The U.S. Navy pumps brine for distillation units, transfers sewage from CHT tanks and pumps wastewater for shipyards. Conventional pumping technology, however, was requiring high levels of maintenance. The U.S. Navy needed an alternative pumping technology that could accomplish these tasks without the downtime or repairs.
Brine pumps have always created a vast amount of problems for distillation units aboard U.S. Navy ships. Conventional centrifugal pumps were constantly being replaced or under maintenance from the high corrosiveness of the 120-deg F brine and cavitation. The combination of these effects caused the impellers to erode relatively quickly. Oversized materials, such as shells, were constantly jamming the impeller and causing severe downtime and motor burnout.
A 6-in alternative technology pump, which harnesses the power of a tornado's eddy effect, has currently been in service on an LHD class ship for 11 years. Much of the success of the pump can be attributed to its unconventional hydrodynamic principles of operation. These principles create energy in the form of a synchronized swirling column through negative pressure, or the "eddy" effect. The need for net positive suction head is not required, and no restraints are caused by cavitation due to excessive vibration. Furthermore, this negative pressure where the rotor shaft penetrates the pump casing eliminates premature packing failure, which often causes motor bearing failure due to exposure from the system fluid or outside contaminant.
Since the installation of this alternative pump, only two pump seals have been changed. There has been zero operational downtime in the 24 hour, 7 days-a-week operational schedule at sea, which adds up to continuous runtimes of more than 14,000 hours of maintenance- free operation. Recent U.S. Navy estimates show that the use of this alternative pump system in the brine pump application can result in a life cycle savings of about $2 million per aircraft carrier.
Centrifugal Discharge Pumps
Centrifugal discharge pumps installed in aircraft carrier sewage Collection, Holding and Transfer (CHT) systems were experiencing a high rate of seal failures shortly after seal installation. A significant maintenance effort was required to replace the failed seals. In addition, the normal operation of these pumps resulted in the leakage of sewage material into the CHT pump room, which violated Navy health regulations. The total carrier fleet maintenance burden to overhaul the pumps and replace seals was estimated to be about $750,000 per year. The Navy's PERA (CV) organization and Newport News Shipbuilding identified the alternative pump used in the brine handling as capable of meeting the zero external leakage health criteria and eliminating the problem of seal failures.
These pump systems have been installed in all the aircraft carriers in the fleet and replaced existing CHT discharge pumps. The new pump system features a 5-ft intake and a 4-ft discharge, a 40-hp motor and a combined electric and low pressure air supply controller. These pumps have been in service on aircraft carriers for 14 years. There has been no seal leakage or need to overhaul the pump systems.
Seal cartridges on one carrier showed increased air flow and were replaced after 18 months of service. These replacements were attributed to a need for clarification of preventive maintenance greasing requirements. The simple design of the pump and the seal cartridge allows for easy service and maintenance if it should be required. Seal cartridges can be economically rebuilt, which provides further cost savings to the Navy.
The ship's force has enjoyed the benefits of zero leakage, low maintenance pump systems: cleaner CHT pump rooms and reduced exposure to sewage. The Navy is realizing significant improvements in CHT system reliability and a substantial reduction in fleet maintenance life cycle costs.
Alternative pump technology has also served in shipyard applications. A large problem in daily operations involved pumping the wastewater from sandblasting ship decks. This wastewater, filled with all types of debris ranging from wood pieces to wire brushes, caused constant clogging of the centrifugal pumps. In response to this problem, a system with two 8 x 6 inch pumps and a fully automated control system alleviated all problems.
The cost for overhauls has also been virtually eliminated with this alternative technology. Without precision machined clearances between the rotor and the internal pump casing, overhauls required when pump capacity decreases due to erosion and wear of the impeller and casing wearing rings are obsolete. A simple seal cartridge design periodically becomes greased through an automated system that utilizes a standard tube of grease, which can easily be replaced. Through this simple seal system and automated greasing system, seal cartridges are achieving life spans of more than seven years with continuous operation.