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:
If fine material overlies coarse material, extend not
less than two feet of the screen designed for the coarse material
into the fine layer.
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
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.
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.
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.
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.
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:
Friction loss from the water flowing through the openings is
negligible;
The rate of build up of incrusting minerals is held to a minimum;
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.
For a relatively thick coarse and aquifer, overlain by fine sand
with clay above that, the length of the screen should be at least one-third the thickness of the thick layer (10)
For relatively thin coarse sand layer overlain by a thick layer
of fine sand with clay above it, the screen length should be nearly as
great as the entire thickness of the coarse layer (10).
For a coarse sand layer overlain by clay and underlain by fine
sand, the length of screen should be about one-half the thickness of
the coarse layer. The well should be completed only to the bottom of
the coarse layer (10).
For a thin fine sand layer between two thin coarse layers, the
length of the screen should be almost the width of the bottom coarse
layer.
Well Screen Slots
A screen that retains 30% of the aquifer material
will allow 70% of the material to pass during development.