IT WOULD BE HIGHLY APPRECIATED IF YOU TELL ME WHAT KIND OF CONSIDRATION WE SHUOULD NOTICE WHEN WE USE PREDEFINED HANGERS?

If you have a system with an existing spring, you need the spring rate and cold load for that spring. These values should be available from the name plate on the spring. This is why this situation is referred to as a "predefined spring".

Considerations:
- is the name plate still there
- was the spring installed correctly
- does the spring match the original design
- was the original design correct
- has the system been modified since the original design

I would add:

- was the travel stop pulled prior to startup?

or the best one.... does the spring can or constant still work???? I have seen coils rust and break... and constants freeze up....

All in all this is one more potential pitfall in doing this work....

What do you mean, John ? yes!! you are right. Most of the existing hangers are really terrible and some can not be used. What should we do next, leaving this job or keeping on? I really wonder that the caesarII program is merely suitable for the new design because in many (unknown)parameters of existing hangers would make the analysis output incorrect....
But I sometimes have to carry on this kind of job by using the existing data (as Richard said)and frankly, i'm not certain about the result at all...
So what about your suggestion???

Regards,

Aye well theres the rub....

Factualy speaking when one gets involved in retrofit work all sorts of problems and difficulties can arise....

I usually try to survey the exsiting cans and supports closely so as to assure myself they have some chnace of being functional.

If I have any doubts I punchlist the item for removal and inspection during the next turn around....

And if nobody wants to do that then I cleary state in the design documents that the following... X, X, X etc. unverified assumptions were accepted by the owner....

One more thing... if a system has been running 30 years through numerous cycles with no overload or maintenance problems appearing, with a shipping stop in place on a can... I would leave it in place...

If you remove the stop you then start a new displacement stress range that the system will have to shake down through....

Even the piping codes shy away from evaluating systems after they are running.

Read B31.3 paragraph 300(c)(2): "This Code is not intended to apply to the operation, examination, inspection, testing, maintenance, or repair of piping that has been placed in service. The provisions of this Code may optionally be applied for those purposes, although other considerations may also be necessary."

John points out one issue - shakedown.

I've had users sing praises to CAESAR II, saying the numbers from the PC matched the field exactly. And I have also heard others complain about how poor the program is at predicting the current (structural) state of their piping system. If all models were accurate, I believe the happy users were looking at new construction while dissatisfied users were looking at systems with some history that included shakedown or other local yielding and/or some unintentional cold spring.