Constant stiffness Hanger vs. Variable stiffness Hanger

I have a question about CSH & VSH
In my piping analysis result included 7 hangers,
3 hangers were CSH and 4 hangers were VSH
I don't know the reason

Could somebody explain this problem?

the answer lays in the definition of your hangers... from the word constant and variable. an example would be if your node is lifting constant support give same effort while in variable, your load decreases... please read your spring hanger catalog..

Regards,

I know the difference between CSH and VSH
I want to know that why caesar2 decide some hangers as constant stiffness hanger and some hangers as variable stiffness hanger.
Is there special criterion?

Where pipe thermal movement is high. Load variation also increase.
In Caesar default setting if load variation is more than 25% then it choose constant spring hanger otherwise select variable spring.
Check model where Caesar pick constant spring, there operating displacement is high.
User also sometime can take constant spring ( user define) even if load variation is not so high but to reduce critical equipment nozzle load.

Dear Aqua Blue:

I agree with Mr. Shr.

Some more information on the topic is as follows,

The selection of CSH & VSH by Caesar-II is based on the type of manufacturer you select in the hanger input data, amount of variation (25%), expansion range, operating load etc, clearance between the structure support point and the pipe (in some cases).

Also, Caesar Hanger data displays a message for the selection of CSH instead of VSH.

I think the above may clarify your doubt of your quote "I want to know that why caesar2 decide some hangers as constant stiffness hanger and some hangers as variable stiffness hanger".

Aqua Blue,

With all due respect, if you do not know the difference between a constant and a variable spring you need help. Talk to an experienced engineer and read a spring vendor's catalogue to understand in particlar, how a constant spring works.

SHR is quite correct, the load variation parameter in the Caesar input will determine which is selected by the program.

Dear Aqua,

Go to here:
http://www.pipingtech.com/products/ptpcat/spring/constants/index.html

I paste some:
Piping Technology & Products' constant support hangers are principally used to support pipes and equipment subjected to vertical movement due to thermal expansion at locations where transfer of stress to other supports or equipment can be critical. The maximum recommended variation according to MSS standard from the operating load is 25% for variable spring hangers. If the variation exceeds 25%, a constant support hanger should be used.

...and try to observe the formula of LOAD variation in Caesar II pdf file located in Caesar directory.

If you set the load variation 25%, the software will search good spring by doing iteration in related spring Fig table to fulfill load variation still below 25%. I suggest you to do hand calculation (iteration)in order to help to imagine how it works...

Take note. Not only the CAESAR will select CSH if the variability is greater than 25% (confirm if this is ot modified on your spreadsheet, of course you can define a lower value).

CAESAR will also select CSH if your "available space" value is lower than the spring can required space (as per vendor specs). If this item is not significant to all your case, leave it blank but confirm later when your done with the spring design.

Goodluck!

aqua blue,

Another parameter that could result in the designation of CSH for the support is the input value you use for 'Available Space' (4th data field), as mentioned by paldex in his response.

For large vertical movements of possibly 2inch (50 mm) or more the double stack spring Fig 98 is required. These could be more than 36 inch (1 m) in height. If the 'space available' is less than the spring hanger dimension, then the selection will default to a CSH. Use a generous number for available space, unless height truly is limited for space. If vertical displacement increases to more than 3 inches (75 mm) then the CSH would definitely be best type of spring support capable of the large travel.

Of course there is also the zero load support, which also defaults to CSH.

aqua,

Forget the ideal world.

Constant is not perfectly constant as mathematics treat it. As engineer you "MUST" know the variation of design. If it falls in that category then it is. again please read vendor catalog..

Regards!

Thank you for your attention

One more question Please...

I had a error as follows

"Note, some designed hangers may be supporting ZERO weight loads!"

Please teach me how to clear this error

Look at your output (hanger report) and find the hanger or hangers that have zero hot load. Remove these from the model.

Caesar is telling me that I need a CSH because there is zero hot load. I don't understand how there could be zero hot load in this case because the spring is compressing almost an inch. The picture is the displacement in the operating case. I understand that cold load = hot load - travel * spring constant. I just don't understand how my hot load could be zero. Please help.

Here is a picture of the whole system. Caesar is telling me I have zero hot load at the two spring cans closest to the bull tee that are supporting the header. There is no issue with the other spring cans

Send the model in to "techsupport@coade.com" and we'll take a look at it.

Guys i have a question to ask

Currently i'm doing stress analysis which is connected to an existing piping. recently i went to the plant and found out that the header is supported with a spring support (can).

at the spring support name plate, i found out that the theoretical operating position is 36mm. however the header was alredy lifted up by 125mm..there are 3 branches connected to this header.

so the header is still supported by the spring support or not?

Look at the can pressure plate ,if the pressure plate is touching the top location of the slot there is chance that the can is not taking any load, can you send some picture of you inspection at the site. What type of can it is ‘H’ OR ‘F’ type