Page 152 - Fluid mechanics, heat transfer, and mass transfer

130 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS
TABLE 5.4 Comparison of Capacities, Heads, and Efficiencies for Positive Displacement Pumps

Pump Type Capacities Heads Efficiencies
3
Axial flow 0.076–376 m /min (20–100,000 gpm) Up to 12 m (40 ft) 65–85
3
Rotary 0.00378 m /min (20–100,000 gpm) Up to 15,200 m (50,000 ft) 50–80
3
Reciprocating 0.0378–37.8 m /min (10–100,000 gpm) Up to 300,000 m (1,000,000 ft) 70% at 7.46 kW (10 HP)
85% at 37.3 kW (50 HP)
90% at 373 kW (500 HP)
3
264 gpm ¼ 1m /min; 3.28 ft ¼ 1m.

& A relief valve, set to open at a pressure below design & It is sometimes better to select a larger pump running
pressure, avoids pipe rupture. at low speeds than a smaller pump at high speeds
. Give a comparison of axial flow, rotary, and recipro- when dealing with viscous liquids.
cating pumps with respect to ranges of capacities, heads & As a general guide, speed is reduced 25–35% below
developed, and efficiencies. rating for each 10-fold increase in viscosity above
& Table 5.4 presents a comparison of capacities, heads, 1000 SSU. Also, generally, the mechanical efficiency
and efficiencies for axial flow, rotary, and recipro- of the pump is decreased 10% for each 10-fold
cating pumps. increase in viscosity above 1000 SSU.
. What type of pump is normally recommended for & Not suitable for liquids with abrasive solids.
hydraulic testing of vessels? Why? & They are usually self-priming, able to handle very
& Direct-acting plunger pump is suitable as it stalls at a low NPSH and reversible.
set pressure and pumps only when pressure falls. & Low cost.
. What type of pump is suitable for pumping light hydro- & Require small space for installation.
carbons from refinery knockout drums? & Low volumetric efficiency.
& Direct-acting positive displacement pump because of & Require overpressure relief protection.
low NPSH and variable speed requirements. & Entrained gases reduce liquid capacity and cause
. “Plunger pumps are suitable for homogenization of pulsations.
food products.” True/False?
& Figure 5.24 illustrates a sliding vane rotary pump.
& True. Milk and food products are pumped to high ➢ Vane pumps are effective for low-viscosity liquids
pressures and throttled through special valves. and when dry-priming is required. They are not
. What are the characteristics and applications of rotary ideal for abrasive liquids.
pumps? Give a diagram of a typical rotary pump. . What are the factors that influence the amount of slip in
& Flow is proportional to speed and is independent of a rotary pump?
DP. & Clearance: Increased clearances result in increase in
& BHP varies directly with speed and pressure.
slip.
& With speed and pressure constant, BHP is propor-
tional to viscosity of liquid.
& Have close clearances.
& Sometimes used as metering pumps.
& Some designs involve rotation in either direction.
& Internal slip reduces efficiency; increases with pres-
sure and decreases with viscosity.
& Self-priming.
& Can be used for liquids of any viscosity but effective
for high-viscosity liquids, greases, paints, and so on.
& Majority of these pumps handle clean liquids with
6
wide range of viscosities upto 10 cPand pressures of
over 70 bar.
& Used for small and medium capacities in the range of
3
0.2–115 m /h (1–500 gpm). FIGURE 5.24 Sliding vane rotary pump.