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The relation between Orifice TurnDown Ratio and D/P cell accuracy
Continuous process industries, DCS questions. topic
Posted by vahid on 16 November, 2010 - 5:19 am
I know that the turndown ration of the Orifice is about 3:1.
Suppose that the orifice is sized for 1000m3/h., then the rate that could sensed accurately would be 1000 to 3000, Am I correct?

Second the accuracy of a D/P cell is expressed in term of Calibrated span or Full range, then what is the minimum rate or Differential pressure that could be measured?

I would thank you for your help.

Regards,
Vahid


Posted by John Catch on 16 November, 2010 - 1:00 pm
Looking at the issue in simplified terms;

One of the great myths is that an orifice plate itself has a turn down ratio. For a plate that has a beta ratio in the acceptable region (typically 0.2 to 0.75), its discharge coefficient is relatively constant, varying generally about +/-4% or less over the complete Reynolds number (flow) range. Application constraints also need to be considered, but in general the plate is not the problem.

What does have a turn down ratio is the DP transmitter. However, again this is often incorrectly stated. The 3:1 rule comes from the old mechanical, pneumatic, and early analog DP devices which had poor relative accuracy. Today, the principal factor in determining transmitter turn down is the acceptable accuracy for the application. With SMART transmitters (typically 0.1% FS or better), far greater than 3:1 is available. For example a SMART 100" WC DP transmitter can operate relatively accurately even at less than 2" WC. If the application can accept 5% uncertainty at the lowest flow, then one might accept this situation. On the other hand, the turn down ratio will decrease as higher accuracy requirements dictate.

The manufacturer of the transmitter can give you the generally accepted minimum DP value vs. accuracy for a given transmitter's range. From that value you can determine whether it is suitable for your needs.

John Catch
jbc [at] inflowinc.com


Posted by vahid on 18 November, 2010 - 6:30 am
Mr. Catch,

Thank you for your concern.

I know that the smart transmitters do cover a wide range such as 100:1 or even more)

I found an explanation regarding to the "Accuracy & Rangeability" of
Diff flowmeters, (Ref: Transactions in Measurement and Control, Vol 4, page 17). What I found is that: "the limitation of the D/P cell accuracy in low ranges would cause the orifice rangeability (turndown ration) to be limited 3:1 or 4:1"

I have involved in a practical problem to read a rate with an orifice, the transmitter does have a good quality and Temperature and pressure are also connected to the flow computer, in low rates our engineer process claims that the low rate does have error. But in moderate rates the reading is accepted.

A solution is to use two D/P cells in parallel, but I would be thankful if you could help me to understand what is stated in the above mentioned paper.

Thank you for your concern.


Posted by dave on 18 November, 2010 - 9:48 pm
say you have a 100:1 turndown, but the accuracy in that range is perfect. In that case you have a flow turndown of 10:1.

Now, if the measurement error is 1% of calibrated transmitter scale, then the d/p measurement error at the lowest turndown is 100% and the flow error equally large. Using split ranged transmitters helps to reduce this turndown error when the transmitter has a finite measurement error.

You also have reynold number corrections to the primary measurement, as well as fluid property corrections to consider.

In sorting out your metering errors, also check to make sure that your square root extraction is being done either in the transmitter or the receiving instrument, but not both!

good luck,


Posted by dave on 16 November, 2010 - 3:59 pm
the turn down you quote is based on the d/p transmitter turn down when they were pneumatic.

For modern transmitters, the d/p error of the transmitter governs as long as the impulse lines are properly installed.

The turn down of an orifice plate depends on the Reynold's number and the variation of the discharge coefficient.


Posted by Rohit Chandak on 20 November, 2010 - 2:56 am
There is a flow element (Primary Device) having its accuracy & turndown specified while there is a DP Transmitter (Secondary Device) having its accuracy & turndown specified. The complete system consists of Primary Device & Secondary Device, once has to calculate the flow meter accuracy based on inaccuracies associated with both this devices. Primary device is very important hence actual rate devices should be purchased over full scale devices so that you can maintain a stable accuracy over the entire flow turndown.

If secondary device is full scale than there is a impact on overall accuracy of the system but the transmitter accuracies this days are so good that the impact on the system accuracy is not much. Proper selection of transmitter range is most important hence select the transmitter range which is closest to DP produced at maximum operating range or full scale range. This will ensure a better turndown on the application. The accuracy would be highest at the maximum flow & will decrease as flow rate/velocity decreases.


Posted by VAHID on 15 December, 2010 - 10:20 am
Refer to Mr. Chandak reply:

May I ask you to explain more about the statement "The accuracy would be highest at the maximum flow & will decrease as flow rate/velocity decreases."?

Thank you for your concern.


Posted by David on 15 December, 2010 - 1:37 pm
One factor, among several is the uncertainty (accuracy or more appropriately inaccuracy) of a DP cell, which is always defined as a percent of full span, not a percent of reading. The closer the reading is to full span, the less percentage the uncertainty percentage is of the reading

example:
uncertainty/accuracy spec (arbitrary for purposes of discussion): 0.1% uncertainty full scale

range: 0 - 250 mm w.c.
0.1% of 250 mm = 0.25mm w.c.

The percentage influence of a 0.25mm w.c. uncertainty at 10 mm w.c. measurement reading is greater than the same uncertainty on a 250mm w.c. reading.


Posted by anynomous on 19 December, 2010 - 8:50 pm
Dear Vahid

We feel op calculation standard , inherent inaccuracy permitted/laid is about 2% & hence xtrs accuracy have no significance here probably.

Turn down ratio mainly connected with Reynolds No & type of region [Laminar, Transition, Turbulent region -----> K constant variation, curve.You do not work with multiple Flow coefficient for calculation. It also works on pressure loss & pressure recovery[re-establish the flow pattern].

It is best practice to have wetted calibration of op with transmitter so that you can work on proper pattern.

The ideal example , which was followed for 1:3 turn down ration was followed for this concept in 3 element drum level control , control loop was switched to single element drum level , by-passing feedforward signal.

We are using op for the first time & we are using it for Reynolds No < 2000 , Where we have opted for conical entrance orifice plate assembly, we have yet to do wetted calibration for the same, we would be able to give feedback in the month of March 2011.

Always associate turn down ration in Multivariable concept wrt other associated variable of unit operation.

Regards
anynomous

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