2240

"Despite the high resistance of this material to cavitation damage, it is not suitable for ordinary use because of its comparatively high cost and the difficulty encountered in machining and grinding.

Source: Reference 69.

"Despite the high resistance of this material to cavitation damage, it is not suitable for ordinary use because of its comparatively high cost and the difficulty encountered in machining and grinding.

Source: Reference 69.

(MDPR, in mm per year) times the thickness of the material in mm. Values of MDPR have been deduced37 from the weight loss of test samples of known diameter in a magnetostriction test (Table 8)38. Unfortunately, MDPR-values obtained in such tests are not the same as those of actual pumps, as the mode of cavitation varies with pump operating conditions and is generally different from the laboratory results. Nevertheless, laboratory results for MDPR have been used to rank the ability of various materials to resist cavitation erosion in pumps. See Section 5.1 for material selection guidelines.

Cavitation-resistant coatings, either metallic or nonmetallic, have found some niche applications. Elastomeric coatings are resilient and resist cavitation through a different erosion mechanism than that of metal. As such, they can be very effective69. At least two considerations are involved in the use of coatings for resisting cavitation damage: 1) even if a contemplated coating demonstrates a reduced damage or erosion rate, this reduction must be enough to justify the cost of establishing a satisfactory bond between the coating and the base metal; 2) erosion of both the coating and the base material must be considered in determining the life according to the above 75%-depth criterion. Therefore, the life in this case would be equal to the MDPR of the coating times the coating thickness plus the MDPR of the base material times the allowable erosion depth of that material.

Inducers It is sometimes difficult or impossible to provide the required NPSH for an otherwise acceptable pump. Besides normal industrial situations that might produce a very low available NPSH, the need to keep the weight down in aircraft and rocket liquid-propellant pumps has led to high rotative speeds, which, for typical values of NPSH, produce extremely high suction specific speeds. The performance-NPSH required by the impeller under these circumstances can be provided by a small, axial-flow booster pump, called an inducer, placed ahead of the first-stage impeller39. Inducers are designed to operate with very low NPSH and to provide enough head to satisfy the NPSH required by the impeller. In fact, long stable cavities are established on the suction sides of the long, lightly-loaded blades of an inducer, which enable it to operate at about twice the suction specific speed of a conventional impeller40,41. At lower than normal flow rates, however, inducers readily produce swirling, destabilizing backflow at the inlet, which can cause excessive pump vibration in high-head pumps. These instabilities can be overcome by various passive design features, such as that described in Reference 39.

The inducers described in Reference 42 (Figure 30) have "constant lead" helical blades. They contribute not more than 5% of the total pump head. Although the efficiency of the inducer alone is low, the reduction in overall pump efficiency is not significant. Because this type of inducer causes prerotation, a careful match between inducer and suction impeller is required. In vertical multistage pumps, where a long shaft can be better supported, a vaned diffuser may be inserted between the inducer and the first-stage impeller. Such an arrangement is very beneficial for operation at reduced flow rate. Reference 42 shows that a suitable inducer-impeller combination can operate at about 50% of the NPSH required for the impeller alone at flow rates not exceeding the normal value. The NPSH requirement increases rapidly for flow rates above normal. Unless a variable-lead inducer is used32,39, operation in this range should be avoided.

Entrained Air Air or other gases may enter the impeller inlet from several sources. The immediate effect usually will be a drop in pump pressure rise, flow rate, and power. This will be followed by loss of prime if more gas is present than the impeller can handle. A typical limit for commercial industrial pumps is an inlet gas-to-liquid volume fraction (GVF) of 0.03, although specialty pumps such as those used in aircraft (Section 9.19) can handle higher GVF. See also Reference 9, Section 2.1. Air may be released from solution or enter through leaks in the suction piping. Stuffing box air leakage may be prevented by lantern rings supplied with liquid from the pump discharge. If the pump takes water from a sump with a free surface, a vortex may form from the free surface to the impeller inlet. The remedy may be the introduction of one or more baffle plates or even major changes in the sump. For information on proper sump design and the prevention of air-entraining vortices, see Sections 10.1 and 10.2, pp. 457 and 460 of Reference 7, and Reference 43. It is sometimes permissible to inject a small amount of air into the pump

FIGURE 30 Pump fitted with inducer (Reference 42)

suction to reduce the noise and damage from cavitation caused by inadequate NPSH or recirculation in the impeller (see Subsection 2.3.2).

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