Impellers in pumps

When the layer of graphite and corrosion products is impervious to the solution, corrosion wdl cease or slow down. If the layer is porous, corrosion will progress by galvanic behavior between graphite and iron. The rate of this attack will be approximately that for the maximum penetration of steel by pitting. The layer of graphite formed may also be effective in reducing the g vanic action between cast iron and more noble alloys such as bronze used for valve trim and impellers in pumps. 

Chemical end uses employ the most exceptional property of PPS, chemical inertness. PPS is almost as chemically resistant as Teflon. It is used in pump impellers and housings and for down-od-weU end uses. New extmsion grades have been developed for potential piping end uses. Coatings of PPS are used extensively. 

For instaUations in which suspended sohds must be handled with a minimum of solids breakage or degradation, such as pumps feeding filter presses, special attention is required either a low-shear positive-displacement pump or a recessed-impeller centrifugal pump may be called for. 

Using a draft tube in the tank for solids suspension introduces another, different set of variables. There are other relationships that are veiy much affected by scale-up in this type of process, as shown in Fig. 18-22. Different scale-up problems exist whether the impeller is pumping up or down within the draft tube. 

FIG. 18-22 Typical draft tube circulator, sbown bere for down-pumping mode for tbe impeller in tbe draft tube. 

Many copper alloys are also used in pumps as bushings, bearings, impellers, and gaskets. Bronzes, brasses, and other copper alloys are frequently used. Thus, pump components are often corroded. 

Similarly, graphitically corroded cast iron (see Chap. 17) can assume a potential approximately equivalent to graphite, thus inducing galvanic corrosion of components of steel, uncorroded cast iron, and copper-based alloys. Hence, special precautions must be exercised when dealing with graphitically corroded pump impellers and pump casings (see Cautions in Chap. 17). 

For the purposes of understanding this concept and formula, there s nothing mathematically significant about the square root of the flow, or the NPSHr to 1 power. These mathematical manipulations simply give us Nss values that are e. y understood and recognizable. For example, the health inspector might j. . a restaurant s cleanliness on a scale from 1 to 100. We might ask you to rate this Lc on a scale from 1 to 10. Those are easy numbers to deal with. How would yc- this book on a scale from 2,369 to 26,426,851 This doesn t make sense. Likewi , the mathematical manipulations in the Nss formula serve simply to convert weird v . into a scale from 1,000 to 20,000 that cover most impellers and pumps. Values at... 

Inadequate Pressure. Not enough velocity. Air or gases in pumped liquid. Impeller diameter too small Worn or damaged impeller Incorrect rotation... 

The theoretical maximum suction lift at sea level for water (14.7 psi) (2.31 fi/psi) = 34 ft. However, due to flow resistance, this value is never attainable. For safety, 15 feet is considered the practical limit, although some pumps will lift somewhat higher columns of water. WTen sealing a vacuum condition above a pump, or the pump pumps from a vessel, a seal allowance to atmosphere is almost always taken as 34 feet of water. High suction lift causes a reduction in pump capacity, noisy operation due to release of air and vapor bubbles, vibration and erosion, or pitting (cavitation) of the impeller and some parts of the casing. (The extent of the damage depends on the materials of construction.)... 

For low available NPSH (less than 10 feet) the pump suction connection and impeller eye may be considerably oversized when compared to a pump not required to handle fluid under these conditions. Poor suction condition due to inadequate available NPSH is one major contribution to cavitation in pump impellers, and this is a condition at w hich the pump cannot operate for very long without physical erosion damage to the impeller. See References [11] and [26]. 

The principle significance of specific speed for the process engineer is to evaluate the expected performance of a second pump in a particular manufacturer s series while basing it on the known performance (or curve) at the point of optimum efficiency of a first and different size pump. In effect the performance of any impeller of a manufacturer s homologous series can be estimated from the known performance of any other impeller in the series, at the point of optimum efficiency. Figures 3-48 and 3-49 represent the standardized conditions of essentially all pump manufacturers. 

Electric motors in pump application never run at the standard rotative design speeds noted above, but rotate at about (with some deviation) 3450, 1750, and 1150 rpm, which are the speeds diat most pump manufacturers use for their performance curves. If the higher numbers were used (motor designated or name plate) for pump performance rating, the pumps would not meet the expected performance, because the motors would not be actually rotating fast enough to provide the characteristic performance curves for the specific size of impeller. 

Titanium impellers have been used in pumps employed for the conveyance of corrosive and erosive ore slurries, for organic chlorides containing hydrochloric acid and free chlorine , for handling moist chlorine gas, and in the wood-pulp and the textile-bleaching industry, particularly with sodium hypochlorite . 

In practice it is expensive, and therefore uneconomic, to produce a pump which operates completely free from cavitation. As a result it is usual for commercial pumps to operate in the NPSH range between inception and a point where erosion damage is unacceptable. The extent of this range may be increased by using impellers made from the more resistant materials shown in Fig. 8.77. The subject of cavitation in pumps has been dealt with extensively by Pearsall and Grist... 

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