Product type and environment must be considered when choosing the proper pump.
Webster’s Dictionary offers a number of definitions for the word harsh, including “having a coarse or uneven surface that is rough or unpleasant to the touch” and “causing a disagreeable or painful sensory reaction.”
When choosing the proper pump for an industrial application, the word harsh has two meanings: the characteristics of the product that will be pumped and the environmental or atmospheric conditions in which the pump will operate. In many situations, harsh products may need to be pumped in harsh conditions.
- When identifying and selecting the proper pump for use with harsh products or in harsh conditions, three questions must be asked:
- What considerations should be taken when choosing a pump for harsh conditions?
- What are some pump problems/maintenance issues that can be encountered in harsh conditions that may adversely affect performance?
- Which pumping technology provides the best answer to the first two questions?
Let’s answer the last question first. For more than 50 years, the pump technology that has proven to perform best in handling harsh products in harsh environmental conditions is air-operated double-diaphragm (AODD) pump technology. AODD pumps are defined as reciprocating, positive-displacement pumps since their operation sees the pump displace fluid from one of its two product chambers upon the completion of each stroke.
AODD pumps have only a few wetted parts—the two diaphragms, two inlet valve balls and two discharge valve balls—while the pumps are air driven by an air-distribution system (rather than by an electric motor). These design characteristics enable the pump to run dry without damage, create a suction lift up to 21 feet (6.4 meters) of water, create positive suction head when necessary, operate while completely submerged and pass solids up to 13/8 inches (34.9 mm). Together, these characteristics make AODD pumps preferable to other pumps in harsh pumping conditions.
Several internal factors must be considered when pumping harsh products.
First, the correct wetted material and the correct elastomeric material must be chosen. For example, if pumping a biocide that can be toxic, an AODD pump that has wetted parts and elastomers that are compatible with that product must be used.
A pump’s diaphragms, valve balls, valve seats and O-rings are collectively known as elastomers. Generally, the three types of elastomers are: rubber compounds, thermoplastics and polytetrafluoroethylene (PTFE), which is more commonly known as Teflon.
Rubber compounds consist of rubber and manmade additives that have been designed to increase resistance to specific types of fluids.
Thermoplastic compounds are constructed entirely of manmade elements that have been designed to increase the tensile strength of the elastomer while maintaining excellent abrasion resistance. They are ideally suited for pumping many acidic and caustic materials.
Teflon is the most chemically inert manmade compound known which expands the range of applications in which AODD pumps can be used. Since Teflon is non-elastic, a backup diaphragm must accompany the Teflon diaphragm. The backup diaphragm provides added support resulting in increased mean time between repairs (MTBR).
Most AODD pumps can be designed to use any of the elastomer types to meet most application requirements. Considerations for specifying elastomers include chemical compatibility, temperature limitations, flex life, abrasion resistance, suction-lift capabilities, sanitary standards and cost.
Another issue to consider is the resistance of AODD pump components to abrasion. When a highly abrasive fluid is pumped, damage to the pump’s internal components can occur if the internals do not possess the level of abrasion resistance needed.
Certain powders, as well as slurries, containing rocks, metal fines or sand, tend to be highly abrasive and can scratch the pump’s internals as they flow through the wetted path of the pump. The internal components most likely to get damaged in these instances are the ball cages and elastomeric components such as the diaphragms, balls and seats and any other wetted components that come into contact with the abrasive material being pumped.