The Flow-Measuring Apparatus is designed to accustom students to typical methods of measuring the discharge of an essentially incompressible fluid, whilst at the same time giving applications of the Steady-Flow Energy Equation (Bernoulli's Equation). The discharge is determined using a venturi meter, an orifice plate meter and a rotameter. Head losses associated with each meter are determined and compared as well as those arising in a rapid enlargement and a 90-degree elbow. The unit is designed for use with the TecQuipment HI Hydraulic Bench, which provides the necessary liquid service and gravimetric evaluation of flow rate. 1.1 Description of Apparatus
Figure 1. Flow measuring apparatus.
The TecQuipment HI0 Flow-Measuring Apparatus is shown Figure 1. Water from the HI Hydraulic Bench enters the equipment through a Perspex venturi meter, which consists of a gradually-converging section, followed by a throat, and a long gradually-diverging section. After a change in cross- section through a rapidly diverging section, the flow continues along a settling length and through an orifice plate meter. This is manufactured in accordance with BSI042, from a plate with a hole of reduced diameter through which the fluid flows.
Following a further settling length and a right-angled bend, the flow enters the rotameter. This consists of a transparent tube in which a float takes up an equilibrium position. The position of this float is a measure of the flow rate.
After the rotameter the water returns via a control valve to the Hydraulic Bench and the weigh tank. The equipment has nine pressure tappings as detailed in Figure 2 each of which is connected to its own manometer for immediate read out.
Figure2. Explanatory diagram of a flow measuring apparatus.
1.2 Installation and Preparation
a) To fit the rotameter float, remove the control valve support nut, top 'U' clamp assembly and bottom pipe clips. Position the float and reassemble. a) Connect the supply hose from the Hydraulic Bench to the inlet of the venturi meter and secure with a hose clip. Connect a hose to the control valve outlet and direct its free end into the central hole in the Bench. Before continuing, refer to the TecQuipment HI Hydraulic Bench manual to find the method of flow evaluation by weighing. a) With the air purge-valve closed, close the apparatus valve fully then open it by about 1/3. Switch on the bench and slowly open its valve until the water starts to flow, allow the apparatus to fill with water then continue to open the bench valve until it is fully open. Close the apparatus valve fully. Couple the hand pump to the purge valve and pump down until all the manometers read approximately 280mrn. Dislodge entrained air from the manometers by gentle tapping with the fingers. Check that the water levels are constant. A steady rise in levels will be seen if the purge valve is leaking. a) Check that the tube ferrules and the top manifold are free from water blockage, which will suppress the manometer level. Ferrules blockage can be cleared by a sharp burst of pressure from the hand pump.
1.3 Routine Care and Maintenance
a) When not in use do not allow water to stand in the apparatus for long periods. After use fully drain the apparatus and dry externally with a lint- free cloth. a) If the control valve shows signs of leaking, the procedure for checking and inspecting is as detailed in the TecQuipment HI Hydraulic Bench manual. a) If plastic manometer tubes become discolored a stain and deposit remover is available.
Figure 3. The steady-flow energy equation
For steady, adiabatic flow of an incompressible fluid along a stream tube, as shown in Figure 3, Bernoulli's equation can be written in the form;
P1ρg+V122g+z1=P2ρg+V222g+z2+∆H12 (1) Where Pρg is termed the hydrostatic head.