LOW PRESSURE GAS MEASUREMENT Class# 1951.1

摘要

Accurate natural gas measurement is challenging for many reasons and uncertainty can be introduced from a multitude of ways.Much has been written over the years about how these challenges may reside in the physical design of a measurement station or in meter selection.Uncertainty also can arrive in choices made with regard to which secondary or tertiary devices to use and even choices made on standard pipeline fittings can affect overall measurement accuracy in some applications such as with torturous tube turns or various combinations of meter tube end treatments.Another way in which un-intentional introduction of uncertainty can be found is with the improper calibration of secondary devices,improper use of calibration equipment where the span of said device is too large or with improper sampling techniques.Sampling is important because the flow calculations for natural gas are dependent on relative density and compressibility which if misrepresented can cause uncertainty.With all things being equal; difficulties also may be experienced in achieving accurate measurement because of the environment given by the operating conditions under which gas measurement occurs.Gas measurement at low pressures specifically can create an environment where any or all of the examples from above can culminate and represent a worst case scenario for errors in overall gas measurement accuracy.This paper discusses those issues which are focused on low pressure gas measurement as it relates to various meter types,procedures for the use of measurement calibration equipment,choices that are made for secondary devices and sampling techniques.The meter types discussed will be Ultrasonic(USM),Turbine,V-cone/Orifice,and Coriolis meters and further discussion will be on the impact of improper techniques for sampling,calibration and proper ranging of secondary devices as well as improper selection of the span and accuracy of hand held calibration equipment.