Floating saddle base for water tank

[Trevorsg80]

I need to design a "floating" base for a 42,000 litre, round, horizontal stainless steel water storage tank. Total weight will be 120,000 lb full and a tad under 20,000 lb empty. One saddle will be fixed but the other saddle will be "floating" to compensate for thermal expansion. Steam is used to sanitize the tank between batches which will bring the tank temperature to 240F. Otherwise when it is full of water it will be at room temperature. This will be my first time I've had to design for this type applications and I am not sure where to start. My expertize is in automation design. Also, these tanks will be in a class 10,000 clean room environment.

Thanks for the guidance

 

[hokie66]

I think that Teflon slide bearings would be appropriate. Check with a manufacturer of bearings in your country.

 

[dhengr]

If you make that tank about 46' in dia. and 1' long, you shouldn’t need the sliding saddle in the long direction, and the tank will have approx. the right volume. Alternatively, you could provide a sketch with some dimensions, and end and side views showing the saddles and the piping. Then we would have some better basis for discussion. Piping near the fixed end is pretty clean, any piping near the sliding end may require some special attention. Does the whole tank really heat up to 240°F, or to something less from the expansion standpoint? Then only from 60° to 200°F? And, the sliding movement is based on length from saddle to saddle not total tank length. The tank is going through this excursion when it only weighs about 25kips, not 120kips, right? Thus, only about 12k reaction at the sliding saddle? Make the saddle base plate wider along the tank long axis, and don’t make the saddle any higher than needed due to the moment it will induces on the tank. Let’s see that sketch.

 

[Trevorsg80]

I've attached a very crude hand sketch of the tank with major dimensions. The tanks have been designed and are being built as we speak so no changes can be made to them. I just started working on this project a few days ago so I was not involved with the design or these considerations unfortunatily.

hokie66 - My first thought was to use a structural bearing pad. Only concern I have is the wear. The tanks will be heated up to three times a day. Will a PTFE pad be able to handle this?

dhengr - Yes, the entire tank will go up to 240F. I will do a thermal mapping of the heating when I do the validation on the tanks to ensure it does. I've calculated the thermal expansion at the pads to be approximatly .288". I did not take into consideration radial expansion though. Don't beleive that is necessary. You are also correct that the tank will only see about a 12k reaction at the sliding saddle.

 

[MiketheEngineer]

How about slotted bolt holes??

 

[Trevorsg80]

The floating saddle has slotted holes so it can slide.

 

[dhengr]

It’s a bummer when these kinds of design considerations are not (may not have been) taken into account during the initial saddle design. You may end up with some difficult things you have to design around for this condition. Poorly located stiffeners or slotted A.B. holes, etc. Your 10' dia. x 20' tank is about what I actually would have expected to see. But, without seeing the details of the saddles, it’s tough to go into much detail, but some random thoughts...
1.) The 12k sliding reaction will induce some moment and concentrated loadings into the tank shell. Check to see that that was accounted for.
2.) For circumferential tank expansion, maybe you shouldn’t weld the tank shell to the saddle top plate over the full arc length.
3.) You can buy the PTFE in various reinforced sheet forms and bond it to bearing plates. I’ve even chrome plated and polished the mating bearing plates.
4.) Put skirts on all four sides of the bottom saddle base plate, hanging down, to protect the bearings from crud and splash, etc.
5.) Remember, the primary bearing happens in 3-5" bands under the webs and/or cross stiffeners on the saddle. Thus you may actually have/want discrete (maybe four) bearing pads about 6" x 16" with the PTFE bonded to them, under cross stiffeners. This is true because the web/stiffener and a few inches of the saddle base pl. are far stiffer than the rest of the 21"x120" base pl. Then you have to size the bearing area for the total static loading condition too. You certainly don’t want these web/stiffener hard spots crossing each other. That is N-S stiffeners in the top structure and E-W stiffeners in the lower structure, that’s a real killer.

 

[Trevorsg80]

I've attached a more detailed drawing of my application.

From my research and doing a little math I think my best option is to us a polish stainless upper plate attached to the saddle and a PTFE bonded to a steel plate on the bottom. I am think that using two smaller pads in lieu of one larger pad. Basically what you said dhengr. You bring up a good point dhengr. I did not think about he cross stiffeners.

 

[dhengr]

Trevor:
1.) What’s the Earthquake situation where this tank is going to be installed? This is related to the A.B’s. and any tie-down needs, and the saddle design for that matter. That’s a large C.G. load about 6' above the fl. Two A.B’s per saddle doesn’t sound like too many to me, if you have to consider any lateral loading on that tank.
2.) Does the top saddle plate account for the .375 on 12 slope of the tank? That’s .625" across that plate at the bottom of the tank. Or, are you making that up at the fit-up btwn. the saddle pl. and the saddle web and their welding? And, you only have two sets of cross stiffeners (in line with the axis of the tank, the tank length) to take that moment up into the tank. Are those stiffeners, the saddle pls. and their welding able to do that? Of course, you have this moment condition at both ends, and it reverses from a tension to compressive condition.
3.) The two bearing pads sounds right given what you have for a saddle design. I’d put them under the cross stiffeners, but maybe slightly off-set outward toward the A.B’s. Then put two more bearing pads (stability pads) outside the A.B’s., but not touching the leveling plate on top of the foundation.
4.) Make your polished SS brg. pl. an inch or so larger on a side than the PTFE brg. pl. so that you always keep the PTFE surface covered w.r.t. dirt and crud. The PTFE should be bonded to its own .375" or .5" base pl. which is then attached to the found. leveling pl.

