Why Should We Use Plastic Casing And Screens

Estimates place the average annual water availability at 1869 BCM out of which the utilizable ground water resources are estimated at 452 BMC. This year our total requirement for utilizable water will be 750 BCM and in the year 2025 it would be 1050 BCM. However, estimates of availability depends on a lot of investment in the development and conservation of water resources.

One of the earth's most important sources of water is groundwater. It provides almost a third of all freshwater on earth. It is threatened by pollution, mismanagement, over exploitation and population. It is more difficult and very expensive to extract ground water.

Excessive withdrawal of ground water. pollution or mismanagement can deprive our future generation from using that  source. It can cause drying out of well and land subsidence.

Deep ground-water is relatively free from pollution in many places and is excellent for drinking. domestic use and industrial purpose. But once the aquifer is contaminated, remedial measures can be long-time and costly, even impossible. The slow penetration of pollutants can be called a ' CHEMICAL BOMB'. It threatens our future generations.

Because ground water is invisible, the dangers to it are often mishandled or totally disregarded. Another danger is of saltwater intrusion i.e. the displacement of fresh water in coastal aquifers by sea water.

Ground water tapping techniques have undergone a sea of  changes in the last 30 years. New materials for casing the tube-well have been in use replacing the conventional steel casing. Use of steel casing and screen are becoming obsolete world-over. Acceptance of thermo - plastic casing and screens is increasing day by day in developed and developing countries alike.

GENERAL INDICATORS OF CORROSIVE WATER

1. Low pH: If the pH is less than 7.0 an acidic and corrosive condition is  indicated.

2. Dissolved oxygen concentration greater than 2 ppm (probably occurs in shallow water table aquifers).

3. Hydrogen sulfide concentration at less than 1ppm.

4. Total dissolved solids greater than 1000 ppm(leads to electrolytic corrosion).

5. Carbon-di-oxide concentration greater than 50 ppm.

6. Chloride concentration greater than 500 ppm.

GENERAL INDICATORS OF ENCRUSTING WATER

1. pH greater than 7.5

2. Carbonate hardness greater than 300 ppm (deposition of Calcium Carbonate)

3. dissolved iron concentration greater than 2ppm.

4. Dissolved managers concentration greater than 1 ppm.

Corrosion is the major problem in steel casing and screens. Corrosion results from chemical and electrochemical processes. Chemical corrosion occurs when a particular constituent is present in water, in sufficient concentration, to cause rapid removal of material over broad areas. Commonly, these constituents are carbon dioxide, oxygen, hydrogen sulfide, hydrochloric acid, chloride and sulfuric acid. Chemical corrosion can cause severe damage in steel wells, regardless of the amount of total dissolved solids.

However, the number of wells affected by the chemical corrosion is small in comparison to wells affected by electro-chemical corrosion.

WHAT IS ELECTRO-CHEMICAL CORROSION ?

Electro-Chemical corrosion can be described as the flow of an electric current which facilitates the corrosive attack on metal. For this to happen, two conditions are necessary. :

Electro-chemical corrosion can be described as the flow of an electric current which facilitates the corrosive attack on metal. For this to happen, two condition are necessary :

1. A difference in electrical potential on metal surfaces &

2. Water containing enough dissolved solids to be a conductive fluid (Electrolyte).

HOW A DIFFERENCE IN ELECTRICAL POTENTIAL ON METAL SURFACES DEVELOP ?

An electrical potential difference may develop between two different kind of metal, or between nearby but, separate areas on the surface of the same metal. Different in potential on the same metal surface such as steel pipe can occur due to following reasons :

1. At heat affected areas around welded-joints.

2. At heated areas around torch-cut slots.

3. At work hardness areas around machine-cut slots.

4. At cut surface of exposed threads at pipe joints.

5. At breaks in surface coatings.

6. At the column pipe/submersible pump casing.

In these cases, both a cathode and anode develop and metal is removed from the anode.Bi-metallic corrosion results when two different metals are in contact and immersed in an electrolyte. A galvanic cell is created and corrosion occurs as the electro-Chemical action proceeds. A well-screen made of two different metals such as low-carbon steel and stainless steel will be damaged because the mild steel portion is corroded by galvanic action.

This is the reason why galvanized iron pipes corrode much faster than M.S.. pipes and hence in tube wells G.I. pipes are seldom used as casing/screen.

WHERE IS IT  LEADING US WHEN USE CORRODIBLE CASING & SCREEN ?

When we arrest the saline zone outside the tube well with clay packing and cement, we think we have prevented the saline zone from polluting the sweet water zone at the bottom. The saline zone at the top comes in to contact with the casing and the chemical corrosion takes places at the welded joints. At a faster rate the joints become leaky and allow the saline water to leak through the joints and pollute the bottom zone from inside the tube well.

This starts an electrolytic corrosion at the screen pipe which starts widening the slots and allows migration of sand and gravel into the well and finally the well collapses. If the collapsed well is not plugged properly the pollution continues and the zone which was giving sweet water, turns into a saline zone in that area and we have to drill deeper in search of sweet water increasing the cost of ground water tapping.

