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Reasons and solutions for errors in measuring natural gas with orifice flowmeters

Author:    Source:    Date: 2021-06-03 14:24:27
1. Reasons for the error of orifice flowmeter measuring natural gas
   The length of the upstream and downstream straight pipes is not enough, and the air flow cannot be fully developed, which brings great errors to the measurement results. The length of the shortest straight pipe section specified by the measurement standard is generally difficult to meet on site, especially due to the gas supply process and other reasons, there are often elbows upstream of the measurement device. In the case of single bend or flat double bend, the measurement result will become higher. When the pipe is bent, the measurement result is lower.

   2. Solution

   A rectifier should be installed before the throttling device to avoid the influence of swirling and eddy currents on the measurement.

   Natural gas quality and gas flow conditions

  1, the cause of the error

   GB/T21446-2008 "Measuring Natural Gas Flow with Standard Orifice Flowmeter" stipulates that the natural gas passing through the orifice is purified natural gas, and the gas flow maintains subsonic speed, stable or slowly changing with time. The airflow is a uniform single-phase Newtonian fluid. As long as the gas contains solid or liquid particles with a mass fraction of 2% or less and is in a uniformly dispersed state, it can also be considered as a uniform single-phase Newtonian fluid. Before the airflow passes through the orifice, its flux must be parallel to the axis of the pipe. The airflow should fully develop turbulence and no turbulence. It is considered that if the vortex angle at all points of the tube section is less than 2, there will be no vortex. The measurement survey shows that the current gas flow conditions at about 50% of the measurement points in the oil field natural gas measurement have the following two characteristics.

  (1) The gas flow rate is unstable and changes greatly with time. It is a pulsating fluid. Causes of pulsation sources (2): Reciprocating compressors, engines and vane-type superchargers; sudden opening and closing of pressure reducing valves or loose and worn valve bodies. A large amount of water or oil condensate works irregularly in the pipeline.

  ) The gas has a large water content, which will involve impurities such as crude oil and trace particles, and coagulation liquid will be generated when the airflow passes through the throttling device. If the temperament and air flow conditions do not meet the requirements specified in the GB/T21446-2008 "Measuring Natural Gas Flow with Standard Orifice Flowmeters", measurement errors will occur. In order to obtain the correct measurement value, it is necessary to suppress the pulsation of the airflow and remove the moisture and impurities in the temperament.

   2. Solution

  1) Install the measuring tube at the inlet of the pressure reducing valve and the place far away from the source of pulsation, etc., where it is helpful to eliminate the pulsation.

  2) In order to reduce the pulsation amplitude, a container can be installed on the line between the pulsation source and the measuring tube, or a special pulsation attenuator can be designed. Installing a throttle valve or restrictor between the pulsation source and the measuring tube is the most effective way to reduce the pulsation, but this method should not be used when the conditions permit because the pressure drop is too large.

  (3) Under the operating premise that the pressure difference is still high, a smaller measuring tube can be used, and the opening diameter of the orifice is almost unchanged.

  4) A gas-liquid separator with a certain volume is installed at the outlets of the pressure pipes before and after the flowmeter, so that the differential and static pressure signals entering the flowmeter are separated and buffered by the gas and liquid before entering the measuring device. )5) Use large-diameter ball valves for the pressure pilot valves upstream and downstream of the pressure pilot pipeline.

  ) 6) In order to prevent the pressure guiding pipe from freezing and clogging, the pressure guiding pipe needs to be insulated, and heating compensation measures should be added.

  7) When the downstream pulsation source causes the airflow pulsation at the metering point, by adjusting (closing) the valve position of the downstream valve of the flowmeter, the airflow pulsation amplitude at the metering point can be reduced. Experiments have proved that the method of closing the downstream valve of the small metering tube has a significant effect on controlling air flow pulsation. In actual use, this method must ensure safe production.

  Through the above method, it is proved that the air flow pulsation can be effectively suppressed, the moisture and impurities in the temperament can be removed, and the additional measurement error of the flowmeter can be reduced.

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