Operating, Design and Overdesign values
Operating, Design and Overdesign values
(OP)
Hello,
When one wants to design any equipment, we define the OPERATING (which are the real ones) and the DESIGN (which are with 5% or 10% more to the operating ones) values of T and P.
I thought that :
OPERATING values = DESIGN values
OVERDESIGN values = DESIGN values + 5/10%
But I think it's false...My boss said to me:
OPERATING values = Real values found in calculations
DESIGN values = OPERATING values + 5/10%
What do you think?
Thanks a lot!
Méli
When one wants to design any equipment, we define the OPERATING (which are the real ones) and the DESIGN (which are with 5% or 10% more to the operating ones) values of T and P.
I thought that :
OPERATING values = DESIGN values
OVERDESIGN values = DESIGN values + 5/10%
But I think it's false...My boss said to me:
OPERATING values = Real values found in calculations
DESIGN values = OPERATING values + 5/10%
What do you think?
Thanks a lot!
Méli





RE: Operating, Design and Overdesign values
Your boss's option is the closest to the generally accepted practice. It has however, room for correction;- the additional "5-10%" is not mandatory, but usually added for increased safety or margin for error in process or unexpected variances. If you have a system in place with sufficient protection to maintain or limit the process conditions, you don't add any extra percentage, which may increase the fabrication costs unnecessary. If that's the case, your assessment is the correct one, not your boss's, ie operating values = design values is the correct one.
The overdesign case is typically a process condition included in the sizing of a process equipment like heat exchangers with several different operating conditions, ergo several design cases, where the basic design case + x% overdesign will cover all those operating cases. One only margin of some percentage can be added to the operating conditions, as design value or overdesign value (same thing), but not additional overdesign percentage on top of the design percentage. That would be an overkill.
Cheers,
gr2vessels
RE: Operating, Design and Overdesign values
Thank you gr2vessels for your explanations...
I think I understood...
But what does "overkill" mean??
Sorry, I'm French
Méli
RE: Operating, Design and Overdesign values
You can kill a fly with a little swat or you can drop a nuclear bomb on it to kill it. That is an OVER_kill. In your case, an additional overdesign percentage put in top of a percentage already added to the process conditions to satisfy the design conditions would be an unnecessary exageration = an overkill.
RE: Operating, Design and Overdesign values
Story
I was involved in an expansion project that involved multiple EPC contractors. During the startup the client found that one of the contractors reduced the capacity of the flow meters because their process group used the term design flow meaning normal flow but it was interpreted as the maximum flow. Fortunately it was another company thus not my problem.
RE: Operating, Design and Overdesign values
Just another question as I am new in this forum:
Who puts a pink star on a topic to mark up it as a helpful topic?
I have sent 2 topics and both of them have a star! ;o)
Méli
RE: Operating, Design and Overdesign values
I suggest that you stay away from the term "overdesign"..
ASME Pressure vessels use the term: MAWP "Maximum allowable working pressure".
When a pressure vessel is designed and specified in the US, the MAWP is typically 10-15% more than the maximum operating pressure expected.
The ASME piping codes use a term MSOP "Maximum sustained operating pressure", this is sometimes also called the design pressure. This should be 10-15% more than the maximum sustained pressure expected.
In my opinion, it is best to use the acronynms and terminology contained in the particular design codes and explain where necessary.
By the way... what gender are you...??
-MJC
RE: Operating, Design and Overdesign values
RE: Operating, Design and Overdesign values
we use MAWP "Maximum allowable working pressure"!!
For MAWP, I have put my "normal or operating pressure" (which in my case is a balanced pressure (une pression d'équilibre) that I found by calculations.
My design pressure is called "internal design pressure" in my data-sheet.
Your opinion?
Méli
RE: Operating, Design and Overdesign values
Operating Value x Safety Factor = Design Value
The Safety Factor varies depending on the state and maturity of the technology in that field, the quality of the data gathered, the uncertainty of the proper application of the technology, etc. S.F.'s of 1.05 to 1.25 are very common. I've seen S.F.'s larger than this used from time to time. This is probably an area that is more art than science, and practical knowledge and experience are probably going to benefit the Engineer more in selecting the "right" S.F. than anything else.
Good luck,
Latexman
RE: Operating, Design and Overdesign values
RE: Operating, Design and Overdesign values
Look at the bottom of each post in any thread and find the little pink star that says "Thank (the poser's name) for this valuable post!"
