No hold downs for equipment skid 1

[WARose]

I have a client who wants to move a piece of equipment from one plant to another (apparently they do this quite a bit)....but one thing they asked me to do: no hold downs for the slab they are going to put it on. To me this definitely not SOP. There is some unbalanced force with this equipment (not a lot though; a little less than 1000 lbs)......but with even assuming a extremely low coefficient of friction, I see that it is ok.

It still bothers me. The potential is there for rattling (for a lack of a better way to put it) and possibly damaging/chipping the concrete as a result. All I can do is warn them. (I even cited a code that recommends it.) What do you think?

 

[XR250]

As long as it is not a safety issue and they know the potential serviceability consequences (in writing) then I would be OK with it. They an always repair it later.
I would be careful to adequately foresee all potential safety issues - such as the machine walking across the slab and crushing a bystander etc.

 

[TLHS]

There are some parts of industry where skidded or mobile equipment is pretty common. Check for stability under seismic/wind, check that it won't flip over if there's an emergency stop or on startup, and make sure nothing bad happens if it walks a touch (i.e. it won't hurt anyone or break anything). Note that you may not be able to count on friction against seismic loads, so you may need positive attachment.

Then there's some amount of judgement involved. A small pump on its baseplate might work fine if it's maintained well and kept balanced, a big pump on its baseplate seems like a terrible idea but a big pump on a skid might work out. Theoretically you can do a dynamic analysis to work some of this stuff out, but it's an odd situation to try and reliably model. It's probably not worthwhile.

If it's sitting on the ground floor, the numbers work, and the owner is okay with all the consequences you list out, then it might be fine. If it's not at grade, I would worry about the possible consequences of this thing hammering at a raised slab or imparting fatigue loads on the system.

On top of structural consequences, make sure the owner is aware that a non-anchored piece of rotating equipment is going to see more wear. It's more likely to fall out of alignment, see fatigue problems, and otherwise have issues due to vibration and flexure in the system.

 

[WARose]

On top of structural consequences, make sure the owner is aware that a non-anchored piece of rotating equipment is going to see more wear. It's more likely to fall out of alignment, see fatigue problems, and otherwise have issues due to vibration and flexure in the system.

That's one of the points I made......but that's kind of hard for me to prove on paper. After all, with a base modeled that cannot move, it doesn't mean anything if it's anchored. Not sure if I could model the "pinch" of anchors....but even if I could, it would probably affect stuff outside of my model.

 

[nickelkid]

Many compressors, even recips, are skid mounted without anchors. Granted these units use generous structure for the baseplate.

No question, secure mounting is preferred, but it can and is done without anchoring.

 

[Compositepro]

Plenty of equipment is designed to operate without being anchored to the floor. On concrete it would be wise to use leveling pads with rubber pads under the skid, otherwise the equipment could walk if it vibrates.

 

[TLHS]

You can't really quantify the wear you're going to see with a structural model. I could probably find a reference, but basically you've just got a risk that your lifespan is going to get reduced. It's the same risk you have with an undersized or badly proportioned equipment foundation with the added problem of having less stiffness to hold the system in alignment.

Your equipment has a larger vibration amplitude than it would if it could activate more mass, which will cause more wear on components along with greater fatigue. Without the added stiffness of a foundation, you can also get some amount of twisting and bending of your baseplate/skid as a dynamic mode, which will help move components out of alignment or balance.

If your equipment is well balanced or is heavy compared to the rotating components you might see almost no vibration and everything will be perfect. I've seen unanchored equipment last for decades with no problems. Basically, though, there is a somewhat increased risk of equipment problems.

Remember, you presumably have the option of tying this thing down later if there are problems.

Basically, I'd write all this down but I wouldn't do it in the "everything will fall down, people will die and then you'll get sued for everything you own" language I'd use to tell someone that what they are doing is dangerous. This isn't even a bad idea in a lot of contexts, it's just a balancing of costs and benefits by the owner. I'd lay out the reasonable structural consequences that you see, tell them to keep an eye on things and mention that there's some unquantifiable risk of additional wear on the equipment. Oh, and they may be killing a warranty if they have one.

 

[WARose]

Thank you much for the feedback TLHS......and everyone else!

 

[BUGGAR]

On another post, I just put shock absorbers around a similar machine so it could move safely.