This article mainly briefly introduces the causes of inaccurate measurement of orifice plate flowmeters and the corresponding solutions.
What Are the Causes of Inaccurate Measurement by Orifice Plate Flowmeters?
There are several methods for measuring the flow rate of media flowing through pipelines, among which the differential pressure flowmeter is the most widely used and prevalent. It is composed of either a throttling device and a differential pressure gauge, or a combination of a throttling device, a differential pressure transmitter, and a secondary instrument. Throttling devices have a long history of application and have been standardized both internationally and domestically. As the primary element in differential pressure measurement, throttling devices are used to create a pressure difference for the fluid within the pipeline. The pressure difference generated before and after the throttling device is transmitted to the differential pressure transmitter via a pressure guiding tube, and then input to the secondary instrument, which displays the instantaneous flow rate or cumulative flow rate of the liquid in the pipeline. Flow rate adjustment can also be achieved using a regulating instrument.
A standard orifice plate is a thin, circular plate with a precision-machined circular hole. The cylindrical surface of its throttling hole is perpendicular to the upstream end face of the orifice plate, its edge is sharp, and the ratio of the orifice plate’s thickness to its diameter is relatively small. The part of the orifice plate located inside the measuring tube must be circular and coaxial with the axis of the measuring tube, and both end faces of the orifice plate shall remain flat and parallel at all times.
Common Issues
1. Orifice Plate Eccentricity
According to the provisions of GB 2624-81, the orifice plate shall be aligned with the straight pipe section in the throttling device. Experiments show that the measurement error caused by orifice plate eccentricity is generally within 2%. The higher the beta ratio (β) (ratio of the orifice hole diameter to the pipe inner diameter), the greater the impact of eccentricity. Therefore, orifice plates with a high β value should not be used.
2. Orifice Plate Bending
Improper installation or maintenance can cause the orifice plate to bend or deform, leading to significant flow measurement errors. Tests conducted on orifice plates with flange pressure tapping indicate that the maximum error caused by orifice plate bending is approximately 3.5%.
3. Sharpness of the Orifice Plate Edge
Blunting of the orifice plate’s inlet edge due to wear, notches caused by corrosion, welds inside the orifice plate pipeline, or gaskets of measuring flanges will all increase the actual flow coefficient and reduce the differential pressure, resulting in an underestimation of the calculated gas volume.
Measures to Improve Orifice Plate Metering Accuracy
1.Elimination of Pulsating Flow in Gas Stream
Due to sudden changes in gas velocity and pressure in the pipeline, pulsating flow occurs, which can cause fluctuations in differential pressure. The flow rate calculation formula of the throttling device is based on the stable flow of the orifice plate. When there is a pulsation phenomenon at the measuring point, the stability principle does not hold, thus affecting the measurement accuracy and causing metering errors.
The total uncertainty of pulsating flow is equal to the combination of the measurement error calculated according to GB/T2624-93 and the uncertainty added by pulsation.
In the formula: ET - uncertainty added by pulsation, dimensionless; - axial time-averaged velocity, m/s; - root-mean-square value of velocity pulsation component, m/s. (Formula application condition ≤ 0.32)
Therefore, in order to ensure the accuracy of natural gas metering, pulsating flow must be suppressed. Commonly used measures are:
(1) Under the condition of meeting the metering capacity, a measuring tube with a smaller inner diameter should be selected to increase the differential pressure and the diameter ratio;
(2) Use short pressure guiding pipelines, reduce the resistance components in the pipelines, and make the lengths of the upper and lower pipelines equal to reduce the occurrence of resonance and the increase of pressure pulsation amplitude in the system;
(3) Eliminate free liquid from the pipeline. Pulsation caused by liquid accumulation in the pipeline can be handled by using an automatic pipeline cleaning system or installing a liquid separator at a low position.
2. The Design and Installation of Metering Devices Shall Comply with SY/T 6143-1996
The fundamental reason affecting the measurement accuracy of orifice plate flowmeters is whether the geometry and flow dynamics of the throttling device deviate from the design standards. Therefore, during use, regular calibration and maintenance of the system must be done. For changes in working condition parameters such as pressure, temperature, and flow rate in actual use, timely corrections should be made. A fully compensated flow computer calculation scheme can be adopted to reduce metering errors and ensure metering accuracy.
3.Avoid Human Metering Errors
Strengthen metering management, improve the technical quality of operators, and actively introduce and absorb advanced natural gas metering technologies from abroad.
A standard orifice plate is a thin circular perforated plate obtained by mechanical processing. Its throttling hole cylindrical surface is perpendicular to the upstream end face of the orifice plate, its edge is sharp, and the ratio of the orifice plate thickness to the orifice plate diameter is relatively small.
