Operating, Design and Overdesign values 2

[Melimelo]

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 )

 

[gr2vessels]

Hi Melimo (Melinda?)
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

 

[Melimelo]

Thank you gr2vessels for your explanations...
I think I understood...
But what does "overkill" mean??
Sorry, I'm French )
Méli

 

[gr2vessels]

Ha ha,
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.

 

[JLSeagull]

These terms confuse many engineers. I received a message last week from a process engineer who suggested to allow at lease 50 degree F above the normal operating temperature for the design temperature of an analytical sensing element. Such criteria is common in the piping design.

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. ;-)

 

[Melimelo]

Hello everybody and thanks for your answers...
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 )

 

[MJCronin]

Fench Meli...

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

 

[Melimelo]

I'm a girl, is it important? )

 

[Melimelo]

Well, I was checking my data-sheet (that I had to complete),
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 )

 

[Latexman]

The terminology I like to use is:

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

 

[SeanB]

You should not be specifying the MAWP. On your datasheet you should fill in your normal operating pressure and your design pressure. Design pressure is some margin above your normal operating pressure. This is not some fixed margin, every vessel has to be considered individually. The MAWP should be specified by the vessel manufacturer - you should not be specifiying this number.

 

[JLSeagull]

Pink Star
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.

 

[Melimelo]

Hello everybody,
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 )

 

[Melimelo]

JLSeagul, thanks for your reply but I didn't understand your last sentence...
Sorry, my English is very bad!!
)

 

[JoeWong88]

Interesting discussion...

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

 

[Melimelo]

Hello,
Do you know how to say "pression de service" in English, is it the same as the operating pressure??
Thanks.

 

[SeanB]

To Joe Wong: it is quite common to have different operating modes, for example, Start of Run vs End of Run conditions on a hydrotreater reactor can vary quite significantly. If I understand correctly what you posted above, Mode 1 operates 1 to 2 months out of the year while the rest of the year the operation is Mode 2. Then doesn't that make Mode 2 your "normal" operation? Also it is not uncommon to specify 2 sets of design conditions for a vessel. If you wanted to be ultra-conservative you can state design conditions as 40 barg at say 350 C. However, these conditions are not actually coincident. So you specify two sets of design conditions 1)20 barg @ 350 C and 2)40 barg @ 150 C. Give those conditions to your vessel designer and let him/her design accordingly.

 

[786392]

Dear Melimelo Hello,
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)

 

[JoeWong88]

SeanB,
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

 

[gr2vessels]

JoeWong88,
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