Flow Measurement Methods
1 Inferential Methods
2 Positive Displacement Type
3 Mass Measurement Type
Inferential: Most flow rates are determined by inferential measurements. Inferential measure means that the flow is not directly measured but are inferred from other quantities that are related. These quantities may be pressure, temperature, force, displacement, velocity etc. the information obtained from the sensor is converted into a velocity value, which in conjunction with cross section area of the tube or pipe, can provide the volumetric flow rate.
Inferential methods of flow measurement are:
Head Type: wherein the flow is inferred from differential pressure across an engineered
obstruction or constriction.
Variable or Rotameter type: wherein the flow rate is inferred from the displacement of an
object that results from a balance of weight against a velocity force.
Magnetic meter: here the flow rate is inferred from velocity. Target meter: the flow rate is
inferred from a force measurement Thermal flow meter: the flow is inferred from change in
thermal conductivity Swirl meter: the flow is inferred from temperature or pressure oscillations
Sonic meter: the flow rate is inferred from noise or Doppler Effect.
Positive Displacement: these type of flow meters basically capture and release a fixed volume of fluid by some type of pumping action. Discrete quantity measurements are used for a small percentage of industrial flow rate applications. This action can be performed by a reciprocating piston, a rotary vane or lobe or a flexible diaphragm.
Mass measurement: mass flow rates can be determined by using volume flow rate meters and multiplying the readings of the latter by the density. It is not easy to calculate the mass flow rate under conditions where temperature and pressure are changing. Hence true mass flow meters are rarely used in industrial applications at present. Mass flow rate meters can be used provided corrections for change in pressure and temperature are applied.
The viscosity of a fluid depends primarily on temperature and to a lesser degree on pressure. Viscosities of liquids generally decrease with increasing temperatures. The effect of pressure on viscosities of liquids is small. Its effect on gases are significant at higher pressures.
The densities of gases and vapors are greatly affected by changes in pressure and temperature. Most flow measurements are made on the basis of volumetric measurements and therefore fluid densities must be known to determine the true mass flow.
Liquids are considered incompressible except at higher pressures. The effect of compressibility need to be considered for most liquid flow measurements. However, in the measurement of gas flows, compressibity is a significant factor.
Venurimeter: it is most widely used device for measurement of discharge through a pipe. A venturimeter is a short pipe which tapers to a small cross section called throat. The diameter is again gradually increased to its original size.
The venture barrel is equipped with piezometer connection in an annular pressure ring for averaging the upstream pressure. A similar ring is designed for the throat where the low pressure connection is made. The differential pressure head is due to the difference in main inlet and throat pressures.
The venturimeter may be used in any position, horizontal, vertical or inclined. In order to measure pressure differentials U Tube manometers are used. The size of the venturimeter is expressed in term of the inlet and throat diameters. The throat diameter is usually between 1/3 and ¾ of the inlet pipe diameter.
The advantage of using small throat diameter is that it leads to higher pressure differentials and hence higher sensitivity and accuracy. But small diameters leads to high velocity and low pressure at the throat and if the pressure falls below vapor pressure, the fluid flowing vaporizes causing cavitation.
Advantages: since these have been used over a long period from the past, the characteristics are well established through practice and can be predicted perfectly.
It has low head loss about 10% of differential pressure head
They have a high co-efficient of discharge on account of low loss, and their capability to measure high flow rates in pipes having diameters of the order of a few meters.
Disadvantages: their large size which renders them unsuitable for applications where space is limited. Also the cost is high on account of large size and the cost of installation and replacement is also high.
There are four types of orifice plates and these are:
1 concentric 2 eccentric 3 segmental 4 quadrant type
Concentric: a concentric orifice plate has a sharp edged concentric circular hole. The standard concentric plate is normally made of stainless steel from 3mm to 12.5mm thick depending upon the size of line. When corrosion is to be prevented materials like nickel or monel are used.
The concentric orifice plate has a high degree of predictable accuracy primarily because it has been standardized as regards its performance data covering a wide range of flow rates, pipe sizes, differential pressures and other factors involves in their use.
Eccentric: these tangents are bored tangential to a circle concentric with the pipe and a diameter 0.98 of the pipe diameter. These plates prove useful when the measure fluid contains suspended material which may tend to build up pressure at the back of a concentric plate. The tendency of such suspended matter to accumulate on the upstream side of concentric orifice plates leads to erratic and false readings.
Segmental orifice plates: a segmentally bored orifice plates are used for applications which are similar to those which use eccentric plates.
Quadrant edge orifice: it is constructed so that the edge is rounded so as to form a quarter circle. The plate has a concentric opening with a rounded edge on the upstream side because this arrangement gives a constant value of discharge co-efficient of the laminar flow. The quarter edge plate may be used when the line Reynolds number from 10000 or above.
These are used for heavy crudes, syrups, and slurries.
Orifice meter works on the principle of venturimeters. The pressure difference is measured between two sections. Vena contracts is the point where the liquid jet issuing from the orifice has the smallest diameter. Hence it is the point where the highest differential pressure is obtained. The jet of the fluid coming out of orifice plate gradually expands from vena contracta to fill the pipe. a part of the kinetic kinetic energy of jet is converted into eddy currents causing dissipation of energy and loss of head.
1 low initial cost
2 easy to install
3 it has simple and inexpensive maintenance as compared to ventruimeter.
4 characteristics of orifice plates are standardized and so the results are more accurate.
5 occupies less space.
1 they have a discharge coefficient of 0.6 which lowers its sensitivityas compared to unity coefficient of venturimeters.
2 the loss of head is almost 60%
3 the inaccuracies result from erosion, corrosion and sealing.
4 flat orifices cannot be used to measure slurries as they tend to clog.