Rubber lining

RUBBER LINING:

Rubber lining is a rubber sheet or membranes used for protection from corrosion in aggressive chemical environments. There are several rubber Lining applications some of which include resistance to acid, chemicals and alkalis, abrasion resistance, infect protection

Types of Rubber for Linings

·     Natural Soft Rubber Linings

·     Hard Rubber Linings

·     Synthetic Rubber Linings

Various Synthetic rubbers  like chloprene rubber, butyl rubber, and hypalon rubber are used for making these types of rubber lining. 

Both the types of rubber- natural Rubber  and Synthetic rubber - are used for making different varieties of rubber linings. The type of rubber depends upon the purpose and application of rubber lining.

Natural soft rubber linings are usually made from various types of poly soft rubber. These types of rubber have such characteristics as low hardness, high resilience, and flexibility. 

They are abrasion resistant linings capable of absorbing and repulsing the impact of abrasive forces of material handled. They are suitable for slurry tank lining, pipe lining, pump lining etc. Natural soft rubber linings are also used for pickling tanks, storage tanks resistance to hydrochloric acid and sulphuric solutions.

Hard rubber linings are generally made from various types of poly hard rubber. They are chemical resistant linings with good flexibility and high impact resistance. Some of them are graphite based rubber linings having excellent chemical resistance at high temperatures and some others are exhaust steam vulcanized rubber linings that are suitable for lining vessels, which can't be cured with pressurized steam. These various types of hard rubber linings are suitable for storage tanks, reactors, plants which use ion exchange membrane process, pipes at high operating temperatures etc.

Butyl Rubber Lining- ButylRubber  has excellent chemical resistance at high temperatures. These rubber linings are also very durable due to the inherent low vapor permeability. Butyl rubber linings are, therefore, good for use in vessels, pipes for acid plants, FGD, hypochloride storage and transport. 

Chlorobutyl Rubber Lining- Chlorobutyl rubber linings made of chlorobutyl have properties similar to butyl rubber lining but are more easier to apply. They have heat resistance upto 200°F. These linings are unaffected by cold weather or rapid temperature changes.

Hypalon Rubber Lining- These types of rubber linings are based on chloro sulphanated polyethylene rubber having superb chemical resistance to oxidizing acids at high concentrations. Hypalon rubber linings are also available in pre vulcanized form. They are good to be used in vessels, pipes for high acid concentration, and hypo chlorite service. 

EPDM Rubber Lining- EPDM rubber has high tensile strength along with excellent resistance to punctures, UV radiation, weathering and microbial attack. As it is highly flexible material with a low co-efficient of thermal expansion and contraction, EPDM lining can be applied in a wide range of temperatures and terrains. These characteristics of EPDM rubber not only makes it suitable for linings but also for rubber  Coating  As far as EPDM rubber linings are concerned, they are suitable for all types of pond lining such as decorative ponds, golf course ponds, irrigation ponds as well as other water features like lakes & streams, canals and channels, reservoirs etc. They are also suitable for lining in landfill caps, waste water treatment facilities, and fish hatcheries. 

Rubber Lining Application

Once the proper rubber for a given application is chosen, it is applied in five basic steps:

     1.      Prepare the surface for rubber application, usually by solvent cleaning, and blast 

cleaning  to  remove all contamination, and to roughen the surface for better adhesion.

2.      Apply the adhesive system to the surface to be lined. The adhesive may be a single coat or a multi coat system.

3.      Apply cut to fit rubber sheets to the surface using rollers and stitchers to press out air 

pockets  and to press the sheet into the adhesive.

4.      Cure, or vulcanize the rubber lining by steam in an autoclave, exhaust steam, or by chemical application to the rubber surface.

5.      Inspect visually for bubbles or blisters, and for pinholes using a high voltage holiday spark 

tester.

A rubber covering it lining may be attached to various kinds of surfaces such as following

Ordinary steel:

This is the most widely used material for lined containers. the normal working adhesion between rubber and steel is around 500 psi.

Stainless steel:

As the percentage of bucket in stainless steel alloys increase the adhesion it is possible to obtain between rubber and alloy decreases.The percentage of chromium in the alloy is not particularly important with respect to adhesion.

Aluminum:

Rubber lining do not adhere to aluminum so well as to steel.

Brass:

Containers made of various kinds if brass but the demand for such construction has been small. Rubber can be bonded to brass having copper percentages runnings from 70 to 80 but the lining materials mist be specially compounded.

Products commonly lined:

Typical industrial equipment that are commonly lined with rubber include railway tank cars, chemical storage tanks, pickling tanks, playing tanks, pipes for carrying chemicals etc.

Lining materials factors:

The selection of the lining materials may depend on several factors such as chemical resistance,workability of compound, cost and temperature to be encountered.

Vulcanization

Vulcaniziation  a critical stage of processing is necessary to enhance the performance properties a  critical stage of processing is necessary to enhance the performance properties of raw 

compounded rubber through cross linking of its molecular structure. 

Cross-linking (copolymerization) is accomplished through a reaction with accelerators like 

organic peroxides or sulphur that are mixed into the raw rubber base. The cross linking process is  called “vulcanization (to over-vulcanization on prolonged heating). 

