Several devices are used for measuring the flow of water in pipes. A few of them, which may find application on the farm, are as follows: 1. Volume Measurement 2. Flow Rate Measurement 3. Measurement of Cumulative Flow 4. Water Level Recording.
Method # 1. Volume Measurement:
A simple method for measuring small irrigation streams is to collect the discharge in a container of known volume for a measured period of time.
The rate of flow (discharge Q in lps) is measured by:
This method can be used determine the discharge rate of pumps and other lifts if a barrel of 150 to 200 l capacity is used.
Method # 2. Flow Rate Measurement:
Devices used in closed conduit (piped) irrigation systems are usually flow meters, which are inserted into the pipeline, typically at the end of the farm or the field unit. They are equipped with gauge showing cumulative discharge in m3.
Venturi Meter:
It is used to measure the flow of water in pipes under pressure (Fig 9.7). It utilises the venturi principle in that the flow passing through a constricted section pipe is accelerated and its pressure head is lowered.
The cross-sectional area of the pipe and the constricted section being known, the flow is determined, measuring the drop in pressure head.
Rate of flow through the meter is computed from the formula:
where, Q = Rate of flow
C = Empirical discharge coefficient (from standard tables)
d2 = Dia of the throat section
h1 = Pressure head measured above the axis of the meter at the straight entrance section
h2 = Pressure head measured above the axis of the meter at the throat
k = Factor corresponding to the ratio of throat dia to dia of the entrance section from standard tables.
Pipe Orifices:
They are usually circular orifices placed within or at the end of horizontal pipe. Head on the orifice is measured with a manometer. The ratio of the orifice dia to the pipe dia should be between 0.5 and 0.83 and is selected in such a way that the pipe flows full.
Discharge is computed with the equation:
where, Q = Discharge
C = Coefficient of discharge (from standard curve)
g = Acceleration due to gravity
h = Head of the orifice measured above the center
A = Cross-sectional area.
Coordinate Method:
It measures the jet or water issuing from the end of the pipe for calculating rate of discharge. Flow from pipes may be measured whether the pipe is discharging vertically upward, horizontally or at some angle with the horizontal. Coordinate methods are used to measure the flow from small pumping plants discharging horizontally and from flowing wells discharging vertically. These methods have limited accuracy owing to problems in making accurate measurements of coordinate of the jet.
To measure the flow from pipes discharging vertically upward, it is necessary to measure inside dia D of the pipe and the height H of the jet above the pipe outlet (Fig 9.8). To measure the flow from pipes discharging horizontally, it is necessary to measure both horizontal and vertical distance from the point on the jet.
For instance, coordinates are measured from the top of the inside of the pipe to a point on the top of the jet. These horizontal and vertical distances are called X and Y ordinates, respectively.
The formula used is obtained by combining the following equations:
Q = A × V
X = V × t or V = X/t
Y = ½ × gt2 or t = √2Y/g
Introducing C, the coefficient of discharge, we have
Q = C × A × X × √g/2Y
The above equation can be written as
where, Q is in lps, X and Y measured in cm, A in cm2 and g taken as 980 cm s-1. Discharge can be obtained from standard tables.
Method # 3. Measurement of Cumulative Flow:
Meters used to measure irrigation water are of the velocity type in canals, streams or contained within pipes or conduits. When the meters are installed in open channels, the flow must be brought through a pipe or conduit of known cross-sectional area.
i. Irrigation Meters:
Irrigation meters, essentially, consist, of a conical propeller connected to a registering head by a gear system. They are operated by the kinetic energy of flowing water. Suspended propeller, facing center of flow is rotated by the flow of water. The speed of the propeller is proportional to the average velocity of flow within the tube and since the cross-sectional area of the tube is known and remains constant, the propeller speed is proportional to the rate of flow.
Meters are given a volumetric calibration test at the factory. Adjustments or recalibration in the field is not normally required. Low pressure line meters, open flow meters and vertical flow or hydrant type meters are the three basic types of irrigation meters.
ii. Low-Pressure Line Meters:
They are used where pipelines are used to distribute irrigation water. They can be installed in concrete, cement or steel pipelines. They are mostly used for measurement of water delivered to individual farms from lateral lines of an irrigation enterprise.
iii. Open Flow Meters:
They are similar to low-pressure line meters in construction. They are used for measuring flow in open channels or gravity flow and closed conduit system. The meter is suspended from a wall or simple support structure into the center of a full flowing submerged discharge end of a pipe, culvert or siphon, which serves as a meter tube.
iv. Vertical Flow Meters:
They are used for measuring flow of water in vertical pipes. Operating principles are similar to those of the other two meters, except that the position of propeller is vertical. Water flows in upward direction, actuates the propeller and is then deflected downward by a bonnet or cover to the outside of the tube. Its principal use is measurement individual farm deliveries, where such deliveries are made in gravity flow pipelines and through pipe turnouts.
Method # 4. Water Level Recording:
Where the flow rates fluctuate with time, continuous recording of head flow in the measuring devise is essential. Water level recording equipment is used for such purposes. Water stage recorder (laminograph) is the commonly used equipment. It consists of a float connected to a recording devise. The float rests on water in a float well, connected to main channel by a pipe or trench. The fluctuations in water head make the float move, which in turn actuates a pen. This pen records the level on a clock driven chart.
Water measuring devises giving instantaneous discharge rates are based on water level or velocity measurements at known cross-sections. Discharge measuring devises are necessary to determine the total volume of water actually used for each period of the year. Canal discharges are commonly measured by weirs, submerged orifices, Parshall flumes and commercial meters.
Current meters are commonly used to measure discharge of streams too large to be measured by weirs or other standard measuring devises. What should be quite suitable in a developed country may be quite inappropriate in a less developed one, especially where irrigated agriculture is not a well-established practice.