Hi all:

I have a situation where "preloading" a spring support in installed condition helps me relieve the nozzle loads, i.e. instead of zero displacement in the Hanger design case; I have custom design spring such that I have a +0.25" displacement in the "W+H" case. Is this acceptable by code? What sort of special safety / installation measures do I need to consider in order to use this option? Do my load cases need to be defined differently? This is like a cold spring so do I define this in the Hanger design case, i.e. "W+H+C"?

Is there a max displacement that's allowed in the hanger design case?

Please advise me in this situation.

Thank you

Anican,

What do you mean by "preloading" in this case? If you mean, you're specifying a user defined "operating Load" , then it is not an issue at all. That doesn't , by any mean ,is cold springing.

If the displacements at spring in hanger design case is zero, then there is a possibility of replacing it with a rigid support.

As far as codes are concerned, B31.3 doesn't have any kind of guidelines for springs.

Just make an additional check in case your line is a liquid line, whether the springs don't exert forces & moments beyond satisfactory limit for empty pipe case.

In the spring datasheet, just mention your preset load, operating load,spring rate, vertical & horizontal movement & available space.That's it.

Hope this helps.

Ask yourself how you'll gonna pull out the pin. Displacement of +0.25 inch in SUS is a pain for it. Some hydraulic devices will help you but this is not common solution.

Dan,

Can you please elaborate?

Because the pins (or lock "rings") are pulled out before the start-up. Anyway, the springs are always in compression, so this issue will always be there.

Am I correct?

I just see that in Mexico too, Pipe Stress is with you !!!!

A nice explanation came from Lisega catalog page 1.24:

http://www.lisega.com/pdf/pg1_us.pdf

The spring is in compression and the pin is jammed. Then you add pipe weight and the pin can be pulled out. If pipe weight is not equal with the spring force, the pin remain jammed. I hope that now is clear.

Really nice here in Mexico Ray please add a sombrero to emoticons.

@Anican,

A spring support appears to be physically a support, however is not more than a force exerted into system.
For a constant supports, this was more evident long time ago when a constant effort support was a pulley carrying the pipe on one side of a wire rope and on the other a mass having weight as desired. The tension in both sides of the rope is equal such that if the pipe moves downwards the weight moves upwards with zero effort or change in rope tension. With than construction, it was quite evident that there is a force rather than a support.
In our days a constant support is more sophisticated, however the principle remains...

I think that keeping in mind that there is not more than a force there, it's easy for you to give yourself the answer to your questions.

Just be sure that the calculated force will give the calculated displacement in piping system.
Sometimes this is not easy, for example you must know accurately a valve weight that might be nearby support. I suggest also to experiment how the nozzle flexibility has an influence in your calculation. When acceptable from stress point of view, a calculated 0.25" displacement at 3 feet near nozzle may be quite sensitive to the nozzle flexibility; a 0.25" displacement in a point at 20 feet is probably not so sensitive in calculation, so case by case you may need or not to evaluate accurately the nozzle flexibility by FEA....

In field is nothing special if your calculated cold load & support installation dimension correspond to the reality.
I suggest you to make clear on piping iso that you want a specific displacement at installation and make clear the install length of support, otherwise there is the risk that they will try an unauthorized adjustment of the support (normally this is not happening with a good Contractor, however...)

After install, spring will provide a force giving the same install displacement and the pin shall be correctly removed, if everything was well calculated.


@dan: I wouldn't say "If pipe weight is not equal with the spring force, the pin remain jammed" because there are few cases for which the spring force is balanced by pipe weight and the elastic force of the piping system.
From my field experience, I would say that the pin remains jammed when the spring force (after installing support) does not fit with what it has been calculated and ordered.
Also IMO, what Liesga presents there is a procedure to handle such cases, i.e. loads adjustment must be done to free the pins and "any load adjustments must as a matter of principle be agreed on with the technical department responsible for the piping system; any new load setting values should be indicated on the load scales and recorded".

