Versatility of Air Operated Diaphragm Pumps in the Chemical Industry
An Air Operated Double Diaphragm pump (AODD) may be one of the simplest pump designs found in an industrial plant. The simple design and ease of use makes an AODD one of the most versatile pumps, especially in a chemical plant.
By Ed Kupp, Business Development Manager, Warren Rupp, a unit of IDEX
The simple operation of an AODD utilizes compressed air to drive the diaphragms that pump the desired fluid. The AODD includes a series of ball or flap check valves that open and close to allow liquid in and out of the diaphragm chamber. The pump can produce liquid discharge pressures equal to that of the air pressure being used to power the pump. As the diaphragms are balanced with air pressure on one side and liquid process pressure on the other, they see very low stresses during operation and can operate for millions of cycles before fatigue failure. During the suction stroke of a diaphragm, the air behind the diaphragm is discharged to atmosphere.
There are a few basic designs for AODD pumps. AODD pumps that utilize check balls are configured with the suction on the bottom of the pump and the discharge at the top of the pump. This can cause problems when the pumpage has heavy solids present. The solids can be held down by gravity and accumulate in the diaphragm chamber. Flap valve pumps, on the other hand, typically have the suction on the top and the discharge on the bottom. This allow solids to follow gravity and flow down through the pump. Flap valve pumps are also ideal for large solids that would not normally pass through a ball check valve. There are also hybrid ball valve pumps that can be down ported to allow gravity to pull the solids out of the diaphragm chamber and resolve the solids build up problem.
Easy to Specify and Install
Because AODD pumps are simple by design, they are also easy to document in an engineering specification. Since there is no electric motor needed to operate the pump, there is no need to provide a motor specification or motor data sheet. To help understand the differences between pump types, Table 1 shows the typical requirements of an AODD and comparing them to needs of other pump types.
When looking for information regarding proper specification or installation of an AODD, please consult with your AODD supplier, see Figure 1 for depiction of AODD installation.
In chemical plants, there are a variety of corrosive and non-corrosive applications. Having a pump that is available in multiple materials of construction and can be easily selected and sourced is very important. AODD pumps are manufactured in a variety of metal and plastic materials to better suit the needs of the application. Metal materials include aluminum, cast iron, stainless steel and higher alloys like Hastelloy-C for corrosive applications. Providing a pump in metal or plastic material with similar dimensions and interchangeable parts is important for a plant’s asset and inventory management. AODD pumps in plastic materials such as Polypropylene, PVDF, PFA, and conductive versions of these materials, allows the pump user to leverage one pump style to support numerous applications.
All pump designs require static seals or other elastomeric wetted components in a variety of corrosive resistant materials. AODD pumps are easy to specify with corrosion resistant diaphragm, check valve/ball and gasket materials. Available materials range from EPDM, Neoprene, FKM, and Santoprene to PTFE and PFA. When specifying the materials for an AODD, it is typical to specify both the body material and the elastomeric material, making it easier to ensure all the right materials are being provided.
Figure 1: Example of a proper AODD installation.
AODD pumps provide a unique combination of safety features for chemical process pumps. Most importantly, AODDs are air operated and require no electricity. By using air alone, if properly grounded, AODDs are intrinsically safe and can be used in environments with flammable liquids. Special care should be taken when using plastic pumps to ensure the plastic material provided is conductive. This can be done by the manufacturer of the pump by adding carbon to the plastic material. If the plastic material is not modified to ensure conductivity, it could develop a static charge that could be released as a spark.
In other applications there could be a risk of the pump running in a dead-head condition. This means that the discharge side of the pump could be closed off while the pump is still energized and operating. When an AODD pump is dead-headed, it simply stops. When the dead-head condition is relieved the pump will start pumping again. This restart does not require special monitoring or controls.
