An in depth Comparison of Four Gas Measurement Technologies:

Orifice, Turbine,Ultrasonic and Coriolis

The aim of this article is to address and hopefully provide some pointers to assist engineers with flow meter selection within the four major categories, say Trilochan Gupta, Tom Mooney & David Fisher.

Introduction
It is widely accepted that global gas demand is set to double in the next ten years with major new upstream developments together with midstream transportation systems and downstream feed stock projects already underway. As this gas revolution evolves, there will be a dra-matic rise in the requirement for high accuracy measurement at every point in the gas value chain, Figure 1. This value chain can be subdivided into four major categories within which, metering is carried out,
Gas Production
Gas Transmission
Gas Storage
Gas Distribution
Within these categories, there is a huge array of different gas metering applications and a similar number of potential solutions. This can lead to confusion when selecting the optimum solution for the application. Two of the traditional approaches that have been used are orifice plates or turbine meters. Over the last few years, however, newer technologies, in particular ultrasonic and coriolis meters, are being utilised more frequently. Being new technologies, many practitioners are unaware of how they compare with traditional technologies such as orifice and turbine meters. In particular, it can be difficult to know what flowmeter is most appropriate for a particular project, application or specific set of circumstances. The aim of this article is to address this issue and hopefully provide some pointers to assist engineers with flow meter selection within the four major categories listed above.

Orifice Meter
International Standards As a result of its longevity and wide spread usage in the industry, the orifice plate is an extremely well documented and regulated measurement device. There are two main standards for orifice metering; ISO 5167:1991[1] and AGA 3:2000[2]. This article is based around the requirements and guidance of ISO 5167:1991.

Orifice Flowmeter Overview
The orifice flowmeter consists of a thin flat plate sandwiched between flanges or installed in a dedicated fitting. The plate has a precise, sharp edged orifice bored concentric with the pipe axis. The flow pattern contracts as it approaches the orifice, the contraction continuing to a distance of about an orifice diameter downstream. This point of minimum cross section is called the vena contracta. Thereafter, the jet diverges to the full pipe section. A mathematical model, generated from experimental data, of the conditions in the meter stream must be applied in order to calculate the flow. Refining this mathematical model is a continual process. The uncertainty in the flowrate measurement can be predicted in accordance with ISO 5167. There are many ways of locating an orifice plate within a pipeline. These range from a simple orifice flange to a more specialised fitting such as the long standing Daniel Senior FittingÔ, which permits removal of the plate under pressure, Figure 2. It should be noted that other manufacturers offer orifice fittings with the similar design objectives. There are also guidelines as to how the orifice flowmeter should be mounted in the pipeline. As the orifice flowmeter is particularly sensitive to flow profile distortions, care should be taken to ensure fully developed flow. ISO 5167 provides details on meter tube design. Figure 3 provides a representation of the‘catch all’ meter tube. This tube incorporates a 2D straightening vane within the 44D of upstream meter tube. throat or downstream side of the plate, Figure 4. The rate of flow can be determined from the measured value of this pressure difference and from knowledge of the flowing gas as well as the circumstances under which the device is being used. Applying the Bernoulli equation to the flow between the inlet and the vena contracta;

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