I have been wondering about how design conditions are set for pieces of equipment. I believe that design pressures are usually related to the shut-in pressures of upstream pumps right? But for design temperatures I am always seeing values that far exceed any operating conditions. For example a tower could have a normal operating temperature of 280 F but have a design temperature set at 500 F. Is this normal? Could someone help with some insight into this? Thanks.
Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations cowski on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
Design conditions of equipment 3
- Thread starter NPC5
- Start date
-
1
- #3
But this is precisely why errors are made when design conditions are set by what the materials are thought to be able to tolerate versus what the process engineers deem necessary for the process. It tends to encourage changes of process conditions without too much thought as to the consequences as the change is believed to be within 'the allowable limits'. Design temperatures should be set by process engineers within the margins that are required for the process not what the material can withstand as other considerations may come into play. Any subsequent change to these design values should then be subjected to a formal management of change process (Centre For Chemical Process Safety - Guidelines For the Management Of Change For Process Safety, 2008)
Steve Jones
Materials & Corrosion Engineer
Steve Jones
Materials & Corrosion Engineer
Mr Jones, et al...
The problem, of course, lies in the term "design temepature". ASME and others have very specific meanings for the term.
Is it the maximum allowed for the process or for the equipment ?
I like the terms "maximum operating temperature/pressure" and "equipment design temperature/pressure
For pressure vessels, ASME defines the design pressure for the component to be not less than the MAOP (maximum allowable operating pressure).
Careful word usage and clearly defined terms/acronyms can help.
-MJC
The problem, of course, lies in the term "design temepature". ASME and others have very specific meanings for the term.
Is it the maximum allowed for the process or for the equipment ?
I like the terms "maximum operating temperature/pressure" and "equipment design temperature/pressure
For pressure vessels, ASME defines the design pressure for the component to be not less than the MAOP (maximum allowable operating pressure).
Careful word usage and clearly defined terms/acronyms can help.
-MJC
Careful word usage and clearly defined terms go without saying as should a clear understanding of the design basis by everyone involved with operating the equipment.
In the example that started the thread
Steve Jones
Materials & Corrosion Engineer
In the example that started the thread
the answer would have to be 'it can be'. Maybe there is an operational requirement to periodically take the tower to a higher temperature (catalyst regeneration or such like as an example); maybe there could be a process fault condition that could dump a high temperature fluid into the tower. Because the steel can stand 500 deg F would not form a good design basis.
Steve Jones
Materials & Corrosion Engineer
SJones said:
I agree, but we should not stop specifying equipment smartly because of failure to follow Management of Change correctly. The equipment spec is not the root cause of the problem; it is human error! The process has its safe operating envelope, and the equipment has its design envelope. Both should be well known and well documented. Specifying equipment in a way that maximizes future operating flexibility and asset utilization is the smart thing to do.
Good luck,
Latexman
I agree with SJones. In fact, I dislike it with a passion that company standard sometimes require me to use a design temperature of 500F minimum.
Flange and instrument ratings decrease below 500F, so it creates a disconnect between the vessel and instrument / piping design temperatures.
Flange and instrument ratings decrease below 500F, so it creates a disconnect between the vessel and instrument / piping design temperatures.
My preference comes from being bitten several times on the cheap end of the spectrum. I've seen too many tanks rated at atmospheric pressure and 100 F! How do you size a relief when the design pressure = atmospheric pressure? You don't.
Good luck,
Latexman
Good luck,
Latexman
-
1
- #9
I tend to agree with Latexman for exactly the same reasons. I'm not saying that everything should be "bomb-proof," but if my design conditions dictated say, 100 psi and 400ºF required, and during the course of equipment design review the PV fabricator tells me that the same vessel design is adequate for 500ºF, why not stamp the vessel for 500ºF? If the service changes down the road, sure, an MOC is still required, but if the vessel is already rated for the new conditions, it saves potentially expensive and timeconsuming engineering work to re-rate the vessel (which was already designed appropriately in the first place) and that time and money can be spent ensuring that the remaining system components are modified appropriately.
