Strange Bearing Question

[jsalas1]

Greetings,

I have a customer whose pump we've overhauled, its a 9 stage boiler feed Sulzer MD 100-300. I wasn't told until now, but we received the pump with a pair of bearings that had been badly wiped. Now we're wrapping up the overhaul and the customer shipped us the spare journal bearings they'd like for us to install.

The coupling end bearing is a plain journal bearing, split geometry with oil grooves at the split line. The thrust end bearing has four grooves in the bearing surface, each 90 degrees apart. Both are oil ring lubricated. I've never seen a bearing like this and I'm thinking it was made this way to lower the bearing stiffness and allow the shaft to find equilibrium using mostly the wear rings and balance drum centering forces (Lomakin effect). A picture is attached.

Have y'all seen anything like this before? Seems like the bearing is just allowing the shaft to whirl in the clearance due to it being virtually unloaded. We sometimes have issues with multi-stage pumps where the bearings end up lightly loaded; our solution is to up the specific load by lowering the bearing L/D ratio (shorter axial length). Never have I seen this arrangement, and the customer claims they've gotten a good reliable run of the pump (~2 yrs), but I'm weary of putting this back into their pump and offering a warranty.

I've heard tell of the mystical four-lobe bearing but never seen them in action. I'm pretty sure I'm not looking at one here though, at least not a proper four-lobe. The clearance is circular, no clearance preload or pad offset (we took a FARO arm to the bearings, they are plain cylindrical).

Let me know what you guys think. I think I'm just looking at a goofy bearing that the OEM decided didn't need real bearing features for the pump to run fine. 2 years of run-time is not amazing, somehow it feels like we're going backwards in our expectations of reliability.. maybe a thought for another time!

 

[JJPellin]

That bearing is unusual. And, their reliability expectations are too low. In BFW service, I would put Vespel wear rings in that pump and expect it to run 20 years between overhauls. I attempted to find pictures of the bearings from some of our similar Sulzer pumps, but was not successful. I will try to find a few minutes later to comment more on this one.

Johnny Pellin

 

[jsalas1]

Johnny,

Thanks for the input. We've had success with Vespel and Nickel Graphalloy stationary wear rings, depending on whether or not the customer can really control their process (and the deepness of their pockets). We find that some of the younger folks running older systems are sort of just hobbling along, overhauling their equipment really often and just assuming its part of life. I was in a discussion group in 2015 where Heinz Bloch was tearing into everyone for this reason and with this younger generation coming in, it seems even more relevant. I want all BFW pumps we touch to be 10 year pumps minimum, but its hard when the customer swings them up and down on the curve, runs them shut-in for extended periods, and pulls the pumps offline multiple times a year.

To add to the original topic, I actually found a bearing picture in a Sulzer presentation on Scribd that shows the same arrangement I just described! Take a look at the attached photo. Pretty weird.