Hotel Service Pumps

FLASH-DISTILLING-PLANT pumps Many steam ships have multistage flash distilling plants that are used to generate freshwater from seawater. Several electric-motor-driven singlestage centrifugal pumps are used with a typical flash unit. Smaller pumps are often provided in a close-coupled configuration.

A distiller-feed pump takes suction from a sea chest and supplies seawater to the distilling plant's first-stage flash chamber. This water often passes through various heat exchangers, such as a distillate cooler, distillate condensers, air-ejector condenser, and sea-water heater before it enters the flash chamber. The distiller-feed pump is usually located sufficiently below the vessel's waterline so it operates with a flooded suction.

The hot high-salinity brine remaining in a flash distilling plant's last stage is typically removed by a brine pump that discharges it overboard. A line is often provided to permit a brine pump's shaft seals to be flushed with seawater discharged from the distiller-feed pump. In addition, a vent line is typically connected from a brine pump's suction to the last-stage flash chamber.

The freshwater or distillate produced by a flash distilling plant is removed from the last-stage distillate condenser by a distillate pump. During normal operation, water discharged from this pump ordinarily passes through a cooler and is then directed to a vessel's distilled-water (reserve-feedwater) or potable-water tanks. A salinity cell in the pump discharge line, however, will typically trip a three-way valve that dumps the distillate to the bilge if salinity of this water is excessive. A vent line may be connected from a distillate pump's suction to the last-stage distillate condenser.

An additional pump is sometimes provided to remove condensate from the hotwell of a flash distilling plant's seawater heater and transfer it to a freshwater drain tank. Alternatively, however, this condensate may be returned directly to a main or auxiliary condenser through a vacuum-drag line. With this latter arrangement, the feed-heater condensate pump is not required.

Because the distillate, brine, and feed-heater condensate pumps each take their suction from a chamber in which the pumped liquid is at or near its vapor pressure, pumps used in these applications should have low NPSH requirements. In addition, to increase the submergence of their impellers, these pumps are often located as far below the distilling plant assembly as practicable.

HEAT-RECOVERY DISTILLING-PLANT pumps It is common for a diesel-propelled vessel to be fitted with one or more heat-recovery distilling plants in which jacket water from the vessel's diesel engines serves as the heating medium. In a typical heat-recovery distilling plant, this jacket water is pumped through either a plate or a submerged-tube evaporator.

Seawater is often delivered to a heat-recovery distilling-plant by a dedicated distiller-feed pump (sometimes referred to as an eductor or ejector feed pump) that takes its suction from a sea chest. In some units, this seawater initially passes through the distilling plant's condenser where it absorbs heat from the distilled vapor generated in the evaporator. A portion of this seawater then enters the evaporator section of the shell as feed. The remaining portion of the seawater frequently serves as the motive fluid for an eductor that removes brine from the evaporator and air from the condenser portion of the distilling plant's shell. After leaving the eductors, this seawater is directed overboard with the brine and air. (In some units, the mixture of seawater, brine, and air passes through the distilling plant's condenser before being discharged overboard.) The feed pump is typically located low in a vessel to enable it to operate with a flooded suction.

A distillate pump is used to remove the freshwater collected in a heat-recovery distilling plant's condenser and discharge it either to the vessel's freshwater tanks or, when the salinity is high, to the bilge. Both the eductor-feed pump and the distillate pump are generally electric-motor-driven single-stage centrifugal-type pumps. In addition, they are often furnished in a close-coupled configuration.

POTABLE-WATER pumps A potable-water pump typically takes suction from a vessel's potable-water or domestic tanks and discharges freshwater either directly to sinks, showers, and other potable-water fixtures located throughout the vessel or to an air-charged pressure tank, sometimes referred to as a hydrophore or a hydropneumatic tank, that is frequently included in a potable water system. Most vessels have at least two full-capacity potable-water pumps. Electric-motor-driven single-stage centrifugal pumps are often used in this application. In addition, regenerative turbine pumps are sometimes used. Smaller pumps are often furnished in a close-coupled configuration. To improve suction conditions, a potable-water pump may be installed as far as practicable below a vessel's potable water tanks.

