Well-Performance Testing Procedures
8-1. Testing Pumps. You will normally use the permanent pump for pump testing. If you use a temporary unit, it must be adequate to draw down the water and hold it at a prescribed flow rate for a period of hours. This test will determine the specific capacity of the well. You can estimate the yield of a small well by bailing water from the well rapidly if no pump is available. You must know the bailer's volume and count the number of times per minute the bailer is brought up full to estimate the GPM of the well. Accurately measuring drawdown is not possible during the test because the water level constantly fluctuates.
a. Permanent Wells. You should use two different testing procedures when a pump is available, depending on the intended use of the well and the available testing time. If the well will be a permanent installation and maintained in the future, you should conduct a detailed test. Measure the static water level in the well before testing, and measure the drawdown during the test. Conduct the test as follows:
- Pump at a rate that will lower the water in the well about one-third of the maximum drawdown possible (one-third the distance from the static water level to the top of the well screen) or about one-third of the rated capacity of the pump.
- Monitor and adjust the flow rate early in the test because as the drawdown increases the flow rate decreases.
- Continue pumping at a constant flow rate until the drawdown remains constant (about 1 to 4 hours).
- Record the flow rate, drawdown, and testing time. Initially, take readings rapidly, and then spread out the readings as the test continues. A reading schedule that doubles the time between readings is preferable. The recommended schedule is as follows: 0 (at the start of the test) 30 seconds, 1 minute, 2 minutes, 4 minutes, 8 minutes, 15 minutes, 30 minutes, 1 hour, 2 hours, 4 hours, and so forth.
- Establish the desired, constant flow rate quickly. You must record the exact time of each reading (not the intended or scheduled time). After the drawdown stabilizes (1 to 4 hours), the pumping rate should increase to a new, constant flow rate, which will produce two-thirds of the capacity of the pump. Do not stop the pump between these test segments.
- Repeat the measurements, noting the exact time that the new flow rate was started. Try to follow the above reading schedule, starting from the time the flow rate was increased. When the drawdown stabilizes, increase the pumping rate to produce the maximum drawdown or about 90 percent of the maximum capacity of the pump. Conduct another reading schedule until the pumping level stabilizes.
You may modify the above procedure depending on well requirements and local site conditions. You should not modify the precision and accuracy of the measurements taken. Test results should become a part of the permanent records. The results are useful for evaluating the efficiency of the well in the future and for determining the need for well rehabilitation. Calculating the GPM per foot of drawdown gives the capacity of the well. You can use this information to estimate production and to regulate the pump's flow rate to prevent dewatering of the well and possible pump damage.
b. Temporary Wells. Conduct a single-stage test rather than the step drawdown test. To establish the flow rate, conduct a 1- to 2-minute test to determine the GPM per foot of drawdown. Let the well return to the original static water level before testing (about 1 hour). Select a flow rate that will produce about two-thirds of the available drawdown but will not reach more than 90 percent of the pump's capacity. Conduct the test as described above, but with only one segment. When the drawdown stabilizes for the selected flow rate, stop the test.
c. Methods. See Chapter 4 for a description of pumps used in testing and well production.
(1) Submersible-Pump Method. Use the submersible pump in well-completion kits to pump test the water well. Set the pump deep enough to attain the maximum pumping rate and drawdown. When testing a well with a screen, set the suction of the pump above the top of the screen to prevent lowering the water level below the screen. When testing a well without a screen, try not to dewater the production part of the aquifer. For proper testing, you must have a reliable power source so that testing will not be interrupted. The power must be sufficient to drive the pump at a rated speed so that full capacity can be developed.
(2) Air-Lift Method. This method is sometimes best for military field operations, especially if the well may produce sand that could damage or reduce the life of a submersible pump. An air-lift pump has two major problems. Air turbulence could make drawdown measuring difficult, and entrained air may cause considerable error in measuring the flow rate. After constructing an air-lift pump, check the pump capacity against the expected well yield. To conduct the test, set the pump according to the readings in Table 4-2.
An air compressor that puts out 350 cfm at 200 psi is suitable for performing most air-lift pumping operations. To determine the amount of air needed for pumping water, use the following equation or refer to Figure 8-1.
V = free air (actual) required to raise one gallon of water, in cubic feet.
h = total lift, in feet.
log = logarithmic value.
H = operating submergence, in feet.
C = constant (Table 8-1).
