Air-operated double diaphragm (AODD) pumps are widely regarded as some of the most versatile and cost-effective pumps available for fluid transfer. Configurable to pump nearly any pourable fluid from caustic acids to abrasive slurries, this configuration flexibility often creates confusion and anxiety when selecting an AODD. In each application, different pump materials will be suited for varying factors such as pH levels, abrasives, temperature and fluid density. This article identifies and recommends popular ball, seat and diaphragm materials (internal pump components) and pump fluid sections (pump bodies) to perform in applications with varying fluid characteristics. These are general guidelines and points to consider when specifying the correct configuration. Prior to purchase or operation, it is important to check the manufacturer’s specifications and chemical compatibility guides.
Specifying a pump to safely handle chemicals with fluctuating pH levels and varied concentrations presents a challenge when configuring an AODD pump, which requires the proper selection of internal pump components and pump bodies to ensure compatibility, avoid pump corrosion and minimize maintenance intervals. In applications in which pH levels change or when concentrations of the chemical may vary, plastic pump bodies such as polypropylene, polyvinylidene fluoride (PVDF or Kynar®) and polytetrafluoroethylene (PTFE) are commonly chosen because of the materials’ chemical inertness.
PTFE offers the highest chemical compatibility and can generally be safely used with the most acidic acids, such as hydrochloric, to the most basic chemicals, such as sodium hydroxide. However, in many cases PTFE is unnecessary when PVDF or polypropylene options are sufficient. To this point, the most important step in the material selection process is to reference the manufacturer’s available chemical compatibility guides for direction on appropriate pump materials for the chemical in question.
Recommendation: Check manufacturer’s chemical compatibility guide.
As mentioned above, PTFE is popular because of its high level of chemical inertness and compatibility in high-temperature applications, but it is often misapplied in abrasive applications, such as chemical slurries. Generally, metal pump bodies such as stainless steel, aluminum and Hastelloy® are best for abrasive fluids. In applications with abrasives, Santoprene® and Geolast® internal pump components will provide longevity and resistance to destructive abrasives. Applications with petroleum-based fluids, such as waste oil, are best paired with Geolast ball, seat and diaphragm materials. Santoprene, one of the most versatile elastomers, offers good chemical compatibility and excellent abrasive resistance. Although all of these materials excel with abrasive fluids, it is recommended to check chemical compatibility.
Recommendation: Stainless steel pumps with Geolast or Santoprene wetted materials.
To expedite production and reduce manufacturing costs, more and more plant processes operate at extreme temperatures. The increase in temperature adds another level of complexity to material selection. In demanding temperature applications, plastic pump bodies are not recommended because of the effects of cold flow and creep that can lead to leaking. As temperatures exceed 180°F, choices of compatible internal components drastically decrease. At these elevated temperatures, materials such as fluoroelastomer (FKM or Viton®), ethylene propylene diene monomer rubber (EPDM), PTFE and PVDF are preferred. EPDM and FKM have the highest heat resistance, rated to 275°F and 320°F, respectively. PTFE and PVDF materials are the next best alternatives, rated to 212°F and 225°F, respectively.
In cold environments such as natural gas fields in the Pacific Northwest, glycol transfer pumps are required to withstand frigid environments that can dip well below zero degrees, thus requiring aluminum, stainless steel, EPDM, FKM and Santoprene. It is important to identify temperature limits of all components in the pump. Just because the internal pump components are rated to an extremely low or high temperature does not mean the pump body material is rated to the same level, and vice versa. Be sure to identify temperature ratings of all components when configuring an AODD pump, which can be found in product manuals.
Recommendation: Aluminum or stainless steel pumps with FKM, EPDM, PTFE or PVDF wetted materials in high-temperature applications. Aluminum or stainless steel pumps with EPDM, FKM or Santoprene wetted materials in low-temperature applications.
Check fluid density
The weight of a liquid, referred to as fluid density, is commonly overlooked when configuring a diaphragm pump. Diaphragm pumps utilize ball checks that rise and fall as pressure changes occur within the fluid chambers of the pump. Many different metal and elastomer configurations are available for ball checks. Each of these materials has a different weight, or specific gravity. It is important to understand how this weight relates to the fluid.
For example, when transferring a low-viscosity fluid, such as sodium hypochlorite, the weight of the ball is not of critical importance because the fluid is not restrictive during the ball seating process. In contrast, a 30 percent slurry has a higher viscosity that requires a heavier ball to “cut” through the restrictive slurry so the ball seats properly. Denser fluids typically require heavier ball checks to seat reliably. If the ball is too light, it will hang up in the slurry and will not seat, leading to poor pump performance that includes priming issues and cavitation.
Recommendation: Depending on the chemical compatibility results, weighted neoprene, stainless steel and PTFE ball checks have the highest specific gravity, or weight, allowing the ball checks to seat reliably through high-viscosity fluids.
Is PTFE really needed, or will Santoprene work just as well with 25 percent sulfuric acid? This is another reason why it is important to reference chemical compatibility guides. More often than not, multiple material configurations can be considered once the chemical compatibility, fluid abrasives, temperature and fluid density are measured. This is the time to think about price. Polyproylene, Geolast, EPDM, TPE and aluminum are ideal for configuring a cost-effective pump. As variables such as abrasives or temperature become more challenging, ultra-high molecular weight polyethylene (UHMWPE), neoprene or nitrile may be required to reliably handle the fluid, increasing the cost of the pump.
Specialty materials intended to handle the most extreme fluid characteristics include PTFE, PVDF, FKM, Hastelloy and stainless steel. These materials add the most cost to the configured pump. Many users believe that during a repair, the same materials must be reordered to ensure the pump has the appropriate materials. Instead, consult a chemical compatibility guide to see if other materials are adequately suited for the same fluid that can reduce the cost of the pump. It is surprising how many AODD pumps are misconfigured from the start.
Recommendation: Polypropylene, Geolast, EPDM, TPE and aluminum are considered cost-effective material options. Midrange materials include UHMWPE, neoprene and nitrile. PTFE, PVDF, FKM and stainless steel are specialty materials at higher price points.
Fluid characteristics such as pH levels, abrasiveness, temperature and fluid density must be considered when configuring an AODD pump. The available choices may seem overwhelming, but the tips above should reduce confusion and anxiety when configuring a pump. Always check chemical compatibility guides found on manufacturers’ websites to ensure pump materials are suitable for the fluid being transferred. In addition, use the tool in Table 1 to identify alternative materials to reduce the overall cost of the pump. Manufacturers’ websites often feature online pump selector tools to assist in configuring a pump. Print catalogs have thousands of part numbers to comb through, but new online tools can assist in configuring a pump in a matter of seconds.
Author’s note: All product names, logos, trademarks and brands in this article are property of their respective owners and are used for identification purposes only. Use of these names, logos, trademarks and brands does not imply endorsement.
Michael Sandlin currently serves as a global product marketing manager in the Process Pump Division at Graco Inc. Graco manufactures premium pumps and spray equipment for fluid handling in the construction, manufacturing, processing and maintenance industries. Sandlin may be reached at email@example.com or 612-379-3701. For more information, visit graco.com/process. Visit graco.com/materialsguide for more assistance with pump materials.