WHAT ARE THE OTHER CAUSES FOR WELL FAILURE ?

The JOINT-by far the site of most casing failure-is the most critical place of the well casing pipe-The proper preparation of the joint and the correct welding method play a major role in the casing's ability to maintain its structure and prevent leakage.

• Low strength materials may fail at the joint under high pressure.

• Incomplete welds (i.e. taking or failure to properly inspect the weld) can also   cause leaks.

• Excess slag resulting from the welding procedure can hinder tool movement inside the well, which can cause damage and wear to the tool and casing.

• If different materials are welded together, chemical reactions can create areas of low corrosion-resistance, causing eventual weakness and failure.

• Non circular casing or dented casing.

• Unknown verging wall-thickness or bad longitudinal welds.

WHAT IS THE ANSWER TO THIS PROBLEM ?

The answer to this problem has been found thirty years back in the from of Thermo - plastic Water-Well casing and screen. These casing and screens were initially used in shallow depth upto 400 feet and when the confidence grew with the use, now they are being used upto 2000 ft. depth without any problem.

WHAT ARE THE ADVANTAGES OF USING THERMO - PLASTIC WELL CASING AND SCREEN ?

1. Totally immune to corrosion-whether chemical or electrolytic.

2. Very smooth inner bore-reduce friction and deposit of scales or growth of iron bacteria.

3. In case of chemical choking of the screen and gravel pack,  rehabilitation of the well is possible with chemical treatment (either acidic or alkaline)

4. Due to horizontal slot pattern, they allow laminar flow into the well and convergence of flow is avoided, thus reducing well losses.

5. The screens are longitudinally ribbed and the slots are below the ribs. These ribs keep the gravel pack away from the slots by 2 mm and provide a continuous 'V' channel in the aquifer zone. This increases the permeability of the screen and reduces entrance losses further.

6. The screens are available with a complete range of slots providing a great latitude for the Engineer to select the screen according to the sieve analysis. The slots are available from 0.2 mm to 3.o mm width.

7. The casing and screen are made available in convenient lengths of 3 m and they are provided with spigot and socket with trapezoidal acme threads which make it easy to joint at site neither wasting much time on welding nor needing any special tools. The rig need not carry a welding-set, generator-set and a skilled welder. On request the screens are made available in 1 m, 1.5m, 2m and other odd lengths also.

8. Cost economy can be achieved by using lesser dia screen due to their excellent permeability. In many cases, it has been proved that a reduction of 25% draw-down has been achieved when compared to conventional M.S. Screens with higher percentage opening (almost double)

*Metal Casing and Screens are costly.

*Due to vertical slot opening-increased entrance resistance

*More than 50% if slot opening is obstructed by Filtering Media (Gravel Pack)    hence low permeability.

*Due to local action and corrosive soil nature the life the tube wells is as low as two years.

*Slot Opening Get enlarged and silting starts due to erosion in the screens.

*Metal casing and screens are heavy, this making installation cumbersome and tedious.

*Available only in 6 m length. Cumbersome and heavy. Requires more manpower at site. more cost.

*Metal Casing require special equipments life welding sets and generator sets for assembly and erection.

*Screen slot widths can not be made to suit aquifer sieve analysis for proper design of the well, which cannot be avoided.

*Due to its weight vertically can not be maintained.

Slotting to be done at extra cost like slitting transport and handling charges. AND RIBBED SCREENS will be substantially cost effective when compared with conventional mild steel casing and screens.

* Due to the horizontal slot openings entrance is very much reduced and also increased the collapse resistance of the screen.

* Due to the screens, the FILTERING MEDIA, is kept 2 mm (Gravel Pack) away from the slot opening and this increases the permeability of uPVC RIBBED SCREENS.

• uPVC WELL CASINGS AND RIBBED SCREENS are manufactured out of quality PVC compound and PVC being and inert material uPVC WELL CASINGS AND RIBBED SCREENS ARE totally unaffected by corrosion.

• Slot opening graving of PVC being 1.4, the uPVC WELL CASINGS AND RIBBED SCREENS weigh only 1/5th of Steel Casings and Screens. This makes it easier to handle and install.

• The specific gravity of PVC being 1.4 the uPVC WELL CASINGS AND RIBBED SCREENS weigh only 1/5th of Steel Casings and Screens. This makes it easier to handle and install.

• Available in different lengths from 3 to 6 m. Very easy to handle at site. Requires minimum manpower. Hence saves cost.

• uPVC WELL CASINGS AND RIBBED SCREENS are manufactured with screwed socket and spigot ends with trapezoidal threads which require no special equipment to assemble and install.

• uPVC RIBBED SCREENS are manufactured with slot widths ranging from 0.2mm to 3mm to suit to suit the aquifer sieve analysis which helps engineers to design the well to give better life and yield.

• uPVC WELL CASING AND RIBBED SCREENS are supplied with centering guides to maintain vertically of the well.

• Ready made factory cut slotted pipes. Saves  extra cost and time.