Lubricating Slide Supports 1

[rts9364]

I am working on a set of double-stacked horizontal vessels; each vessel has one fixed and one sliding base plate. The top vessels bolt to the bottom vessels as the bottom vessels are bolted to the foundations. There is a significant amount of thermal growth and during our fatigue analysis, concerns have been raised about a "slip/stick" phenomenon occuring at the steel/steel sliding interface, especially between the top and bottom vessels. We are at a point where we are looking into lubricating the slide plates. Furthermore, we are also thinking about adding grease zerks so the end user can provide lubrication throughout the life of the vessels.

Is there another way to address this problem? The end user is not interested in PTFE pads. If lubrication is the way to go, is there a certain grease I can recommend they use? Is adding grease zerks in the sliding base plates a reasonable step to take (if they actually get used 10 years from now is unknown!)?

 

[pennpiper]

You are going to need to supply much more information about the situation before you will get any real help.

Bottom Vessel ----
How big is the vessel (Diameter, Length, Wall thickness. etc) ?
What is the material of the vessel?
What is the Operation Temperature of the Vessel?
What is the Operating Pressure?
What is the Commodity in the vessel?
What is the specific gravity of the commodity?
What is the total Operating Weight of the vessel?

Top Vessel -------
How big is the vessel (Diameter, Length, Wall thickness. etc) ?
What is the material of the vessel?
What is the Operation Temperature of the Vessel?
What is the Operating Pressure?
What is the Commodity in the vessel?
What is the specific gravity of the commodity?
What is the total Operating Weight of the vessel?

Support Structure ----
Are these vessels supported on simple "T" supports?
or
Are these Vessels supported in a more complex (four leg,multi-level) structure?

Other -------
Are there any direct piping connections between these two vessels?
What is the location/country of installation?

 

[rts9364]

The top and bottom vessels are identical 48" ID x 2.375" wall nitrogen vessels, SA-738-B material. They are 85'-0" long supported on (2) traditional "T" saddles (centered web). Operating weight is about 115,000 lb each.

The bottom vessel has a full encirclement "wear pad" and "T" supports on top and bottom. The "T" support on the top vessel rests on the top support of the bottom vessel.

The vessels are to be installed in the U.S. and the piping to each vessel runs from a separate manifold system.

The support/foundation design is complete. We are just trying to find a way to ensure this possible slip/stick sliding condidtion is never realized. Grease seems to be a simple solution. Thanks.

 

[pennpiper]

Try here:

http://www.hipertech.com/spip/pdf/AFG2EN.pdf

or

Google for others

 

[gr2vessels]

rts,
For this application the grease alone won't be enough, the asperities on the contact surfaces will in time lock together due to extruding of the grease and you'll be left with full contact of carbon steel plates. Typically, a polished thin plate (could be stainless steel) is inserted between the top and bottom suport/base plates, including lots of grease;- you might want to consider side guides on the bottom vessel support plates, in order to minimse the bolting torque down to finger tight (on the sliding side only!)
cheers,
gr2vessels

 

[racookpe1978]

Rather than attack the whole problem, why not replace the "skid plates" (between top vessel and lower vessel, and between lower vessel and deck/concrete/pad) with rollers of equal capacity?

The weight transfer is then straight up and down, with no real moment loads to be passed from the top vessel to the bottom vessel. The weight of the top vessel can be carried by (non-pressure bearing) simple structural legs coming up from the lower vessel, and extending down to the concrete. Rolling distance for an 85 foot tank is 2 - 4 inches at the legs, so the plates need to be slightly longer than that, which is easy to plan for.

Greasing the rollers, and preventing rust from freezing movement, is simpler than trying to maintain a "perfect" greased surface between two large plates.

 

[4Pipes]

From my farming days, I would have thought grease is a waste of time. Grease will only work if you can get it onto 100% of the working surface and you have enough movement to keep it spread it around to maintain film thickness. If you put a grease nipple onto the middle of the bearing, the chances are that the grease will squidge out at one point leaving the other parts dry. This often happens on shafts and king pings on farm machinery - or it did 40 years ago. One end gets the grease, the other end none.

I would have thought a pukka ptfe or self lubricating metal bearing would be the only option.

 

[boilerone]

What is the disadvantage with PTFE slide pads that your client is concerning about, except for temperature limitation? I have no problem using on boiler component bottom supports with hundreds of tons of loads plus quite high temperatures.

 

[rts9364]

Thank you for all the responses. You have caused me to think about this some more and I have to agree - imagining the grease providing any significant advantage after the initial installation is tough to do. Also, as I was trying to imply in my OP, I also have trouble convincing myself that an application of grease through zerks at some frequency later on would be beneficial at all. My colleagues and I will talk about this again.

boilerone, I am not entirely sure. We suggested PTFE and I think there was some concern about having to possibly replace them after x number of years. We may be able to present some of our fatigue data and steer them back down that road. Any opinions about a PTFE pad vs. a thin SS plate?

Thank you all,
RTS

 

[MJCronin]

Who says that the PTFE plate has to be replaced after a number of years ? Fluorogold plates have been around for at least 40 years....people are still using/specifying them

Why are these plates good enough for bridges/boilers etc etc....but not good enough for your client..???

http://www.sepbearings.com/fluorogold.pdf

There are structural/pipe support applications that were designed in the 1970s and earlier that are in great shape.

(Note: I have always been an strong advocate of mandatory drug testing for clients and thier reps)

-MJC

 

[eadwine]

Ahh... the great Slide Plate debate once again. As for the workability and reliability of various types of slide plates,is dependent on the preferences (and mental state) of the Project Engineer in charge. Some just don't like "Fluorgold" Plates for reasons that escape me. If they don't like the fluorgold versions, suggest "Babbit Plates". Made from babbit inserts in the basic plate structure.

If they don't like these, then you will have to reinvent the wheel. MJCronin's response is apropos.

 

[pveng6224]

Another possibility: bronze slide plates. They require no maintenance or grease and last forever. Contact Lubron (

http://www.lubron.com)

or Con-Serv (

http://www.con-servinc.com).

They can analyze your situation and recommend a solution.

pveng6224

 

[unclesyd]

We have numerous jacketed vessels 5' to 6' in dia. from 16' to 24' long that are supported on 4 slides. The rear slides (2) are fixed in the axial direction and front slides (2) allow both radial and axial movement. Our movement, approximately 5/8" occurs on heat up, ambient to 285C. We use bronze bearing pads. The pads are slotted for guidance and have lubrication grooves on all moving contact surfaces. Some of these guides are over 50 yrs. old and are still operating as designed.

We had another system were we used a similar design using Cast Iron instead of bronze. If I recall correctly the sliding plates were CS running against the CI.

Here are some wear plates with lubrication grooves. They also have graphite groves and plugs.

http://www.spectrummachine.com/liner_wear_plate_bronze.html