Use of Vortex Flowmeters for Gas Measurement
Authors: Scott Phillip G. , Webster Andrew T. 
Published: December 2007

There is an increasing need for reliable and accurate gas flow measurement. The principle of vortex shedding (first studied in the early 1900's) has been incorporated into a flow measuring device. The vortex flowmeter compares favorably with other types of flowmeters used for gas measuring because of its accuracy and simplicity of construction.

Introduction

The industrial world has an increasing need for accurate gas flow measurement - for the process engineer, it is valuable for material and energy balance to improve plant efficiency; for the production manager, it provides accurate accounting production manager, it provides accurate accounting for the operation of a facility.

Gas that would have been flared before is being gathered and processed or used as fuel for manufacturing plants now. In addition to the accuracy requirement, there is a need for reliability and the ability of a flowmeter to withstand the harsh environment that may accompany shoreside, onshore, and offshore installations. To be most effective, a flowmeter must be designed for available maintenance expertise and resources.

Background

Traditionally, the gas industry has used head-type flowmeters (e.g., orifice plates, nozzles, venturi tubes, pitot tubes, etc.), velocity-type flowmeters such as the turbine flowmeter, displacement-type flowmeters such as the diaphragm flowmeter, and variable-area flowmeters to measure and control the flow of gas. All these types of flowmeters have major drawbacks in the measurement and control of gas flows.

Head-type flowmeter design is based on the fact that differential pressure or velocity pressure caused by the flowing gas is related to the square of the flow rate. This limits the range of flow rates for which a given head-type flowmeter can measure the flow of gas accurately. Maintenance of the head type, although it is a rugged and simple device, can be very time consuming and require skilled employees with detailed knowledge of the flowing process. The primary device (placed directly in the flow line) must primary device (placed directly in the flow line) must be kept clean, and all critical surfaces kept in proper order; the impulse lines (differential-pressure transmission lines) must be kept clean, leak-tight, and free from extraneous materials; and the differential pressure device, integrator, and recorder must be maintained in good working order. All this hardware must be mounted in the facility such that all the components work in conjunction with one another. If any one fails, the flow measurement becomes unknown.

Velocity- and displacement-type flowmeters, although highly accurate with a high flow rangeability, are not rugged and have moving parts inside the flow stream that must be kept in working order. Again, skilled employees are required to maintain these types of flow measuring devices.

The vortex flowmeter has been designed to take advantage of the positive features of the head-, velocity-, and displacement-type flowmeters. The vortex flowmeter is a rugged device that can be designed without moving parts. It can be of one-piece construction that requires no impulse lines or special pressure taps into the pipelines to measure pressure taps into the pipelines to measure differential pressure. It is a highly accurate device with a large flow rangeability. The principle of vortex shedding has been used for flow measuring for more than 10 years.

JPT

P. 2082