Gas is a kind of gas and is the most important fuel for urban residents and many enterprises and institutions. Some are also used as raw materials for chemical products.

There are a large number of gas meters in all aspects of gas production, transportation and distribution, some are used for general monitoring, some are used for trade settlement, among which the measurement system for trade settlement, the national standard GB/T17167 stipulates its accuracy requirements.

(1) Characteristics of gas flow measurement

a. The static pressure of the fluid is low, the flow rate is low, and the pressure loss is small, and it is generally not allowed to increase the flow rate by reducing the pipe diameter.

b. The fluid humidity is high, and some measurement objects also carry a small amount of water, which flows in layers at the bottom of the pipe.

c. Some measurement objects have high hydrogen content and low fluid density, and the signal is weak when measured with a vortex flow meter.

d. The gas produced by gas generators, coke ovens, etc. generally contains tar and other viscous substances, and some also carry a certain amount of dust.

e. When the measurement point is located at the outlet of the compressor, there is a certain flow arterial motion.

f. The fluid is a flammable and explosive medium, and the instrument has explosion-proof requirements.

g. Various pipe diameters are available from small to large.

h. The difference between the minimum flow rate and the maximum flow rate is very large.

i. The system used for trade settlement requires high measurement accuracy; as a general monitoring and process control system, the accuracy requirements are lower.

(2) The main contents of national standards 

 In 2000, the State Administration of Quality and Technical Supervision issued the first part of GB/T18215.1 "Flow Measurement of Main Pipelines of Urban Artificial Gas", which uses the method of standard orifice throttling device to stipulate the relevant technical issues in gas flow measurement, including:

(1) Requirements for fluids: "It should be a uniform and single-phase (or can be considered single-phase) fluid".

(2) The gas is washed during the purification process, so the general moisture content is saturated, and the relative humidity is 100%.

(3) The accuracy of the measurement system used for trade settlement should generally be better than level 2.5. The basic margin of error is expressed as a percentage of the value.

(4) Gas flow is defined as the dry part of the wet gas.

(5) The measurement result is expressed in volumetric flow and converted to the standard state. In addition to the general definition of 101.325kPa and 20°C, the definition of the standard state also takes into account the tradition of the gas industry, and can also take other temperature, pressure and humidity negotiated by the supply and demand sides.

(6) The throttling device adopts the form of multi-pipe parallel connection.

(7) In the presence of flow arterial motion, the following measures are proposed for measuring the average number.

a. Adopt attenuation measures on the pipeline and install filters (composed of containers and pipe resistors).

b. Instrument testing parts should be kept away from pulsating sources as much as possible.

c. Use the largest β and Δp to reduce the pipe diameter at the measurement point.

d. Pipeline and instrument bracket are firmly installed.

e. The resistance of the two differential pressure primer pipes is symmetrical.

3) Types and uses of instruments 

There are many types of flow meters that can be used to measure gas, but the ideal meter for measuring gas flow is almost impossible to find, which is mainly determined by the characteristics of gas. Due to the presence of viscous substances such as tar, it is difficult to use rotary flow meters. Due to the small density and low flow rate, vortex flow meters are difficult to use. Due to the abundance of water vapor and the variation in gas composition, thermal flow meters are not ideal. In the end, the differential pressure flowmeter, which has been used for decades, plays the leading role.

(1) Orifice differential pressure flow meter

a. Interchangeable orifice plate throttle device.

The international GB/T18215-2000 stipulates standard orifice plates. If the measuring point fluid is dirty, an interchangeable orifice throttle device is required. In this way, under the condition of non-stop air, the throttle parts can be cleaned, repaired and replaced. The typical structure of the interchangeable plate throttle device is shown in Fig. 3.18.

b. Round missing hole plate.

Round notch plates are special orifice plates designed for the measurement of dirty fluid flow. The opening is part of a circle (circle missing part) with a diameter of 98% of the inner diameter of the pipe. The arced part of the opening should be precisely positioned so that it is concentric with the pipe.

(2) Differential pressure flow meter of equal velocity tube.

Standard orifice differential pressure flow meters play an extremely important role in gas flow measurement and have a long history of use. As long as the design, processing, installation, inspection and use according to the standard can achieve the specified accuracy, so it is very convenient and widely used. However, when the pipe diameter is large, the price of a set of interchangeable plate throttling device is quite considerable, so if the measurement data is only used for process monitoring and the accuracy is not high, then the average velocity tube differential pressure flow meter can be selected.

The key to the success or failure of the application of the average velocity tube differential pressure flow meter in gas flow measurement is that the pressure pipe should not be blocked by water droplets. Because most of the shut-off valves with the shaped average speed pipe products are needle valves, the diameter is small and the water vapor in the fluid is condensed into water droplets, if the needle valve is not handled well or the slope of the pressure pipe is not reasonable, this droplet is very easy to seal the passage.

The differential pressure signal output by the differential pressure equalization tube is generally very small. When the fluid is air at room temperature and pressure, if the flow rate is 10 m/s, it can only reach a differential pressure of 62.5 Pa [28]. In this way, a drop of water seals the differential pressure transmission channel, which is enough to cancel out all of this differential pressure. Some manufacturers change the positive and negative pressure shut-off valves to through gate valves with larger diameters, which creates conditions for ensuring the reliable use of the instrument.