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Tutorial on flow metering (Part 1)

17 Jul 2012  | Mohit Arora, Prashant Bhargava

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At higher velocities or low viscosities the flow breaks up into turbulent where the majority of flow through the pipe has the same average velocity. In the "turbulent" flow the fluid viscosity is less significant and the velocity profile takes on a much more uniform shape. Turbulent flow is represented by Reynolds numbers above 4,000. Between Reynolds number values of 2,000 and 4,000, the flow is said to be in transition.

So Reynolds (Re) number is a quantity that engineers use to estimate if a fluid flow is laminar or turbulent. This is important because increased mixing and shearing occur in turbulent flow that results in increased viscous losses, which affects the efficiency of hydraulic machines. A good example of laminar and turbulent flow is the rising smoke from a cigarette. The smoke initially travels in smooth, straight lines (laminar flow) then starts to "wave" back and forth (transition flow) and finally seems to randomly mix (turbulent flow).

Types of flow meters
A flow meter is an instrument used to measure linear, nonlinear, mass or volumetric flow rate of a liquid or a gas. There are many ways to measure flow of a liquid, gas or steam; and every type of flow meter can be assigned to one of two categories: traditional or mechanical flow meter, and new technology or electronic flow meter.

The following list shows some of the popular flow meter types:

Mechanical flowmeters:

 • Positive Displacement
 • Paddle Wheel
 • Variable area
 • Differential Pressure
 • Turbine
 • Open channel
New technology flowmeters:

 • Coriolis
 • Magnetic
 • Pulse based
 • Vortex
 • Ultrasonic
Traditional flow meters were introduced many years ago, and their performance characteristics, such as accuracy, are lower than their newer counterparts and require greater maintenance. For instance, the orifice plates in differential pressure meters are subject to wear and can be knocked out of position by impurities in the flow stream. The various technologies represented in this group have been slow to incorporate recent advances in communication protocols such as HART, Foundation Fieldbus, and Profibus.

New technology or electronic flow meters have the advantage of microprocessor based technology. Their construction and operating principles avoid some of the problems inherent in the traditional meter types and they generally out perform their precursors, particularly in their accuracy levels of ±1.0% or better.

In this part of the series we are going to cover some of the popular mechanical flow meters to make readers aware of the ongoing challenges with mechanical meters and later we willcover the details of the more popular microprocessor, pulse based as well as ultrasonic flow meters in Parts 2 and 3 of this series.

Mechanical flow meters

a) Positive displacement flow meter
Positive-displacement (PD) meters mechanically make direct measurements of the fluid by separating the fluid into segments of known values (figure 1). Fluid goes through a chamber with a unit that repeatedly fills and discharges a fixed volume.

Figure 1: Positive Displacement Flowmeter for petroleum production. (Reference: Atchafalaya Measurement Inc.)

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