AUTOCLAVE CURE

This method involves placing the object(s) to be covered or lined with rubber inside a pressure vessel called an autoclave. A controlled and continuous flow of steam from a boiler is introduced into the vessel. At the start of the process, air is purged from the autoclave. The temperature and pressure is carefully monitored. Because an autoclave offers the best form of control over the vulcanization process, it offers the greatest potential for producing uniform and high-quality products. However, to achieve these goals, it is recommended that the autoclave be equipped with precise controls for air pressure and steam. 

Variations in steam pressure and temperature will lead to undesirable anomalies in the cured rubber. Once started, an autoclave cure should never be interrupted. Prompt cool-down at the termination of cure is also important. Proper cool-down of the autoclave will prevent post-curing and preclude the possibility of blistering and cracking hard rubbers.

While autoclaves can offer precise management of the vulcanization process, they are size-limited. Large tanks, water boxes, and other process equipment cannot fit inside. Rubber applied to these units must be vulcanized by other means.

INTERNAL STEAM CURE

Internal steam pressure vulcanization is used on vessels that are designed to operate at elevated pressures and are too large to be placed in an autoclave. By this method, units can be installed and the linings cured in place. Internal curing should be accompanied by the installation of drains and traps to collect steam condensate. Recording thermometers and pressure gauges installed near the bottom of the vessel are used to monitor the process. Blind flanges should be installed at other openings. 

Bottom outlets should initially be left open to purge the vessel of air. Boiler capacity should be sufficient to raise the temperature from ambient to cure in a relatively short period. Long runs of uninsulated pipe between the boiler and lined vessel should be avoided. Once started, the vulcanization process should not be interrupted. Once the endpoint of the cure is reached, the vessel is cooled down by the introduction of air. Any steam-induced curing cycle should include an appropriate vacuum-break device to prevent a vacuum collapse of the vessel.

ATMOSPHERIC STEAM CURE

Atmospheric steam is normal used for field vessels that have open tops or bottoms, or cannot withstand elevated pressures. With the case of open tanks, suitable temperature-resistant, steam-tight covers must be fabricated for all openings. Steam is introduced through an insulated steam line through a piping connection or opening in the top of the vessel. Provisions must be made to drain all condensate that collects at the tank bottom. Other outlets should be closed with blind flanges drawn up to within 1/32” of the flange rubber. 

Vulcanization at atmospheric pressure will allow entrapped air under linings to form blisters. Some procedures call for a pre-cure steaming period of one hour, followed by an internal inspection of the tank to located and repair spontaneous blisters. In this method, entrapped air is removed by deflating the blister with a hypodermic needle and applying a small patch of uncured rubber. The vulcanization process then proceeds to completion.

AMBIENT (CHEMICAL) CURE

Rubber is cured at ambient temperatures when heat cannot be applied to surfaces to achieve vulcanization. The method is most commonly used when patches must be applied to vulcanized rubber surfaces. A chemical cure may also be used when surfaces like flanges will be shielded from steam exposure. 

These methods expose the raw rubber to a sulphur-bearing chemical for a sufficient period to complete cure. The chemicals include carbon disulfide and sulphur dichloride. They are used as either a solution applied to the rubber surface or vapour form. Vulcanization is accomplished in periods of up to one week. If the vessel will operate at temperatures of up to 150ºF, the chemical cure can be shortened.

SELF VULCANIZED (COLD BOND)

In the case of some on-site rubber linings where other curing methods are impractical, self vulcanized rubber sheets have been developed. The rubber in these sheets, specific soft rubber mixtures commonly used with a partial cured polychloroprene bottom layer for greater adhesion strength. These linings are ideal for chemical and high abrasion patch repair work. Self-vulcanized sheets can also be stored indefinitely, and installed as fully vulcanized linings. As such, they will be immediately resistant to the operational environment.

INSPECTING RUBBER LINING WORK

Inspection before Lining

Surfaces to be covered with rubber should be cleaned by abrasive blasting to achieve a cleanliness equivalent to Steel Structures Painting Council Specification SSPC SP-5, “White Metal Blast Cleaning”. The cleaned surfaced should have a minimum surface profile of 2 mils. The surfaces to be coated should be free of all oil, grease, mill scale, rust, corrosion products,

oxides or other foreign matter. Primer should be applied immediately after cleaning and before any visible surface oxidation has occurred. Precautions must also be taken to avoid condensation when applying primers and cements to metallic surfaces. No operations should be conducted when the metal temperature is within 5ºF of the dew point.

Inspection During Lining Operations

The installation of a rubber lining system involves a series of steps. Each one requires surveillance to ensure that the specification representing the manufacturer’s recommendations is being followed. Liquid primers-used to prepare the metal surface, and liquid cements-to create an adhesive bond at the metal-rubber interface, and rubber-rubber interfaces, are generally applied by hand brush. Different rubber systems require different primers and cements. Separate brushes and rollers should be maintained for each type of primer and cement used in the installation. In this work, the sequence of application, drying time and selection of the mating surfaces are critical to the success of the work. The manufacturer’s recommendations should be diligently enforced.

