Several methods are available for measuring flow in pipes. Only a few of them which can be used on the farm are discussed here:
1. Volumetric Measurements:
Flow measurements by this method are made by measuring the time required for the flow to fill a container of known volume. Volume divided by time is equal to the rate of flow. This method is very simple and requires little equipment. But where the flow rate is not uniform over a period of time, frequent measurements are to be taken to get accurate data.
2. Flow Rate Measurements:
The different methods commonly used for measuring the rate of flow in pipes are as follows:
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i. Venturi Meters:
The Venturi meter consists of converging and expanding section of short length. It is useful for measuring the flow of water in pipes under pressure. It utilises the principle that the flow passing through a constricted section of pipe is accelerated and the pressure head lowered.
Knowing the cross- sectional areas of the pipe and the constricted section, the flow is determined by measuring the drop in the pressure head. The drop in the pressure head is measured by providing openings in the Venturi meter at the points shown in the figure and connecting these openings to a U- tube manometer.
Considering points (1) and (2) with cross- sectional areas A1 and A2 respectively and assuming horizontal pipe, from Bernoulli theorem, neglecting friction, we have-
Where, Cd is the coefficient of discharge. Cd values are found to be between 0.97 to 0.99. Accurate Cd values are obtained by experimental procedures.
A similar device for measuring discharge in a pipe-line is an orifice plate. Its function is similar to the venturi meter. The same discharge equation therefore applies. However, the major difference between the devices lies in the fact that, downstream of the orifice plate, the flow area expands instantaneously while the fluid is unable to expand at the same rate.
This creates a ‘separation zone’ of turbulent eddies in which large energy losses occur. Consequently, the coefficient of discharge is considerably lower than that for the Venturi meter (typically about 0.65). The advantages of the orifice plate are its lower cost and its compactness.
The orifice can be put at the end of the pipe and is known as pipe orifice. The ratio of the orifice diameter to the pipe diameter should be between 0.5 and 0.8 and is selected in such a way that the pipe flows full. The discharge is computed using the equation.
Where a is the area of the orifice and h is the head of flow measured from the center of the pipe. Cd values are to be experimentally determined.
The pipe orifice assembly can conveniently be jointed to the outlet pipe of a pumping installation for measuring discharge.
ii. Pitot Tube:
The pitot tube is an open L shaped tube (Fig. 4.17) useful for measuring velocity of flows in an open channels as well as in pipes. The equation for velocity is obtained using Bernoulli equation,
The velocity at point 2 immediately at the nose of the tube is zero as water is at rest here. The Pitot tube gives the velocity at the point at which the open end is placed.
The use of the Pitot tube for measuring flow when the pipe is flowing under pressure the static pressure of the water is to be taken into consideration. Fig. 4.18 illustrates how this is done using an inverted U-tube. The velocity of flow is given by the equation-
Where, C is known as the Pitot tube coefficient, a value to be determined for the particular tube (usually 0.95 to 1.0). Knowing the area of cross-section, the rate of flow can be computed.
iii. Elbow Meters:
Another type of differential pressure flowmeter is an elbow meter. Pressure differences between the outside and inside walls of an elbow are related to volumetric flowrate. Eq. 4.25 is used to compute volumetric flowrate when the pressure difference and cross-sectional area of the elbow are known.
As shown in Fig. 4.19, the elbow flow coefficient, Ce, ranges between 0.63 and 0.83, depending on size, shape, and type of elbow.
iv. Co-Ordinate Method:
In this method, measurements are taken of the jet of water issuing from the end of the pipe and these are used for calculating the rate of discharge. This method can be used whether the pipe is discharging vertically upwards, horizontally, or at some angle to the horizontal. Methods for measurement, when the pipe is discharging horizontally and vertically are outlined here.
To measure the flow from pipes discharging horizontally, the horizontal distance (X) and vertical distance (Y) from some point on the end of the pipe to a similar point on the jet are measured.
The formula to be used is obtained by combining the following equations:
Values of the coefficient of discharge C for a pipe flowing completely full at the end are given in Table 4.4. D is the internal diameter of the pipe.
These methods have limited accuracy due to the errors in measuring the coordinates of the jet. Errors up to 10 per cent in the measurements can be expected. For a greater accuracy, X/D should not be less than 3.
When water is discharging from a vertical pipe into the air, the discharge can be estimated by measuring the height to which the water rises above the end of the pipe. The height of water is to be measured carefully to get the accurate measurements. Proper height measurement consists in measuring the height at which the water stays the longest time.
The discharge is estimated using the equations given by Lawrence and Braunworth.
The equations are:
Eq. 4.31 is valid for weir flow (h ≤ 0.37D) and Eq. 4.32 is valid for jet flow (h ≥ 1.4D). For heights between 0.37 D and 104 D, the flow is somewhat less than given by either of the equations. Here, D and h are in meters and Q is in m3/s.
The above equations are obtained through actual measurements. The practical range of the pipe diameters is 0.025 m to 0.609 m and the head is 0.0025 m to 4.0 m. Pipes should have clear cut edges and a constant diameter over at least a length of 6 D. They should also be vertical for at least a length of 6 D from the top of the pipe.
3. Meters for Measuring Cumulative Flow:
i. Propeller Meters:
These meters record the cumulative flow of water. These are widely used in USA in the farm irrigation systems at the canal outlets. The flow from the canal outlet is allowed to pass through a pipe into a basin.
A propeller which rotates due to the flow of water is installed at the pipe outlet (Fig. 4.22). The number of rotations indicated by a counter will give the cumulative water flow. Calibration and maintenance are important to get accurate readings.
ii. Deathridge Meter:
This device used in Australia operates on the same principle as the propeller meter but the construction is different. Fig. 4.23 indicates its constructional details. The size of the rotating wheel depends upon the discharges to be measured. The number of rotations are recorded on a counter. The device is to be calibrated in situ to obtain accurate readings.
iii. Water Meters:
These are designed for measuring pipe flow. They are generally used for measuring municipal water supplies and are rarely used for measuring water on the farm. These consist of a multiblade propeller made of metal, plastic or rubber, connected to a counter by means of a gear system.
The counter readings are calibrated to give the volumetric flow in the desired units. Water meters are made in different sizes to suit the different pipe diameters and ranges of flow.
The basic requirements for using the water meters are:
(1) The rate of flow should be within the designed range,
(2) The pipe must always flow full and
(3) The water should not contain any debris.
4. Water Level Recording Equipment:
Where the rate of flow fluctuates with respect to time, it becomes necessary to continuously record the head of flow in the measuring device used. Water level recording devices are used for such a purpose. Such a device (Fig. 4.24) consists of a float suitably connected to a recording device.
The float rests on the water in a float well or a stilling well which is connected to the main channel by a pipe or a trench. The fluctuations in the water level make the float move which in turn actuates a pen. This pen records the level on a clock driven chart.
