WATER WELL SCREEN & CASING

SYSTEMS

Water Well Design

Well design is the process of specifying the physical materials and dimensions for a well. The

principal objectives of good design should insure the following:

• The highest yield with minimum drawdown consistent with aquifer capability

• Good quality water with proper protection from contamination

• Water that remains sand free

• A well that has a long life (25 years or more)

• Reasonable short – term and long – term costs

Well casing Design

Choosing the proper casing diameter for the well is important because it may significantly

affect the cost of the structure, depending on the type of drilling equipment used.

The diameter must be chosen to satisfy two requirements:

1- The casing must be large enough to accommodate the pump, with enough clearance

for Installation and efficient operation

2- The diameter of the casing must be sufficient to assure that the up hole velocity is

5 ft/sec (1.5 m/sec) or less.

Anticipated Well

yield (m³/day)

Nominal Size of

pump bowls (mm)

Optimum size of

well casing (mm)

Smallest size of

well casing (mm)

Less than 545 100 150 ID 125 ID

409 to 954 125 200 ID 150 ID

818 to 1910 150 250 ID 200 ID

1640 to 3820 200 300 ID 250 ID

2730 to 5450 250 350 OD 300 ID

4360 to 9810 300 400 OD 350 OD

6540 to 16400 350 500 OD 400 OD

10900 to 20700 400 600 OD 500 OD

16400 to 32700 500 750 OD 600 OD

Casing for water wells is constructed from various types of fiberglass-reinforced plastic

materials.

Fiberglass casing is resistant to most forms of corrosion, is not conductive, and for its weight

has the strength of steel.

Casing must have:

• Collapse strength

The following collapse formula provides the user practical tools to estimate collapse

strengths of various types of casing and screen. It is important that one pays particular

attention to the material and manufacturing standards for the casing and screen specified so

that acceptable tolerances are considered and adequate safety factors applied.

Timoshenko's Formula, shown below, is commonly accepted to be the most accurate

method available for estimating the collapse strength of pipe with diameter/thickness ratios

common to those used in the construction of water wells.

Where: Pcr = Theoretical collapse strength of a round tube

Where: Pe = Collapse pressure with ellipticity

E = Young's Modulus

u = Poisson's Ratio

Do = Outside diameter of casing

t = Wall thickness

S = Yield strength

e = ellipticity, frequently assumed as 0.01

• Tensile strength

The axial tension loads imparted to the casing string during installation come

primarily from the weight of the suspended pipe and screen, and also from the

additional drag forces that could be transferred to the string assembly during gravel

packing. The weight of a single –diameter casing assembly is easily calculated using

equation below:

�� = ������ − ����

Where:

Wt: Total weight casing

t: Wall thickness of pipe

D: Outside diameter of pipe

L: total length of pipe

k: 2.7 for US. Customary & 0.62 SI

An equation commonly used for determining casing tensile strength is shown below

(AWWA Standard for Water Wells, ANSI/AWWA A100-97, Appendix K, K.3)

��� = ��������� − ��2000

Where:

Cts : Casing tensile strength

Do: Casing outside diameter

St: Ultimate tensile strength

t: Casing wall thickness

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• Column strength

If the downward compressive force on a laterally unsupported casing assembly exceeds the

yield strength of the material, then the casing will buckle.

The axial compressive (column) strength of casing can be calculated using equation below

(which also is published in the AWWA standard for water Wells, ANSI/AWWA A100-97,

Appendix K, K.3).

��� = ���������� − ��2000

Where:

Cas: Casing axial compressive strength

Srp: Material yield strength

Do: Casing outside diameter

t: Casing wall thickness

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Well casing joint

Bell and Spigot Lock Joint

The Bell and Spigot Joint with 2 O-rings and Locking Key is the optimal solution for the water

wells field. The bell is integral at one end of the pipe and the spigot is the other end. The

hydraulic sealing is performed by means of two rubber O-rings. The mechanical coupling is

performed by a Locking Key that is inserted through the bell opening into a groove. The

joining system is very easy and can reduce installation’s time up to 50%. While drilling

through the float shoe and the cement, the casing is subject to a rotational force. This could

result in a relative rotation of two consecutive sections of casings. To avoid this, the joint can

be manufactured upon request with a special “Anti-Rotational” configuration.

Well Screen Design

Waverail manufactures two types of GRP screens:

• Cut Slot Screens

Cut slot screens are produced by cutting directly the GRP pipe wall. The cut slots are in

staggered rows to reduce the negative effect on the screen’s strength. The slot sizes are

between 1 and 3.5 mm and can give open areas ranging from 5% to 15%. As compared to

PVC slotted screens, GRP cut slots screens have much lower wall thickness and therefore

have less clogging problems.

These screens are recommended for:

- Consolidated formations or formations with large particles sizes.

- Shallow and medium-depth wells.

• Pipe Based Screens

Pipe-based screens are used in water wells in many part of the world because of their

strength and durability.

The assembly for this screen is made by perforating a base pipe and then mounting a

continuous-slot screen over it by slipping a separate continuous-slot well screen

jacket over a pipe (screen jacket).

The pipe based screen has two sets of openings, the outer continuous slot and the

holes drilled in the pipe base. The open area of the pipe base generally is less the

outer screen open area, and the inner pipe has a maximum open area of 10%.

Hydraulic performance of the assembly depends on the open area in the pipe base.

The external GRP screen jacket is obtained

by a special injection molding technique

which gives smooth and tapered slots. The

slot openings are horizontal and V-shaped

inwards to prevent clogging. The wires

have non-modified triangular shape with

round edges to guarantee laminar flow.

Continuous slot screen design

The first step in selection of slot sizes for naturally and filter –packed well is conducting a

grain size analysis.

Screen strength

Collapse strength of continuous slot screen is dependent on diameter of the screen, size, shape, and material type of wire used in its manufacture, and the slot size. The following formula can be used to estimate the collapse strength of continuous slot screen.

��� = 24������ + ����

Where: I = Moment of Inertia

� = ℎ���! + 4�" + "!�

36�" + ��

Where: Pcs = Collapse pressure of continuous slot screen

E = Young's Modulus

w = Width of wire on external face

s = Slot width of screen

D = Mean diameter of the screen

h = Wire height

b = Width of minor base of wire

Area/Flow Calculations

Well Production at AWWA Standard of 1.5 feet per second: To calculate the flow in gallons per minute per foot of screen, first you must calculate the percentage of open area of the screen, then use that result to calculate the flow. Determine the open area for the type of screen used then convert area to a percentage.

Calculation of open area for continuous slot screen:

% = & ��� + �� 37.7�) −& �

�� + �� *+12)

Where:

A = Open Area

s = Slot size

w = Wire width

Do = Average outside diameter of screen

d = Diameter of rods

n = Number of rods

Calculation of the flow in gallons per minute per foot of screen:

- = 4.8�%�12��� Where: F = Flow

Ap = Open Area (percent %)

Do = Average outside diameter of screen

= pi or 3.14159