Using Plastic AODD Pumps for Chemical Applications

Two steps forward, one step back. That is what it feels like to run an operation with a leaky pump, which inevitably leads to unscheduled downtime and increased maintenance needs. This stalls productivity and can cost companies hundreds of thousands of dollars per hour of downtime. When hazardous chemicals are involved, the situation is even more serious. Aggressive acids and slurries can cause pumps to leak much faster, and they create an unsafe work environment. Air-operated double diaphragm (AODD) chemical pumps offer many benefits to manufacturers that require the use of hazardous materials. Most other pump designs rely on mechanical seals for chemical containment, but mechanical seals are known for being prone to chemical erosion and leaking. AODD chemical pump designs eliminate the need for mechanical seals by using bolts to secure the pump together and create an effective seal. The AODD pump’s diaphragms move in gentle cycles to minimize turbulence and protect the liquid from shearing. In many instances, these chemicals can be as fragile as they are hazardous, and any mild turbulence could alter their chemical properties, wasting the material and costing money. Because AODD pumps are powered by compressed air rather than electric motors, they can run dry and deadhead without the risk of burning, seizing or harming their components. The problem with AODD chemical pumps is that their designs can vary in quality and effectiveness. The crucial factors of the AODD chemical pump selection process are chemical containment capabilities, corrosion resistance and icing prevention.

Leaky AODD Chemical Pumps

Safety is the primary concern when moving dangerous chemicals. This requires a pump with a leak-free design. Unfortunately, even some of the best AODD pumps struggle with leaks over the course of the pump’s life. A few design features stand above the rest to address certain factors that lead to leaking. In the case of plastic AODD pumps, leaking is often the result of cold flow and creep, which are caused by high temperatures, amount of time under load and level of torquing stress. While metal pumps are rarely damaged by loads at low temperatures, plastic pumps can be damaged by continuous loads at fluctuating temperatures, which is called cold flow. As temperature increases, the effects of cold flow on a plastic AODD pump escalate. Cold flow causes deformation of the plastic pump. The pump’s components release against compression points and begin to leak. Creep refers to the total deformation of the pump beyond the immediate strain of loading and can cause significant leakage. Polytetrafluoroethylene (PTFE) is highly susceptible to leaking caused by cold flow because PTFE tries to escape the load as it gets warmer. PTFE is also one of the most chemically compatible materials for resisting corrosion. Ultra-high-molecular-weight polyethylene (UHMWPE) is second to PTFE in terms of chemical compatibility, but it is also susceptible to cold flow leaking, although at a lower rate. In addition, the violent forces of operation and continuous loading can eventually loosen the AODD chemical pump’s bolts and create leaks. End users are advised to re-torque bolts regularly. Some AODD pumps require even further leak prevention steps, such as the installation of reinforcing metallic plates to evenly distribute the load from the housing bolts.

Leak Solutions

One solution for leak prevention is a machined body reinforced with a dual-force, tie-bolt design. Overall, machined housing components are essential for providing a precision fit that will distribute the load evenly and greatly reduce leaking. With this design, components made with PTFE and UHMWPE, which are highly susceptible to creep and cold flow, are held securely by tie-bolts that are resistant to cold flow and violent forces. This allows the pump design to make use of these chemically compatible materials without leakage occurring because of cold flow. Pumps that have exposed metal or wet parts composed of metal are highly vulnerable to corrosion, so AODD chemical pumps designed with metal-free exteriors are more durable in corrosive environments. Similarly, PTFE and UHMWPE fluid paths can ensure chemical compatibility and prevent interior corrosion. This means that the entire fluid path is either PTFE or UHMWPE, so there is no need to worry about incompatible chemicals eating away the pump’s interior. Because AODD pumps are operated by compressed air, they are subject to rapid changes in temperature. The air is very hot when it is compressed and cools rapidly as it expands. Air motors operate compressed air at about 20 to 180 pounds per square inch (psi) (1.4 to 12.4 bar) and exhaust air at an atmospheric pressure of approximately 14.7 psi (1.0 bar). This is a high percentage of pressure decrease occurring in a short timespan. Because the exhaust air will be below freezing temperatures when it exits, moisture in the air freezes and crystallizes inside the air valves. This ice can cause the air valves to stall, creating unplanned downtime and increased maintenance needs. One possible feature of an AODD pump for icing-resistant air systems includes parallel pilot shifting valves. This stall-free design reduces icing and maintenance needs while keeping production moving.

Technology Replacement

Several visual clues can indicate whether it is time to replace an old or deteriorated pump with current technology. These include a corroded pump exterior, leaks around the manifolds, worn internal fluid bowls and discoloration on the floor caused by severe leakage. End users can prevent premature deterioration by carefully selecting the best quality pump for operational needs. A dual-force, tie-bolt design and precision-machined components offer a leak-free precision fit for effective chemical containment and safe operations.