Causes of measurement errors of orifice flowmeters and methods to improve their accuracy

Orifice flowmeter is a commonly used flow measurement instrument, widely used in process control and measurement in the fields of petroleum, chemical industry, metallurgy, electricity, heating, water supply, etc. Orifice flowmeters may also encounter inaccurate problems in measurement. What is the reason for this? The following editor will specifically introduce the causes of orifice flowmeter measurement errors and methods to improve accuracy, hoping to help everyone.

Causes of inaccurate measurements

1. Orifice plate eccentricity

According to GB2624-81, the orifice plate should be aligned with the straight pipe section in the throttling device. Experiments show that the metering error caused by orifice plate eccentricity is generally within 2%. The higher the aperture ratio β value, the greater the eccentricity effect, and orifice plates with high values ​​should not be used.

2-hole plate bending

Due to improper installation or maintenance, the orifice plate is bent or deformed, resulting in large flow measurement errors. When tested on an orifice plate with flange pressure, the maximum error caused by orifice plate bending is about 3.5%.

3. Sharpness of the plate edge

If the edge of the orifice plate entrance is worn and blunt or notched due to corrosion, or if there are welds inside the orifice plate pipe or metering flange gaskets, the actual flow coefficient will increase and the differential pressure will decrease, resulting in a smaller calculated gas volume.

Measures to improve measurement accuracy

1. Eliminate pulsation in the air flow

The sudden changes in gas flow rate and pressure in the pipeline cause pulsating flow, which can cause fluctuations in differential pressure. The flow calculation formula of the throttling device is based on the stable flow of the orifice plate. When there is pulsation at the measuring point, the stability principle cannot be established, which affects the measurement accuracy and produces metering errors.

The total uncertainty of pulse flow is equal to the synthesis of the measurement error calculated according to GB/T2624-93 and the pulsation additional uncertainty.

Where: ET-pulsation additional uncertainty, 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 include:

(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 aperture ratio;

(2) Use short pressure-inducing pipelines to reduce resistance components in the pipelines and make the lengths of upstream and downstream pipelines equal to reduce resonance and pressure pulsation amplitude increase in the system;

(3) Eliminate free liquid from the pipeline. The pulsation caused by liquid accumulation in the pipeline can be handled by an automatic pipe cleaning system or by installing a liquid distributor at a low location.

2. The design and installation of the metering device shall comply with SY/T 6143-1996

Since the fundamental reason that affects the measurement accuracy of the orifice flowmeter is whether the geometric shape and flow dynamics of the throttling device deviate from the design standard, the system must be regularly calibrated and maintained during use, and changes in operating parameters such as pressure, temperature, flow, etc. in actual use should be corrected in a timely manner. The integration scheme of the fully compensated flow computer can be used to reduce the measurement error and ensure the measurement accuracy.

3. Avoid human measurement errors

Strengthen metering management, improve the technical quality of operators, and actively introduce and absorb advanced foreign natural gas metering technology. The standard orifice plate is a thin plate with circular perforations obtained by machining. Its throttling hole cylindrical surface is perpendicular to the upstream end face of the orifice plate, its edge is sharp, and the ratio of orifice plate thickness to orifice plate diameter is relatively small.