孔板流量计量表测量天然气误差与解决方案

一、孔板流量计测量天然气误差原因

上下游直管段长度不够，气流得不到充分发展，将使计量结果造成较大误差。计量标准规定的zui短直管段长度，在现场实际中一般很难得到满足，特别是由于输气工艺等原因，计量装置的上游往往都存在弯头。如果是单弯头或平面双弯头将使计量结果偏高;对于多个弯头，将使计量结果偏低。

二、解决措施

应该在节流装置之前加装整流器，避免旋转流、涡流对计量的影响。

天然气的气质和气流条件

1、产生误差的原因

GB/T21446-2008《用标准孔板流量计测量天然气流量》标准中规定，通过孔板的天然气是经净化处理后的天然气，气流的流动应是保持亚音速的、稳定的或仅随时间缓慢变化的。气流是均匀单相的牛顿流体。若气体含有质量分数不超过2%的固体或液体微粒，且成均匀分散状态，也可以认为是均匀单相的牛顿流体。气流流经孔板以前，其流束应与管道轴线平行，气流流动应为充分发展紊流且无旋涡，管道横截面所有点上的旋涡角小于2°即认为无旋涡。

通过计量调查发现，目前在油田天然气计量中约50%的计量点其气流条件具有以下两个特点。

（1）气体流量不稳定，并随时间周期性地大幅度变化，属脉动流体。产生脉动源的原因[2]：①往复式压缩机、发动机和叶片式增压机;②调压阀猛开关或者阀芯松动和磨损;③大量水或油的冷凝物在管线中不规则运动。

（2）气质中含水量大，并夹带原油及微量颗粒杂质，气流流经节流装置时产生凝析液。气质、气流条件达不到GB/T21446-2008《用标准孔板流量计测量天然气流量》标准中规定的要求，必将产生计量附加误差。为了获得准确的计量值，必须抑制气流的脉动及消除气质中的水分和杂质。

2、解决措施

进入流量计的气体必须先通过气液分离器、过滤器。

（1）把测量管装在对消除脉动来说较为有利的位置，例如装在调压阀的入口或远离脉动源处。

（2）为了减少脉动幅度，可在脉动源和测量管之间的管线上加装一个容器，或者设计专门的脉动衰减器。把节流阀或限流器装在脉动源和测量管之间是减少脉动的zui有效方法，但是这种方法压力下降过大，除非条件许可，否则不宜采用。

（3）在差压仍然保持较高的操作前提下，可用较小的测量管，而孔板开孔直径基本不变。

（4）在流量计前、后取压管出口处分别加装一个具有一定容积的气液分离器，使进入流量计的差、静压信号达到先气液分离及缓冲后再进入计量装置。

（5）导压管路上的上、下游导压阀应选用大通径的球型阀。

（6）为了防止导压管的冻堵，必须对导压管路进行保温，并增加加热补偿保温措施。

（7）对于因下游波动源而引起计量点气流脉动的情况，可通过调整流量计下游侧阀门的阀位（关小），使计量点气流脉动幅度减小。试验证明，采用关小计量管下游阀的方法来控制气流的脉动，效果是十分明显的，此方法在实际使用时，必须在确保安全生产的情况下进行。

试验证明通过上述方法能有效地抑制气流的脉动、消除气质中的水分及杂质，减少流量计的计量附加误差。

英文版本：

A, orifice plate flow meter in measuring gas error

The upstream and downstream straight pipe length is not enough, the airflow is not fully developed, will make a larger error caused by measurement results. Measurement standard shortest straight pipe length, in the field in general difficult to meet, especially due to gas transmission process, the upstream metering devices often have elbow. If you are single or double elbow bend plane will make the measurement result is high; for more than one elbow, will enable the measurement results.

Two, measures

Should be installed before the rectifier in the throttling device, avoid the influence of rotating flow, vortex on the measurement.

The temperament of gas and air condition

Reason 1, error

Regulation of GB/T21446-2008 "by flowmeter measurement of natural gas flow standard" standard orifice plate, the plate of natural gas is purified natural gas, the flow is subsonic, maintain stable or only slowly varying with time. The airflow is Newton homogeneous single-phase fluid. If the gas containing solid or liquid particles concentration of not more than 2%, and the dispersed state, can also be considered as a homogeneous single-phase Newton fluid. Before the gas flows through the orifice plate, the beam should be parallel with the pipe axis, air flow should be fully developed turbulent flow and vortex vortex free, all points on the tube cross section angle less than 2 degrees that no vortex.

The survey found, about 50% measurement points in Oilfield Metering of natural gas in the air condition has the following two characteristics.

（1） gas flow instability, and with time periodically changes greatly, belonging to pulsating fluid. [2] causes of pulsating source: ① reciprocating compressor, engine and the blade type supercharging machine; the pressure regulating valve with switch or spool loosening and wear; condensate and large amounts of water or oil in the pipeline in irregular motion.

（2） temperament big water content, and some oil and trace impurities, the condensate gas flows through the throttle device. Temperament, air conditions are not up to the GB/T21446-2008 "requirements for flow meter measurement of natural gas flow standard orifice plate" in the standard of measurement, will bring additional error. In order to obtain the exact value of measurement, pulse must restrain the flow and eliminate the temperament in the water and impurities.

2, measures

Gas enters the flowmeter must first through the gas-liquid separator, filter.

（1） the measuring tube is arranged in the elimination of pulsating relatively advantageous position, for example, installed in the entrance pressure regulating valve or away from the pulsating source.

（2） in order to reduce the fluctuation amplitude, a container may be fitted between the pulsating source and the measuring tube lines, or special design pulsation attenuator. The throttle valve or a current limiter is arranged between the pulse source and the measuring tube is the most effective way to reduce the fluctuation of pressure drop, but this method is too large, if conditions permit, or should not be used.

（3） in the differential pressure remains high operation premise, the measuring tube can be smaller, and the hole diameter ratio unchanged.

（4） the pressure at the outlet with a certain volume of gas-liquid separator in front of flowmeter, after entering the flowmeter, differential pressure signal, achieve again into the metering device to gas-liquid separation and buffer.

（5） tube on the downstream pressure guide valve, should choose the type of large diameter ball valve.

（6） in order to prevent the pressure guide tube blockage, must be on the tube for thermal insulation, heat preservation measures and increase the heating compensation.

（7） for the downstream source of fluctuation caused by gas pulsation measurement points, can be adjusted through the flowmeter downstream side of the valve the valve position （Guan Xiao）, the measurement point of the flow pulsation amplitude decreased. Test shows that, by using the method of closing small pulsating metering tube downstream valve to control the flow, the effect is very obvious, this method in actual use, must be carried out to ensure safety in production situation.

Tests show that the pulse, through the above method can effectively suppress the airflow to eliminate moisture and impurities in the reduction of temperament, additional error flowmeter measurement.