In a typical potable-water system in which a hydrophore is connected to the pump discharge line, a potable-water pump may be cycled on and off automatically by a pressure switch installed on the hydrophore. During periods when the pump is not running, any potable-water usage will result in a drop of the water level within the hydrophore, the expansion of the compressed air located in the upper portion of this tank, and a reduction in the hydrophore pressure. However, the force exerted by the compressed air ordinarily prevents the potable-water system pressure from dropping too suddenly. If the potable-water usage continues, the pressure exerted by the compressed air within the hydrophore will continue to drop until it reaches the cut-in pressure for the potable-water pump's motor. At this point, the pump will be started and the water level and pressure within the hydrophore will normally increase. This will continue until the pressure switch's cut-out pressure is reached and the potable-water pump is stopped.

Although it is less common, on a vessel with a high potable-water demand, a potable-water pump may be operated continuously. When a potable-water pump operates continuously, however, a recirculation line is typically provided to prevent the pump from overheating during periods of low water usage.

hot-water circulation pumps Some of the freshwater discharged from a vessel's portable-water pumps is circulated through a heater and is then directed to sinks, showers, and other hot-water fixtures located throughout a vessel. One or more hot-water circulation pumps are ordinarily used to recirculate unused water contained in the hot-water distribution piping through the heater so the water will remain hot. Electric-motor-driven single-stage centrifugal pumps are typically used in this application. In addition, most hot-water circulating pumps, which generally operate continuously, are furnished in a close-coupled configuration.

sanitary pumps Seawater is sometimes used as the flushing medium in bathrooms on a vessel. When this is the case, a sanitary or flushing-water pump may be provided to take suction from a sea chest and discharge seawater to the vessel's flushometer valves. Two electric-motor-driven single-stage centrifugal pumps are frequently used in this application. When each pump is sized to meet peak-demand requirements, one pump can be cycled on and off automatically by a discharge-pressure switch while the second pump serves as a standby unit. To reduce the number of sanitary-pump starts and stops, an air-charged pressure tank similar to the potable-water-system hydrophore described previously is often included in a sanitary system. A sanitary pump is frequently located low in a vessel so it can operate with a flooded suction.

sewage pumps When a vessel has a sewage holding tank, a sewage pump typically takes suction from the holding tank and discharges the sewage and waste water, based on applicable regulations, overboard or to an above-deck connection for transfer ashore. Electric-motor-driven single-stage centrifugal pumps are frequently used in this application. Some of these pumps have casings with large waterways and are fitted with special non-clog impellers. Alternatively, some vessels have sump-type centrifugal pumps that are submerged within the sewage tank and are driven by submersible motors or through vertical line shafting by above-tank motors. With either arrangement, two pumps are often provided for each holding tank.

Instead of a sewage holding tank, many vessels now have an onboard sewage treatment plant, referred to as a marine sanitation device (MSD). At least two pumps are ordinarily provided with each MSD to discharge the treated effluent overboard. Standard electric-motor-driven single-stage centrifugal pumps are frequently suitable for this purpose.

Some vessels also have one or more lift stations in which sewage and wastewater is collected before being transferred to an MSD or a larger sewage holding tank. Macerator pumps are frequently used to transfer a lift station's contents to the MSD or holding tank. A macerator grinds the sewage and breaks up solids into small particles that are more easily treated. Lift-station transfer pumps are usually started and stopped automatically by a float switch in the lift station. Two pumps are often provided for each lift station.

air-conditioning CHILLED-WATER pumps Some vessels utilize freshwater as a secondary refrigerant for air conditioning. With this arrangement, a chilled-water pump circulates the fresh water through a chiller where it is cooled by the system's primary refrigerant. The water then passes through duct-mounted cooling coils, where it absorbs heat from air being supplied to temperature-controlled spaces located throughout the vessel. The chilled water may also be used to cool electronic components. Electric-motor-driven single-stage centrifugal pumps are frequently used in this application. A pressurized or elevated expansion tank that is typically installed on the suction side of the system maintains a minimum pressure at the inlet to the pumps.

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