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INTRODUCTION

Description

As its name implies, an orifice meter utilizes an orifice for its basic component in the measurement of natural gas. The placing of an orifice in a pipe in which there is gas flow creates a pressure difference across the orifice. This pressure difference is measured, along with static pressure and other variables, and then translated into flow rate through the use of flow equations.

When natural gas (or any other fluid) flows through an orifice, the stream continues to decrease in cross section for a short distance downstream of the orifice as shown in Figure 3.1.1. The area of the minimum cross section is known as the "vena contracta." Strictly speaking, the area of the vena contracta should be used in the flow equation. However, there is no satisfactory method for measuring this minimum jet area, whereas, measuring the diameter of the orifice and, thus, calculating its area is a relatively simple matter. By experiment, the amount of contraction has been found to depend primarily upon the orifice to mete tube diameter ratio (beta ratio) and the properties of the fluid. With any one fluid, and the same differential pressure, the relative amount of jet contraction increases as beta decreases. This is because as the fluid particles near the wall of the meter tube converge toward the orifice bore, they obtain a greater radial velocity inward when beta is small than when beta is large. Thus, the orifice coefficient will increase for the same size orifice placed in a smaller diameter meter tube. It has become customary to consider a complete orifice meter as being composed of the two major elements, the primary and the secondary element.

The first of the two major elements is the differential pressure producing device called the "primary element."

This primary element is composed of

Meter Tube-A length of special straight pipe upstream and downstream of the orifice through which the fluid flows.

Plate Holder-The orifice plate holding and positioning device (an orifice flange or and orifice fitting) installed as an integral part of the meter tube to hold the orifice plate in a position perpendicular and concentric to the flow of fluid.

Orifice Plate-A flat with a concentrically bored, sharp edged orifice machined to a specific dimension which forms a calibrated restriction to the flow of fluid through the meter tube and causes the pressure drop which results in differential pressure.

Pressure Taps-Precisely sized and located holes through the pipe walls or orifice plate holder, through which the gas pressure on each side of the orifice plate is sensed.

Straightening Vanes-A precisely manufactured device which, if used, must be carefully located in the upstream section of the meter tube to reduce swirling in the gas stream.

The other of the two major elements is termed the "secondary element." It has many names such as the "differential gage," "flow recorder," or more commonly, the "meter." It is the device for measuring the pressure and is connected with pipe or tubing to the upstream and downstream pressure taps of the primary element. One part of the meter indicates or records the difference between the pressures on each side of the orifice plate and the other part records static pressure at one of the two pressure taps, Recording differential and static pressure gages, using charts with printed scales, are extensively used and constitute a record of flow. Other necessary information, such as temperature, relative density and other variables, may be similarly recorded through use of char recorders. Other types of instruments are available for recording flow from simple indicating gauges to elaborate electronic pressure and temperature transducers whose output may be fed directly to a flow computer permitting flow calculations to be performed "real-time," on site.