Groundwater Wells

Site Selection Foremost among the factors that must be considered in selection of a site for a proposed well are local regulations governing proximity to such potential contaminants as cesspools, privies, animal pens, or abandoned wells. Also to be considered is the effect on adjacent wells—if wells are located too close together, excessive head losses may result.

Types of Pumps Used The two basic types of pump used in groundwater wells are vertical turbine, line shaft pumps, and submersible pumps. The vertical turbine pump consists of three basic components: the driving head, the column-pipe assembly, and the bowl assembly. The driving head is mounted above ground and consists of the pump discharge elbow, the motor support, and the stuffing box. The column-pipe assembly (consisting of shaft, bearings, and bearing retainers) and the bowl assembly (consisting of a suction head, impeller or impellers, discharge bowl, and intermediate bowl or bowls) are suspended from the driver head. Use of multiple bowls and impellers results in a form of series operation and permits pumpage against very high heads.

The submersible pump utilizes a waterproof electric motor located below the static level of the well to drive a series of impellers and produce a series operation similar to that of a line shaft pump. However, the length of shafting required is greatly reduced, and thus the shaft losses and total thrust are minimized. As a result, the submersible pump becomes economically competitive with the line shaft pump at great depths.

Air-lift pumps, which operate on the principle that a mixture of air and water will rise in a pipe surrounded by water, may be used in some cases. Such pumps are easy to maintain and operate and can be used in a crooked well or with sandy water. However, they are relatively inefficient (usually 30 to 50%) and allow very little system flexibility.

Reciprocating pumps are also used in some cases where small capacities are required from deep wells. Such pumps can be driven by electric motors or windmills, but they are generally noisy and are more expensive than centrifugal pumps.

Determining Pump Capacity The capacity of any well is dependent on such factors as screen size, well development, aquifer permeability, recharge of groundwater supply from rainfall and streams, and available head. The basic procedure used in sizing a pump for well service involves drilling the well and performing a test operation. First the static head, or elevation of the groundwater table prior to pumping, is determined. Water is then pumped at various rates and the drawdown associated with each pumping rate determined. A plot of drawdown versus pumping rate can then be derived. Pumping rate is usually measured by a weir, orifice, or pilot tube, and drawdown is determined with a detector line and gage or with an electric sounder.

From the test data and from a preliminary layout of discharge piping, a system-head curve can be derived, with drawdown added to frictional losses for each pumping rate. The pump characteristic curve can then be superimposed on the system-head curve to determine the capacity that can be attained with a specific pump. It should be noted that pump curves for line shaft pumps are based on the results of shop tests, which do not allow for column frictional or line shaft and thrust losses. Consequently, the laboratory characteristic curve for any line shaft pump must be adjusted to field conditions.

Field pumping head can be determined by subtracting column frictional losses from the laboratory head. Field brake horsepower (brake kilowatts) is determined by adding shaft brake horsepower (brake kW), which depends on shaft diameter and length, and on rotative speed, to laboratory brake horsepower (brake kW). Field efficiency is determined from the formula

In USCS units Field efficiency :

In SI units Field efficiency :

gpm X field head in feet 3960 X field brake horsepower m3/h X field head in meters 367.5 X field brake kW

Because thrust loads cause additional losses in the motor bearing, it is necessary to determine the additional power required to overcome thrust losses. Total thrust load is equal to the sum of the shaft weight and the hydraulic thrust (which varies with laboratory head for any particular impeller), and losses due to thrust amount to approximately 0.0075 hp/100 rpm/1000 lb (0.00126 kW/100 rpm/1000 N) of thrust. Motor efficiency is then calculated by dividing the motor's full load power input (without thrust load) by the sum of full load power input and loss due to thrust. Overall efficiency then equals the product of field efficiency and motor efficiency.

As a result of the efficiency losses produced by shaft weight and length in line shaft pumps, it is usually more economical to use a submersible pump at depths of more than about 500 ft (150 m). Sizing a submersible pump requires calculations similar to those for a line shaft pump. However, the submersible pump installation requires a check valve in the column pipe, which must be considered in the determination of frictional losses. Moreover, the efficiency losses resulting from the motor cable (expressed as a percentage of input electric power) must be considered in determining overall efficiency, which can be calculated from the formula:

water power X (% motor efficiency — % cable loss) Overall efficiency =-

where in USCS units Water power in hp : in SI units Water power in kW :

shop brake power X 100

gpm X field head in feet 3960

m3h X field head in meters 367.5

Cable size must be selected on the basis of motor power and motor input amperes, voltage, and cable length.

Well Stations A typical well station generally includes a small building for housing the pump, pump controls, metering and surge-control facilities, and chemical feed equipment. Submersible pumps do not require a pump house for protection, but if pump controls or chemical feed equipment are provided, an enclosure of some type is required. Because well water supplies are often pumped directly into the distribution system, a differential producer is usually installed for metering. Moreover, chemicals may be added at the well station to minimize corrosion, control bacteria, decrease hardness, and inject fluorides into the water supply. Surges are usually controlled by installation of a surge valve in the pump discharge line. Controls may also be installed to permit starting and stopping of the pump from remote central locations and to provide for measurement and control of well drawdown.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

Get My Free Ebook

Post a comment