DESIGN MARGIN 1

[BZLAM21]

HI,
I am a beginner in the study of engineering and i need your help
Is it wise to consider 20% over design for all utility to cover any change expected in process during engineering?
Please advice.

Best regards

 

[1503-44]

Not wise, certainly not as a blanket statement.
On large pipeline systems, that might mean hundreds of million dollars and kill your project before ever leaving the starting gate. On an item by item basis, it could easily shift pump and other equipment sizes to uneconomic cost, or compromise their best operating ranges and wind up increasing operation and maintenance for the life of the system.

It should be evaluated carefully, probably on a unit by unit basis and with regard to stage of the project, degree of project definition (potential for change), total project cost and staged investment criteria (build-now costs verses costs of future add ons).

My strategy is to design for specified design conditions first, then check for what maximum capacity can be achieved with that design, then lastly identify potential bottlenecks and estimate costs of any further increase in capacity.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[MJCronin]

Yes, - 20% is correct ... _ You have discovered our secret !!!

A 20% margin on everything, in all cases, for any application, for any utility ....

... Yup, that's what we all do !!

.... and as a heavily medicated retired Senior Engineer and PE .... I highly support the 20% on everything !!

MJCronin
Sr. Process Engineer

 

[1503-44]

Except taxes.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

It used to be accepted that there was always a bit of "margin" on something like a large process plant so that, e.g. a plant with a nameplate capacity of say 100 MMSCFD would actually be able to do 110 or maybe 115 on a good day.

But that was before things could be analysed to the nth degree and now if you want a plant to do 100, it will do 100.5 before multiple items reach the end of their design rate / power etc and the thing starts tripping or bottle necking all over the place.

The days of a spare 20% are gone. If you get 5% I think you're doing well.

That's why getting a good design basis is key to any project and then understanding that if you change something critical in it, like composition, flowrate or pressure then you start the engineering design again.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Depends on what you mean by "design capacity". Sometimes, the design flow already has a margin over annual average. This margin over annual average would account for time taken for annual shutdown, other unexpected trips, surges in flow etc. This is for the process plant.

For utilities, the sum total of design margins for each utility must generally reflect the design margins used for the process plant for utilities directly related to plant process, such a process cooling water, hot oil, steam for process and for drivers, flare etc. For utilities unrelated to the main process, each must be looked at individually.

Operators always welcome a healthy design margin, but these parameters usually invite scrutiny from project managers who have a finger in the CAPEX pie during design phase, and margins are usually kept to a minimum unless there are plans for things like future capacity debottlenecking.

 

[MortenA]

You have to consider that over capacity costs money to build - so while you might need it at one stage you will have to pay for it when you build it.

--- Best regards, Morten Andersen

 

[MJCronin]

An excellent discussion on a topic that has plagued me for decades ... !!!

Great advice on Design Basis Development from LI ....

That's why getting a good design basis is key to any project and then understanding that if you change something critical in it, like composition, flowrate or pressure then you start the engineering design again.

MJCronin
Sr. Process Engineer

 

[shvet]

@BZLAM21

They will continue to do this untill you specify specific parameters for design margins.

There is no a widely accepted practice for design margins as it is person dependent. Even within one process facility design margins change during decision makers rotation. It is subject to approval for a particular project by a decision maker / principal or similar.