While carrying out flexibility analysis of one of our existing Boiler Feed
Water (BFW) lines operating at 74 kg/cm(superscript: 2) pressure (design
pressure 90 kg/cm(superscript: 2) ) and 182 deg C following are the
salient features of the line :

The line was installed around 20 years back by a consultant. The
flexibility analysis report of that time shows maximum displacement as
128 mm in the axial direction and corresponding transeverse & vertical
displacements are 108 and 31 mm respectively. The whole line is
resting on the spring hangers. The specifications of the hangers are not
available now.

Later on the length of the line on the upstream side was increased and
the operating temperature remained same. The spring hangers provided at
certain locations were found damaged apparently due to excessive
displacement.The pipe at these locations got sagged and now resting on
the pipe rack at around 1 1/2 ft below original position.

This line with modified layout was analysed using CAESAR II with new
hanger as recommended by the s/w. The deflections at these points(where
spring aer found brocken) were as high as 162 mm and 145 mm in axial
and transverse directions respectively.

In order to solve above problem it was tried to provide cold spring(
cut short of 50 mm) in the piping between Node Nos 392 & 395. In this
configuration the deflections came down to the level of 80-90 mm at
spring hanger locations.

Many other options like providing expansion loops, anchors etc. to bring
down the displacement were tried out, however, they didn't yield
satisfactory result.

My queries are as follows :

What is the maximum limit for such displacements.

Of the following methods, which method should be used for the analysis
of above case & why :
a) By using cut-short(material -18) for cold spring or
b) By changing the Alpha tolerance as 1.1 & setting the first available
temp. field for this segment as -1.

Is there any problem in providing cold spring alongwith spring
hangers?

What should be the procedure / sequence for installation of such
piping.


I would be thankful, if anybody could answer to above queries. The input
file is enclosed herewith for the reference

Should anybody require further details, please feel free to contact.

Hi Bhargava ,


I the absence of the model its very difficult to give a specific solution to this problem. Firstly my understanding is that cold spring is used to reduce loads on the terminal points and not as a solution to reduce large displacements. So consider review your decision seeking other opinions . It seems a vertical leg is causing this problem?. A layout and supporting solution is best advised.


To answer your queries please note that by using cold spring you cannot take credit for expansion stresses as per code bcos these are stress ranges and not absolute stresses.Be carefull to check your loads in cold case bcos there are possibilities that the terminal loads could overshoot the allowables in the ambient conditions itself.

1) The maximum limit of displacements in operating state assuming that the code stress check is satisfied is governed by pipe spacing and supporting considerations. We limit the axial displacement of pipes into an expansion loop to 100mm.The transverse movements to 25 mm. But these are not hard and fast rules.

Please note while giving the cold pull the length should be 2/3 rd the actual length to meet code requirements.

2)Since you have a Spring Hanger coming into the picture you have to use the Alpha Tolerance method.This is basically to define the load cases to meet code requirements by segregating the external spring forces from the cold spring forces in the load case F.The cold spring forces are defined as another temperature field using alpha tolerances as you have seen in page 6-6 of Technical Reference Manual.

3)Giving cold spring at locations close to Spring Hanger will definitely affect the hanger design since weight loads are affected .Also note stakes during the the fabrication stage becomes still higher.

4)Though I am not experienced here , Guess you have to fabricate the pipe and then position it and then place the hanger. A second opinion is invaluable.


Hope this is usefull.Feel free to contact me.

Shivaprakash C Rao
www.edeleanu.com.sg

shivaprakashcrao@hotmail.com

Hello,

Shivaprakash has made many excellent comments. The first thing that I would like to suggest reinforces what Shivaprakash essentially has already said: DO NOT TRY TO SOLVE THIS PROBLEM WITH COLD SPRINGING.

As Shivaprakash has said, there are no absolute limits on thermal deflection. The design of this piping for expansion will be driven by the primary and secondary stresses. Presumably, the damaged ("sagged") piping will be repaired and the badly distorted bits will be replaced with new pipe. You probably would be best served by a complete analysis of the entire system (as modified). To limit the axial expansion displacement in any local area consider intermediate anchors (as close as practical to six degrees of fixity) and expansion loops AND GUIDES. Accommodating the expansion between the intermediate anchors with loops may mean installing some new piping in the existing system but it provides a "real" solution to the problem - cold spring is only a theoretical answer. Please read a previous thread regarding cold spring in an existing system.

I think it would be wise to do the design / analysis in steps. Try to manage the axial displacement (e.g., put in some expansion loops and guides)and then address the hanger design. Also consider that the system has a fair amount of "self springing" in it now as it has "relaxed" due to creep in local components. When you cut the piping to modify it you will release the residual cold stresses and you will be left with a system that is no longer cold sprung. PLEASE REMEMBER TO WARN THE TRADESMEN TO PLACE THE TRAVEL STOPS IN THE HANGERS BEFORE THE PIPE IS CUT - there is a lot of stored energy in those compressed springs and it can be dangerous. Also, restrain the pipe for safety while it is being worked on.

Good luck with your project.

Regards, John.