Another example of an upset condition a pump might experience is a lack of fluid being fed to the pump, or dry running. This could happen for a number of reasons. Most pump types will become damaged during dry run conditions, or can become vapor locked and cease to pump. An AODD pump can be run dry and when fluid is reintroduced to the pump, it will continue pumping. AODD pumps are resistant to vapor lock and can even re-prime themselves when drawing from sumps located below the pump. These two aspects make the AODD ideal for unloading, transfer, sump and tough process applications.
When most engineers in a chemical plant hear the term “seal-less” they think of magnetically driven centrifugal pumps or other rotating pump styles. The term “sealless” specifically refers to the pump not having a mechanical or dynamic style of seal that will leak small amounts during operation and large amounts when the seal fails. AODD pumps do not have mechanical seals and are considered ‘seal-less’. Under normal operation, the fluid being pumped will not leak out as liquid or vapor which is an advantage over mechanically sealed pumps that leak vapor under normal operation. For instances where leakage during failure must be avoided, the AODD pump can be fitted with a number of solutions to ensure that the fluid is contained. The simplest way to do this is to pipe the air exhaust back to the liquid source. This way, the leakage in a failed pump will simply go back to the source and will not leak to the environment. An additional way to provide secondary containment of the liquid being pumped is to fit your AODD pump with a second set of diaphragms and a barrier fluid in-between the primary and the backup diaphragm. This barrier fluid can be monitored so when the primary diaphragm fails, that failure can be detected, contained and fixed without leaks to the environment. This is referred to as a containment duty pump, see Figure 2.
A third way to provide a secondary containment in the event of an AODD diaphragm failure is to provide an external containment vessel that will trap any leakage coming out the exhaust and can also be monitored to provide an alert if a failure occurs. These devices can be provided by the AODD supplier or a third-party provider.
When adding a new process to a chemical plant, multiple application aspects need to be considered. Often you may be pumping clean liquids at ambient temperatures. However, conditions can vary greatly throughout a chemical manufacturing process, requiring different pump styles for different applications. If you are looking for one versatile pump that can handle a wide variety of applications and conditions, first consider an AODD. Chemical reactions can increase solids content, viscosity, specific gravity and other fluid characteristics. While an AODD can handle all of these changes, they do have limitations. All application aspects should be checked against the AODD manufacturers’ published limits. The most common reasons to not use an AODD pump in a specific application would be processes containing very high temperatures or very high pressure. Examples of these applications would be boiler feed pumps or hydrocarbon processing.
For specific applications with a high percentage of solids or large solids, there are flap valve or hybrid down ported ball valve AODD designs. The down ported design will allow for solids to leverage gravity to move out of the pump and into the discharge piping. Without this design, solids can build up in the diaphragm chamber and cause damage to the pump shaft or diaphragm plate.
When unloading tanks or totes in a chemical plant, an AODD can be used without complicated control systems. By simply opening a valve or a fill nozzle that is fed by the AODD, product can easily be transferred. When the transfer process is complete, the valve can be closed and the pump will automatically stop and be at the ready for the next time you need to transfer fluid.
There are many applications in a chemical plant where chemicals and other materials are collected in sumps and need to be pumped out. The AODD is ideal for these sump applications since it can handle solids that might get into the collection area, it can run dry and self-prime when needed again.
In the chemical processing industry, pumping needs can vary greatly. The fluid being pumped could be corrosive, hazardous, viscous, contain solids, or be as basic as clean water. The pump application itself can have demands that could require a wide range of pumps. Some applications require secondary containment. Others might have upset conditions like dry running or dead heading. Although the AODD pump may not be the first pump to come to mind, its versatility to handle all these applications might mean it is the perfect fit.
ABOUT THE AUTHOR:
Ed Kupp is a Business Development Manager at Warren Rupp, a unit of IDEX. He has 30 years of experience in the pump industry. Throughout his career he has held roles in Engineering, Sales, Marketing and Product Management. He holds a Bachelor’s degree in Mechanical Engineering and a Master’s degree in Business.