Design Temperature of Piping System 1

[alex2010]

Hello,

I would like to know how the design temperature of a piping system is chosen?

Is the temperature of the working fluid or the smallest temperature rating of each component chosen as the design temperature?

Thank you,
Alex

 

[zdas04]

The short answer is "choosing design parameters is what the design Engineer gets paid to do".

If it matters (i.e., you are designing close to some limit) then you had better either use modeling or other calculation methods.

Generally you just make sure that the material properties are appropriate for a range that is far wider than your expected range. If you have a material that starts to have brittle problems at -120F and strength problems at +500F then using a design temperature of winter ambient (call it -45F in Northern Alberta) or summer ambient (+120F in Death Valley, CA) is appropriate--use whichever one gives you the lower MAWP. You can look at the design temperature of the components you've selected (use the most restrictive), but I contend that I'd better know design pressure and temperature before I begin selecting components.

On the other hand, if you are designing steam piping that is going to exit a building on Alaska's North Slope then you had better be doing some serious thermal modeling. Rules of thumb will end up killing some caribou.

David

 

[ColonelSanders83]

zdas04's advice in on the money. Thats what we are paid the "big bucks" for.

The Casti Guidebook on B31.3 Process Piping has some helpful guidence.

We usually tack 50 F onto the highest temperature the pipe will encounter and use that for our design temperature. The low end is usually specified from coldest ambient at location.

Just my two cents worth.

Always remember, free advice is worth exactly what you pay for it!

 

[SJones]

It is all too common to see the second approach adopted, i.e. what is the material listed in B31.3 as being capable of - OK that's the minimum design temperature. So, because somebody sees a process temperature at say -30 deg , they select A333 Gr 6 and, bingo, the minimum design temperature automatically becomes -46 deg C. Why? Not because the process engineer deemed it but because the piping engineer wrote the datasheet! I'm not saying it's a bad thing but it does cloud the issue of trying to ascertain fitness for service in the future. It also doesn't help when dealing with linepipe specs that demand Charpy testing at 20 deg C below the minimum design temperature and somebody sets a minimum of -50 deg C!

Just a heartfelt plea for the rational and transparent engineering derivation of design temperatures.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[BigInch]

Design temperatures are the minimum and maximum temperatures (+ safety margin) you must have to do the process. Select materials that have a wider range than the design temperature range. Same for pressures.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[rneill]

With regards to ambient temperatures, there is not much guidance in the piping or vessel codes on that subject so I usually default to the local building code (and I use the minimum ambient temperature for the location specified therein). However, there are also some temperature isotherms published in a chart in API 650 which I think are often used for selection of minimum ambient temperatures for design purposes in the United States.

I won't get into the subject of how the minimum ambient temperature relates to your minimum design metal temperature other than to repeat the prior comment that the CASTI book "Guide to ASME B31.3 Process Piping" does do a good job of discussing this topic

 

[rconner]

Not going to profess specific expertise here (and there is no doubt more erudite or defensible explanation in some code somewhere), but I guess in answer to the second original inquiry question it is often said with regard to many matters "a chain is only strong as its weakest link" (so one would think that answer to that is likely YES.)

Beyond that and directed toward other discussion, I also guess some might argue there could conceivably be some danger in just mentioning “ambient” as the only measure, at least in the case of pipelines carrying fluids such as very high pressure gases. E.g. if the pipeline carries a gas under high pressure, and the line somehow contains or develops a breech for whatever reason (to where there is a leak), as reported at

http://aiche.confex.com/aiche/s06/preliminaryprogram/abstract_40438.htm

there can be due to “unsteady state fluid dynamics and heat transfer effects” (or adiabatic?) extremely low temperature developed locally in the adjacent pipe wall (and the subsequent danger of a “running fracture”). [Incidentally, I don’t think this tragic incident in Belgium is the first time something like this may have been suspicioned over the pond.]

 

[BigInch]

BTW to be clear, my "do the process" temperature are all temperatures which the fluid can reach; including from environmental heating and cooling effects.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

rconner,

While joule-thompson effects can be important and could have contributed to the severity of the case you mention, the initiation of the leak was most probably due to construction damage as reported in the following.

Fluxys reportedly told the public RTBF television station that gas was not flowing in the pipeline on July 30 because the terminal at Zeebrugge was temporarily closed. Fluxys allegedly said that “with gas neither entering nor leaving the pipeline, energy losses due to friction disappeared and the pressure in the conduit rose”. The pipeline then ruptured at a point where it had been damaged. Grooves scored in the pipeline were reported to be up to 10mm deep.

It was alleged that the manager of the building site that was destroyed by the gas explosion was warned of damage to the pipeline days before the tragedy. The revelation emerged during an inquiry in Tournai into the explosion that left 24 people dead and injured a further 132 injured, 62 of them seriously. A building site worker at the plant said he warned his boss that he had damaged a gas pipe when digging up the land, but his boss decided to take no further action.

Asked what official statement she could make regarding the pipeline, she replied: “It does not seem that any inherent defect in the pipeline led to the explosion. These are the first facts which have been brought to the attention of the juge d'instruction by the experts. And I think that now we must have a little patience and await the submission of this report.”

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

This is going along the same path of another thread being discussed right now, so I'll add a comment I made there to this one too.

B31.3 permits variations in temperatures and pressures from operating levels, provided certain conditions are meet. Joule-Thompson cooling IMO would usually be considered outside the normal operating temperature levels, so a JT temperature need not be the "design temperature". Non-the-less, the design should accomodate the lowest temperatures that could be reached under the increased design limit provisions for transient occurrences and either by choosing a material listed for a lower temperature, by qualifying a choosen material for lower temperature use by specifying appropriate toughness requirements, or by providing crack arrestors.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[alex2010]

Thank you all for your replies.
Alex

 

[4Pipes]

Whatever you do, it must be realistic, logical and justifiable. Just adding margin for its own sake and for personal comfort, as is often done, can be horrendously expensive. Over design can often increase risk because of more exotic materials, added weight etc. I have just come across a small drain tank that should have had a design pressure of "full of water". Design using a tank code and "good engineering practice" would have been safe and adequate. Some well meaning process engineer converted the pressure to absolute and then applied a "safety factor" of 1.5. Designing an open tank to be 50% over filled might have been justifed on the basis of adding a bit of rigidity. However, adding a 50% safety margin to atmospheric pressure is not!!!!????. The safety margin converted what was no more than a gloried water butt costing a few dollars into a coded pressure vessel costing $1000s.

 

[BigInch]

Yes, but that one isn't an example of more safety factor. I'm afraid you just have to call that one a BIG mistake any way you look at it.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[zdas04]

It is in the scary category that I call "well meaning mistakes". This is the same kind of stuff that makes some Oil & Gas companies build wellsites to ASME B31.3 with dozens of drawings and Engineering costs higher than the cost of equipment.

I recently saw it in a PSV study I did. We had a well that could produce 100 MCF/d into 20 psig. At 45 psig the flow rate was near zero. We had a 350 psig MAWP on a vessel. I did a credible-scenario analysis and said that the required flow rate at set pressure was a pretty small number (thermal case was the only one credible). One of the teenaged engineers (lower case) that sat in on my presentation said that he wasn't comfortable with saying that the required capacity was very low--since 2-inch pipe at 350 psig can move 3 MMCF/d and that is the required capacity. The room agreed with him and the PSV went from fitting in the space provided to requiring 3 sizes bigger valve that required a code modification to the vessel. A costly "well-meaning mistake".

David