For constant & variable spring supports,put spring indicator to cold load position before start up by rotating turnbuckle or load column for bottom spring.

Otherwise, piping stress analysis cold loads will not be achieved in reality.


reg,
sam

There is still no comment on this issue which is very common to observe.

With much care we design piping & spring supports and align equipment nozzles with spring supports locked.

After unlocking of springs, we have CL at some other location from the original specified CL white mark in spring scale.

So long the new CL and postulated new Opr load positions remain within spring operating range, no problm !

But, when spring lift off or bottom down after unlocking, what is the way out ?

My solution is to bring back spring CL & opr load position within working range in any way - have an additional spring to take additional load for bottom out spring, use lighter spring in case of lift off spring. Tune adjacent support location if it helps!

Otherwise simulate in CII the lift off or bottom down spring - if every requirement qualifies - that is our new normal!

Any other solution - please share - which you can prove to solve this commonplace problem.

reg,
sam

Its a good practice to check the impact of spring stiffness in two stages in analysis using Caesar II:

1) Predefine Spring hangers i.e. Spring hanger stiffness (K) and Theoretical Cold load (CL) in Caesar II using some standard vendor catalog in analysis prior to release of MR to spring hanger vendor

2) Run final analysis after updating all weights and details (as accurately as possible based on availability of input) after finalization of spring hanger sizes

The above would give a fair idea on the selection adequacy e.g. whether Operating and Sustained loads are within Normal working range of spring size finalized as well as displacements at nozzle connection location are within acceptable limits.

Your opinion right!

But, many times weights of valves, flanges & piping are not very accurately avaiable when we are specifying the springs! In later stage, we cannot change spring size post order.

Similar is the case for support friction coefficients, support gaps and gaps occuring between pipe shoe and structure at site which we can not very accurately predict beforehand.

Such uncertainity of inputs and sensitivity of spring movemen based on these input variation from those considered in modelling can give rise to lift off or bottom down - some situation when spring stiffness changes from no support in case of lift up to rigid support in case of bottom down.

If the impact of these occurances are analysed and can not be tolerated for protecting equipment from nozzle load etc, we ask for change in spring size.

Here comes the role of engineering integrity!

Will you expose the deficiency in design in time to rectify or wait for some incident of, say flange leakage, to happen!

reg,
sam

You're right sam. Spring is sensitive to weight and movement. I have even experience one constant spring at the top of the Vertical Vessel (High Temperature Tower) where client noticed that the spring gauge is already the top most (above the hot load setting) almost acting rigid. Upon checking the calculation it was noticed that the thermal profile of the Tower has something unusual in it. There is one line at the middle of the tower which is not a main line (the purpose of which is some sort of cooling process). It was used to get the average temp. on the vertical vessel and apparently the difference of the temp. to the main lines are significant. Ultimately, the spring was replaced with a new one (higher movement) for reliability purpose. So if spring is present in the system, we really need to be cautious on the weights and movement of the piping. Unlike normal supports where we can easily impose factor on weights (if information is preliminary) for structural design. So in my current project which we just finish, I make sure that the spring is purchased where the weights of control valve are certified final and piping layout is fixed. And this can be done with educating other department that their input is important for spring design and might cause additional cost & delay (because approximately it takes 4 months to purchase a spring) to the project if spring is not sized properly.

Any other opinion is highly appreciated.

Cheers!!!

Dear Borzki, "make sure that the spring is purchased where the weights of control valve are certified final and piping layout is fixed." is a very good suggestion!

But, where such care is not followed in design stage, we carry out audit of springs in selective sensitive locations before start up at ambient condition after unlocking of springs and after operating condition is reached to know whether lift off or bottom out has occured.

Next comes the question of decision - can we wait for next shutdown, add another spring nearby with piping in operation or have a shut down to change to a different size spring.

I prefer the second option most of the time as a "Tubelight" (slow learner) I leant multiplication as repeated addition in the childhood.

Why change size 8 with 10, when 8+2 = 10 can be an answer in bottom out case, too! Spacing nearby support out stategically can allow the existing spring to work within range in case of lift off springs.

reg,
sam

Hi sam.

Thanks for sharing your experience. That's the hard truth in the industry that we worked for. There will always be inevitable situations like the one you describe and sometimes we are challenged on solving those kind of problems without sacrificing schedule of the project (like avoiding shutdown.)

You're right. Things will not always be an ideal one. Anyway, as long as we uphold safety in all our decisions, then we are ok.

Cheers!!!