Hi everybody,

i have few bypass lines in my model. i have a doubt that should i consider these bypass lines for operating case analysis?, should i have another operating load case for the bypass lines?





Thanks & Regards
Baskar

Dear baskar1982
Make load case in such a way that all main lines operating and all bypass stand by, and another case with all bypass operating and all main lines stand by.

Hi Stress engineer thank you for your reply.

i have a doubt on load case and equipment conditions(Working & standby)

a) The line which i consider as the bypass lines i should not enter the operating temperature data right? and also same for the main line when i consider the bypass line as operating line?.

or

b) is there any option to isolate the lines from analyis in caesar II?

c) i have the conditons on equipments that one stand by and two is working but i prefer to do analysis considering that all the equipments are working, will it be better? beacuse if i consider the condiotns of equipments i need to run few more analysis.

Thanks & regards
Baskar

hi stress engineer,
what will be the temperature of by-pass when it is stand by and what temperature should be put in main line when main line is bypass.

Dear baskar1982,

a) OPERATING OR DESIGN temperature can be used .

b) Semi anchors can be used to isolate lines.

c) Make one case with all lines working @ Design temperature, and another case for equipment stand by and working.

Hi learner 2011,
For stand by condition use ambient temperature and for main line use Design temperature.

Basker1982,
Your question did not give a clear picture of what your analysis is covering. You did not include important information such as the line size or the commodity (Liquid or Gas phase),but specifically the length of the bypass from the upstream main run to down-stream main run and the length of the main line from the bypass to the bypass.
If these lengths are short (as from your elbow to the tips of your finger tips) then the bypass is the same temp as the main line. On the other hand if the bypass is a "mile" long then there will be a temperature gradient and most of the bypass will be something less than the the full operating temperature.
As for using the "Design" temperature, I think this is folly and a pure waste of money. The system will NEVER "Operate" at the "Design" condition so why do an analysis at a temperature that will never happen. You are cheating your Clients.

Yes, just my opinion.

Dear Stressengineer/Learner 2011/Jop & all, thank you very much to share your knwoledge and views.

HI Jop
here the deatails:
a) line size is 24inch
b) Commodity: Gas
c) Bypass Lengths between upstream and down-streams are around 10 & 16meters

yes it is true that system will never operate at the design conditions but some clients are demanding analysis for design conditions also.

nobody having the intention to cheating someone thts why we are using this forum to clarify our doubts and sharing our knowlegde & experience prior to proceeding for analysis.

thank you very much for your input.

regards
Baskar

Dear Mr. JOP,
As you said the system will never "operate" at the "Design" condition. Then why Design conditions are used in wall thickness calculation?
why can't we go for "Operating" conditions for wall thickness calculation( Instead of cheating client by using design conditions).

Yes, Just my opinion.

The design condition is to prove the system is safe according to ASME B31.3 code requirement. It mention that the worst case should be taken into analysis. In my opinion, the design condition is from process design. It may be at occasional case such as steam in-out, shut down or start up condition or system failure condition. All depend on Process Design.

I confident that the design condition is required to perform stress analysis.

TH Engineer.

Hi Stress Engineer,
I think putting ambient temperature into by-pass line is bit conservative.If we have high a design /operating tempearaure (for example of 350/300 Degree centigrade) then putting ambient temperature of 21 degree centigrade in bypass line require a good amount of flexibilty in bybass.
I think we should adopt a more realistic approach and develop a temperature profile for by-pass .

Some BIIIG companies still believes that after few days, the by-pass will be cold enough to create problems. If they know something we don't?

Regards,

stressengineer:
Using "Design Conditions" for wall thickness selection is a totally different issue than doing stress analysis related to thermal expansion. The temperature the line will "see" is only the maximum operating condition. So there is no valid reason to use a temperature that is 20%, 30% or 40% higher.

Basker1982
With the pipe size you have, the operating temperature you have and the length of the bypass then you must explore options.

Here is what I recommend. Make the bypass run at the same temperature or near the same) as the normal flow line.

To do this install a small valved bypass or jumper line (DN 20 or DN 25) around the throttle valve in the primary bypass line. This jumper line would have a small globe valve in it and the connections to the primary bypass line would be as close to the primary bypass throttle valve as possible. You might even consider tapping the body of the primary throttle valve up-stream and down-stream of the seats.

The small throttle valve in this secondary bypass line would be normally open and the setting of the main line control valve would be adjusted as required to maintain the proper flow. With this properly in place the bypass line will be at or near the same temperature as the main line.

Now rerun your stress analysis with one temperature for the whole system. Use the Design or the Operating temperature it won't matter and I am not your boss.

baskar1982,
Please don't leave us hanging!

What have you done to solve your problem?

Hi Jop,
The methodology you are suggesting to analyze a system can be very dengerous.
First of all, design and occasional conditions have been defined and cleared in the code. It is out of an stress engineer's hands to modify them of ignore them.

Secondly, adding a warm up component to a system or TAPPING THE THROTTLE VALVES, is the exact chaeting the client if I use your wording. It's a process design decision to have this line or not.

Stress analysis job is nothing more than making sure that the system works safely based on the design data, process requirements and stress analysis specificatins. This makes the job tracable, and reliable of course.

Cheers

Baskar,
With due respect!
On your question: "c) i have the conditions on equipments that one stand by and two is working....."

You should not prefer doing unrealistic analysis just because you need to run couple of more load cases. Since you are aware that one equipment is standby and others operating, you should consider this in your analysis. Have a word with process for precise temperature distribution in case you are having tough time passing the system.

Jop,
Have you applied the solution cited by you in a live project?

Rajinder

Yes, I have enough of a faint memory of such a situation from some past project that I recognized it from the expanded data the OP finally gave us here. However I do not remember the specific Client, Unit or Line Number. Sorry!

I say get the right people in a room and discuss the problem. The solution is really very simple.
A. When the main line and the bypass are Cold there is no differential expansion and no adverse stress problems.

B. Find a way to make the main line and the bypass the same (or very nearly the same) operating temperature and there is no differential expansion and no adverse stress problems.



"What you are trying to do may be noble, but the way you do it should remain simple" (Jop)

I agree with Jop. Of course you have to obtain process engineer approval, nobody can add lines at their will. If you can solve the problem this way and there are no other things that forbid you to do this, why not?

Many times piping engineers modify the P&ID's (if process engineer agree) because of many factors, but this is normal during project development.

Example: If you have a 150# flange at limit regarding capability of taking any external load, you can ask for a 300# flange.

Regards,