Joints are usually necessary in lining work, because many sheet rubber materials are supplied in widths up to only 48”. Four methods of construction are typically used to join rubber panels with the selection determined by the type of elastomeric used.

Butt Joint – A joint where the two rubber panels are laid edge to edge without any tapered mating surface. Butt joints are usually covered with an additional cap strip.

Lap Joint – A joint where one rubber panel overlays the next. A minimum overlap of 2” is recommended at the joint.

Skive Joint – A joint where one rubber panel is butted against the next piece. Skives should be cut at a 45º angle between abutting pieces.

Closed Skive Joint – A butt joint where a reverse 45º angle cut is made between the abutting rubber panels. The reverse skive is the recommended joint for all rubber panels. It is required whenever multi-layer rubber sheet containing a tie gum is used. The reverse cut allows the installer to stitch down the cut edge so that the tie gum is protected from chemical attack.

Stitching is a method of joining two pieces of uncured rubber. It utilizes a hand-held tool called a stitching roller. The tool is a narrow wheel with a serrated edge that applies a continuous line of localized compression points to drive the rubber panel into the adhesive layer.

Joint construction should comply with both the specification and the manufacturer’s recommendations for the rubber product used in the installation.

After the rubber sheet is applied to the cured cement, it must be rolled tightly against the metal to remove any trapped air and ensure intimate contact across the metal-rubber joint. Joints should be examined to verify that the seams are straight and all edges are stitched down tightly.

Inspection before cure

Immediately following the application of the rubber to the metallic surfaces. The lining should receive an inspection to ensure that the following parameters have been met:

The lining should be checked to assure that the physical dimensions of protected surfaces and the thickness of linings comply with the requirements of the specification. Modifications are more easily made before the system has been cured (vulcanized). Unacceptable areas should be marked with chalk.

Prior to vulcanization, all lined surfaces should be inspected for blisters, wrinkles, pulls, lifted edges (bond failures), or surface defects. All splices (joints) in the lining system should be inspected for integrity and uniformity. Whenever layered rubber stock is used, stitched-down splices are necessary to protect the tie gum.

Air trapped under the rubber sheet should be located and removed. Air bubbles can be located by lighting the surface from an oblique angle and looking for shadows cast by the high spots. The air can be released with a hypodermic needle and the site stitched-down or patched. This step is especially important when atmospheric cures are used with no applied pressure to force out air or flatten the lining.

The lining should be examined for pinholes, punctures, and cuts with a high-voltage spark tester. Both the Rubber Manufacturers Association (RMA) and the American Society for Testing and Materials (ASTM) have written standards for the spark testing of elastomeric sheet linings. High frequency, AC-type spark testers, capable of producing sufficient voltages to achieve proper calibration should be used. Voltages should comply with those recommended by the rubber manufacturer and will vary with thickness and rubber type. The inspection voltage range for most rubbers is between 10,000 and 15,000 volts. Lined tanks should be purged of all volatile vapours and solvents before high voltage spark testing is applied.

Post Cure Inspection

Following completion of the vulcanization process, the rubber surfaces should be inspected for signs of obvious imperfections. These may take the form of loose splices, trapped air blisters and breaks in the lining.

The hardness of the cured rubber should be checked with an appropriate durometer in accordance with a standard test procedure. Durometer (indentation hardness) measurements should cover the entire rubber surface. One reading per 100 square feet of surface should be taken and recorded. The measurements should fall within the tolerances recommended by the

manufacturer’s specifications for the vulcanized product. Areas of the lining that fall below the manufacturer’s tolerances should be isolated and cured through localized exposure to additional hot air or steam.

The rubber surfaces should again be inspected using a high voltage spark tester that is calibrated and adjusted in accordance with the rubber manufacturer’s guidelines. Pinholes and flaws located by spark testing should be repaired in accordance with specified procedures and vulcanized through exposure to hot air, steam or infrared lamps. If cold patches are utilized, they should receive an appropriate chemically induced cure.

Post-Installation Inspection of Rubber Linings

Periodic inspections of rubber linings are usually conducted to determine if there are any flaws or lining anomalies that have developed during the course of operations within vessels, tanks, pipelines and other lined equipment. Among the areas that can be investigated are:

Appearance – A visual inspection can be conducted to locate such lining anomalies as blisters, bubbles, open or loose seams and patches, pinholes or discolouration. In steel tanks, breaks in the lining are often associated with corrosion products that cause localized disbondment and staining.

Hardness – Hardness is measured by using either the Shore “A” or Shore “D” Durometer on the cured (vulcanized) rubber system. Durometer measurements can be made to determine if there has been a significant change from the rubber hardness after vulcanization and before vessel operation. These could include hardening form overheating or softening form exposure to aggressive chemical.

Electrical Integrity - A high voltage inspection can be conducted of the rubber lined surface. However, such an inspection should only be made with the concurrence of the rubber manufacturer, using recommended voltage levels. Some rubber linings that have been in immersion or chemical service will undergo resistivity changes from absorbed moisture and/or chemicals. These changes wither preclude the use of high voltage inspection or require reduced voltages.