Q. I see that many pumps for pumping slurries are made of either hard metals or elastomers. Why is this and which is better?
A. A large variety of metals and elastomers are used for slurry pumps because of the diverse range of applications. Slurries can be erosive, corrosive or erosive/corrosive. Proper material selection depends on the properties of the mixture to be pumped and the pump design. Figure 18.104.22.168a may be used to rank purely erosive wear. Table 22.214.171.124 is a selection guide for materials commonly used in these services along with their appropriate erosive wear service classes.
Metals resist erosion through a combination of proper hardness and toughness. Toughness is defined as the ability of a material to absorb energy and even deform plastically before fracturing. Hardness provides resistance to sliding wear. Toughness diminishes crack formation and propagation encountered in impacting wear situations, providing resistance to impact fracture. Harder materials are better choices for sliding wear services. A very hard, brittle material that fractures easily may not perform as well as a softer metal that resists brittle fracture. Erosion resistance should not be judged only on the hardness of the material.
Material selection is further complicated when corrosive carrier liquids are involved. Materials that are highly resistant to erosion are usually not highly resistant to aggressive corrosion. Material selection is a compromise between erosion and corrosion resistance properties to achieve optimum wear life for any specific installation.
Metals resist corrosion by forming a passivated surface layer that protects against further corrosion. Effectiveness is determined by how tough the passive layer is and how fast it forms. In slurry services, the passive layer is continually being worn away and reformed so corrosive attack is accelerated.
Elastomers resist erosion through resilience and tear resistance. They are soft, and the solid particles rebound without damaging the elastomer by abrasion or fracture. Large or sharp particles may tear the elastomer, so material selection must be carefully matched to the slurry.
Elastomers do not depend on a passivated layer to resist corrosion. The basic chemical resistance is a function of proper material selection and is not significantly changed by exposure to erosive environments. Slurry pumps usually have thicker liners than other elastomer-lined pumps. Experience has shown that when lining thickness is increased, wear life increases by a factor of approximately 2:1, within the limits of a practical liner thickness.
Elastomers can be easily bonded to metals to combine the strength and rigidity of the metal with the elasticity of the elastomer. They can also be bonded to materials such as fiberglass-reinforced plastic (FRP) or thermosetting phenolic/nylon cloth to stiffen liners to prevent collapsing during process disruptions, such as cavitation or surges.
They can be bonded to ceramics to take advantage of the best of both materials. Process and environmental temperatures must be considered, because some elastomers do not perform well above 80 degrees C (180 degrees F).
Erosive and erosive/corrosive wear may occur under different mechanisms. Because of the complex nature, the wear results may vary substantially from case to case. Experience with similar applications is always the best guide for selecting materials. If there is inadequate experience, wear testing can be performed to help evaluate the level and characteristics of wear factors. Some typical wet wear tests include the ASTM G75-01 Miller procedure, slurry-jet wear testing and Coriolis wear testing.
Corrosion, erosion and corrosion/erosion testing may be required to analyze erosive/corrosive applications in which experience is not available. ANSI/HI 12.1-12.6 Rotodynamic (Centrifugal) Slurry Pumps for Nomenclature, Definitions, Applications, and Operation has recently been updated and published.
Q. What is suction specific speed, and how is it used?
A. Suction specific speed is an index of pump suction operating characteristics determined at the best efficiency point (BEP) rate of flow with the maximum diameter impeller. (Suction specific speed is an indicator of the net positive suction head [NPSH] required for given values of capacity and provides an assessment of a pump's susceptibility to internal recirculation.) Suction specific speed is expressed by the following equation:
S = n(Q)0.5 / (NPSH3)0.75
S = suction specific speed
n = rotative speed, in revolutions per minute
Q = flow rate per impeller eye, in cubic meters per second (U.S. gallons per minute) using total flow rate for single suction impellers and one half total flow rate for double suction impellers
NPSH3 = net positive suction head required, in meters (feet), that will cause the total head (or first-stage head of multistage pumps) to be reduced by 3 percent
Note: Suction specific speed derived using cubic meters per second and meters, multiplied by a factor of 51.6, is equal to suction specific speed derived using U.S. gallons per minute and feet. The U.S. customary symbol Nss is sometimes used to designate suction specific speed.