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a_m_shiha
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Join Date: Nov 2008
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Minimum Differential Pressure Across Orifice Plate

04/05/2015 8:17 PM

hi,

trying to evaluate existing orifice plate if it is still working under the new operating condition (temperature , flow ,pressure) , we used Daniel orifice free calculation the result was as follow

1- when fix the orifice bore diameter ( like the existing ) , the differential pressure result was too low 57.36 m bar based in the new operating conditions instead of 250 mbar according to the old operating condition .

my question is ; if these low value is reasonable for flow measurement and what the effect on accuracy

generally in case of measuring gas flow using DP transmitter across orifice plate what is the minimum DP that shall be achieved for measuring the flow within the accuracy

I searched a lot in the web and all I found is related to the maximum DP no mention for the minimum

thanks
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Tornado
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#1

Re: Minimum Differential Pressure Across Orifice Plate

04/05/2015 10:03 PM

Please accept our condolences.

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Iris
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#2

Re: Minimum Differential Pressure Across Orifice Plate

04/05/2015 10:13 PM

I assume you're saying that the maximum DP for the new maximum flow rate would now be 58mbar rather than the original 250mbar.

Traditionally, a 4:1 turndown for DP was considered as acceptable accuracy at 25% of max flow rate. At that flow rate, the DP is only 6.25% of maximum DP.

(the table below shows percentage of max DP vs the max flow rate at that DP, the blue line shows 25% of max flow rate)

58mbar is 23% of the former 250mbar URV. If you accept 4:1 turndown, then the new max flow rate is at the original turndown limit. (FYI, the concept of turndown relates to the percentage of max flow at which the accuracy remains acceptable. Acceptable turndown is an opinion based on what accuracy level is acceptable, it is not a fixed ratio defined by some ethereal 'standard'.)

If you leave the transmitter ranged for the original 250mbar, then your measurement is now working in a marginal range that would be considered unacceptable for those using a 4:1 turndown limit. The values your transmitter will see will be at or below the blue line.

If you re-range the transmitter for a max 58mbar URV, then the turndown becomes dependent on how the new 53mbar URV relates to the transmitters maximum URV. As a transmitter's range is turned down, its accuracy declines and all the major transmitter vendors provide a calculation in their spec sheet for determining accuracy at any URV throughout the configurable range. The new URV is likely near or in the range where you need to run a calculation to determine the actual accuracy.

All that said, an orifice plate (OP) is about the least expensive primary flow element out there. You haven't mentioned line size, but $100 buys a lot of orifice plate. And you get a new plate with a sharp edge. Orifice plates do wear in service and produce erroneous low DPs as the edge dulls.

If there's a question about the diminished accuracy at the reduced URV, consider replacing the OP with a new one with a different bore.

58 mbar is about 23 inches w.c.
250mbar is about 100 inches w.c.
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Wal
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#6
In reply to #2

Re: Minimum Differential Pressure Across Orifice Plate

04/07/2015 12:19 AM

That was a sweet tutorial. Thanks.

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IdeaSmith
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#3

Re: Minimum Differential Pressure Across Orifice Plate

04/05/2015 11:43 PM

Would you mind sharing the operating conditions with us? Kind of could make all the difference.

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a_m_shiha
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#4
In reply to #3

Re: Minimum Differential Pressure Across Orifice Plate

04/06/2015 2:37 AM

Pressure Max.= 98 bar

Temperature Max.=80 C

Density = 20.04 Kg/m3

Flow Max.=23.76 kg/s

Molecular Weight=17
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IdeaSmith
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#5
In reply to #4

Re: Minimum Differential Pressure Across Orifice Plate

04/06/2015 3:56 AM

Aha. I guess max had not changed.

what is the actual change? What are the old or new conditions? Where is max pressure coming from?

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Crabtree
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#7
In reply to #4

Re: Minimum Differential Pressure Across Orifice Plate

04/07/2015 2:37 AM

Next time, try putting that stuff in the original post, Mildred. You'll get a better response faster.

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#11
In reply to #4

Re: Minimum Differential Pressure Across Orifice Plate

06/20/2015 5:09 AM

Why don't you go the whole hog and tell us what the fluid is?

The molecular weight and density don't appear to add up. Presumably that is the actual density (at the temperature and pressure). It's usual to give density at some standard conditions, typically 1 atm, 15°C.

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Codemaster
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#12
In reply to #4

Re: Minimum Differential Pressure Across Orifice Plate

06/21/2015 6:14 AM

From the molecular weight I guess it's ammonia, Is that correct?

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HiTekRedNek
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#8

Re: Minimum Differential Pressure Across Orifice Plate

04/07/2015 10:32 AM

When you understand the basic principles of orifice plate flow metering, variables such as you have encountered will not puzzle you.

I have provided a link to enhance your understanding so that you may confidently answer any challenge presented.

If you know how to perform the calculations from scratch and understand the reasons for the variables, instead of using a calculator, you will be more comfortable and valuable in your job.

Good luck in your new career.

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IdeaSmith
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#9
In reply to #8

Re: Minimum Differential Pressure Across Orifice Plate

04/07/2015 12:03 PM

Link is not working

But if any there is plenty of Orifice calculations available that OP would have used already. No?

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Iris
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#10

Re: Minimum Differential Pressure Across Orifice Plate

04/07/2015 1:17 PM

The OP started a separate thread on turn-down, and some people reading this thread might miss the other thread.

Because turndown is part and parcel of working with sensing instruments, I've posted a representative uncertainty/accuracy graph for a DP flow meter showing percent error (Y) as a function of percent flow rate (X).

As you can see, the uncertainty escalates quite rapidly as the flow rate decreases.
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HiTekRedNek
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#13

Re: Minimum Differential Pressure Across Orifice Plate

06/21/2015 8:39 AM

Here is a link with calculator:

Educate yourself:

http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm

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