Self-priming pumps

These pumps are low in cost, require small space, and are self priming. 

Positive displacement pumps are self-priming by their normal operating action and are designed to cope with mnning dry. Centrifugal pumps are not inherently selfpriming, and need to be provided with assistance in the form of ejectors, as in Figure 32.56 or dry vacuum pump systems. In these systems, a dry vacuum pump allows air to be drawn out and where an automatic valve prevents... 

Pumps are self-priming to a depth of 22 ft and can be effective to depths of 27 ft, providing suction is not broken. 

Not self-priming the pump must be started with the housing full of fluid and free of air... 

The turbine impeller of Figure 7.10(h) rotates in a case of uniform diameter, as in Figure 7.12(j). As Figure 7.4(a) demonstrates, turbine pump performance resembles that of positive displacement types. Like them, turbines are essentially self-priming, that is, they will not vapor bind. 

Although centrifugal pumps are the major kinds in use, a great variety of other kinds exist and have limited and sometimes unique applications. Several kinds of positive displacement types are sketched in Figure 7.12. They are essentially self-priming and have a high tolerance for entrained gases but not usually for solids unless they may be crushed. Their characteristics and applications are discussed in the next section. 

Figure 17.11. Types of contactors for reacting gases with liquids many of these also are suitable for reacting immiscible liquids. Tanks (a) with a gas entraining impeller (b) with baffled impellers (c) with a draft tube (d) with gas input through a rotating hollow shaft, (e) Venturi mixer for rapid reactions, (f) Self-priming turbine pump as a mixer-reactor, (g) Multispray chamber. Towers (h) parallel flow falling film (i) spray tower with gas as continuous phase (j) parallel flow packed tower (k) counter flow tray tower. (1) A doublepipe heat exchanger used as a tubular reactor.

One reason for the popularity of peristaltic pumps is their versatility they can be programmed for low or high speeds. Tubes vary in diameter to cover a variety of flow rates. The various sizes of tubes are color coded for diameter and output rate. Another advantage is ease of cleaning After a run, just throw out the inexpensive tubes and put in clean ones. The tubes can also be capped and preserved with the container of color to be used the next time it is run. Peristaltic pumps are self-priming, and a cycle is included in the programming of the pump to fill the output tube up to the extruder or molder quickly. 

Type of flow distribution. If nonpulsating flow is required, certain types of pumps, such as simplex reciprocating pumps, may be unsatisfactory. Similarly, if operation is intermittent, a self-priming pump may be desirable, and... 

Positive-displacement pumps are self priming, which is the ability of a pump to lift liquids from a level below the center line of the pump. This characteristic of positive-displacement pumps is attributed to the tight seal between the discharge and suction sides of the pump. Thus, at startup air is compressed and... 

Roller pumps are among the least expensive and most widely nsed of all sprayer pumps. They provide moderate volumes (8 to 30 gpm) at low to moderate pressure (10 to 300 psi). Often used on low-pressure sprayers, roller pumps are positive-displacement, self-priming pnmps. The rollers, made of nylon, teflon, or rubber, wear rapidly in wettable powders but are replaceable. A pump that will be subjected to such wear should have a capacity at least 50 percent greater than that needed to supply the nozzles and agitator. This reserve capacity will extend the life of the pump. The pump case is usually cast iron or a nickel alloy. Roller pumps are best for emulsifiable concentrates, soluble powders, and other nonabrasive pesticide formulations (Figure 14B). 

Gear pumps are used on sprayers with low operating pressures. They provide low to moderate volume (5 to 65 gpm) at low to moderate pressures (20 to 100 psi). Often used on special-purpose sprayers, gear pumps are positive-displacement, self-priming pumps. The self-priming ability is rapidly lost as the pump wears. These pumps are designed for oil solution formulations and wear rapidly when suspensions of wettable powders are used. The parts are generally not replaceable. The pump is not affected by solvents. 

Piston pumps are the most expensive of the commonly used sprayer pumps. They deliver low to medium volumes (2 to 60 gam) at low to high pressures (20 to 800 pi). Used for high-pressure sprayers or when both low and high pressures are needed, piston pumps are positive-displacement, self-priming pumps. They have replaceable piston cups made of leather, neoprene, or nylon fabric which make the pump abrasion-resistant and capable of handling wettable powders for many years. The cylinders are iron, stainless steel, or porcelain-lined. The pump casing is usually iron. 

---