Pumps & Systems, February 2013
Hawker—located in Ooltewah, Tenn.—is a manufacturer of industrial, lead-acid batteries. Hawker manufactures batteries for applications as diverse as telecommunications and aircraft, uninterruptible power supply systems and submarines. Industrial lift trucks of different sizes and capacities are also a major application.
“The sulfuric acid we used to make the batteries was an extremely corrosive liquid,” says Kelly Hogan, plant manager. “The conventional metal pumps we previously used to handle it required frequent maintenance, resulting in downtime and lost production. Since we replaced them with thermoplastic cantilevered run-dry pumps (see Image 1), we have saved an average of $3,000 to $5,000 per month on maintenance and parts replacement. Not only is the solid polypropylene of which they are constructed completely inert to sulfuric acid, but their cantilevered design eliminates immersed bearings and allows run-dry operation for extended periods of time without damage.”
Each application requires a battery with different characteristics. Pallet jacks and light lift trucks, for example, require a lower power output than those that make long runs and high lifts under harsh conditions or have attachments that require additional power. For applications with limited accessibility, Hawker’s batteries require watering just four times per year. Its valve-regulated, lead-acid batteries contain a gel electrolyte in a sealed chamber that never requires adding water.
The power output of a lead-acid battery is determined by several factors, including the number and thickness of the lead plates it contains.
Another factor is the specific gravity of the sulfuric acid that reacts with them. The higher the specific gravity, the stronger the acid and the higher the energy output of the battery.
The acid is used to fill the individual battery cells and is pumped through the cells containing the plates during the curing process to dissipate the heat generated by the reaction between lead oxide and the acid.
How Lead-Acid Batteries Are Manufactured
“Lead alloy ingots are formed into grids. Lead oxide, a powder, is made into one of two different kinds of paste,” says Hogan. “A mixture of lead oxide powder, water and sulfuric acid produces a positive paste, and the same ingredients in slightly different proportions with the addition of an expander (generally a mixture of barium sulfate, carbon black and organics) make the negative paste. Machines then force these pastes into the gaps between the grids, which are made into plates.”
The uncured plates are sealed into plastic jars to form individual cells, each containing an average of 17 plates. The jars range from approximately 2 inches (5 centimeters) to 15 inches (38 centimeters) in diameter and from 15 inches (38 centimeters) to 30 inches (76 centimeters) tall, depending on the size of the cell. The jars are then lowered into the appropriate size battery case, which is loaded into a formation system.
A top with two hoses—one for inlet and the other for outlet—is then sealed to the top of each jar, and sulfuric acid is circulated through it. The tops are then removed from the jars, which remain in the battery case. Each formation system holds an average of 96 cells, or a total of 1,530 plates. Depending on the size of the battery, however, the number of plates can vary from 300 to 3,300.
The chemical reaction between the lead oxide and the sulfuric acid starts in the paste, and the mass gradually cures and hardens, liberating heat. As the curing process continues, needle-shaped crystals of lead sulfate (PbSO4) form throughout the mass.
To provide optimum conditions for the curing process, the plates are kept at a temperature near 90 F (32 C) for about four days and are then allowed to dry under ambient conditions.
“The exact composition of the paste is different for each type battery we produce,” Hogan says. “Each requires sulfuric acid with a different specific gravity, which is stored in its own 150-gallon (568-liter) holding tank. To remove the heat generated by the curing process, we circulate sulfuric acid with a lower specific gravity. This is when we use the thermoplastic pumps.”
Image 1. Cantilevered thermoplastic sump pumps |
Each 150-gallon (568-liter) holding tank is equipped with its own cantilevered pump. “During the forming process, we circulate the acid continuously from the holding tank through the formation system and back to the holding tank,” says Hogan.
“When the forming process is complete, the spent acid is pumped into what we call a reclaim tank, where it is combined with spent acid from other formation systems. This acid solution is then filtered and either concentrated or diluted to make each of the specific gravities that we need.”
Thermoplastic Pumps Eliminate Corrosion
Every wetted surface in the vertical pumps that transfer the sulfuric acid is made of solid polypropylene to eliminate corrosion and minimize abrasion, resulting in lower maintenance and longer pump life (see Image 2).
The heavy-duty stainless steel shaft is completely isolated from the corrosive fluid by a heavy-sectioned polypropylene sleeve.
No metal comes in contact with the pumped sulfuric acid solution. The cantilevered, large diameter shaft design eliminates the need for any immersed bearings and permits dependable operation without damage under dry running conditions caused by sudden fluid stoppage for short, extended or indefinite periods.
With dynamically balanced 6.3-inch (16-centimeter) diameter injection-molded polypropylene impellers; 1.5-inch (3.8-centimeter) flanged discharge ports; and 2-horsepower (1.5-kilowatt), 1,750-rpm chemical duty motors, they are 32 inches (81 centimeters) long and operate at a constant temperature near 90 F (32 C), against a maximum 3 feet (91 centimeters) of head.
Mounted on a polypropylene cover plate measuring 34 inches (86 centimeters) long by 18 inches (46 centimeters) wide by 1 inch (2.5 centimeters) thick, the thermoplastic pumps have required little maintenance since they were installed more than three years ago.
“We grease the ball bearings in each pump twice per month,” Hogan says. “We have had to paint the metal stands with epoxy paint only once in three years to keep the corrosion down, and we have never had to replace a single internal part.”
The wastewater generated by the battery manufacturing process is sent to a treatment facility, where it is purified using an advanced iron co-precipitation process.
“The proprietary process effectively removes heavy metals, which in our case means lead, down to very low levels,” Hogan says. “The sodium sulfate brine water left after filtration is processed through a crystallizer to produce clean water and salt crystals. The clean water is used within the plant, and the salt crystals are disposed of properly.” P&S
