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Join Date: May 2008
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PRESSURE

05/21/2008 1:46 AM

WHAT IS THE REAL DIFFERENCE BETWEEN DESIGN PRESSURE AND OPERATING PRESSURE ,IF WE CONSIDER FOR ANY PARTICULAR INSTRUMENT.

For eg: I f the pressure gauge has the operating pressure of 100-psig and the design pressure of 150-psig what does it mean

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Guru
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#1

Re: PRESSURE

05/21/2008 3:15 AM

The answer is at your question. In a word, design pressure is - what we are assuming the gauge could be affected for its life time period or period between repairs (if it's possible). During design we should keep in mind that gauge might be significantly overloaded time to time .

Operating pressure is what get every gauge in normal maintenance conditions.

That's why design parameters (not only for pressure and not only for gauges) are some higher than real operating one. As nobody can forecast inevitable and plausible overloading of gauge sensing element we should to secure to keep one workable no matter what happen.

How to get operating data? You can calculate it theoretically. You can to get data, instrumentally logged for similar gauge operated at similar conditions.

How to get design params? You can use theoretical approach based on several disciplines math statistic include.

You can use (as Vulcan said) "thumb rule" i.e. "stupidity" multiply operating params on 1.5 or 2.

I often use just 1.5 design coefficient as 2 can cause gauge's worse sensitivity and lack of accuracy for low level pressures.

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#3
In reply to #1

Re: PRESSURE

05/21/2008 4:43 AM

I AGREE THAT THE DESIGN PRESSURE NEEDS TO BE 1.5 OR 2 TIMES LARGER THAN THE OPERATING PRESSURE , BUT WHAT I WANT TO ASK IS EVEN IF I PROVIDE BOTH THE INFO TO THE VENDOR ,HE WILL ONLY PROVDE THE INSTRUMENT WITH THE RANGE ACCORDING TO THE OPERATING RANGE ...WHAT FEATURE CAN HE GIVE IN THE INSTRUMENT TO DEAL WITH THE DESIGN PRESSURE.

For eg: If the operatng pressure is 20 psig and design pressure is 180 psig ..and the gauge selected has the callibration range as 0-30psig then..wat will happen if the pressure finally reaches the design pressure(180-psig).

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Guru
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#4
In reply to #3

Re: PRESSURE

05/21/2008 11:21 AM

My apology as I should go out and couldn't reply in time. So if it's still actual ... I assuming that design pressure is term that has right to exist. In may land its named "project pressure", "operating pressure" is named as "working" pressure, respectively. When you're developing your measurement system you can call what you're developing as design pressure.

In my land there is so called 2/3 (two third) rule which means that indicating arrow should "roam" nearby 2/3 of maximum (full) pressure range to deliver best performance in terms of accuracy and reliability. It's just the same what said PWSlack at his post with technical details.

I some confused but I couldn't find out any mentions about "design pressure" at vendors' sites. Only operating, general, absolute, maximum(full) range etc.

In a word to my opinion it would be pretty enough if you give the vendor information about "operating pressure" and "maximum pressure range ". I suppose even only info about desired "maximum pressure range" would be enough. You can call out to vendor for resolving any term's contradictions at all.

Regards, caramba.

P.S. Just small remark, every pressure gauge should endure short pressure impacts 5-6 times over its operating range (arrow is laid on "red line"). I am sure you needn't inform ever about this your vendor as it presumed as default.

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Guru
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#2

Re: PRESSURE

05/21/2008 3:30 AM

Ideally the gauge should be selected for the design pressure of 150psig, so that at the operating pressure of 100psig the needle is still at an intermediate point on the scale.

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#5

Re: PRESSURE

05/22/2008 7:45 AM

Your Eg

The Guage is designed to operate at a MAX pressure of 150 psi.This is not the MAWP or Max allowable workin pressure though. MAWP is still higher than design pressure. And design pressure higher than operating value( ussually)

Operating pressure is the nornal operating pressure of a guage at which you would expect to operate.

Normaly when you choose any inst you would preffer to keep the max operating value at around 70 to 75 % of the full range & min around 20 % .

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Guru
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#6

Re: PRESSURE

11/04/2008 11:02 PM

The design pressure is the Max. Allowable Working Pressure MAWP, and it is taken into consideration mainly while calculating the wall thickness of a pipe or vessel from such equation : t = P D/2 S E.

As you see, the wall thickness t will be increased as you use a higher pressure P. And P = MAWP is not an assumed or arbitrary figure without a reference, but it represents the higher operating pressure expected to be attained in certain process (the worst case at emergencies), see CR4 Thread Working Pressure over Design Presure for some details.

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Guru
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#7
In reply to #6

Re: PRESSURE

11/05/2008 7:55 AM

I think you've hammered last nail into this. It's obvious and logically founded to take a Max allowable pressure as design one.

best regards,

caramba

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Guru
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#9
In reply to #6

Re: PRESSURE

04/19/2024 7:17 AM

Only, <...MAWP...> varies with time, downwards, according to what is found at the vessel's periodic insurance inspection, which will indicate its condition at that moment. A plot of <...MAWP...> against time will indicate its deterioration and predict its replacement date, for expenditure purposes.

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#8

Re: PRESSURE

11/23/2010 1:56 AM

Simple :

Design pressure is Pressure that we calculated for the first time we make a design...

Operating Pressure : it's Pressure which working for that time, and it's usually same or less than design pressure.... ;)

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Users who posted comments:

Abdel Halim Galala (1); bLankz_ajjah (1); caramba (3); naveen_menon (1); p_42 (1); PWSlack (2)

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