Flow Measurement
FLOW MEASUREMENT
The measurement of fluid movement is of major importance to the efficient operation of a industrial processes. It is, for example, essential to very accurately measure and monitor the flow of water to a boiler and the flow of steam from the boiler to the turbine. It is also important to measure the rate of air flow through the gas passages of the furnace.
In general, there are two ways in which flow can be measured. They are:
1. The total quantity of fluid which passes a given point.
2. The rate at which a fluid passes a given point.
Some flowmeters measure both of these values.
Quantity meters fall into two categories:
1. Quantity meters for liquids.
2. Quantity meters for gases.
Quantity meters which measure total flow employing the filling and emptying principle are manufactured to a high degree of tolerance and are therefore very accurate (fig 2:1). They are generally used for accounting purposes, for example, domestic water and gas meters. Other types of quantity meter, measure velocity and infer the quantity (fig 2:2). This type of meter is less accurate and has to be calibrated using a flow rig.
Rotating Vane Flow Meter
If a multibladed impeller is set in the flow stream of a pipeline, so that at any given interval at least one arm of the impeller is presented to the flow, the force, which will cause a pivoted vane to deflect, will now cause the impeller to rotate.
The speed or rate of rotation of the impeller will be related to the velocity of the flow, and a suitably geared mechanism can be taken off the impeller spindle to an indicator - intergrator.
The meter is available in two body designs - for horizontal and for vertical pipework, it is interesting to note that the impeller is normally made from plastic material.
Quantity meters are generally found in the water treatment plant on power stations for measuring liquid flows.
Units of quantity are:
Liquid meters -------- Litres.
Gas meters -------- Cubic meters.
Fig. 2.2
MECHANICAL FLOW METERS
Sparling propeller - type flowmeter. (Hershey - Sparling Meter Co.)
Body Parts 1 Body Flanged BST C and D Flanged BST E Flanged ASA 125 or 150 Flanged BSS. 4622/4504, NP 10 , NP 16 To DIN 19625 Flanged DIN 2533 To DIN 19625 Flanged DIN 2532 2 Top Cover Top cover with regulator plug and regulator sealing ring 3 Top cover plate 4 Joint plate 5 Joint plate gasket 6 Joint plate screws 7 Top cover sealing ring 8 Body bolt 9 Body bolt nut 10 Body bolt washer 11 Regulator plug 12 Regulator plug sealing ring 13 Joint breaking screw 14 Counter box screw Measuring Element Parts 15 Measuring Element (including front vane bearing) 16 Element securing screw 17 Element securing screw washer Front vane bearing bush Front vane bearing Thrust jewel 18 Back bearing cap Assembly (including back vane bearing and thrust jewel) 18A Bearing cap screw 19 Back vane support (not including bearing cap assembly) 20 Tubular dowel pin 21 Vane complete 22 Worm wheel 23 Vertical worm shaft 24 First pinion 25 Drive clip 26 Regulator assembly 27 Regulator assembly screw 28 Undergear complete Metric Imperial gallons Cubic feet 29 Undergear securing screw
Helix 2000
Water Meter
40mm (1 1/2in) to 200mm (8in)
Helix 2000
Water Meter
40mm (1 1/2in) to 200mm (8in)
ROTATING OSCILLATING PISTON
Operating Principle
The measuring cycle
(1) The inlet port in the base of the working chamber is open to the inside wall of the piston and in-flowing water - shown dotted - causes the piston to start its semi-rotary oscillatory movement sliding upon the division shutter. Simultaneously, exhaust water - shown cross hatched - in the remaining part of the piston is being expelled through the outlet port in the top of the working chamber. The neutral water in the chamber is shown as tinted.
(2) The piston has moved round a quarter of its path and the in-flowing water continues filling the dotted area inside the piston and commences to fill the area outside the piston. The neutral water of the first diagram is now shown as cross hatched since it is being expelled through the outlet port.
(3) The piston has now moved round half of its path. In-flowing water is shown on the inlet side of the chamber. Neutral water inside the piston is cut off from both ports and the exhaust water continues to be passed through the outlet port.
(4) With three-quarters of the cycle completed the piston is just starting to open to the inlet port for the beginning of another cycle. The neutral water of the previous diagram within the remaining part of the piston has now become exhaust water and the dotted area in the chamber will soon become neutral water as in the first diagram.
ROTATING OSCILLATOR PISTON
FLOW METER TEST RIG
Flow Test
A calibrated tank ( a convenient capacity being 450 litres (100 gallons)) is required for flow testing. This tank is normally gauged by means of a glass tube of not less than 9.53 mm (3/8in) diameter, and graduated scale mounted on the side of the tank indicating the level of water inside the tank.
The initial calibration of the tank must be accurate to within ± 0.1% in order to test meters accurately to within the permitted tolerance. Kent Meters Limited supply specially designed test plants a typical example of which is shown in the diagram.
