PRIVATE WATER SYSTEMS - Design - Well Screen


Well Screens

Screen Diameter

The screen diameter selected should be the smallest practicable diameter consistent with the casing diameter and the expected yield of the well.

Screen Length

The screen length is determined by the total area of screen openings required to keep the entrance velocity of the water at or below 0.1 feet per second. For high capacity public wells, the entire width of the aquifer is screened. For private wells having limited yields, however, it is inefficient to screen the entire width, and shorter screens are used.

Definite rules on the positioning of short screens in aquifers do not exist, but some examples are given here.

While doubling the diameter of the screen will increase the yield of the well by 10 to 15%, doubling the length of a short screen will also double the yield. Therefore it is easy to see that the flexibility in designing a well screen to provide the maximum yield while supplying enough open area to minimize head loss lies in determining the length of the screen.

Slot Size

For naturally developed wells, the size of openings in the screen should be determined from grain size analyses of the water-bearing formation. The slot size chosen should be within the range of sizes that will keep from 30 to 50% of the aquifer material from passing the screen. When the water is considered to be corrosive and the aquifer consists of fine uniform sands, the slots should be sized to retain 50% of the aquifer material. Conservative selection of the slot size is also appropriate when there is some doubt as to the reliability of the sample, when the aquifer is overlain by fine-loose material, or development time is at a premium.

Slots that retain only about 30% of the aquifer material should be chosen when the aquifer consists of coarse sand and gravel.

In general, the coarser and more homogeneous the aquifer material, the greater the acceptable range of screen slot sizes. It is also helpful to keep in mind that the larger the screen slots, the more material will pass and the longer it will take to develop the well. On the other hand, larger slot sizes allows for development deeper into the aquifer thus increasing the specific capacity and efficiency of the well.

Multiple Well Screens

In inhomogeneous aquifers, a screen having slot sizes corresponding to the gradation of grain size in the aquifer may be installed. There are two rules to follow in designing multiple well screens:

  1. If fine material overlies coarse material, extend not less than two feet of the screen designed for the coarse material into the fine layer.
  2. If fine material overlies coarse material, the slot size for the screen section to be installed in the coarse layer should not be more than double the slot size for the overlying fine layer

Applying these two rules reduces the possibility of pumping sand in case the logging was inaccurate and the depths of the strata are not precisely known.

Screen Types

  1. Continuous Slot: Continuous slot screens provide more intake area per square foot of screen than any other type. They are constructed by winding cold-drawn wire, spirally, around a circular array of longitudinal rods.

    As a result of the extensive open are, a shorter (and therefore cheaper) screen may be used in place of a more expensive, longer screen having a less open area.

  2. Louver (or Shutter Screens): The shutter-type screen has openings that are really like rows of louvers. The shape of the louvers is such that these screens cannot be used successfully in naturally developed wells, and the openings become blocked during the development procedure. Also, sizable blank spaces must be left between the openings for strength, so the open area is limited.

    Shutter screens commonly come in 5 foot lengths that can be welded together to make longer screens.

  3. Pipe-base well screens are made by winding wire directly around a perforated steel pipe forming a continuous-slot screen jacket around a core of steel perforated pipe. The steel lends strength to the screen, while the jacket prevents the perforations from becoming clogged.

    The open area of the pipe is less than that for the jacket, so the hydraulic efficiency of the screen is determined by the pipe. As a result, the efficiency is usually quite low.

  4. Drive Well Points: There are several choices of drive points available:
    • Continuous-slot screen with a forged steel point is the most hydraulically efficient. It will also withstand hard driving but should not be twisted;
    • The Brass-Jacket type of well point consists of a perforated pipe covered with a bronze wire mesh. The mesh is in turn covered with a perforated brass sheet to protect it from damage. The forged steel point at the bottom has a widened shoulder designed to push gravel or stones to the side and reduce the danger of ripping or puncturing the jacket.
    • The brass tube type consists of a slotted brass tube slipped over a perforated steel pipe. This has about the same intake area as the mesh-covered well point and is not as easily ripped.

  5. Slotted Pipe: Pipes with slots cut with torches, or punched through may be used as a substitute for screens. There are many limitations:
    • Structural strength requires wide spacing between slots;
    • Varying opening size;
    • Very low percentage of open area;
    • Difficult to impossible to cut openings small enough to retain fine sand;
    • Jagged edges around slots are more susceptible to corrosion.


Entrance Velocity (10)

Lab tests and field experience show that if the entrance velocity of the water entering the well screen is 0.1 ft/sec or less:
  1. Friction loss from the water flowing through the openings is negligible;
  2. The rate of build up of incrusting minerals is held to a minimum;
  3. and Rate of corrosion is held to a minimum.

Calculating Entrance Velocity

To calculate the velocity (assuming the pump is located above the screen) of the water as it enters the wells screen, divide the expected yield of the well by the total area of the openings in the screen. If the figure is greater than 0.1 ft/sec, the diameter of the screen should be increased to provide a greater open area.

Examples

There are four different examples.

Well Screen Slots

A screen that retains 30% of the aquifer material will allow 70% of the material to pass during development.

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