Methods of measuring flow rates
One important requirement of any pumping test is the ability to accurately measure the rate or flow from the bore. This can be done in several ways.
1. Measurement a container of known volume: This method relies on the time taken to till a container of known voluine such as a 20 L (5 US gal) drum; from this the flow rate in Litres per second (L/sec)(Gallons per minute)can be calculated. As a general rule, a 20 L (5 gal)drum is sufficient for flow rates up to 0.7 L/sec (11 US gpm), however for higher yields, a larger receptacle such as a ICOL or 200 L(25 or 50gal) drum may be required. It would also be advisable to accurately measure the internal dimension of the vessel and calculate its exact volume, rather than just assuming it. When flow rates are sufficient to quickly fill the container, a stop watch should be used. However, for longer periods, an ordinary wrist watch may be satisfactory.
2. Weir boards: These are suitable for measurement of flows from a bore, when, due lo the arrangement of the headworks, use of an orifice or flow type meter is not practical. A weir board can be an accurate way of measuring flows under good conditions; however, its use in open channels introduces certain chances of error;
* Loss of water through soakage into the earth between the bore head and the weir
* time lag variations in flow at the bore and over she weir
* scream line constriction due to the size and shape of the channel near the weir.
Weir boards are generally available in 300 mm 112 in) and 600 mm (24 in) rectangular weirs and as a V notch. They are made of steel plate, with the edge of the notch bevelled to a sharp edge towards the approach channel side. Measurement is made on the principle that the volume of water flowing over the weir depends on the height of water over the crest of the weir. So that this height can be measured, two lugs are provided upstream of the weir. The upper faces of these lugs are on a level with the crest of the rectangular weir and on the point of the V in the V notch weir. By use of a scale, the height of the water level above these lugs and therefore above the crest or V can be measured.
Measurement should not be made until it is certain that all the water from the bore has reached the weir and has filled up the pools above the weir. As in all cases, the field notes and the report should show the actual observations i.e.. the height of water and size of weir and not just the discharge taken from the tables.
3. Orifice bucket: The orifice bucket is a cilindrical container about 460 mm( 18 in)in diameter and 600mm (24 in) high (these dimensions may vary somewhat). It has a solid base into which a number of orifice pipes are set. The discharge to be measured flows into the top of the bucket and out through the orifices in the bottom.
Baffle and diffuser plates, or plastic fluming on the inlet pipe, are provided lo streamline the flow and minimise turbulence. A stand is necessary so that the water can fall freely through the orifices and run away. The discharge through any orifice depends on the height of water above the orifice, and the total discharge is the sum of the discharges through all of the orifices. This height is measured in a clear plastic tube set water level inside the cbucket. A scale is attached from which the height of this water level can be measured.
The most convenient way to provide the openings in the bottom of an orifice bucket is to weld short 20 mm (3/4 in) pipes into holes cut through the bottom. Each pipe must be exactly the same length and the same inside diameters as the others. The ends of the pipes must be square, free from burrs, and all at the same level.
Te water level scale should be calibrated to show the rate of discharge, preferably through a single orifice.
The flow rate taken from this scale multiplied by the number of orifices used then gives the total rate of discharge through the device for any given water level reading.
An orifice bucket made with several orifices can be used to measure a considerable range of pumping rates since one ore more of the opening can be plugged, leaving as many open as required to handle a given flow rate.
4. Orifice meter: The orifice meter is one of the most accurate of the devices available to the driller for the measurement of discharge. The orifice meter should be used in preference to a weir board whenever possible.
As with a weir board, the discharge depends on the head of water above the sill of the weir, so in the orifice meter the discharge depends on the head of water above the orifice. This head is measured in a plastic water lube inserted into the side of the meter at some distance (about 0.6 m or 2 ft) behind the orifice plate and standing above the meter. The height to which the which stands in this tube is observed and the corresponding discharge read of the calibration charts using the curve for the particular diameter of orifice being used.
It is often necessary to maintain a specified flow through the orifice meter and to allow this lo be done the meter is usually equipped with a gate valve in addition to a short length of pipe with internal vanes to streamline the flow. By the manipulation of the valve, it is possible to maintain a constant discharge. i.e., a constant head of water in the measuring tube, over given periods of time.
Essential requirements are:
* the meter must be level both ways
* the orifice plate must at all times run lull of water
* the diameter of the orifice plate and barrel must be recorded
* any gas in the flow and any signs of drilling mud or cuttings should be reported
* all leakages must be reported
* all actual observations must be reported • not just derived or computed results.
Calibration curves for all orifice plates should be carried with the meter at all times
5. Flow meters: These are very accurate devices for measuring both low and high volumes. As with the method of measuring flow rate by timing the filling of a container of known volume, a watch is usually required to time the flow through the meter; however, meters are available to record flow per unit of time.