Design Temperaturevs Operating Temperature
Design Temperaturevs Operating Temperature
(OP)
In order to complete some integrity assessments, I have been reviewing some information concerning operating temperature of the pressure vessel. I noted that some of them appear to be running close to the design temperature. Some engineering practices state and recommend the maximum operating temperature should be 50 F below design temperature. The challenge is that this difference on temperature(design and operating)trends to decrease over the time.
Which experience do you have on this matter?
As per codes, Standards and engineering good practices, What is the difference between operating and design temperature to guarantee the integrity of the vessel is not compromised?
I am absolutely clear the vessels need to be assessed. I am trying to find out what code, standards and best practices say on this regard. It is very likely I will know whether a re-rate is required, or a new vessel with more capacity needs to be required
I will be waiting for your usual and experienced help
Thanks
Which experience do you have on this matter?
As per codes, Standards and engineering good practices, What is the difference between operating and design temperature to guarantee the integrity of the vessel is not compromised?
I am absolutely clear the vessels need to be assessed. I am trying to find out what code, standards and best practices say on this regard. It is very likely I will know whether a re-rate is required, or a new vessel with more capacity needs to be required
I will be waiting for your usual and experienced help
Thanks





RE: Design Temperaturevs Operating Temperature
I don't see a problem with vessels running close to their design temperature, I guess when a vessel is at the design stage there is always a margin between what the vessel operates at compared to the design criteria etc. but I'm not sure whether there is a dedicated margin between the two.
One thing to bear in mind when using any code is that there is always a safety factor built into the allowable stresses, pressures and temperatures.
RE: Design Temperaturevs Operating Temperature
Design prudently within the code. IF you can stay in the code AND increase safety margins (temperature in your case, not pressure) AND stay within the customer's and your company's budget while doing so, ALWAYS make a margin for expansion, future excesses, future greater flow or temperatures or pressures, and greater safety.
RE: Design Temperaturevs Operating Temperature
Reality of operating needs to take account of the rate of change or the parameter to know if it can suddenly increase or not and what sort of protective devices are in place. Spurious or regular trips cause economic loss and time loss so need to be minimised and setting operating levels very close to design / trip levels is one cause.
However each situation is individual and is why design exists to take account of these individual difference which no blanket margin between design and operating can ever take account of, which is why you won't see it written down other than company practices and "normal procedures". Even those though usually have way of getting that margin reduced if it can be justified and shown that the temperature or pressure does not exceed the design limit or cause excessive trips to occur.
After all, if you buy a car capable of doing 100mph, why would you limit it to 90 mph ( other than the speeding fine, but lets not go there). If the manufacturer / designer says it will do it continuously then its time to believe him.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Design Temperaturevs Operating Temperature
RE: Design Temperaturevs Operating Temperature
This notion of ok to exceed design conditions for limited periods of time is unique to piping (e.g. B31.3 302.2.4(f)). As for vessels, ASME Section VIII Div. 1 and Div. 2 (I don't know about Div. 3) do not have any allowances for excursions beyond design. Perhsps some of the other vessel codes allow for excursions. Running with a hot spot on a "cold wall" vessel as you might see on an FCC which has spalled some refractory is ok - but not under Section VIII, rather this would be in a FFS situation.
Agree with the notion of generally having a design margin on temperature in company spec's for new construction. This allows for some operational flexibility at relatively little expense in most cases. By definition, then, this design margin is ok to infirnge upon when operational needs dictate. It is thus very prudent on the part of the person who is specifying the vessel's design conditions to evaluate the likelihood of a temperature excursion. For example, an atmospheric tower is not likely to suddenly operate above its design temperature. A reactor with an exothermic reaction is an entirely different critter; adding more than a nominal 50°F may be a very economical - and safer - approach. I mention safety since let's keep in mind that transient operations such as shutdowns (particularly unplanned, quick shutdowns) and startups (hopefully these are planned...) are inherently more hazardous than steady state operations. Thus, giving the operators time to regain control of something like a runaway exothermic reaction before shutting down an entire unit can be far safer.
RE: Design Temperaturevs Operating Temperature
RE: Design Temperaturevs Operating Temperature
A properly designed NFPA85 control system will alarm and trip the burner before you reach the excessive steam pressure rating of your pressure relief valves, the expanded water will still contract and trip or at least alarm for low level but you will not lift your safety reliefs.
Darrell Collins
http://www.collinscombustion.com
RE: Design Temperaturevs Operating Temperature
All answers really interesting and I see they are clearly coming from experienced people, and I am happy with your support. However in somehow in the real engineering world we need to asses a pressure vessel operating at 30°F hotter than before, therefore the margin between design and operating is getting down. One of the challenges is to evaluate whether the active damage mechanism of the vessel can be increased to the point where I am reducing the remaining life of the vessel. It does not mean, because it was designed to 850°F you have to operate it at 850 °F. Probably, vessel exposed to moderate temperature fluctuation needs certain temperature margins to guarantees safe and adequate remaining life. So, at this level of concern, safety can play an important role, and as LittleInch says, there will be technical reasons of why 50°, 40°, 20°
From this debate I can conclude there is nothing is clearly stated in code for pressure vessel concerning margin between operating and design temperature. At this time, looks to me it has to treat it as particular case with all technical variable involved. Probably Hazop carried out during design stage of the unit, revealed the need of certain temperatures on certain circuits. I will go deeper with the original design criteria. If something relevant shows up, it is very likely I get back to you guys with an updating to enrich this debate
Thank you so much for your valuable support, and have a great day
RE: Design Temperaturevs Operating Temperature
MAWT means what it says - Maximum Allowable. Period.
If operations needs the extra 30° or 50°, you are required to rerate the vessel. Have thicknesses taken, and perform an API-510 assessment of the current condition of your vessel. Then have a "Come to Jesus" meeting with Operations and determine what they really need for a revised MAWT. Recalc the vessel based on the new temperature and the decreased max. material stress.
Usually, you will be pleasantly supprised and find that your vessel calc's out fine at the upwardly-revised MAWT. Then, depending on who your Boiler & Pressure Vessel authority is, fill out the appropriate "Alteration" paperwork. Most areas in the USA, this will require a "U" or "R" Code-stamp.
RE: Design Temperaturevs Operating Temperature
MJCronin
Sr. Process Engineer
RE: Design Temperaturevs Operating Temperature