- FLOW MEASUREMENTS
Positive Displacement Method
Positive displacement meters are direct flow measuring devices. This means that the flowing stream to be measured is divided into small segments of known volume, and the total flow is determined by adding together the total number of each segments.
All positive displacement meters consist of an external housing, an internal measuring unit and a mechanism which drives a counter or indicator.
In the example of a rotary sliding vane meter, we see that as the fluid enters the meter, it is separated from the main part of the stream in a measuring chamber. As the fluid flows on , out of the meter, a drive mechanism is activated which causes an indicator to add another volume unit to it’s total. Additionally, the flow of fluid through the meter causes the mechanism to turn, continuously capturing volumes of fluid to be measured.
(SLIDING VANE TYPE )
Orifice Plate Meters
These types of meters measure flow indirectly by measuring the pressure difference caused by a fluid flowing through a plate with a precisely cut hole that is set in a pipe. By relating the difference between the upstream pressure and the downstream pressure to the size of the particular orifice plate, the flow rate for the fluid in the pipe can be determined.
The orifice is a round, stainless steel plate with a round hole in the middle, having a diameter smaller than the diameter of the pipe. This plate is clamped between two flanges which are provided with connections for differential pressure gauge. A handle is welded to the orifice which remains partially outside the flanges.
Certain information is given on this handle, e.g. the diameter of the pipe and the orifice, the type of material and the words up stream must be facing the direction from which the flow is coming. This is very important because the front and back of the orifice are not the same (see the cross section of an orifice). The edges of hole must be sharp and never rounded. The orifice must always be placed in a straight part of the pipe where no valves or connections are located.
A typical example is given below. This meter station consists of an orifice plate installed in a line, a mechanical bellows installed to detect changes in differential pressure.
ORIFICE PLATE METER
Venturi tubes and flow nozzles are the other two devices that help measure fluid flow rates by using indirect method.
Venturi tube and the flow nozzle work in a similar manner like an orifice plate. They create a pressure difference in an area of the flow line by restricting fluid flow in a precise way.
Turbine Flow meters
As the rotor spins, the tips move past a magnetic pick-up coil, which registers each time a rotor tip passes. The frequency of the rotor tips passing the pick-up coil can be used to calculate the speed or velocity of the fluid moving past the rotor. The cross-sectional area of the pipe can be used to determine the volume of fluid moving past the rotor. With these two pieces of information – the velocity of the fluid and the cross-sectional area – the flow rate of the fluid can be calculated.
Changes in temperature also affects the accuracy.
THE CORIOLIS EFFECT METER
Operation Principle
This is the new favourite of the metering world and proving to be very accurate and reliable.
The Coriolis Meter is named after the principle on which it works, the “Coriolis Effect”.
Imagine yourself in the middle of a revolving circular platform, such as shown in fig above, to get off the revolving platform, you would aim for the edge of the circle. Even though you try to walk upright, you will experience a force that is trying to push you over, and you have to lean over to prevent yourself falling. This is not the same as the centrifugal force, that is trying to throw you away from the centre.
Both pickoffs – the one on the inlet side and the one on the outlet side – generate sine wave current continuously when the tubes are oscillating. When there is no flow, the sine waves are in phase.
V Cone
The cone is differential pressure type flow meter. The principal theory is Bernoulli’s theorem for the conservation of energy in a closed pipe. This states that for a constant flow, the pressure in a pipe is inversely proportional to the square of the velocity in the pipe. Simply, the pressure decreases as the velocity increases. For instance, as the fluid approaches the V-Cone meter, it will have a pressure HP. As the fluid velocity increases at the constricted area of the V-Cone, the pressure drops to LP, as shown in Figure. Both HP and LP are measured at the V-Cone’s taps using a variety of differential pressure transducers. The ∆p created by a V-Cone will increase and decrease exponentially with the flow velocity. As the constriction takes up more of the pipe cross-sectional area, more differential pressure will be created at the same flow rates.
Rota-meters
- It consists of a tapered glass tube with a metal float or otherwise called a plummet.
- As the fluid flow rate
- increases, the plummet rises higher in the tube.
- A scale on the side of the tube indicates the flow rate.
Flow Measuring units
- Barrels/day
- Barrels / hour
- Gallon per minute
- M³/hour
- MMSCFD ( million standard cubic feet per day )
- MSCFD ( Thousand standard cubic feet per day )
Three reasons for measuring fluid flow rates.
- Process control.
- Product quality control.
- Safety (pipeline leak detection and surge prevention)
What have you learnt?
- Fill in the blanks & True and false:
1 Flow takes place from ___________ pressure to ___________pressure.
- An orifice plate creates ______________pressure.
- Coriolis meter is used to measure pressure. (True/False)
- Flow is a measurement of temperature. (True/False)
2. List the types of flow measuring devices.
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- List flow measuring units.
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- In a petrol pump what measuring system is used to fill your car?
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- Is it possible to calculate the time required to fill a tank if the flow rate is known?
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