*TM 5-813-1/AFM-88-10, Vol. 1
The coarse aperture size should not be greater than
(3)
Diameter. The screen diameter shall
twice the fine size. For a gravel packed well, the screen
be selected so that the entrance velocity through the
should retain 85 to 100 percent of the gravel. Screen
screen openings will not exceed 0.1 foot per second.
aperture size should be determined by a laboratory
The entrance velocity is calculated by dividing the well
experienced in this work, based on a sieve analysis of
yield in cubic feet per second by the total area of the
the material to be screened. Consult manufacturer's
screen openings in square feet. This will ensure the
literature for current data on screens.
following:
(2)
Length. Screen length depends on
(a) The hydraulic losses in the
aquifer characteristics, aquifer thickness, and available
screen opening will be negligible.
drawdown. For a homogeneous, confined, artesian
(b) the rate of incrustation will be
aquifer, 70 to 80 percent of the aquifer should be
minimal,
screened and the maximum drawdown should not
(c) the rate of corrosion will be
exceed the distance from the static water level to the top
minimal.
of the aquifer. For a nonhomogeneous, artesian aquifer,
(4)
Installation. Various procedures may
it is usually best to screen the most permeable strata.
be used for installation of well screens.
Determinations of permeability are conducted in the
(a) For cable-tool percussion and
laboratory on representative samples of the various
rotary drilled wells, the pull-back method may be used. A
strata. Homogeneous, unconfined (water-table) aquifers
telescope screen, that is one of such a diameter that it
are commonly equipped with screen covering the lower
will pass through a standard pipe of the same size, is
one-third to one-half of the aquifer. A water-table well is
used. The casing is installed to the full depth of the well,
usually operated so that the pumping water level is
the screen is lowered inside the casing, and then the
slightly above the top of the screen. For a screen length
casing is pulled back to expose the screen to the aquifer.
of one-third the aquifer depth, the permissible draw-down
(b) In the bail down method, the
will be nearly two-thirds of the maximum possible
well and casing are completed to the finished grade of
drawdown. This drawdown corresponds to nearly 90
the casing; and the screen, fitted with a bail-down shoe is
percent of the maximum yield.
Screens for
let down through the casing in telescope fashion. The
nonhomogeneous water-table aquifers are positioned in
sand is removed from below the screen and the screen
the lower portions of the most permeable strata in order
settles down into the final position.
to permit maximum available drawdown. The following
(c) For the wash-down method, the
equation is used to determine screen length:
screen is set as on the bail-down method. The screen is
lowered to the bottom and a high velocity jet of fluid is
directed through a self closing bottom fitting on the
screen, loosens the sand and allowing the screen to sink
where:
to it final position. If gravel packing is used, it is placed
L = length of screen (feet)
around the screen after being set by one of the above
Q = discharge (gpm)
methods. A seal, called a packer, is provided at the top
A = effective open area per foot of screen
of the screen. Lead packers are expanded with a
length (sq. ft. per ft.) (approximately one-half of the
swedge block. Neoprene packers are self sealing.
actual open area which can be obtained from screen
(d) In the hydraulic rotary method of
manufacturers.)
drilling, the screen may be attached directly to the bottom
V = velocity (fpm) above which a sand particle
of the casing before lowering the whole assembly into
is transported; is related to permeability as follows:
the well.
2
P (gpd/ft )
V (fpm)
e.
Gravel packing.
Gravel packing is the
5000
10 (Max)
process by which selected, clean, disinfected gravel is
4000
9
placed between the outside of the well screen and the
3000
8
face of the undisturbed aquifer. This differs from the
2500
7
naturally developed well in that the zone around the
2000
6
screen is made more permeable by the addition of
1500
5
coarse material. Gravel-pack material must be clean
1000
4
and fairly uniform with smooth, well-rounded grains.
500
3
Gravel shall be siliceous material.
0-500
2 (Min)
5-13
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