The pressure required to start pumping will be equal to the depth of water over the submerged end of the air pipe. After pumping has started, the water in the well will draw down to a working level. The air pressure required will be the total lift, in feet, from the working water level plus the friction loss in the airline. Conduct the test and try to measure flow rate and drawdown quickly. Pumping creates turbulance in the well. Use the air-line method (paragraph 8-2c) to try and measure drawdown. Because of entrained air, use the measured-container method (paragraph 8-3a) to obtain flow-rate measurements.
a. Electric-Line Method. Water levels can be measured accurately with a two-conductor, battery-powered indicator known as an M-Scope (Figure 8-2). Well-completion kits usually contain an M-Scope. The M-Scope is a battery and a meter connected in series. When the upper wire on the tip of the M-Scope in the well touches the water, the circuit is completed and the meter gives a steady reading. Measure the amount of wire in the well to determine the depth to the water level. The wire is marked at 5-foot intervals for easy measuring.
- Chalk one end of a weighted steel tape with carpenter's chalk. Lower the tape (Figure 8-3) into the well to a depth of 1 or 2 feet past the chalk. (You can use soluble felt-tip markers as an alternative to chalk.)
- Measure the wetted length of the tape and subtract the amount from the total length lowered below the reference point to obtain the water depth. This test is accurate to within 0.01 foot.
c. Air-Line Method. You can measure the water level with an air line to follow drawdown and confirm a stable head during a test (Figure 8-4). The air line is usually 1/8- or 1/4-inch copper tubing or galvanized pipe that is long enough to extend below the lowest water level you are measuring. Fasten the air line to the pump bowls or cylinder. Install the airline with the pump. The pipe must be airtight; makeup all joints carefully. Measure the vertical length of the airline from the pressure gauge to the bottom of the line at the time of installation.
Attach a pressure gauge to the airline at the surface with an ordinary tire valve so you can pump air into the line. Pump air into the line until you get a maximum reading. The reading should be equal to the pressure exerted by the column of water standing outside of the airline. Subtract the reading from the total vertical length of air line to get the depth to the water below the center of the gauge. Readings are measured in feet, so you may have to convert your figures.
a. Measured-Container Method. You can determine the flow rate from a well or pump by measuring the time required to fill a container with a known volume. With this method, use small containers for early measurements and large containers for later measurements. Also, use an instrument, such as a stop watch, for accurate time measurements. Use the following equation:
FR = flow rate, in GPM.
v = volume, in gallons.
T = time required to fill container, in seconds.
b. Flow-Meter Method. A turbine-type flow meter will give an acceptable flow-rate reading. These meters are used by civilians. You can also use a totalizer-type water meter when the yield is low. Use these meters to measure the total gallons pumped and determine the flow rate. To do this, record the number of gallons that have flowed within a set amount of time and compute the flow rate.
c. Circular-Orifice Method. A circular-orifice meter (Figure 8-5) is a device you can make to measure discharge rates. This device gives good results and is compact and easily installed. The meter consists of a sharp-edged circular orifice at the end of a horizontal discharge pipe. The orifice is from one-half to three-fourths the diameter of the pipe. The inside of the pipe must be smooth and free from obstructions for a length of 6 feet from the orifice. The discharge pipe has a small hole on one side with a rubber-tube connection. The pipe is designed so that you can measure the pressure (head) in the discharge pipe at a distance of 2 feet from the orifice.
The length of hose and ruler depends on the pipe size you use (Table 8-2). The discharge pipe must be horizontal, and the stream must fall free from the orifice. The orifice must be vertical and centered in the discharge pipe. The combination of pipe and orifice diameters for a given test should be such that the head measured will be at least three times the diameter of the orifice.
d. Open-Pipe Method. With this method, the pipe is fully open and you measure the distance the water stream travels parallel to the pipe at a 12-inch vertical drop (Figure 8-6). Use the following procedure:
- Step 1. Measure the inside diameter of the pipe and the distance the stream travels parallel to the pipe at a 12-inch vertical drop. Your results will be in inches.
- Step 2. Estimate the flow from the pipe diameter and the distance the stream travels (Table 8-3). Your results will be in GPM.
For partially filled pipes, measure either the water depth or the freeboard. Divide the diameter by the water depth to get a percentage ratio. Measure the stream as above and calculate the discharge. The actual discharge will be, approximately, the value for a full pipe of the same diameter multiplied by the correction factor from Table 8-4.
|Join the GlobalSecurity.org mailing list|