequent oil analysis is needed to know when to change out oil prior to the onset of varnishing.

Question: if the bearing is a plain bushing and tilt-pad, why are you reporting vibration in ips? Are you taking readings at the casing with accelerometer? It will be tough to diagnose this type of bearing without installed orthogonal prox probes, and by looking at the shaft orbit. We've seen smeared babbitt failure modes that cannot be detected early without shaft orbit data. Even oil analysis can sometimes miss that because the babbitt does not break off into discrete particles in the oil, and thus can't be picked up by oil analysis.

Also, I think RTS_Dave may be onto something with his question. When flows exceed design in a heat exchanger, sometimes poor heat transfer is seen at the tube surface and can reduce heat transfer efficiency. Also, varnish and lubricant degradation products from the high temperatures can accumulate on the outside of the heat exchanger between the fins, limiting heat transfer as well. Need to make sure that the oil isn't allowed to oxidize (may need to be testing for anti-oxidant additive levels with RULER, or perhaps a varnish tendency test) and that the cooler fins are thoroughly cleaned.

Rich Wurzbach
Maintenance Reliability Group

Thanks for the comments.

We do housing vib measurements for convenience. Don't have the luxury to monitor all these with prox probes. We use fishtails occasionally for troubleshooting. We will rely on temperature and possibly lube oil for bearing monitoring.

We initially heard from one or two people on site and also the motor OEM the same comment that temperature might be higher than normal due to too much cooling water flow. I didn't agree. We talked it through and the OEM came around to my way of thinking. More flow will have two effects: 1 - reduce the temperature of water within the tubes everywhere except the inlet where it remains the same; 2 - reduce the fluid boundary layer thickness which makes the thermal resistance lower (transfer more heat for given deltaT). Both factors help keep the bearing cooler.