Introduction

Industrial flow measurements include measuring of flow rate of solids, liquids and gases. There are two basic ways of measuring flow; one on volumetric basis and the other on weight basis. Solid materials are measured in terms of either weight per unit time or mass per unit time. Very rarely solid quantity is measured in terms of volume. Liquids are measured either in volume rate or in weight rate. Gases are normally measured in volume rate. In this chapter, the flow measurements of liquids and gases will be discussed in detail rather than that of solids.

Fluids are classified into two types, namely incompressible and compressible. Fluids in liquid phase are incompressible whereas fluids in gaseous phase are compressible. Liquid occupies the same volume at different pressures whereas gases occupy different volumes at different pressures. This point has to be taken care of while calibrating the flow meters. The measurements taken at actual conditions should be converted either to Standard temperature (0°C) and pressure (760 mm Hg) base (STP base) or to Normal temperature (20°C) and pressure (760 mm Hg) base (NTP base).

Units of Flow

The units used to describe the flow measured can be of several types depending on how the specific process needs the information.

• Solids. Normally expressed in weight rate like Tonnes/hour, Kg/minute etc.
• Liquids. Expressed both in weight rate and in volume rate. Examples:Tonnes/hour, Kg/minute, litres/hour, litres/minute, m3/hour etc.
• Gases. Expressed in volume rate at NTP or STP like Std m3/hour, Nm3/hour etc.
• Steam. Expressed in weight rate like Tonnes/hour, Kg/minutes etc. Steam density at different temperatures and pressures vary. Hence the measurement is converted into weight rate of water which is used to produce steam at the point of measurement.

Measurement of Flow

Flow meter is a device that measures the rate of flow or quantity of a moving fluid in an open or closed conduit. Flow measuring devices are generally classified into four groups.

They are:

1. Mechanical type flow meters. Fixed restriction variable head type flow meters using different sensors like orifice plate, venturi tube, flow nozzle, pitot tube, dall tube, quantity meters like positive displacement meters, mass flow meters etc. fall under mechanical type flow meters.

2. Inferential type flow meters. Variable area flow meters (Rotameters), turbine flow meter, target flow meters etc.

3. Electrical type flow meters. Electromagnetic flow meter, Ultrasonic flow meter, Laser doppler Anemometers etc. fall under electrical type flow meters.

4. Other flow meters. Purge flow regulators, Flow meters for Solids flow measurement Cross-correlation flow meter, Vortex shedding flow meters, flow switches etc.