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Temperature range

Temperature range

Temperature range

(OP)
What are rules for deciding the lower limit of piping design temperature range?

Is it the lowest possible temperature the pipe could see in it's life. ie shut down in the coldest winter or can more a resonable temperature for the annual shut down be used.

The systems we design are base load and only shut once a year. (a lot less than 7000 cycles)

Thanks

RE: Temperature range

Here's how I determine a project minimum design temperature, wave height, maximum rainfall rate, flood level or any other statistically determinate minimum or maximum constraint (when such is otherwise not specified by code or regulation).   What you can do is go to the nearest weather station and pick up the data for the minimum temperatures for the last 50 or 100 years or so.  From that you can develop a "Return Frequency Histogram".  What that will tell you is a specific probability of reaching any given temperature in any given year.  That is where the "50" or "100 year" storm figures come from.  Then you look at your power supply contract and see what level of outage you are required to deliver.  Say you need to produce power to a reliability level of 97.5 % of the time.  Then you cross-reference that % reliability with the % confidence of not exceeding some given temperature during the life of the contract, or your plant's or pipeline's design life or whatever your local building or insurance company requires and you have your design temperature (or flood height, maximum wind etc.  Say your power contract is for 5 years and you must deliver to 97.5% reliability.  From your histogram, you might have something like this,

-2 ºC 80%
-5    85%
-10   90%
-12   95%
-15   98%
-16   99%
-19   100%

If that is the case, you should design for -15ºC and if the temp drops to -16, you can continue to operate if you can, but if you can't, you can still claim you did perform to the minimum contract delivery requirements.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Temperature range

BTW, that was a 5 year Frequency of Return Temperature table, for which I totally made up the figures, so don't use it for your or any other project.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

RE: Temperature range

Its a combination of stress' and temperature that needs to be taken into account.  By stress, hoop stress, shock, bending.  Carbon steel can be subjected to -100 F and not be a hazard if the hoop stress is lowered and the pipe protected from other forces.

If the line is always in service, then the fluid will keep it warm in most situations too.  When its out of service, the stress' are lowered and that is acceptable. Just have a proceedure to bring the line back up to temperature as you put it back in service.

RE: Temperature range

(OP)
We need to decide on the lowest tempeture for the calculation of thermal stress range. The codes allow this stress range to cycle less upto 7000 times.

In our case we may only see maximum temperature (brust disk pressure) a few times in the life of the plant and the low tempetures will only happen once a year at shut down.

If we take the lowest possible temperature during the shuts we are being very conservate.

I have seen a calculation in another code (AS4041) where the you can calculate the equivant number of cycles based on the frequency of the different temperatures. And also Increase the allowabe stress range for when the cycles are less than 7000.

RE: Temperature range

What is your design code?  "AS" = Australian, no?

Temperature cycles are based on process start-ups going from ambient temp to normal operating temp and back, i.e. not based on excursions of ambient temperatures superimposed on a constant normal operating temperature.  On a well insulated line, an ambient temperature excursion to a 500 yr Lo temperature may not affect the temperature of the piping material at all.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

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