Differential pressure flowmeters
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Transcript of Differential pressure flowmeters
Differential Pressure
Flowmeters
https://pursuitengineering.blogspot.com/2017/03/differential-pressure-flowmeters_51.html
Introduction When certain flowmeters 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.
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 flowmeter.
Such devices are known as differential pressure flowmeters.
Introduction
Introduction The pressure difference is measured using a manometer connected
to appropriate pressure tapping points.
The pressure is seen to be greater upstream of the flowmeter than downstream, the pressure difference being shown as h.
Examples of differential pressure flowmeters commonly used
include
(a) Orifice plate (b) Venturi tube(c) Flow nozzles (d) Pitot-static tube
Orifice plate
Construction
An orifice plate consists of a circular, thin, flat plate with a hole (or orifice) machined through its centre to fine limits of accuracy.
The orifice has a diameter less than the pipeline into which the plate is installed
Orifice plates are manufactured in stainless steel, monel metal, polyester glass fibre and for large pipes, such as sewers or hot gas mains, in brick and concrete.
Orifice plate
Principles of operation When a fluid moves through a restriction in a pipe, the fluid
accelerates and a reduction in pressure occurs. The position of minimum pressure is located downstream from the orifice plate where the flow stream is narrowest.
This point of minimum cross-sectional area of the jet is called the
‘vena contracta’. Beyond this point the pressure rises but does not return to the original upstream value and there is a permanent pressure loss.
This loss depends on the size and type of orifice plate,
Orifice plate
Advantages of orifice plates(i) They are relatively inexpensive.(ii) They are usually thin enough to fit between an existing pair of pipe flanges
Disadvantages of orifice plates(i) The sharpness of the edge of the orifice can become worn with use, causing calibration errors.(ii) The possible build-up of matter against the plate.(iii) A considerable loss in the pumping efficiency due to the pressure loss downstream of the plate.
Orifice plate
Applications
Orifice plates are usually used in medium and large pipes and are best suited to the indication and control of essentially constant flow rates. Several applications are found in the general process industries
Venturi tube
Construction
The Venturi tube or venturimeter is an instrument for measuring with accuracy the flow rate of fluids in pipes.
The entrance and exit diameter is the same as that of the pipeline into which it is installed. Angle β is usually a maximum of 21°, giving a taper of β/2 of 10.5°.
The length of the throat is made equal to the diameter of the throat. Angle α is about 5° to 7° to ensure a minimum loss of energy but where this is unimportant α can be as large as 14° to 15°.
Venturi tube
Venturi tube
Pressure tappings are made at the entry (at A) and at the throat (at B) and the pressure difference h which is measured using a manometer, a differential pressure cell or similar gauge, is dependent on the flow rate through the meter.
Usually pressure chambers are fitted around the entrance pipe and the throat circumference with a series of tapping holes made in the chamber to which the manometer is connected. This ensures that an average pressure is recorded.
Venturimeters are usually made a permanent installation in a pipeline and are manufactured usually from stainless steel, cast iron, monel metal or polyester glass fibre.
Venturi tube
Advantages of venturimeters
(i) High accuracy results are possible.(ii) There is a low pressure loss in the tube (typically only 2% to 3% in a well proportioned tube).(iii) Venturimeters are unlikely to trap any matter from the fluid being metered.
Disadvantages of venturimeters
(i) High manufacturing costs.(ii) The installation tends to be rather long (typically 120 mm for a pipe of internal diameter 50 mm).
Flow nozzle
The flow nozzle lies between an orifice plate and the venturimeter both in performance and cost.
Pressure tappings are located immediately adjacent to the upstream and downstream faces of the nozzle (i.e. at points A and B).
Flow nozzles are suitable for use with high velocity flows for they do not suffer the wear that occurs in orifice plate edges during such flows.
Flow nozzle
Pitot-static tube
A Pitot-static tube is a device for measuring the velocity of moving fluids or of the velocity of bodies moving through fluids.
It consists of one tube, called the Pitot tube, with an open end facing the direction of the fluid motion, shown as pipe R in Figure, and a second tube, called the piezometer tube, with the opening at 90° to the fluid flow, shown as T in Figure.
The difference in pressure (pR −pT), shown as h in the manometer of Figure , is an indication of the speed of the fluid in the pipe
Pitot-static tube
Pitot-static tube
Below Figure shows a practical Pitot-static tube consisting of a pair of concentric tubes.
The centre tube is the impact probe that has an open end which faces ‘head-on’ into the flow. The outer tube has a series of holes around its circumference located at right angles to the flow, as shown by A and B in Figure.
The manometer, showing a pressure difference of h, may be calibrated to indicate the velocity of flow directly.
Pitot-static tube
Pitot-static tube
Applications A Pitot-static tube may be used for both turbulent and non-turbulent
flow.
Often used for making preliminary tests of flow rate in order to specify permanent flow measuring equipment for a pipeline.
The main use of Pitot tubes is to measure the velocity of solid bodies moving through fluids, such as the velocity of ships.
Pitot-static tube
Advantages of Pitot-static tubes
(i) They are inexpensive devices.(ii) They are easy to install.(iii) They produce only a small pressure loss in the tube.(iv) They do not interrupt the flow.
Disadvantages of Pitot-static tubes
(i) Due to the small pressure difference, they are only suitable for high velocity fluids.(ii) They can measure the flow rate only at a particular position in the cross-section of the pipe.(iii) They easily become blocked when used with fluids carrying particles.
Mechanical flowmeters
With mechanical flowmeters, a sensing element situated in a pipeline is displaced by the fluid flowing past it.
