Sealless Eccentric Pumps for Isocyanates
In the early hours of Dec. 3, 1984, water entered a tank containing 42 tons of methyl isocyanate (MIC). The resulting exothermic reaction increased the temperature inside the tank to more than 200 C (392 F) and dramatically raised the atmospheric pressure.
As a result, about 30 metric tons of MIC escaped the tank into the atmosphere in less than one hour. The gases were blown by southeasterly winds over the Indian city of Bhopal, and the chaos that followed resulted in the greatest industrial disaster in history.
The Bhopal gas tragedy, which originated at the Union Carbide India Limited (UCIL) plant in Bhopal, Madhya Pradesh, India, was reportedly responsible for 3,000 deaths within weeks of the leak. Another 8,000 people have since died from gas-related diseases. A 2006 government affidavit estimated that the toxic substance caused more than half a million injuries.
Theories vary on how water originally entered the tank. Some believe a disgruntled worker introduced water directly into the tank as an act of sabotage. Others cite management negligence. One thing is undeniable—the Bhopal gas tragedy demonstrated the destructive power of isocyanates if they are not properly handled and contained.
Isocyanates
Isocyanates are a family of highly reactive, low molecular weight chemicals. Methyl isocyanate is a monofunctional isocyanate, while the most widely used compounds are diisocyanates and polyisocyanates. Diisocyanates contain two isocyanate groups while polyisocyanates are usually derived from diisocyanates and may contain several isocyanate groups. The most commonly used diisocyanates include methylenebis (phenyl isocyanate), known as MDI, and toluene diisocyanate (TDI).
Eccentric disc pumps consist of a cylinder and pumping element mounted on an eccentric shaft. As the eccentric shaft is rotated, the pumping element forms chambers within the cylinder, which increase in size at the intake port, drawing fluid into the pumping chamber. The fluid is transported to the discharge port where the pumping chamber size is decreased. This action squeezes the fluid out into the discharge piping.
MDI and TDI are often used in the polyurethane industry, including the manufacture of flexible and rigid foams, fibers, elastomers and coatings—such as paints and varnishes. Isocyanates are being used more frequently in the automobile industry, auto body repair and building insulation materials.
Spray-on polyurethane products containing isocyanates have been developed for a wide range of retail, commercial and industrial uses to protect cement, wood, fiberglass, steel and aluminum, including protective coatings for truck beds, trailers, boats, foundations and decks.
Isocyanates are used in paints because they provide a cross-linking mechanism (chemical bonds) that is different from other coatings. As a result, isocyanates help create more durable coatings with outstanding aesthetic properties. These coatings provide films that cure quickly at low temperatures. They also have excellent application properties, producing smooth films and high-gloss coatings.
Isocyanates must be properly handled throughout the supply chain because the risk of overexposure can lead to a variety of short- and long-term health problems.
Exposure Risk and Results
Isocyanates are powerful irritants to the mucous membranes of the eyes and gastrointestinal tracts. Direct skin contact can also cause noticeable inflammation. Isocyanates can sensitize workers, subjecting them to severe asthma attacks if they are exposed again.
The most common occupational exposure to isocyanates is inhalation of the vapor or aerosol, although exposure may occur through skin contact during the handling of liquid isocyanates. Exposure typically occurs during the production and use of isocyanates, particularly during the manufacturing process in the polyurethane foam industry. This includes the pumping of isocyanates.
Isocyanates are toxics and pollutants, but they can also be flammable. While their flammability is low, specific concentrations, the presence of explosive vapors or other flammable liquids on the same site can create a hazardous environment for workers and the community, as in the Bhopal tragedy.
To safely transfer and contain isocyanates, operators must choose the correct pumping solution.
Given their unique properties, isocyanates require a pump that handles the product gently (low shearing), has strong suction capability and offers a low-maintenance, wear-compensation design. Different technologies can be used to transfer isocyanates, but the safest and most efficient solution that meets all these criteria is the eccentric disc pump.
Advantages of Eccentric Disc Pumps
Eccentric disc technology provides less risk and more efficiency than other pumps, including mag-drive gear pumps, which are commonly used for handling MDI. The advantages of using eccentric disc over gear are its design, strong suction, low shear and wear compensation.
Design
Eccentric disc pumps consist of a stationary cylinder and mobile disc. The disc is moved by an eccentric shaft housed inside a bellows end, which holds the disc. As the eccentric shaft is rotated, the disc forms chambers within the cylinder, increasing at the suction port and decreasing at the discharge port. During operation, the discharge pressure exerts itself against the eccentric disc, preventing it from slipping. The low slip between the disc and cylinder gives eccentric disc pumps their ability to strip or clear lines of product—a key advantage over gear pumps.
Isocyanates are toxic by nature, so they require sealless pumps to eliminate leaks and provide maximum product containment. Eccentric disc pumps and mag-drive pumps are sealless. Mag-drive gear pumps use a magnetic coupling to create a static shaft seal rather than packing or mechanical seals. Eccentric disc pumps have no magnetic couplings, packing or mechanical seals, which eliminates a potential leak point that could lead to failure.
Strong Suction
When loading or unloading MDI, having a pump capable of clearing the lines is important. The strong suction of eccentric disc pumps makes loading and unloading much safer and cleaner because hoses and pipes are emptied prior to hose disconnection at the end of the transfer. Eccentric disc pumps also feature superior suction at low speeds, ensuring that a minimum of product is left in the piping prior to any maintenance.
Low Shear
Isocyanates are dilatant liquids, meaning that when they are subjected to high shear and agitation, their viscosity tends to increase. Gear pumps by themselves shear more liquid than eccentric disc pumps, which can lead to overheating and blockages. The end result is pump failure and downtime. This problem is more common with gear pumps, but cannot happen with eccentric disc pumps because of their low shear rate and consumption of more energy.
Wear Compensation
Most positive displacement pumps do not have wear-compensation systems, which means normal pump wear will lead to progressive flow rate losses. Operators can try to compensate for wear by accelerating the pump, but that will begin a dangerous cycle of running the pump faster, resulting in a pump wearing faster and consuming more energy.
The unique design of eccentric disc pumps allows them to self-compensate for mechanical wear, giving the pumps the ability to maintain consistent flow rates over time. Eccentric disc pumps do not wear as quickly as gear pumps. From the first time a gear pump is turned on, the gears begin to wear, which creates wider clearances in the liquid path and less consistent flow rates and product containment. Eccentric disc pumps have a total wear-compensation system that keeps running despite wear and without the need for speed increases.
Conclusion
The Bhopal disaster demonstrated the hazards involved with handling isocyanates. In today’s manufacturing environment, particularly in the polyurethane industry, isocyanates are one of the most important compounds within the production process. Many short- and long-term risks are involved with handling isocyanates. Therefore, operators need the peace of mind that only eccentric disc pumps can deliver. Eccentric disc pumps are low maintenance and deliver the superior wear compensation, strong suction and low shear that is mandatory when handling this dangerous chemical.