Pumps & Systems, March 2007
Q.
What are the key requirements for good suction piping design for rotodynamic pumps?
A.
Good suction piping design must eliminate air entrainment in the liquid, minimize friction loss, provide straight and uniform flow at the pump inlet and avoid excessive forces due to pipe strains at the pump.
Inlet flow disturbances, such as swirl, unbalance in the distribution of velocities and pressures, and sudden variations in velocity, can be harmful to the hydraulic performance of a pump, its mechanical behavior, and its reliability. Usually the higher the energy level and specific speed of a pump, and the lower the NPSH margin, the more sensitive the pump's performance is to suction conditions.
All inlet (suction) fitting joints should be tight, especially when the pressure in the piping is below atmospheric, to preclude air leaking into the fluid. Any valves in the inlet (suction) line should be installed with stems horizontal to eliminate the possibility of air accumulation. For pumps operating with a suction lift, the inlet (suction) line should slope constantly upwards toward the pump, with a minimum slope of 1 percent.
In general, as liquid travels through a piping network, entrained air tends to rise to the highest point. If the pipeline slopes upward, then the velocity of the liquid will move the air bubbles towards this high point. In contrast, if the pipeline is fairly flat and the inside surface of the pipe is very rough, or the pipeline slopes downward, the fluid velocity may not be sufficient to keep the air bubbles moving. As a consequence, it is possible for a pocket of air to collect at high points and gradually reduce the effective liquid flow area, which can create a throttling effect similar to a partially closed valve.
The suction pipe should be at least as large as the pump suction nozzle. Valves and other flow-disturbing fittings located in pump inlet (suction) piping should be at least one pipe size larger than the pump inlet (suction) nozzle, with the exception of continuous-bore, 100 percent open valves (such as full-ported ball valves). The maximum velocity at any point in the inlet (suction) piping is 8-ft/s. For fluids close to the vapor pressure, the velocity must be kept low enough to avoid flashing (cavitation) of the liquid in the piping, especially when fittings are present.
The most disturbing flow patterns to a pump are those that result from swirling liquid that has traversed several changes of direction in various planes. Liquid in the inlet (suction) pipe should approach the pump in a state of straight steady flow. When fittings, such as tees and elbows (especially two elbows at right angles), are located too close to the pump inlet (suction), a spinning action, or swirl, is induced. This swirl may adversely affect pump performance by reducing efficiency, head, and NPSH available, and potentially causing noise, vibration, and damage. It is therefore recommended that a single uninterrupted section of pipe be installed between the pump and the nearest fitting to allow the flow to straighten itself.
The suction pipe design must also be designed and built to minimize forces or stains on the pump suction nozzle. The pump must not be used as an anchor to close gaps due to construction errors or to withstand forces from pipe expansion due to temperature changes during operation. See HI Standard ANSI/HI 9.6.2 Centrifugal and Vertical Pumps for Allowable Nozzle Loads.
Q.
What are the minimum requirements for pumps to be used in boiler feed service?
A.
The type of boiler feed pump required by a generating plant is determined by the maximum boiler flow (capacity), the suction conditions (NPSHA), total pressure (head) required to be generated, and the operating temperature.
For low flow, low-pressure boiler feed systems, it may be possible to fulfill flow and head requirements with a single stage pump. In most cases, pressure (head) requirements are such that multistage pumps are necessary. In these cases, the pump can be one of several types and construction:
- Low- to medium-pressure/temperature systems may require a pump of ring-section construction, where the individual stages are made up of impellers and segmental rings (or casing sections, which include collectors to lead the flow from one stage to another), held together with tie rods. End heads contain the pump suction and discharge nozzles (see Figure 1.21 in ANSI/HI 1.1-1.2[Hyd Inst1] ).
- Medium-pressure/temperature systems may require axially split or ring-section pumps. Axially split pumps, unlike the ring-section pumps described above, may be of either back-to-back or in-line impeller construction and use cast casings, the lower half of which contains the pump suction and discharge nozzles. These pumps can be of either diffuser or volute construction. A back-to-back impeller pump design with volute construction is shown in Figure 1.20 in ANSI/HI 1.1-1.2[Hyd Inst2].
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- 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.
Q.
What are the most commonly used methods for priming a pump?
A.
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.