Examples of mechanical flowmeters commonly used include:
(a) Deflecting vane flowmeter (b) Turbine type meters
Deflecting vane flow meter
The deflecting vane flowmeter consists basically of a pivoted vane suspended in the fluid flow stream
When a jet of fluid impinges on the vane it deflects from its normal position by an amount proportional to the flow rate.
The movement of the vane is indicated on a scale that may be calibrated in flow units. This type of meter is normally used for measuring liquid flow rates in open channels or for measuring the velocity of air in ventilation ducts.
The main disadvantages of this device are that it restricts the flow rate and it needs to be recalibrated for fluids of differing densities.
Deflecting vane flow meter
Turbine type meters
Turbine type flowmeters are those that use some form of multi-vane rotor and are driven by the fluid being investigated.
Three such devices are the cup anemometer, the rotary vane positive displacement meter and the turbine flowmeter.
a) Cup anemometer An anemometer is an instrument that measures the velocity of moving gases and is most often used for the measurement of wind speed.
The cup anemometer has three or four cups of hemispherical shape mounted at the end of arms radiating horizontally from a fixed point. The cup system spins round the vertical axis with a speed approximately proportional to the velocity of the wind.
Turbine type meters
(b) Rotary vane positive displacement meters measure the flow rate by indicating the quantity of liquid flowing through the meter in a given time.
A typical such device is shown in section in Figure and consists of a cylindrical chamber into which is placed a rotor containing a number of vanes (six in this case).
Liquid entering the chamber turns the rotor and a known amount of liquid is trapped and carried round to the outlet. If x is the volume displaced by one blade then for each revolution of the rotor in Figure the total volume displaced is 6x.
This type of meter in its various forms is used widely for the measurement of domestic and industrial water consumption, for the accurate measurement of petrol in petrol pumps
Turbine type meters
Rotary vane positive displacement meters
Turbine type meters
c) A turbine flowmeter contains in its construction a rotor to which blades are attached which spin at a velocity proportional to the velocity of the fluid which flows through the meter.
A typical section through such a meter is shown in Figure. The number of revolutions made by the turbine blades may be determined by mechanical or electrical device enabling the flow rate or total flow to be determined.
Applications include the volumetric measurement of both crude and refined petroleum products in pipelines up to 600 mm bore, and in the water, power, aerospace, process and food industries
Turbine type meters
A turbine flowmeter
Float and tapered-tube
Principle of operation The float in the tapered tube produces a restriction to the fluid flow. This
reduction in area produces an increase in velocity and hence a pressure difference, which causes the float to rise.
The greater the flow rate, the greater is the rise in the float position, and vice versa. The position of the float is a measure of the flow rate of the fluid and this is shown on a vertical scale engraved on a transparent tube of plastic or glass.
For air, a small sphere is used for the float but for liquids there is a tendency to instability and the float is then designed with vanes that cause it to spin and thus stabilize itself as the liquid flows past. Such meters are often called ‘rotameters’.
Float and tapered-tube
Float and tapered-tubeAdvantages of float and tapered-tube
(i) They have a very simple design.(ii) They can be made direct reading.(iii) They can measure very low flow rates.
Disadvantages of float and tapered-tube
(i) They are prone to errors, such as those caused by temperature fluctuations.(ii) They can only be installed vertically in a pipeline.(iii) They cannot be used with liquids containing large amounts of solids in suspension.(iv) They need to be recalibrated for fluids of different densities.
Float and tapered-tube Practical applications of float and tapered-tube meters are found
in the medical field, in instrument purging, in mechanical engineering test rigs and in simple process applications, in particular for very low flow rates.
Many corrosive fluids can be handled with this device without complications
Electromagnetic flow meter
The flow rate of fluids that conduct electricity, such as water or molten metal, can be measured using an electromagnetic flowmeter whose principle of operation is based on the laws of electromagnetic induction.
When a conductor of length L moves at right angles to a magnetic field of density B at a velocity v, an induced e.m.f. e is generated, given by: e = BLv.
Electromagnetic flow meter
With the electromagnetic flowmeter arrangement shown in Figure, the fluid is the conductor and the e.m.f. is detected by two electrodes placed across the diameter of the non-magnetic tube.
Rearranging e = BLv gives
Thus with B and L known, when e is measured, the velocity of the fluid can be calculated.
Electromagnetic flow meter
Advantages(i) Unlike other methods, there is nothing directly to impede the fluid flow.(ii) There is a linear relationship between the fluid flow and the induced e.m.f.(iii) Flow can be metered in either direction by using a centre-zero measuring instruments
Applications of electromagnetic flowmeters are found in the measurement of speeds of slurries, pastes and viscous liquids, and they are also widely used in the water production, supply and treatment industry.
Hot-wire anemometer
A simple hot-wire anemometer consists of a small piece of wire which is heated by an electric current and positioned in the air or gas stream whose velocity is to be measured. In practice there are various ways in which this is achieved:
(i) If a constant current is passed through the wire, variation in flow results in a change of temperature of the wire and hence a change in resistance which may be measured by a Wheatstone bridge arrangement. The change in resistance may be related to fluid flow.
(ii) If the wire’s resistance, and hence temperature, is kept constant, a change in fluid flow results in a corresponding change in current which can be calibrated as an indication of the flow rate.
(iii) A thermocouple may be incorporated in the assembly, monitoring the hot wire and recording the temperature which is an indication of the air or gas velocity.
Hot-wire anemometer
Advantages (a) Its size is small
(b) It has great sensitivity