Introduction

In this chapter, we are concerned with devices defined in international standard documents. Among these, the Venturi meter is one of the oldest industrial methods of measuring flow, although other methods may be more common. Chapter 8 deals with other devices such as variable area meters, target meters, averaging pitot tubes, Dall meters, V-cone meters and other Venturi, nozzle and orifice-like devices.

As in the case of the orifice plate, the reader should have access to a copy of the latest version of the ISO standard (5167). I have refrained from repeating information that should be obtained from the standard, and include only the minimum information to provide an understanding of the discharge coefficients, the likely uncertainty of the various devices and the type of material from which they may be constructed. There is a growing interest in the use of some of these devices for difficult metering tasks and a re-evaluation of their behaviour. Recent publications are briefly reviewed in Appendix 6.A.

Spink (1978) suggested that a standard orifice should be used unless:

1. • The velocity is such as to require the value of β to exceed 0.75: if higher differential pressure or larger pipe is not possible, he suggested considering a flow nozzle.

2. • The fluid contains a second phase of some sort – particulate, bubble or droplet – in suspension: either a Venturi or a flow nozzle with vertically downward flow, an eccentric or segmental orifice, or a different type of device such as a target meter or electromagnetic flowmeter should be considered.

3. • The fluid is very viscous or results in low Reynolds number flows: the meters already mentioned may be suitable, or a quadrant or semi-circular orifice may be worth considering.

4. • The flow is very low and the fluid very clean: an integral orifice or a small calibrated meter run may be appropriate.

5. • Pumping costs are a major consideration: in which case go to a lower-loss device.

If a liquid or a gas flows down a pipe of decreasing cross-section, the velocity (provided, in the case of a gas, that it is below the speed of sound) increases as the area decreases, and the pressure also decreases with the increase in velocity. This behaviour allows the measurement of the change in pressure as a means of obtaining the flow rate in the duct.