Each of us can choose to thank the original poster for the topic or thank a responder. Within that thread the pink star shows up for each subsequent post too. In this case I gave a star to GR2V for his original reply.
BTW, gender is not particularly important in the context of this forum - but gender often provides an interesting perspective.
RE: Operating, Design and Overdesign values
First, thank you very much for all these answers.
Well, one of my colleagues advices me to forget the "over-design" value and to work just with normal or operating value and design value which has a safety margin.
Thanks again for your help!
Hello from Paris!
Méli
RE: Operating, Design and Overdesign values
Sorry, my English is very bad!!
RE: Operating, Design and Overdesign values
JLSeagull... your experience is really unique. Design flow = normal flow... this really a disaster. But no joke. I have been working with contractor from a country. The design flow in their language is mean the expected normal flow. Over design is their way to define the "design flow" in our "language".
Let me create more confusion in this topic..
A system with has two operating mode. Operating mode 1 is 30 barg and 100 degC whilst operating mode 2 is 15 barg with 300 degC. What is the normal operating pressure ?
The operating mode 1 is at 30 barg but with some special campaign (may be 1 in 2 months) the operating pressure need to be adjusted to obtain higher yield. Possible operating pressure is 35 barg. Operating pressure could be 35 barg subject campaign.
The designer say due to possible heating flow fluctuation, reaction, etc, the pressure could fluctuate plus-minus 3 bar. Then the expected maximum pressure can be seen is 38 barg.
Then the operator set the high pressure alarm to 2 bar above the expected maximum pressure to avoid alarm being triggered continuously. This make 40 barg.
Following the "norm"
Design pressure = Operating pressure plus a margin
What is operating pressure to define the design pressure ?
Let increase the level of confusion...
Operating mode 1 : 35 barg at 100 degC
Operating mode 2 : 15 barg at 300 degC
Let take the worst case for operating mode 1 : 40 barg at 100 degC.
After we add the design margin, the second operating mode could be determining case due to the temperature ?
So, what the operating pressure to define the design pressure now ?
JoeWong
Chemical & Process Technology
RE: Operating, Design and Overdesign values
Hello,
Do you know how to say "pression de service" in English, is it the same as the operating pressure??
Thanks.
RE: Operating, Design and Overdesign values
RE: Operating, Design and Overdesign values
I am somewhat new to these Fora's.
However I may add my little bit hoping it is useful.
Since basically we are not having fixed operating parameters all the time i.e. pressure& temperature. As usually this is a range and we have to know or obtain from similar processes operating (maximum values encountered in real scenarios without majour upsets)
In this way we get 'Normal Operating values',
Now based upon possible upset conditions envisaged and extent of exposure in such conditions helps in deriving design values.
This in addition with the material strength give-away cosiderations during service provides final Design and/or Maximum Allowable Working Pressure(MAWP) value.
Hope this narration helps.
Best Regards
Qalander(Chem)
RE: Operating, Design and Overdesign values
Thanks for your response.
Infact the Mode 1 in my example is continuous whiel mode 2 is 1 in 2 months...anyway it is just an example.
The main objective of my example is to trigger engineer to understand that there is COINCIDENT or MULTIPLE design condition i.e.
i) low design pressure with high design temperature
ii) high design pressure with low design temperature
Second is really to understand WHAT's the meaning of OPERATING pressure.
Your response is what i want. Thanks.
JoeWong
Chemical & Process Technology
RE: Operating, Design and Overdesign values
It's all going far out of the original topic and is creating now a bit of confusion;- the temperature and pressure excursions are pretty common to any process, but are not necessarily taken in consideration as design conditions. The process engineer normally decides to specify several operating conditions and select one design condition. Under normal mechanical design, the short process excursions may be absorbed by the generic material properties, without increasing the design conditions and obviously the plant cost.
Also, once the design conditions have been established and the plant built to that specification, is rather unusual changing the operating conditions of that plant and quite scary to exceed those design conditions without proper re-engineering, to squeeze out a higher output. Campaign for two months? No more penalties for blowing up a process plant?
The two design conditions you have mentioned seem to be in fact only one condition, since the level of stress generated in the plant equipment during low pressure/high temperature and high pressure/low temperature could be the same.
