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Accuracy and Rangeability of Orifice Metering Systems.The performance of the orifice meter system just like other differentialpressure flow meters depend on the precision of the orifice plate and theaccuracy of the differential pressure sensor. Orifice plate accuracy is

rated in percentage of actual flow rates whereas the differential pressuretransmitters have their accuracy rated in percentage of calibrated span.Due to the fact that flow rate is proportional tothe square root of the differential pressure for an orifice plate, at lowflow rates, the errors in differential pressure measurement leads to largeerrors in the actual flow rates. As a result, orifice metering systems arelimited to aturn down ratio or rangeabilityof 3:1 or 4:1

As shown above, Orifice plate flow meter rangeability can be increased intwo ways:

(a) Operating two or more meter runs in parallel. When the total flowrate is great, all meter runs are placed into service and their respective

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flow rates summed to yield a total flow measurement. When the totalflow rate decreases, individual meter runs are shut off, resulting inincreased flow rates through the remaining meter runs.

(b) Another option is to stack two or more differential pressuretransmitters in parallel onto the same orifice plate element, one for lowflow(say 1-30%), the other for high flow (say 30-100% )of full scaledifferential pressure produced.

-->A combination of the two approaches above is the common practice todayused to improve the rangeability of the orifice plate meter in order toincrease its accuracy. However one drawback of these techniques is thatthey are complex and expensive to implement. To reduce the complexity

of these systems and to further improve accuracy, the use of intelligentor smart differential pressure transmitters is becoming the norm. A smarttransmitter with the ability to switch between two calibrated spanseliminates the need for two differential pressure transmitters across theorifice plate flow element.

Straight Run Pipe RequirementMeasurement errors can be introduced into an Orifice metering systemwhen there is insufficient straight run pipes upstream and downstreamthe orifice plate. The pipe runs are required for laminar flow to beestablished before the flow element is encountered in the flow stream.The amount of straight run pipes required depends on both the beta ratioof the orifice plate installation and on the nature of the upstreamcomponents( like elbows, tees, valves etc) in the metering setup.

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Straight run requirement can be eliminated in the metering setup by

using flow conditioners or straighteners. Flow conditioners adjust the

velocity profile, ideally eliminating or greatly reducing the magnitude of

the flow distortion caused by the upstream piping configuration.

Read more: http://instrumenttoolbox.blogspot.com/2013/04/basics-of-

flow-measurement-with-orifice_11.html#ixzz2UNi3xgly

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