Chemical transfer applications demand maximum performance and superior chemical containment. ChemSafe pumps feature 100% PTFE and UHMWPE fluid paths and are chemically safe. You can use these pumps with a variety of materials, plus they deliver unmatched chemical compatibility with challenging corrosive acids and destructive slurries. The precision-machined body features a tie-bolt design that ensures reliable sealing surfaces free of leaking.
Two steps forward, one step back. That’s what it feels like to run an operation with a leaky pump. Leaky pumps inevitably lead to unscheduled downtime and increased maintenance needs. This stalls productivity and costs the average company hundreds of thousands of dollars per hour of downtime. Now add hazardous chemicals into the equation. Aggressive acids and slurries can cause pumps to leak much faster, and they create an unsafe work environment.
While these chemicals are necessary for the production of pharmaceuticals, printing inks, steel production, ceramic finishing, and more, they are also a huge liability. Selecting and maintaining the proper equipment to control hazardous materials is crucial to the long-term success of a business, but it isn’t easy. With so many different pump designs on the market, end-users are having a tough time distinguishing between the pumps that will cost them and the pumps that will save them.
AODD Chemical Pumps
Air-Operated Double Diaphragm Chemical Pumps (AODD) offer many benefits to manufacturers who require the use of hazardous materials. While most other pump designs rely on mechanical seals for chemical containment, mechanical seals are known to be 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.
In addition, the AODD pump’s diaphragms move in gentle cycles to minimise turbulence and protect the liquid from shearing. In many instances, these chemicals can be as fragile as they are hazardous, and any mild turbulence can alter their chemical properties, wasting the material and costing money. Lastly, because AODD pumps are powered by compressed air rather than electric motors, they can dry run and deadhead without the risk of burning, seizing, or harming internal components.
The problem with AODD chemical pumps is that their designs can vary in quality and effectiveness, which means that end-users must select their pump carefully. 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. Chemicals must be properly contained in order to fulfil their intended purpose without harming employees. 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 lifespan. Although leaking may seem natural or unavoidable, costs of employee injury and wasted materials are too great to ignore. 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 a result of cold flow and creep, which are caused by high temperatures, amount of time under load, and level of torqueing 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 termed, “cold flow.”
As temperature increases, the effects of cold flow on a plastic AODD pump escalate. Cold flow causes deformation of the plastic pump over time, causing its components to 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.
PTFE is one pump material that is most susceptible to leaking caused by cold flow because PTFE tries to escape the load as it gets warmer. PTFE is one of the most chemically compatible materials for resisting corrosion, which would make it a desirable pump material if not for its susceptibility to cold flow.
The material, UHMWPE, is second to PTFE in terms of chemical compatibility, but it is similarly 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 leak paths.
End-users are recommended to re-torque bolts regularly. Some AODD pump designs require even further leak prevention steps, such as the use of reinforcing metallic plates in attempt to evenly distribute the load from the housing bolts. While these quick fixes may work in the short term, constant checkups and reinforcements are far from ideal when controlling hazardous fluids in a fast-paced work environment.
Solution to Creep and Cold Flow
A proven solution for leak prevention is a machined body reinforced with a dual force, tie-bolt design. Overall, machined housing components are an essential feature to provide 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 the effects of cold flow and violent forces. This allows the pump design to make use of these chemically compatible materials without allowing leakage due to cold flow.
Pumps that have exposed metal or wet parts composed of metal are highly vulnerable to corrosion. Therefore, AODD chemical pumps designed with metal free exteriors will be more durable in corrosive environments. Similarly, interior corrosion can be prevented using PTFE and UHMWPE fluid paths to ensure chemical compatibility.
This means that the entire fluid path—from entering the inlet, passing the first check ball, leaving the fluid chamber through the second ball, and finally exiting the outlet of the pump—is either PTFE or UHMWPE, so there is no need to worry about incompatible chemicals eating away the pump’s interior.
Air Motor Icing Stalls Production
Because AODD pumps are operated by compressed air, they are subject to rapid changes in temperature. The air is very hot when it’s compressed and cools rapidly as it expands. Air motors operate compressed air at about 20 to 180 psi (1.4 to 12.4 bar) and exhaust air at atmospheric pressure, approximately 14.7 psi (1.0 bar). This is an extremely high percentage of pressure decrease occurring in a short time span.
Since the exhaust air will be below freezing temperatures when it exits, moisture in the air freezes and crystallises inside the air valves. This buildup of 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.
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 discolouration on the floor caused by severe leakage. Premature deterioration can be prevented 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. Wherever harmful chemicals are present, business owners must be mindful of the proper safety precautions and continue to educate themselves on current trends for chemical handling.
AUTHOR: Michael Sandlin – Graco Inc.