Almost one-third of all wastewater used in oil and gas production is reused. Last year, one of the nation’s largest energy producers treated and recycled more than 8 billion gallons of wastewater in California alone. Depending on the intended use—from boiler feeds to drinking water—different processes require different types of pumps.
Choosing between a reciprocating pump and a rotary gear pump should not be a spontaneous decision. To make a proper selection, end users should consider factors such as initial cost, pump capabilities, performance, reliability and maintenance costs over the life of the pump.
At the simpler end of the spectrum are applications for treating process water that will be reused for industrial purposes, such as boiler feeds, cooling water or water needed to dilute chemicals for other processes. For these activities, bleaching chemicals, cleaning agents and corrosion inhibitors are used to remove toxic elements. Few of these applications require high-pressure chemical injections, and most of the chemicals used are dispersed on a high-volume, continuous basis. For these reasons, fully functioning metering pumps may be considered extravagant, and lower-priced rotary gear pumps can do the job successfully.
The highest end of the water treatment spectrum involves treating water for human consumption. Municipal water treatment plants deploy a combination of reciprocating and rotary pumps to accomplish the multitude of stages that are required to make drinking water safe for human consumption. They use metering pumps and rotary gear pumps to administer a wide variety of chemicals:
- Disinfectants, such as chlorine or sodium hypochlorite, are used to improve odor and kill unwanted microorganisms.
- Coagulants are used to collide millions (or billions) of miniscule particles that are too small to be filtered (finer than 1 micrometer). These particles remain in motion because of negative electrostatic charges that cause them to repel one another. Positively charged flocculants, such as polymers, are injected in order to bring tiny colloidal particles together to form flocs, or larger and heavier aggregates, that become easier to filter.
- Chemicals such as sodium hydroxide and calcium carbonate control alkalinity. Diluted sulfuric or hydrochloric acids are used to neutralize other acids and correct pH levels. In some cases, a uniform and high-volume flow of these neutralizing agents is required. In other cases, the required amounts of chemicals to be dosed can vary depending on the pH of the untreated water.
Perhaps the middle ground for the water treatment industry involves treating wastewater that will be reused for irrigating parks, golf courses and, most important, crops. In drought-affected places like California and the Pacific Southwest, reusing wastewater for these purposes has become a necessity as a result of the costs and limited availability of freshwater.
The level of treatment required to process wastewater for these purposes comes close to the requirements for treating drinking water. Disinfection, dechlorinating, denitrification, coagulation and flocculation, algae control, odor, and color control are critical. As more recycled water is used for these purposes, a higher level of scrutiny and monitoring will be required for these applications.
The companies treating these new sources of water must be mindful to strike the balance between efficiency and effectiveness. So which pumps are best-suited to do the job?
Pumps for Metering Use
Reciprocating pumps work by using a piston or a diaphragm to push the fluid out. The reciprocating action alters the volume of the pumping chamber, which changes the pressure exerted on the fluid being pumped. A pair of unidirectional check valves controls the pumping action, and the plunger cross-sectional area, stroke length and cycle rate determine displacement.
Reciprocating pumps can deliver precise volumes for applications where high pressure is required. Sometimes, these pumps can be expensive because they require a complex drive for varying stroke. Reciprocating pumps may not be ideal for applications requiring excessively high volume flows, and they can be sensitive to changes in liquid viscosity, which can be an issue in dynamic processes where the viscosity changes with temperature variations.
Reciprocating pumps can require complementary parts, such as a pulsation dampener to smoothen the pulsed flow of injected chemicals, an elevated reservoir, or a pressurized supply tank for liquids with low vapor pressures that do not flow freely.
Despite these issues, reciprocating pumps are perceived to be higher-end products than rotary pumps. Their costs and capabilities may render them overbuilt for certain applications.
Rotary pumps can overcome many of the challenges posed by reciprocating metering pumps, and they are usually less expensive. The cost factor is even more significant when pumped liquids require corrosion-resistant materials.