Groundwater Technology Handbook by A.R. Mahendra

Author:A.R. Mahendra [Mahendra, A.R.]
Language: eng
Format: azw3
ISBN: 9781482812633
Publisher: PartridgeIndia
Published: 2013-11-14T05:00:00+00:00

Fig 14: Set-up for a well test; includes systems to measure water levels, discharge pressure and flow rate

Preparations for the test:

1. Static water level (non-pumping level) in the well is accurately measured and recorded.

2. The throttle valve is kept ½ to ¾ ths open, with pressure gauge reading zero.

3. For measuring output, the discharge pipe with the required ‘bend’ is centered on the measuring drum; any drum of known capacity, viz., 220 lts drum will serve the purpose for low yielding wells. (See chapter on ‘measuring water flow’). Discharge measurements could also be made by a flow-meter. A manometer with a synchronized orifice plate is an integral set-up to monitor output from high yielding wells.

4. An electric depth sounder, or a steel tape with chalk rubbed on its surface to a length of about 10m is inserted into the airline, or directly into the well, to a depth of 6m below the static water level. The tape reading at the top casing head, minus the ‘wet cut’ (the watermark on the chalked surface) indicates the depth to water level.

Well test:

The primary objective of the well test is to determine the yield capacity of the well, and the drawdown for that discharge, to facilitate selection of an appropriate pump. Procedure involves pumping water, with the throttle valve fully open, under a reasonably constant pressure, and to determine the dependable yield and the maximum drawdown in the well, (which is the ‘head’ against which the pump will be pumping water). A throttle valve is installed, to control output, in case of poorly yielding wells. In low yielding wells, the throttle valve is maneuvered to stabilize the output and water levels to a constant rate.

1. The pump is switched on; and the exact time of starting the pump is noted against the respective column.

2. Reading on the pressure gauge is monitored. If, and as the gauge tends to record a continuous rise, the valve is opened appropriately to stabilize the reading. Reciprocally, if the gauge tends to recede, the throttle is gently closed to restrain further decline. This opening and closing of the throttle valve is a continuous process until a steady pressure and output are achieved. At regular intervals, pressure gauge readings are noted in the appropriate column.

3. A steady state of flow, with a balanced drawdown, if and when achieved reflects balanced recharge and discharge conditions in the well. A pumping well so stabilized to yield a steady output should be pumped for 12 hours, or as long as possible to confirm its yield capacity. In case the output / pressure tend to decrease, or increase, time of such change is of importance to project the eventual potential of the well. A fully open throttle valve and a steady gauge reading signify a good production well. An erratically behaving pressure gauge indicates rapid decline in water levels reflecting an unbalanced inflow and output. It may become necessary to continuously throttle the flow to maintain pressure, and a steady flow. Such a

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.