babbit bearing temp limits - especially Renk UpperGuide brg 5

[electricpete]

We purchased and just installed a new 3000 HP 1200 HP 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 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 25-30 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

http://www.renk.biz?

Does anyone have contact info for Renk technical support accessible to end users?

Thx in advance.

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[electricpete]

Correction: 3000 HP 1200 RPM

btw - oil is VG68.

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[Carburize]

The problem with babbitt is rapid creep deformation at high temperatures but you are correct that compositions such as ASTM B23 Grade 2 are capable of exposure to higher temperatures. Many machines now have a limit of 250 F and depending on the control system the alarm is set between 220 and 240F

 

[electricpete]

If there is very little static load in this application (radial bearing of vertical motor), then maybe creep wouldn't be much of a problem?

I can see if the material is softer when heated that might make it more susceptible to damage from a variety of other factors: denting in presence of oil particle contamanation, fatigue in presence of dynamic vibrating load (which this application may have due to imbalance etc).

Also I can believe corrosion damage mechamisms would be accelerated.

Any other thoughts?

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[aolalde]

A good quality equalizing thrust bearing could work satisfactory up to 235-245 °F.
The cut-off recommended temperature is 265°F, the Babbitt starts becoming plastic or creep at 320°F (160 °C). The thrust load must almost double for the temperature to increase from 265°F to 320°F, this when the oil is properly cooled...
Note that the temperature sensor must be embedded by the bearing manufacturer, touching the Babbitt at the hot spot location (75° - 75° from both shoe edges).
The oil viscosity affects the temperature rise too. The higher the viscosity the more heat is generated but more thrust load can be handled. Follow the bearing manufacturer specified oil grade, which is matched for the thrust load and speed.

By the way, your radial guide bearing should not experiment major temperature changes from no load to full load, except for the oil temperature increase due to the thrust load.

 

[vanstoja]

Elwell,R.C.,1971, "Thrust Bearing Temperature", Pts. 1&2, Machine Design, 6/24/71,pp.79-81 and 7/8/71,pp.91-94 discussing Babbitt surfaced thrust bearing temperature tests states that "The generally accepted limit of 265F(221.4C) was used in these tests." His Fig.6 shows Babbitt stress in psi vs temperature in F where 265F corresponds to ASTM alloy 2 and 3 stresses of about 3800 and 2800 psi,respectively. He recommends dividing Babbitt stress value by 3 to estimate an average bearing specific load in psi. This gives 1267 and 933psi as specific load limits for these two Babbitt alloys. These values are much higher than typical oil-lubricated thrust bearing design specific load limits usually ranging from 200psi to 500psi depending on bearing size and running speed. Consequently, the alarm temperatures for oil lubricated babbitted thrust bearings are apparently not controlled by damage to the load surface material. A more likely controlling parameter is the minimum fluid film thickness associated with the specific load into which lubricant temperature enters importantly by affecting fluid viscosity.
Peak temperatures in sector shaped thrust bearing pads usually occur at the peak scecific load point which is generally taken to be the 75%-75% location circumferentially and radially from the leading inner edge of the pad. I would think the same minimum film thickness-based criterion apply to the babbetted oil-lubricated radial bearing even though the vertical motor orientation will produce much lower loading than seen by the thrust bearing.

 

[electricpete]

Good point about viscosity as a limiting factor in general.

VG68 95VI oil
80C -> 14cSt
100C -> 7.5 cSt
120c -> 5.5 cSt

As you mention I suspect it would not be a key factor for an unloaded bearing.


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[vanstoja]