A supply of water having been connected to the inlet side of the meter, and the outlet side connected to the calibrated tank, about five gallons should be passed through the meter to flush out the meter and eliminate all air from the system. This also ensures that all backlash in the gearing is taken up in the correct direction. Drain the tank to zero. A convenient quantity of water (say 100 gallons) is then passed through the meter into the tank. The gauge shows the amount of water passed on the tank and any difference between the quantity and the quantity registered on the meter is the meter registration error. To calculate the percentage error, substitute the appropriate values in the following formula: -
Percentage error (Quantity registered - Quantity passed) x 100
Quantity passed
A control valve between the meter and the tank can vary the rate of flow. On the
MAGNETIC FLOW METERS
Electromagnetic Flowmeter
The measurement of liquids containing suspended solids such as sewage or the feed to paper mills presented considerable problems until the advent of the electromagnetic flowmeter. As its name implies, it can be used to measure the flow of any flowing material which is electrically conductive. The meter can be regarded as a section of pipe which is lined with an insulating material. Two saddle coils are arranged opposite each other, and electrodes diametrically opposed are arranged flush with the inside of the lining. If the coils are energized, the moving liquid, as a length of conductor, cuts the lines of force, resulting in a generation of emf which is picked up by the electrodes. By suitable circuitry and, amplification an electrical signal proportionate to flow can be obtained.
The diagram gives a general idea of the construction of such a meter. The principle on which the meter operates is that of the d-c generator. The generator rotor is replaced by the pipe between two magnetic poles. As the fluid flows through the magnetic field an emf is induced in it and can be picked up by the electrodes. This can be expressed mathematically as follows:
E = Blv x 10-8
where E = emf, volts
B = field strength, cgs units
l = conductor length, cm
v = velocity of conductor, cm/sec
In the case of the electromagnetic flowmeter, the flowing liquid represents the conductor and the internal diameter corresponds to the length l. If the field strength B is maintained constant, the only variable is v; hence the emf is proportional to the velocity.
The electromagnetic flowmeter has the advantage of causing no drop in the pressure of the fluid and having a very large range. It is not suitable for low velocities, the smallest range possible being 2 ft/sec for full scale or 1 ft/sec with the sacrifice of some accuracy. It is limited to the measurement of fluids having a conductivity in excess of 10 micromhos/cm. Of great importance is the fact that the readings are unaffected by variations m viscosity, density, temperature, pressure, or conductivity. The electrodes must, of course, be kept clean, and this can present a problem in the measurement of sewage, in which grease, a poor electrical conductor, may be present.
Electromagnetic flowmeter. (Mawdsley’s Ltd.)
(C) VENTURI TUBE CORNER PRESSURE TAPPINGS (A) ORIFICE PLATE PRESSURE TAPPINGS (B) NOZZLE PRESSURE TAPPINGS
Primary Elements Fig. 2.3
The differential pressure producer, reduces the cross sectional area of the pipe and causes an increase in the velocity of the fluid flowing through it. This increase in velocity is a transformation of energy and some of the pressure energy is converted into kinetic energy. The net result of this transformation is a pressure drop across the restriction. The difference between the pressure before and after the restriction being proportional to the square of the flow (fig 2:4).
Fig. 2.4
Differential pressure flowmeters
The simplest method of measuring a differential pressure is by using a manometer (fig 2:5). obviously a glass manometer has very serious limitations in this application, particularly when used on high static pressures.
The trend over the last few decades has been to dispense with liquid filled manometric meters and replace them with bellows and diaphragm types. Measuring elements of this type are sometimes referred to as d.p. cells (fig 2:6).
Fig. 2.6
Liquids being incompressible, are unaffected in their volume by changes in pressure and their flow is usually expressed in terms of volume, that is to say litres/sec. The only significant exception to this is feed flow which is expressed in mass flow units to allow direct comparison with steam flow i.e. Kg/s. Gases which includes steam, being both compressible and having volumes sensitive to changes in temperature, are not usually expressed in terms of volume. mass steam flow as already mentioned is measured in Kg/s whereas air flow is measured as a percentage of full flow.
DALL TUBE TYPICAL HORIZONTAL T - TYPE (LIQUID) TYPICAL HORIZONTAL W - TYPE TYPICAL HORIZONTAL FH - TYPE TYPICAL HORIZONTAL HP - TYPE TYPICAL HORIZONTAL GP - TYPE (LIQUID) DALL - PITOT TUBE TYPICAL PIPEWORK LAYOUTS KJ AND KG Detecting Element Above Instrument
ORIFICE PLATES
ORIFICE PLATES
Installation of primary element and fittings for steam flow through a vertical line
Installation of nipples, valves and reservoirs for steam flow
through a horizontal line
VARIABLE AREA FLOW METERS
In this type of meter, as opposed to differential pressure flow meters, the differential is fixed and is in the form of a float. The variable in this case is the area.
The commonest type of variable area meter is shown in the diagram where the flow is passed through a tapered glass tube housing a float. This float, being of a greater density than the fluid being metered, offers resistance to the flow and at the base of the tube is a close fit.
As the flow increases the float will rise until it reaches a point where, for a given rate of flow, it comes to rest. During the motion of the float the annular area, presented to the flow, between float and tube, increases. The head of the float is usually surmounted by a thin disc, which gives a point of reading through the glass tube scale.
Square law correction is effectively carried out by the tapered nature of the glass tube. This type of meter is suitable for most liquid flows including those of a corrosive nature. However the pressure range of this instrument is obviously limited due to the glass.
Metal Tube Type
This meter works on the same principle as the glass tube variety and has been produced to meter flows of higher static pressures. The obvious difference between glass and metal tube types lies in the indicating devices used in the metal tube type.
(a) The float contains a magnet and attracts an external magnet attached to a driving link.
(b) The float has an extension rod attached to the end of which is an iron core which affects the inductance in windings fitted externally. (This type of meter will register flows as low as a half a gallon an hour and is accurate to ± 1%.)
(c) The float has an extension into an external indicator.
METAL TUBE TYPE
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