You can include however, in agreement with the process designer / plant operator, a provision for further plant expansion or throughput increase, but this has nothing to do with the "overdesign" percentage.
cheers,
gr2vessels
RE: Operating, Design and Overdesign values
Hello everybody,
Here is an interesting discussion I have found on the same subject (on another site:
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It is not unusual for engineers to get confused with the terms "design pressure" and "Maximum Allowable Working Pressure" (MAWP). The two terms are not related mathematically; rather, they are related in a practical procedure that takes place during the actual fabrication of a pressure vessel.
Every Chemical Engineer - sooner or later - will have to deal with a pressure vessel fabrication or operation and it is sound and good advice that he/she should familiarize themselves with engineering terms employed and the logic of their application. In dealing with or specifying a pressure vessel, an engineer must resort to filling in or using a Vessel Specification Sheet - much like the one I have included in the attached Excel Workbook that gives you a host of Specification Sheets that you should be familiar with and employ in the course of your work.
The "Design" pressure is that pressure that the engineer decides is the value of the pressure at which the vessel will normally operate (or which it must withstand under operating conditions). This value must include any normal excess pressure that can occur during the vessel's operation. This is a discretionary value that depends on the background and experience of the design engineer. Sometimes the design value can be 10% over the pressure calculated (as in a simulation) or as much as 25% more. Good engineering judgment is employed in arriving at this design figure.
Once the Specification Sheet is received by the vessel fabricator, mechanical fabrication design takes place in which alloys, fabrication techniques, available materials, and other factors are taken into consideration to generate a fabrication drawing. Although the design pressure given is employed to generate the required vessel physical characteristics, some practical factors - such as available materials, fabrication efficiency factors, and alloys employed - will result in a vessel that not only meets the required design pressure, but often EXCEEDS it. This is a fortunate and conservative procedure because it ensures that the vessel will meet pressure safety expectations. The Maximum Allowable Working Pressure (MAWP) is a result of back-calculating the ultimate resulting fabricated vessel and is the prime factor in setting the pressure at which the corresponding vessel Safety Relief Devices will be activated. I consider the MAWP the most important pressure value attached to a vessel and one that should be clearly understood and stamped on the vessel for all to clearly read. The MAWP will change with time (as will the related design value) due to wear, corrosion, and vessel fatigue. This is why it is so important to religiously keep and maintain current and accurate data sheets and calculations on all pressure vessels as they are inspected and repaired through the years of service.
When you have a need to set a PSV on a vessel and you don't have its MAWP figure, you can employ the "design" pressure value - as long as it can be proven that the vessel is in as good a physical condition as the day it was fabricated. Note that I'm going to lengths to define the physical condition of the vessel. We often neglect to mention that we are ASSUMING that the physical condition of the vessel doesn't change from the day it was fabricated. This can be a dangerous assumption that doesn't necessarily apply. A vessel can undergo corrosion and wear as well as other chemical attacks through its use and lifetime. Physical and meticulous inspections and reports are essential to ensure that the vessel can be safely applied to a process --- especially to a high-pressure application. And I consider any pressure over 50 psig as HIGH PRESSURE. When a vessel explodes, it isn't the pressure that kills you; it's the amount of shrapnel and steel pieces that are blown about that do the damage. And even 50 psig can cause a considerable amount of serious damage if allowed to trigger a vessel failure.
I have never come across the term "Maximum Operating Working Pressure" and can only presume it means the same thing as MAWP. People are forever changing the writing of terms in order to suit their own likes and dislikes. MAWP was first described and is still employed by ASME in the USA and is the term I have always used to define what I have described in the above.
I hope this helps you understand the terms you have been confused with.
RE: Operating, Design and Overdesign values
I have two comments about your most recent posting. First I have to disagree with your contention that the MAWP is based on the vessel being in a new/good as the day it was fabricated condition. Not true. We typically specify a corrosion allowance for a vessel, and the vessel can withstand the MAWP at the design temperature as long as it has not corroded in excess of the allowance. Thus a new vessel may be able to withstand quite a bit more pressure than a corroded vessel, but the corroded vessel can still withstand the full MAWP (as long as the corrosion is <= the allowance).
My second comment is that the MAWP is a true characteristic or property of the fabricated vessel. The design pressure is a minimum requirement established during the design phase. It is independent of the mechanical construction and pressure withstanding capability of the vessel and is really a non-issue once the vessel has been properly fabricated.