There are multiple reasons that facility managers, process engineers and maintenance personnel choose air operated double diaphragm (AODD) pumps for their facilities and processes. A few include the pump’s ability to improve downtime due to issues such as leaks or diaphragm ruptures, which cause more maintenance intervention and increase the overall total cost of ownership. Some of the main reasons for AODD selection include:
- Low total cost of ownership: From initial acquisition cost to service and repair, AODD pumps are often lower cost.
- Portability: As long as there is an air-line drop and an inlet and outlet hose, an AODD pump can be used most anywhere in a facility or within a specific process.
- Shear sensitivity: The pump usually will not shear or separate the fluid being pumped.
- Submergibility: The pump can be fully immersed as long as the exhaust is piped above the liquid line.
- Deadhead and run dry capability: If the discharge outlet is blocked or stopped or if the supply of fluid is empty, the pump is designed to continue cycling without damaging the pump or the product being transferred.
- Sealless design: There are no seals to replace due to excess heat or if the pump were to run dry.
- Self-priming: The pump will draw fluids into the pump in either a dry (unprimed) or wet (primed) application.
- Variable flow control: Because AODD pumps are considered 1:1 ratio pumps, the amount of the air inlet pressure going into the pump is directly related to the fluid pressure at the discharge outlet of the pump.
One of the less obvious reasons that AODD pumps are selected is the range of chemicals that can be transferred, injected, batched or filled during a process or application. There are hundreds, if not thousands, of configurations for AODD pumps to handle almost any chemical. A typical AODD original equipment manufacturer (OEM) might have as many as 1,700 viable configurations based on size, wetted material, elastomeric options, center body and hardware materials.
Chemical identification is where the selection process starts and where most sales professionals will inquire first, right before asking for application details such as head pressure requirements or flow rate needed and availability of compressed air. Some chemicals are more straightforward than others. Chemicals are required to be marked with their specific chemical name and their chemical formulation. In some situations, it could be a proprietary chemical that a company has developed and its formulation is not as easily gleaned. In those cases, a copy of the material safety data sheet (MSDS) would be required to understand main chemical ingredients and concentrations.
Once the chemical has been determined, it is identified in the AODD manufacturer’s Chemical Compatibility Guide. It is important to remember to use the guide from the AODD OEM brand from where you will be acquiring the pump. Although the guides may look similar and often cover the same range of chemicals, some OEMs offer different grades of metals or plastics. Some are proprietary to them and may not be equally compatible with the chemicals in the process.
A typical chemical guide will list all of the wetted pump material options across the chart top. This generally includes the outer fluid chamber and inlet/discharge manifold material options along with all of the elastomeric material options that the OEM offers, such as diaphragm, seat and ball material options. Table 1 includes a list of chemicals that have been tested by the OEM or by the resin supplier in various chemical concentrations.
Once the chemical is located in the guide, it is rated according to which wetted pump and elastomer materials are most or least compatible or not to be used. In some cases, there are conditions included in the rating such as maximum temperature or concentration that the material is still compatible. A rating of A/65 degrees means that the material is compatible up to 65 C or a rating of A/70 percent means the material is compatible up to 70 percent chemical concentration and after that temperature or concentration is exceeded, the compatibility changes. It is always recommended that only A-rated materials with no conditions called out be selected.
In most cases several materials can be acceptable, so it is up to the end user to determine what configuration is best suited for them. If the pump is configured against the recommendations of the chemical guide, it can void the manufacturer’s warranty, cause premature pump failure and possible chemical leakage.
Because of the wide range of chemicals that AODDs can pump, it is imperative to select the right materials of construction for the specific chemistry involved in the process. Misdiagnosed pump configuration can cause more than maintenance and process interruption; it can be a safety issue. Even household chemicals can be dangerous if exposed to a team member when fluids are transferred in the chemical process of diluting them to consumer concentration.
It is an essential, if not mandatory, step to identify the chemical being used, including the concentration and the temperature limits so the product being transferred does not cause the pump to prematurely fail. Failure can happen either by leaking or by diaphragm breach, causing increased maintenance intervals, added production costs for downtime and additional repair part costs, which ultimately increases total cost of ownership. Extra work at the beginning allows more uptime, more production and fewer headaches.