As for initial determination of "design temp" I don't find it that uncommon for there to be a wide margin. If the vessel or column was specified taking into consideration potential overpressure scnarios (for example, external fire or abnormal heat input scenarios), a much higher maximum allowable temperature based on the vaporization temperature of the vessel contents at the MAWP may be required.
As for initial determination of "design temp" I don't find it that uncommon for there to be a wide margin. If the vessel or column was specified taking into consideration potential overpressure scnarios (for example, external fire or abnormal heat input scenarios), a much higher maximum allowable temperature based on the vaporization temperature of the vessel contents at the MAWP may be required.
skearse, all the piping, gaskets and instrumentation on the vessel in your example are still only good for 400F. Designing the vessel only for anything else just create confusion, which means higher risk.
Another minor point, is that we do not include the fire case temperature in setting design temperatures.
Latexman, I agree with your comment on 100F. But the preferred way is to use a more reasonable minimum (say 250F to cover steamout) and use that for all components (vessel, instrument, etc).
Another minor point, is that we do not include the fire case temperature in setting design temperatures.
Latexman, I agree with your comment on 100F. But the preferred way is to use a more reasonable minimum (say 250F to cover steamout) and use that for all components (vessel, instrument, etc).
Latex....
I agree completely with your point about atmosheric tanks, but this is a problem that was started by the tank specifier, not you.
I have been in exactly the same circumstance
If a tank has to be "retrofit" with a vacuum/conservation device, it means to me that the tank is undergoing a change in service. The choice of reusing that tank should have involved an evaluation of it's pressure/vacuum capability. (a duty that was probably ignored by management)
Some major chemical/petrochemical companies have "minimum specifications" for flat bottomed tanks that may change service.
The min specs include +10/-5 IWG pressure design; specific gravity minimum of 1.10; extra nozzles etc...etc
-MJC
I agree completely with your point about atmosheric tanks, but this is a problem that was started by the tank specifier, not you.
I have been in exactly the same circumstance
If a tank has to be "retrofit" with a vacuum/conservation device, it means to me that the tank is undergoing a change in service. The choice of reusing that tank should have involved an evaluation of it's pressure/vacuum capability. (a duty that was probably ignored by management)
Some major chemical/petrochemical companies have "minimum specifications" for flat bottomed tanks that may change service.
The min specs include +10/-5 IWG pressure design; specific gravity minimum of 1.10; extra nozzles etc...etc
-MJC
-
1
- #12
The pressure vessel code typically (depends on the material) has no derating for temperature until you hit 500F. That's why a lot of pressure vessels will be designed for 500F because it's free (I believe in the past it used to be 650F, at least that is what I was told, which is why a lot of older equipment is designed for 650F).
One exception are the flanges, those are still governed by B16.5 so by automatically going to 500F you could bump yourself into the next flange class, you need to look at it on a case by case basis.
It's also possible the tower was designed for a higher temperature as a result of some upset condition the process engineer identified.
One exception are the flanges, those are still governed by B16.5 so by automatically going to 500F you could bump yourself into the next flange class, you need to look at it on a case by case basis.
It's also possible the tower was designed for a higher temperature as a result of some upset condition the process engineer identified.
"but if my design conditions dictated say, 100 psi and 400ºF required, and during the course of equipment design review the PV fabricator tells me that the same vessel design is adequate for 500ºF, why not stamp the vessel for 500ºF?"
A similar logic is why some companies require the vessel designer to calculate the limiting component (flange, body, head, etc) and stamp the vessel for that pressure. It avoids having to re-rate it later if requirements change.
A similar logic is why some companies require the vessel designer to calculate the limiting component (flange, body, head, etc) and stamp the vessel for that pressure. It avoids having to re-rate it later if requirements change.
djack77494
Chemical
MJC,
Your statement,
"The min specs include +10/-5 IWG pressure design; specific gravity minimum of 1.10"
What a great idea. Will cost little or nothing for all but the largest of tanks and would save countless trouble and aggrevation. Wish this would be widely adopted.
