- Higher pressure/temperature systems require the use of pumps with confined, controlled-compression gasketed joints. This can be accomplished by selecting pumps of either barrel or ring-section design to ensure containment of the high-pressure/high-temperature boiler water, and to resist the considerable nozzle loads that can be imposed on the pump as a result of temperature changes. Barrel-type pumps (see Figure 1.22 in ANSI/HI 1.1-1.2[Hyd Inst3] ) are often chosen for ease of maintenance and the double case construction feature.
The user must determine the pump type that is appropriate for the system. Such decisions as pump construction (axially split, radially split casing, ring-section or barrel casing), rotor construction (back-to-back rotor arrangement or in-line rotor arrangement, with balance drum or balance disk), bearing type and pump support system requirements are usually made in conjunction with the pump manufacturer and take into account the manufacturer's specific installation and service experience.
What are the most commonly used methods for priming a pump?
If the pump is located below the source of liquid, it is only necessary to open the suction valve and let liquid enter the pump. However, when necessary, pumps may be primed by one of the following methods.
When steam, pressurized water, or compressed air is available, the pump may be primed by attaching an air ejector to the highest points in the pump casing. The ejector will remove the air from the pump and suction line, provided a tight valve is located in the discharge line close to the pump.
As soon as the air- or steam-driven ejector waste pipe exhausts water continuously, the pumps may be started. After starting, a steady stream of water from the waste pipe indicates that the pump is primed. If this stream of water is not obtained, the pump must be stopped at once and the process of priming repeated. A foot valve is unnecessary when this kind of device is used.
When it is not practical to prime by ejector or exhauster, a foot valve in the suction inlet will prevent liquid from running out the suction inlet, and the pump can be completely filled with liquid from some outside source. Vents on top of the pump should be opened during filling to allow the air to escape. A tight foot valve will keep the pump constantly primed so that the pump may be used for automatic operation. The valve must be inspected frequently, however, to see that it does not develop leaks and thus allow the pump to be started dry.
The pump may also be primed by the use of a vacuum pump to exhaust the air from the pump casing and suction line. A wet vacuum pump is preferable, as it will not be injured if water enters it. When a dry vacuum pump is to be used, the installation must be such as to prevent liquid being taken into the air pump. The manufacturer's instructions should be followed.