2 replies to this topic

[jprocess]

Dear All,
I need some information regarding overdesign factors that applied for equipment design like heat exchangers or cartridge filters,for example:when and why sholuld we apply them?their typical values and etc.
Thanks in advance.
Best Regards.

[Nirav]

Your question is very much general in nature.

I would like to point out as to "why" the margin or over design needs to be applied. It may depend on more than one variable.

[1] The fluctuations in service expected or anticipated with respect to "normal" operation.
[2] Provision for future developments, expansion etc.
[3] Decrease in efficiency of equipment over a period of time between change over or shut-down (equipment out of service).
[4] This one is last but not the least in my list. For a specific project or design of unit, general philosophy or preferences of process plant owner dictates the same (even though fluctuations are not anticipated). This could be based on point no[2]. e.g. Plant should be designed at 110% capacity. In such cases, you are bound to ensure "minimum" of 10% margin over normal operation. It can go higher based on other factors but not less than 10% in any case.

I just tried to list down whatever came in my mind as to why I am taking margin when I'm preparing specifications for equipment or instrumentations etc. But remember, taking into account "unnecessary" margin may create problem during 'normal' operation.

Apart from this, I would like to mention two concerns from engineer's point of view.

[A] Sometimes we use approximate values or past experience etc during calculations. As an engineer, you are supposed to give "guarantee" for design margin. Therefore, you would also like to take into account the "precision" of calculations. Even though it is not always, but sometimes, you may keep "internal" margin (to be on safer side) over guaranteed figure. It could be in the range of 2-3% but it is NOT reported.
e.g. Consider a heat exchanger. The normal heat duty is 100%. Heat duty to be guaranteed is 110% (based on points [1] to [4] I have listed above). I would prefer my calculations to give me 2% higher heat duty over 110%. But I may not like to report it. Similar is the case with many suppliers when they are reporting their guaranteed value for a specific "proprietary" items.

[B] Sometimes, we have to purchase equipment or its internals as a standard "of the shelf" product from its supplier. It could save cost many times. Since they are not "tailor made". In such cases, you select the next higher capacity over the required (calculated) value. So, you will have some margin depending on selection.

I have tried to explain whatever I think about design margin. If you have very specific query, reply back
Any more opinions?
Thank you,

[Sandeep01]

Nirav

Though design practices vary to a certain extent from company to company, i would be of the opinion that items 1 to 4 in your list are all quantifiable and should be considered/accounted for prior to applying hte design margin.

Personally I see the design margin as a function of robustness and confidence on the design, gained over years of experience. This, of course is pretty well brought out in Item A of Nirav's posting.

Regards
Sandeep