Best regards.

We are talking two different things. Question was about preloading the spring in cold condition as such the spring will push up the pipe +0.25".

This mean that after you pull out the pin (if you can), the spring force, that is much higher than the pipe weight will move up the pipe by +0.25".

If you can not remove the pin, because the spring force is not equal the pipe weight, at limit you can use the hydraulic extractor (e. g. Lisega).

You can also readjust the load, but in this case it will not match your calculation. Guess now is more clear.

what about to push up the pipe +0.25" (let say by a hydraulic jack) and installing the support in that position?

Yes, it is easier, but is more dangerous for maintenance. The pin can be easily inserted (assumed that the site people realy have done that). But imagine what will happen when they will dismantle or cut the pipe. "Tweaking" the spring can only be done within some limits.

One interesting note here is that when you use "free nozzle" option, Lisega do not change that much the initial choice as other springs.

In fact, I don't like very much such solutions even I applied them in few (desperate) cases.
Any other alternative in layout (if any) is better than this one.

0.25" is within construction tolerances, so erection Contractor should first understand what you intend. They could understand and erect what you want, but the maintenance subcontractors would forget how is make it in few months after commissioning.

By the other hand, you should be sure that such solution is more than a math trick implemented with hardware...

Best regards.

@mariog and danb,

I want to understand your point of view, because I was expecting somebody to tell Anican that he does not need to worry about the cold condition displacement (as far as equipment loads are concerned). He only needs to specify the Cold Load and the Hot Load and the travel between them. That is what we specify to our spring hanger vendors.

Next, after the springs are manufactured, locked, and shipped to site, the field personnel do the following:
1. Install the locked-springs to their specified locations.
2. After all locked-springs and other supports have been positioned in place, a hydrotest is performed (if required).
3. After hydrotest is successfully performed, the locks on the springs are removed according to the procedure set by the supplier (which is usually the same).
4. Finally, all the springs are adjusted to their cold loads by means of a turnbuckle.

That is the common procedure that I am familiar with (and please correct me if I am mistaken). With that said, the displacement that we see in the SUS case in the Caesar output is meaningless because it does not concern us (except if you are checking slopes). We are mainly concerned with the loads that are being transferred to the equipment. So if we modify the spring loads in our analysis (what we call "preloading") and then check the reactions at the equipment nozzles and are happy with it in Cold and Hot condition, then there is no problem.

This discussion about 0.25 inches in Cold condition should not really be an issue.

Do you agree with me?

AbdulRahman,

Indeed, the discussion was not so clear.

If you are looking to my first post, what I tried to say was that in field will be nothing special (vs. Anican12's questions) if the calculated cold load & support installation dimension correspond to the reality.

I guess here the problem was "theoretically" complicated because we expect that providing the cold load, the system will respond with a supplementary deflection of +0.25" and Anican12's question was to model or not a "piping cold spring" there.

Remaining at theoretical level only, my point is that promoting such solution - increasing cold load (and consequently assuring 0.25" displacement in system)- we still be sure it is what the system needs in order to save the nozzle. What I suggested is reconsidering the layout and/or experimenting the sensitivity of such solution to be sure is not a math trick.

Yes, in practice shouldn't be a problem, It is true also I've seen contractors quite afraid when the cold load would deflect "too much" the system, asking for field assistance for this reason.
I think the problem is complicated sometimes by Spring support Vendor's instruction as "After unblocking of all spring supports the Load Indicator should show Cold Blocking Load. A difference must be eliminated by the regulation turnbuckle. Should the difference be bigger than ± X mm, consult the project engineer."
I cannot say it is incorrect, I would prefer to say more clear the goal is to assure Cold Load.

I agree with you, this discussion about 0.25" in Cold condition should not really be an issue, first because 0.25" is not "too much" and second because ideally the contractor should assure cold load as specified, not to worry about the cold condition displacement.

About the problem of famous Lisega pin for constant support, my position is that always there is a way in field to remove it...

Best regards.