Correction to 3/4 post-265F is 129.44C not 221.4C.
Leopard,A.J., 1976, "Tilting Pad Bearings-Limits of Operation", Lubrication Engineering, V.32#12,Dec.pp.637-644, Figure 2 shows that the range for safe operation of tilting pad bearings (shaft speed abscissa, load ordinate) is bounded by lines of film thickness limit (positive slope), pad temperature limit (negative slope) whose intesection at an apex is truncated by a horizontal (constant load) mechanical limit, the latter related to pad surface crowning, pivot stress/fretting/binding, etc. Consequently, either or both film thickness and mechanical factors may account for temperature alarm limits well below babbit surface damage criterion. Leopard also provides figures showing minimum allowable film thickness for tilting pad thrust and radial (L/D=0.4) bearings and maximum pad temperatures vs specific load at three sliding speeds for a 1.59 in. pad of area 2.54in^2. Pad to pad temperature variations in an 18 pad thrust bearing are also shown to range over 20F. This article has a wealth of information useful for evaluating film thickness limits.
Do you know for sure that TH and UG temperatures are actually near-surface pad measurements? Could they be oil flowpath temperatures possibly downstream of these components? It seems dangerous to put thermocouples or RTDs into tilting pads of a production pumpmotor going into service since the electrical leads could restrain the motion of the pads and lead to false loading conditions or even worse damage to the pads. Such instrumentation is usually confined to laboratory bench tests or to a dedicated engineering test machine of high volume output or critical applications such as your Reactor Coolant Pumps(RCP). Such special manufacturer's testing of combined babbitted dual-direction thrust/radial bearings in an RCP is described in Marr,A.H.C. etal,1980, "Loss of Component Cooling Water Capability of a PWE Reactor Coolant Pump, ASME Paper 80-C2/PVP-28. Here RTDs were imbedded into the pads at locations estimated to give temperature about 50F less than the surface temperature. Remarkably, normal temperatures of the upthrust bearing and the upper guide bearing had a differential of 27F which is close to your 25-32F range in several tests. However the gradient is reversed with the upthrust bearing having the higher temperature. A sketch of the pump showing one oil line connection into the thrust runner does not reveal the direction of the oil flow. The tested unit was a 7200HP(hot) B-J pump driven by a Siemens-Allis motor.

 

[motorman]

The whitemetal used on these bearings is Renk metal “therm89”. This material is similar to ASTM B-23 Grade 2. The melting temperature is 233 to 360°C with a pour temperature of 460°C.

The Renk applications manual say that the admissible temperature can be set up to 125°C with embedded type temperature detectors that are within 1 to 3 millimeters from the bond surface between the steel hell and the whitemetal.

So the problem or limit is probably dictated by what oil you will be using since it must be able to take these temperatures. That is why it can sometimes be a little difficult to get a motor supplier to nail down precise temperature limits and times between oil changes. Since motors ship without oil, the motor designer does not know what specific oil will be used at the site. Also, the designer does not know what each site considers a reasonable amount of time between oil changes is. A motor supplier could easily lose an order for say a motor driving a pump 24/7 for three years on a pipeline if he were to state that his motor needs oil changes every 12 months while the competitors are saying once every 3 years. See if you can get your hands on a copy of the book: Lubrication: A Tribology Handbook by M.J. Neale. It contains graphs showing that even mineral-based oil containing anti-oxidants can handle +100-degree C temperatures with from 3000 to 5000 hours between changes.

Also, for what it is worth, I have seen many times where the guide bearing part of the upper bearing runs hot during startups, whatever the make. The gaps are set too small when they leave the factory... Ok, I said it and excuse me for jumping on my soap box but I blame this on the manufactures wanting a simple one-size-fits-all gap setting. They are so concerned about oil whip that they keep the clearance tight. Then when the bearing does run hot they think it will soon become lower after the bearing is “run-in” for a 100 hours are so. That is why we so often in the field in up opening the clearance up. Hmmm… maybe that’s why the bearing designers make it realatively easy to do with those nice adjusting screws they provide.

 

[Mendit]

Renk have a very experienced bearing person, Klaus Bock working from their office in Duncan, South Carulina. I don't have his number at home but I think his local is 12, Richard Ninnes Parts coordinator can also be helpfull.

You did not mention if the temperature sensing is by embedded RTD or Thermocouple. The embedded type can allow you to reach 125C but be sure as the derate factors can be high for non specific locations, 10% for not being embedded,a further 10% for not being at the hottest point of the bearing and a further 5% for not being spring loaded.

Renk also make thrust pads of different shapes, Round pads mounted on belville type washers, kidney shaped pads with a raised bars across the back (bi-directional) and a kidney shaped pad with a 2mm step across the back to allow the pad to tilt with the direction of rotation.

The UG is hotter because it is above the thrust bearing and poor circulation and or cooling will allow the heat to rise slowly in the bearings.
Keeping these bearings cool can often be a problem. We rebuilt a large Toshiba motor with a similar bearing and the thrust collar was drilled to act as a centrifugal pump and circulate the colling oils around the housing.

Vanstoja, stated embedded RTDs are only for laboratory stuff, Sorry most Turbomachinery has embedded RTDs. API has a standard for the placing of just such devices.

 

[electricpete]

Thanks for the great responses from all.

The sensor (thermocouple) is not embedded but put inserted into a hole in the pad. I did not look closely regarding whether it is spring-loaded or not but I would hope it would be spring loaded rather than relying on the mounting collar to keep pressure on the t/c through varying temperatures (seems like an impossible task). Hope and reality may be two different things, though. I will follow up on that.

These are the round pads on Belleville washers.

I had the opportunity to meet with Klaus - he was very helpful and knowledgeable as you say.

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