Pumps & Systems, March 2007
What are the key requirements for good suction piping design for rotodynamic pumps?
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.
What are the minimum requirements for pumps to be used in boiler feed service?
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].