14-07-2012, 11:25 AM
Flow Measurement
L-07 Flow Measurement.pdf (Size: 249.17 KB / Downloads: 189)
Introduction
Accurate measurement of flow rate of liquids and gases is an essential requirement for maintaining the quality of industrial processes. In fact, most of the industrial control loops control the flow rates of incoming liquids or gases in order to achieve the control objective. As a result, accurate measurement of flow rate is very important.
Needless to say that there could be diverse requirements of flow measurement, depending upon the situation. It could be volumetric or mass flow rate, the medium could be gas or liquid, the measurement could be intrusive or nonintrusive, and so on. As a result there are different types of flow measuring techniques that are used in industries. The common types of flowmeters that find industrial applications can be listed as below:
(a) Obstruction type (differential pressure or variable area) (b) Inferential (turbine type), © Electromagnetic, (d) Positive displacement (integrating), (e) fluid dynamic (vortex shedding), (f) Anemometer, (g) ultrasonic and (h) Mass flowmeter (Coriolis).
In this lesson, we would learn about the construction and principle of operation few types of flowmeters.
Obstruction type flowmeter
Obstruction or head type flowmeters are of two types: differential pressure type and variable area type. Orifice meter, Venturimeter, Pitot tube fall under the first category, while rotameter is of the second category. In all the cases, an obstruction is created in the flow passage and the pressure drop across the obstruction is related with the flow rate.
Basic Principle
It is well know that flow can be of two types: viscous and turbulent. Whether a flow is viscous or turbulent can be decided by the Reynold’s number RD. If RD > 2000, the flow is turbulent. In the present case we will assume that the flow is turbulent, that is the normal case for practical situations. We consider the fluid flow through a closed channel of variable cross section, as
Version 2 EE IIT, Kharagpur 3
The channel is of varying cross section and we consider two cross sections of the channel, 1 and 2. Let the pressure, velocity, cross sectional area and height above the datum be expressed as p1, v1, A1 and z1 for section 1 and the corresponding values for section 2 be p2, v2, A2 and z2 respectively. We also assume that the fluid flowing is incompressible.
Orifice meter
Depending on the type of obstruction, we can have different types of flow meters. Most common among them is the orifice type flowmeter, where an orifice plate is placed in the pipe line, as shown in fig.2. If d1 and d2 are the diameters of the pipe line and the orifice opening, then the flow rate can be obtained using eqn. (3) by measuring the pressure difference (p1-p2).
Orifice Plate, Venturimeter and Flow nozzle
The major advantages of orifice plate are that it is low cost device, simple in construction and easy to install in the pipeline as shown in fig.3. The orifice plate is a circular plate with a hole in the center. Pressure tappings are normally taken distances D and 0.5D upstream and downstream the orifice respectively (D is the internal diameter of the pipe). But there are many more types of pressure tappings those are in use.
Permanent Pressure