Basics of Flow Measurement with the Orifice Flow Meter II ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Basics of Flow Measurement with the Orifice Flow Meter II

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Accuracy and Rangeability of Orifice Metering Systems
The performance of the orifice meter system just like other differential pressure flow meters depend on the precision of the orifice plate and the accuracy of the differential pressure sensor. Orifice plate accuracy is rated in percentage of actual flow rates whereas the differential pressure transmitters have their accuracy rated in percentage of calibrated span. Due to the fact that flow rate is proportional to
the square root of the differential pressure for an orifice plate, at low flow rates, the errors in differential pressure measurement leads to large errors in the actual flow rates. As a result, orifice metering systems are limited to a turn down ratio or rangeability of 3:1 or 4:1

Orifice plate flow meter range-ability can be increased in two ways:
(a) Operating two or more meter runs in parallel. When the total flow rate is great, all meter runs are placed into service and their respective flow rates summed to yield a total flow measurement. When the total flow rate decreases, individual meter runs are shut off, resulting in increased flow rates through the remaining meter runs. The diagram below illustrates this technique:

(b) Another option is to stack two or more differential pressure transmitters in parallel onto the same orifice plate element, one for low flow(say 1-30%), the other for high flow (say 30-100% ) of full scale differential pressure produced. The diagram below illustrates the combination of (a) and (b) technique that is now standard industrial practice:

A combination of the two approaches above is the common practice today used to improve the rangeability of the orifice plate meter in order to increase its accuracy. However one drawback of these techniques is that they are complex and expensive to implement. To reduce the complexity of these systems and to further improve accuracy, the use of intelligent or smart differential pressure transmitters is becoming the norm. A smart transmitter with the ability to switch between two calibrated spans eliminates the need for two differential pressure transmitters across the orifice plate flow element.

Straight Run Pipe Requirement
Measurement errors can be introduced into an Orifice metering system when there is insufficient straight run pipes upstream and downstream the orifice plate. The pipe runs are required for laminar flow to be established before the flow element is encountered in the flow stream. The amount of straight run pipes required depends on both the beta ratio of the orifice plate installation and on the nature of the upstream components( like elbows, tees, valves etc) in the metering setup.

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.

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