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After a well is constructed and developed, it should be tested for its yield and the corresponding drawdown.
The equipments required for conducting an yield test are:
(i) A test pumping unit consisting of a submersible pump (with 3-phase motor) and a diesel generating set (since power may not be available at the time of test near the well site).
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(ii) Measuring devices for time, discharge and water level.
The pump and power unit for an yield test should be capable of operating continuously at constant discharge rate for long periods of time. Preferably, the well should be pumped at a slightly higher rate than the anticipated routine pumping rate. Even before the well is completed, some indications as to the probable yield of the completed well is usually obtained.
In cable tool drilling such indications are given by the sudden changes in water level, rate of recovery after slush bailing, caving, etc. In hydraulic rotary drilling, mud loss, mud thinning, caving etc., are indicative of the water bearing properties of the formation drilled, while in air rotary drilling, the driller can practically observe how much water is being blown out of the well along with the cuttings.
In fact in down-the hole hammer drilling, the air compressor is continuously operated for 3-5 hours after completion of the total depth of drilling, and the water blown out is measured over a 90° V-notch, which gives an approximate yield. It has been found by experience of drilling in hard rock areas of southern India that the actual yield of the well exceeds the measured yield over a V-notch by about 10%.
Once a rough idea of the probable yield is thus obtained, a suitable test pumping unit may be selected depending upon the size of the well, the yield range and the probable heads or drawdowns in the operating area. A quotation for two test pumping units suitable for 10 and 15 cm diameter wells drilled in hard rock areas of the southern India is given in the following.
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Quotation for Test Pumping Units:
The discharge rate should preferably be kept constant throughout the test. The flow rate is measured at regular intervals and adjustment is made to keep it constant. This can be done by adjusting a valve on the delivery pipe rather than by changing the speed of the pump, since a valve gives more accurate control. Small discharges can be accurately measured by noting the time taken to fill a container of known volume.
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Some of the other methods of measuring the discharge are given below:
(i) Integrating Type of Water Meter Installed in the Pipe:
The pipe should run full and so it is given a vertical bend beyond the meter position, Fig. 13.1. The dial of the meter indicates the total volume discharged through the meter. By noting the time lapse between two dial readings the flow rate can be calculated.
(ii) Orifice Meter:
An orifice meter is installed in the horizontal length of the delivery pipe, Fig. 13.2. From a piezometer inserted in the pipe ahead of the meter, the pressure head (h) can be read. By noting the horizontal and vertical distances (x, y) to a point on the falling jet of water (trajectory) and the area of the orifice (A0), the flow rate (Q) can be calculated as-
Where Dc = the diameter of the jet at the contracted section (veena contracta)
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D0 = the actual diameter of the orifice
(iii) 90 ° V-notch- If the flow is led to a field channel in which a 90° V-notch is installed, Fig. 13.3, the flow rate (Q) can be determined by measuring the head (H) over the V-notch as-
Q = 1.38 H2.48 …(13.2)
Where,
H = head in metres
Q = flow rate in m3/sec.
