Design Temperaturevs Operating Temperature 3

[leont]

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

 

[desertfox]

Hi leont

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.

 

[racookpe1978]

And that safety margin is written in the blood of victims who were too close when an operator decided to exceed that safety margin ....

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.

 

[LittleInch]

The same issue exists with pressure and any other design limit. The design codes allow use of vessels and associated piping up to the design pressure and temperature and depending on your code, some excursions above this for limited periods of time. If it has been designed correctly for a given temperature why operate it below it's limit?, why 50F (clearly just an arbitrary number), why not 40F or 20C or 10C??

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

 

[moltenmetal]

There is no requirement in ASME VIII-1 for temperature relief analogous to the pressure relief passive layer (relief valve or rupture disc) which IS mandatory. Instrumented overtemperature protection is considered adequate in industry, and there is no requirement to set that protection at any temperature below the MAWT. Owner/operators are of course free to set such protection anywhere below the MAWT at their discretion. There are also provisions in the code for "short term" operation beyond the MAWT unless I am mistaken.

 

[jte]

... depending on your code, some excursions above this for limited periods of time.

... There are also provisions in the code for "short term" operation beyond the MAWT unless I am mistaken.

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.

 

[moltenmetal]

jte is correct: the short term operation provisions are in B31.3 for piping, not in section VIII-1 for vessels. However, as I mentioned, Section VIII-1 is silent on requirements for overtemperature protection. Deliberate, protracted operation of pressure vessels with a mean metal temperature beyond the MAWT is clearly unacceptable, and a design margin above maximum operating is therefore required rather than just being prudent. However, the magnitude of design margin above max operating temperature, and the design and operation of overtemperature protective equipment, is an owner's judgment call rather than something which can be designed by code rule.

 

[CollinsCombustion]

Temperature is related to pressure. If you reach your design temperature, you must be reaching your design pressure as well. If you go above your design pressure, you will lift your safety relief valves, loose your steam too quickly for the feed water to keep up and trip on low level cutout.

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

 

[leont]

Thanks !!!

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

 

[Duwe6]

If the river regularly gets too high, just raise the bridge.

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.

 

[MJCronin]

I strongly agree with jte and MM.....

MJCronin
Sr. Process Engineer

 

[moltenmetal]

CollinsCombustion: not every ASME VIII vessel contains a saturated liquid. There are plenty of pieces of equipment in flowing services where temperature and pressure are almost completely decoupled, i.e. you can adjust one without affecting the other much at all. It is therefore not just possible, but EASY, to cause a vessel to fail- rapidly- due to overtemperature alone, even when the vessel is fitted with a correctly sized pressure relief device. Although ASME VIII is silent on the issue, overtemperature protection as part of safety system design is very important. As to the design of a proper control and interlocking system with control over the heat source, that's one important aspect of the overall safety design of an engineered works and there are lots of others, including design margin selection.