 

[Trevorsg80]

dhengr
1) The tanks have been designed for UBC seismec zone 3. (Just to clarify when you say A.B. do you mean attachment bolt?)
2) The saddle plate does account for the slope. The slope will be compensated with the fitment of the saddle to tank interface. I will have to confirm the design with the stiffeners to ensure they adaquate. Knowing how this project was run I would not be surprised it's not. Considering that the surface finish of the steel was spec'd for food grade and not pharmaceuitical the enigineer responsible may have not communicated the requirments correctly. That may be one of the reasons the guy got fired.
3) Good information. I did not think of that. I will confirm the design to ensure that this is necessary or not.
4) Already considered. I was picturing the upper element of the bearing pad as an "umbrella" to cover the teflon pad.

A couple more thoughts I have.

1. In my research I've been reading that the coefficient of friciton of teflon actually decreases when the weight increases. Is there a minimum presssure that I should be designing the pads to withstand?

2. In order to prevent bio burdan building up with these pads I want to enclose the pads to prevent any water from getting under there. The solution that will be in these tanks is a perfect medium for bacteria to feed and grow off of. Are there any curtains out there that will ensure the pad is sealed?


Thanks for all of your help on this. First time I have done any design in this sort of application. It has been fun.

 

[dhengr]

Trevor:
1.) The idea of Coef. of Friction improving a bit with increased pressure probably has to do with material flow and creep; with the Teflon material flowing to the contact surfaces under compressive pressure. It’s been a long time since I’ve looked at that data, I’d talk with a supplier who you would buy from. I’d be inclined to go with the std. CoF and try to deal with the forces that causes, unless the product manuf’er. gave me a very compelling explanation and data on this coef. improvement, for this low/slow motion condition. Because on the one hand you have to design for sliding at 12k reaction, but then also contend with a 60k static reaction for 90% of the time. I would also want my brg. pads to be large enough in width and length to be sorta self equilibrating during the sliding action. Thus, my earlier 6" wide (across the tank) by 12-16" long (along the tank length). I’d have very low brg. pressures, but know that the sliding brgs. were very stable. The exact size is, of course, an engineering judgement call, so 6x16 may be larger than needs be. The outer stability brg. pads actually have a .25" +/- gap with the found. leveling plate, no Teflon, so they can be washed out during cleaning. They just prevent the tank from rolling too much.
2.) This was the reason for my earlier comment about skirts all around the bearing pads, in my second post, #4.), 3JUL, 13:54. My problem is usually keeping dust, grit and rain water out of the bearing, not bacterial growth or build-up and high pressure washing solution out. I would think you would want to try to keep this material out of the bearing, but you still probably want it to drain too. And, you probably want to be able to open it up fairly easily for inspection and periodic cleaning. There are wipers, similar to a window washer’s squeegee. You’ll see these in various forms on machine tool beds and the like. There is rubber sheet material for skirting. It is held to the two surfaces with some sort of steel bar flange, screwed down. Or, imagine something akin to... a bicycle inner tube inflated btwn. the two surfaces, how you put a new one in is another matter. What about long rod/sq. stock of 2" square closed cell rubber foam material, compressed and fitted into the space btwn. the two surfaces, bottom saddle pl. and found. leveling pl.? This would just shear and roll with tank movement, but be easy to remove for cleaning. Maybe the easiest solution would be a rubber sheet skirt around the entire 21" x 120" saddle bottom pl., hanging down over the found. leveling pl. This could easily be lifted for cleaning. It might just sit on the bottom saddle pl. with some gasketing at that pl. and hanging down over the found. leveling pl.
3.) A.B’s = Anchor Bolts, in my world. Sorry about the abbreviations, because I sure bitch about them and acronyms often enough. This kind of design is fun design for Struct. and Mech. Engineers, at least I really enjoy it. I like the mix of struct. and mech. and it’s much better than just picking parts out of a catalog, watching parts come off a production line or some such, and calling that engineering and design.

 

[Trevorsg80]

Thank you guys for all of your help on this. I think I've gotten my design and have found a supplier to make it for me. The company recommends that I have a minimum applied force of 75 psi on the pads during the expansion cycle though. Does this sound correct? Under these guidelines I will use two pads at 80 in^2 each. Right in line with what you were saying dhengr. Just looking for a sanity check.

One other item. Would you think my application would also be appropriate using an rubberized or neoprene type bearing pad? I have not looked into this option as I did not want to deal with the cycle nature of the compression loading. I've gotten some push back from the facilities manager here. He's an old duffer who can be difficult to deal with. He was planning on placing a couple steel bars under the floating saddle and I had concerns with that. Hence the bearing pad.

Thanks again. Glad I found this forum.