In any large installation of air-operated double-diaphragm (AODD) pumps, a relatively small portion of pumps contribute to higher than average maintenance costs, often due to misapplication. Since the majority of AODD pump applications involve no more than simple process fluid transfer, many pumps are mobile and transferred from one location to another. An operator may attribute a “good” pump installation to the pump, and not the installation.
The operator may transfer the “good” pump to a new installation and soon after, the “good” pump becomes a significant maintenance problem.
Worse still, the high maintenance may become routine, resulting in substantial costs, in relation to the pump. Since most AODD pump applications are either simple transfer or filter press feed, following a few necessary steps can avoid costly mistakes.
System properties, such as piping, can contribute significantly to AODD pump application issues, particularly on the suction side of the pump.
Most diaphragm pumps have dry lift capability in the range of 15 to 20 feet. One way to think of this is the literal lift capabilities of the pump. Another way is to think of this specification as the maximum frictional pipe loss that a particular pump can handle.
In other words, if a pump has a dry lift capability of 15 feet, then it cannot tolerate more than 6.5 pounds per square inch (psi) of pipe loss when priming the pump with a particular process fluid. Consider the viscosity of conventional latex house paint, which is approximately 1,500 centipoise (cps). When transferred through a 5-foot-long, 1-inch line at 20 gallons per minute (gpm), the frictional losses are around 34 psi, which is well beyond the capabilities of any AODD pump. Increasing the inner diameter of the suction line to 1.5 inches reduces the pipe loss to 6 psi, well within the capabilities of most AODD pumps.
2. Chemical Compatibility
Although a simple step in AODD pump specification, chemical compatibility is often overlooked when moving pumps within a facility. A common mistake is to assume that a more expensive material affords better chemical compatibility. Consider an 80 percent solution of sodium hydroxide (NaOH). Commonly available compatibility charts list ethylene propylene diene monomer (EPDM) rubber as A-rated and the more expensive fluroelastomer (FKM) as D-rated. Another example is isopropyl acetate. In the case of this process fluid, the less expensive polypropylene is often listed as A-rated, while the more expensive polyvinylidene fluoride (PVDF) is commonly listed as D-rated. The pump manufacturer should provide charts or tools to assist with this process.
Two considerations must be taken into account when considering temperature and AODD pumps: temperature limits of the pump’s material of construction and additional limitations due to process fluids. AODD pump manufacturers will often publish temperature limitations of the pump’s various materials of construction. These temperature limits are solely based on mechanical stress, thus other considerations must be taken into account when deciding on the limitations of a particular pump application.
Certain process fluids can reduce the maximum temperature limit even further. Consider an application that seeks to transfer 10 to 75 percent of sulfuric acid (H2SO4) with a polytetrafluoroethylene (PTFE)-lined polypropylene pump. Common compatibility tables list both PTFE and polypropylene as A-rated for H2SO4 (in certain concentrations). However, many compatibility tables list 72 F as an upper limit for polypropylene. Thus, should the temperature of the process fluid (H2SO4) exceed 72 F, the material of the pump should be changed to a more appropriate material. In this case, PVDF is a suitable alternative to polypropylene.
Abrasives can have a detrimental effect in many pump applications. Several steps can be taken to mitigate the effects of abrasion. First, a material’s physical properties should be considered. Thermoplastic elastomers have abrasive resistance properties. In practical applications, an abrasive resistant elastomer that is B-rated for a particular process fluid may outlast an elastomer that is A-rated but has poor abrasive resistance. Second, consider fluid velocity. The relationship between a fluid’s velocity and the abrasive effect on a pump’s material of construction is not linear. A small increase in fluid velocity can have a disproportionate impact on the abrasive wear of components. When pumping abrasive fluids, transfer the liquid at the slowest rate possible. In extreme cases, consider oversizing the pump so fluid moves slower.
5. Air Issues
Due to the adiabatic expansion involved in operating a pump on compressed air, one must consider moisture, both in the process and ambient since the temperature of the exhaust air is well below the freezing point of water. Ensure that the pump is operated on clean, dry air, and in cases where this is not possible, be aware of muffler blinding. Consider a pump’s placement in the facility when evaluating air issues—i.e., is the pump close to an open door? Make sure the airline supplied to the pump is adequately sized. An undersized airline can result in chattering, underperformance or unwanted noises. Consult with the manufacturer as to the recommended airline size.
Develop a preventative maintenance program to torque all external hardware to the recommendations, particularly when using plastic pumps. Follow the manufacturer’s recommendations for frequency and technique.
These items are often overlooked. Transfer applications are simple, and the basics are often ignored or assumed to play an insignificant role. When in doubt, consult the manufacturer, who should be able to offer valuable advice for implementation.