I want to measure a fluid flow whose range-ability is 20:1 (max flow:min flow = 20:1) using ORIFICE. How many dP tx can i use for the measurement?

 

It's a while since I have seen two DP transmitters on one Orifice Plate, in the days of 4-20 mA transmitters they sort of made sense. If it's Foundation Fieldbus the accuracy is probably sufficient with just one since the signal is transmitted in engineering units.

Is there not a better way to measure the flow like Magnetic flowmeter?

>I want to measure a fluid flow whose range-ability is 20:1
>(max flow:min flow = 20:1) using ORIFICE. How many dP tx can
>i use for the measurement?

 

Emerson claims a 0.5% accuracy at 14:1 turndown

http://euedocs.emersonprocess.co.uk/groups/public/documents/markcom/500754.pdf

and I suspect the other majors are pretty close, if not equivalent (paper is 10 years old).

I have seen most people doing this with DP go to some flow element other than an orifice plate - Accelabor/Verabar or a V-cone.

So, I'd probably say 2 DPs would do it.

Here's some links on stacked DPs:

Dual (stacked) DP Flow Transmitters, thread 698-205770
http://www.eng-tips.com/viewthread.cfm?qid=205770

Orifice Plate Flow Meters - Measurable Range , thread 378-241081
http://www.eng-tips.com/viewthread.cfm?qid=241081

Orifice Rangeability, Liptak in Controlglobal
http://www.controlglobal.com/articles/2010/orificerangeability1002/

 

Hi Chirag,

>I want to measure a fluid flow whose range-ability is 20:1
>(max flow:min flow = 20:1) using ORIFICE. How many dP tx can
>i use for the measurement?

I'm a bit rusty with this stuff, so others may have more up to date information. Firstly are you asking for advice on actual makes and models of DP transmitter or whether to use two or more transmitters to measure this flow range?

If you are wanting to use only a single conventional DP transmitter to cover a 20:1 flow range through an orifice plate then: <b>I wouldn't!</b>

When I was doing this stuff regularly. we wouldn't use an orifice plate/DP transmitter for any flow range larger than 10:1, preferably less. For larger turn-down ratios, we'd use linear measurement devices such as vortex, magflow etc.

As you are probably aware, flow through an orifice plate is proportional to the square root of differential pressure developed across the plate. Your minimum flow will be 5% (1/20th or 0.05) of the maximum. At 5% flow (0.05), the DP will be 0.05^2 = 0.0025 or just 0.25% of the transmitter span. For a 4-20 mA transmitter, the transmitter output will be just 4.04 mA - not much and likely to be 'lost' in process and electrical noise, zero drift etc.

Perhaps it may be possible using a smart transmitter with square-root extraction in the TX and digital transmission. Others on here will have better information on this approach.

It would also be possible to use two transmitters, one ranged to the maximum flow and a second ranged to, say 20% of maximum flow. At flow rates from 20 to 100% the first TX is used. Below 20% of max. flow the second TX is used and with 10:1 turndown on this TX, the minimum usable flow would be about 1/10 * 20% - 2% of max. flow = 50:1 turndown overall.