Your statement,
"The min specs include +10/-5 IWG pressure design; specific gravity minimum of 1.10"
What a great idea. Will cost little or nothing for all but the largest of tanks and would save countless trouble and aggrevation. Wish this would be widely adopted.
Sorry guys, missed the responses on this one...
CJK-to clarify, I'm not saying design for the actual fire temperature, but rather, the relieving conditions of the vaporizing liquid in the vessel during a fire case.
As far as increasing the risk...OK, so if you have someone that is going to base rerating a process simply based on the vessel ratings...yes, that's a major issue, but one that the process engineer, or someone else in the design review and MOC process, should know better.
The other one that kills me (that happens quite a bit around here, unfortuantely), is equipment designed for full vacuum and NO PRESSURE RATING (atmospheric). I cannot even begin to think about how much money we're going to have to throw at PV engineers to re-rate this equipment just to find out that, oh, by the way, the original vessel design would've been good for 50 (or whatever) psi...and if it wasn't originally built to ASME...uggggh. Sorry, to close to the heart for me on this one.
CJK-to clarify, I'm not saying design for the actual fire temperature, but rather, the relieving conditions of the vaporizing liquid in the vessel during a fire case.
As far as increasing the risk...OK, so if you have someone that is going to base rerating a process simply based on the vessel ratings...yes, that's a major issue, but one that the process engineer, or someone else in the design review and MOC process, should know better.
The other one that kills me (that happens quite a bit around here, unfortuantely), is equipment designed for full vacuum and NO PRESSURE RATING (atmospheric). I cannot even begin to think about how much money we're going to have to throw at PV engineers to re-rate this equipment just to find out that, oh, by the way, the original vessel design would've been good for 50 (or whatever) psi...and if it wasn't originally built to ASME...uggggh. Sorry, to close to the heart for me on this one.
moltenmetal
Chemical
latexman: how can you design a relief valve for a tank rated for atmospheric pressure and 100 F?
EASY: it's a piece of pipe connected to the atmosphere!
Going somewhere else with the vent? You need to design for that from the get-go!
Designing for later re-use is a mug's game. Re-use for what purpose, by whom? I agree that vessels should be stamped with their actual calculated hot and corroded MAWP rather than some lower "design" figure- to do otherwise is to throw away money. But designing for some MAWP or MAWT higher than needed is seldom a zero cost proposition.
I can't count how many times a client ended up with significant additional project cost as a result of scope creep related to a desire for "potential" re-use of the equipment. They started out with a defined project need for a non-code atmsopheric storage tank and ended up with a pressure vessel- with all associated piping, instrumentation and valves now designed to the MAWP of the "vessel", just in case... Not that I mind taking the profit on the extra money- if the project isn't killed entirely due to excessive cost, and if the right tools for the particular process job are not rendered unfit for purpose due to the new, unnecessarily high MAWP!
+10/-5"w.g. plus s.g. 1.1 on the other hand, I can live with!
EASY: it's a piece of pipe connected to the atmosphere!
Going somewhere else with the vent? You need to design for that from the get-go!
Designing for later re-use is a mug's game. Re-use for what purpose, by whom? I agree that vessels should be stamped with their actual calculated hot and corroded MAWP rather than some lower "design" figure- to do otherwise is to throw away money. But designing for some MAWP or MAWT higher than needed is seldom a zero cost proposition.
I can't count how many times a client ended up with significant additional project cost as a result of scope creep related to a desire for "potential" re-use of the equipment. They started out with a defined project need for a non-code atmsopheric storage tank and ended up with a pressure vessel- with all associated piping, instrumentation and valves now designed to the MAWP of the "vessel", just in case... Not that I mind taking the profit on the extra money- if the project isn't killed entirely due to excessive cost, and if the right tools for the particular process job are not rendered unfit for purpose due to the new, unnecessarily high MAWP!
+10/-5"w.g. plus s.g. 1.1 on the other hand, I can live with!
- Status
Similar threads
- Locked
- Question