Determining NPSH3, Comparing Axial Thrust & Evaluating Boiler Circulating Pumps

Q. What is NPSH3, and what are the methods for determining the NPSH3 of a rotodynamic submersible pump? A. NPSH3 is the net positive suction head available to a pump under test at a constant rate of flow when the pump head is decreased by 3 percent as a result of cavitation caused by a decreasing available suction head. Sometimes NPSH3 is referred to as net positive suction head required (NPSHR); however, a pump's NPSHR must be higher than the NPSH3 for the pump to operate without head reduction, and it may need to be higher than the NPSH3 for long-term reliable operation. For more information on the required margins above NPSH3, refer to ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin.Figure 11.6.7.2a. Suction throttling NPSH test setup This simple arrangement usually is satisfactory for NPSH greater than 3 meters (10 feet); however, the turbulence at the throttle valve tends to accelerate the release of dissolved air or gas from the liquid as the pressure on the liquid is reduced. A test made with this arrangement usually indicates higher NPSHR than what would be expected with deaerated liquid. Figure 11.6.7.2b shows a second option. The pump is supplied by a sump in which the liquid level can be varied to establish the desired NPSHR. Be careful to prevent entrained air or vortexing as the liquid level is varied. The priming connection should be installed above the eye of the impeller, either in the discharge pipe or on the pump.

For more information on test methods for rotodynamic submersible pumps, refer to ANSI/HI 11.6 Rotodynamic Submersible Pumps for Hydraulic Performance, Hydrostatic Pressure, Mechanical, and Electrical Acceptance Tests. Q. How does axial thrust compare among impeller types for a rotodynamic vertical pump? A. The net axial downthrust force is carried by the pump shaft. The shaft will stretch under this load. Before the pump starts, any stretch that occurs is the result of the sum of the static forces. The thrust load will increase after the pump starts because of the addition of dynamic forces. The dynamic forces creating thrust on a vertical turbine pump enclosed impeller result from the difference in pressure distributions on the upper and lower shrouds along with the force from the change in momentum of the flow through the impeller (see Figure 2.3.3.2.3a). The semi-open impeller has only an upper shroud (see Figure 2.3.3.2.3b). The difference in pressure distributions along the shroud's backside and vaned side is typically greater than between the upper and lower shrouds of an enclosed impeller. Semi-open impeller axial thrust is higher than that of the enclosed impeller. The axial flow pump impeller has no upper or lower shroud; vanes are attached directly to the hub. The axial thrust generated is primarily from dynamic forces created by interaction of the propeller vanes with liquid. The impeller back ring with balance holes configuration reduces axial thrust (see Figure 2.3.3.2.3c). Back rings may be cast integrally into impellers with a top shroud. They are used when pump total axial thrust requires reduction. The flow through balance holes in the impeller hub shroud, combined with the leakage past the balance ring, reduces efficiency. Increased leakage through clearances that results from wear of the back ring arrangement may cause an increase in downthrust. For more information about axial thrust for vertical rotodynamic pumps, see ANSI/HI 2.3 Rotodynamic (Vertical) Pumps for Design and Application. Q. What information is available regarding boiler circulating pumps for combined-cycle power plant service? A. Boiler circulating pumps circulate water within the boiler to enhance boiler operation. They take suction from a header connected to the bottom of the boiler drum and discharge through additional tube circuits. This means the water pumped is at boiler temperature and pressure. For this reason, standard boiler circulating pumps must be designed for high temperature (usually between 300 and 600 F [150 and 315 C] depending on boiler size and rating) and high pressure (corresponding to boiler temperature and water vapor pressure). For small boilers, with relatively low temperatures and pressures, conventional overhung pump designs may be suitable for boiler circulating service. Boiler circulating pumps must develop enough head to overcome the friction of the tube circuits; however, the combination of high temperature and pressure results in sealing conditions that require special sealing systems. Because of the relatively low head requirements, the pumps are single-stage with single-suction impellers and a single seal chamber. This creates a problem of unbalanced axial thrust, which may require special pump bearing systems or balancing arrangements. An alternate solution is to use pumps of wet-motor construction, where the pump and the motor are inside the pressure vessel, eliminating sealing and unbalanced axial thrust issues. Such special pumps are welded into the boiler piping. For higher temperatures to 685 F (365 C) and pressures from 1,800 to 2,800 pounds per square inch (psi) (124 to 193 bar), special pump designs are required. For more information about pumps used in combined-cycle power plants, see HI's guidebook Power Plant Pumps: Guidelines for Application and Operation to Maximize Uptime, Availability, and Reliability.