fluid flow
The measurement of fluid flow is of great importance in many industrial processes, some examples including air flow in the ventilating ducts of a coal mine, the flow rate of water in a condenser at a power station, the flow rate of liquids in chemical processes, the control and monitoring of the fuel, lubricating and cooling fluids of ships and aircraft engines, and so on. Fluid flow is one of the most difficult of industrial measurements to carry out, since flow be haviour depends on a great many variables concerning the physical properties of a fluid. There are available a large number of fluid flow measuring instruments generally called flow meters, which can measure the flow rate of liquids (in m3/s) or the mass flow rate of gaseous fluids (in kg/s). The two main categories of flow meters are differential pressure flow meters and mechanical flow meters. This chapter also contains calculations on Bernoulli’s equation and the impact of a jet on a stationary plate.
At the end of this chapter you should be able to:
• appreciate the importance of measurement of
fluid flow
• describe the construction, principle of operation, advantages and disadvantages, and practical applications of orifice plates, Venturi tubes, flow nozzles and Pitot-static tube and describe the principle of operation of deflecting vane and turbine type flow meters
• describe the construction, principle of operation, advantages and disadvantages, and practical applications of float and tapered tube flowmeters, electromagnetic flow meters, and hot-wire anemometer
• select the most appropriate flow meter for a particular application
• state the continuity equation (i.e. the principle of conservation of mass)
• state and perform calculations on Bernoulli’s equation
• state and perform calculations on the impact of a jet on a stationary plate
Differential pressure flow meters
When certain flow meters are installed in pipelines they often cause an obstruction to the fluid flowing in the pipe by reducing the cross-sectional area of the pipeline. This causes a change in the velocity of the fluid, with a related change in pressure. Figure 23.1 shows a section through a pipeline into which a flow meter has been inserted. The flow rate of the fluid may be determined from a measurement of the difference between the pressures on the walls of the pipe at specified distances upstream and downstream of the flow meter. Such devices are known as differential pressure flow meters.
The pressure difference in Figure 23.1 is measured using a manometer connected to appropriate pressure tapping points. The pressure is seen to be greater up- stream of the flow meter than downstream, the pressure difference being shown as h.
Calibration of the manometer depends on the shape of the obstruction, the positions of the pressure tapping points and the physical properties of the fluid.
In industrial applications the pressure difference is detected by a differential pressure cell, the output from which is either an amplified pressure signal or an electrical signal.
Examples of differential pressure flow meters commonly used include:
(a) Orifice plate (see Section 23.2)
(b) Venturi tube (see Section 23.3)
(c) Flow nozzles (see Section 23.4)
(d) Pitot-static tube (see Section 23.5)
British Standard reference BS 1042: Part 1: 1964 and Part 2A: 1973 ‘Methods for the measurement of fluid flow in pipes’ gives specifications for measurement, manufacture, tolerances, accuracy, sizes, choice, and so on, of differential flow meters.