Thanks heaps for the support and feedback from everyone. Much appreciated and surely makes you feel that you are not alone.

I have another one. I have a set of design temp/pres : 100C/20MPa as well as an operating temp/pres for a system: 20C/30MPa. Furthermore, I have a collected data from several instruments along the system of temp/pres that varies throughout the day. The max actual temp/pres is 90C/35MPa. What I am compelled to use as the temp/pres for the analysis are the maximum from these values being the design temp and actual max pres 100C/35MPa. Would this be a correct assumption? Should I specify a temperature per area of the system that has a different temp/pres max? I'm just thinking though that this will yield a lot of data and case sets. Is obtaining a maximum combination of all expected temp/pres be the correct approach? Thanks!

Dear Pickles

although it will be very time consuming but the correct approach will be to have design temp and design pressure for each section of ur system and model it appropriately. taking highest temp and highest pressure of the system may not always give conservative results from the flexibility point of view.
assume two vessels (at different temps) connected by a pipelength having a U-type layout with a valve at the middle. in this case the colg leg may restrain the expansion of hot leg. whereas if the two legs are asummed to be at the same temp the expanion will not be restricted leading to unconservative results.

I disagree entirely with RSP! The B31 codes use the terms of design temperature and pressure together exclusively and they also pertain SOLEY to pressure wall thickness requirements!!!!!!

To illustrate this we even showed this quite clearly in B31.3 2004 Appendix S!

The thermal displacement strains being that must be accounted for, should be those thermal displacements that are the maximum.

Sometimes this occurs when the maximum operating temperature occurs. Sometimes this happens when the piping is at ambient temperature and is displaced by hot equipment.

Footnote: When Task Group B of B31.3 put this example together and this subject came up.... we were all quite vexed over people using the design temperature for analysis. In some cases the piping specification is written based soley on its components and the piping specifications design temperature is based on a component rather than the maximum or minimum operating temperatures.

Often time I have found ambient lines being dragged around by hot or cold equipment overstressed!

Thank You Very Much John,

I was also using design conditions for my flex. analysis problems. My problems are related to B31.1 code and there i could not explicity get this point. I will look into the example of B31.3.but i used to check for the various conditions a system sees as mentioned by u.

Just i have some doubt in the sentence-
"The thermal displacement strains being that must be accounted for, should be those thermal displacements that are the maximum."

i think it should be-
"The thermal displacement strains being that must be accounted for, should be those thermal displacements that result in maximum stresses."
because the system may be having unrestained strains which may be highest but the stresses can be highest at some other location.
m i misunderstanding something ?

Thanks again,

After going through B31.1 again, i would like to correct my statement that B31.1 code does not explicity points to use the operating temp for flex. analysis. In fact in para 119.6.1 of B31.1 the code requires that the thermal expansion data shall be obtained for temps corresponding to operating and shutdown condtions.

Also Sh is defined as the basic material allowable stress at the maximum(hot) temp (and not at design temp).

One interesting point i would like to highlight is that in determining SL the 'P' in the pressure term is the design internal Pr whereas limit for SL is Sh which is determined at operating temperature.

be very carefull.
the temps/pressures required for analysis are often misunderstood.

design parameters are not valid.
Ask the process engineer what the max temps pressures are likely to be and then sense check these.
by example:
Often, condensate lines are give temps of below boiling point by the process eng. however, if these condensate lines are subject to back pressure, or are part of the lines to the trap, the temp can rise considerably above boiling.

these things are (usually )part of a stress critical line list (SCLL) the SCLL is created from the IFD line list which should contain the max/min OPERATING temps and pressures.


for some lines,all is not what is seems, on thermal relief lines for example. the temp of the fluid relieved may well be high, but the quantity released is so small as to not have enough energy to heat the discharge pipework up by any reasonable
amount.

for PRV systems, the normal OP temp is ambient, but under upset conditions, may rise . (fire cases for example)


tread carefully...........

After seeing a dozen springs on lines that are basically at ambient temperature... (the lines are thermally displaced by hot equipment) all I can say is think and look at what is going on!

Power Plant work seems more straight forward in this regard versus the idiosyncracies of process plants.

When in doubt think and ask questions!!!