Pump suction decrease

When liquids are being pumped, it is important to keep the pressure in the suction line above the vapor pressure of the fluid. The available head measured at the pump suction is called the net positive suction head available (NPSHA). At sea level, pumping 15°C (60°F) water with the impeller about 1 m below the surface, the NPSHA is about 9.1 m (30 ft). It increases with barometric pressure or with static head, and decreases as vapor pressure, friction, or entrance losses rise. Available NPSHA is the characteristic of the process and represents the difference between the existing absolute suction head and the vapor pressure at the process temperature. The required net positive suction head required (NPSHR), on the other hand, is a function of the pump design (Figure 2.121). It represents the minimum margin between suction head and vapor pressure at a particular capacity that is required for pump operation. Cavitation can occur at suction pres-... 

If the similarity laws are applied to a pump whose impeller diameter is increased, be certain to consider the effect of the higher velocity in the pump suction line. Use the similarity laws for any liquid whose viscosity remains constant during passage through the pump. However, the accuracy of the similarity laws decreases as the liquid viscosity increases. 

If the head losses in the pump suction piping can be reduced, the NPSHa wiU be increased. Various methods for reducing head losses Include Increasing the pipe diameter, reducing the number of elbows, valves, and fittings in the pipe, and decreasing the length of the pipe. 

They decrease considerably the sodium leakage from pressure pipes to the pump suction ... 

Capillary Suction Processes. The force needed to remove water from capillaries increases proportionately with a decrease in capillary radius, exceeding 1400 kPa (200 psi) in a 1-p.m-diameter capillary. Some attempts have been made to use this force as a way to dewater sludges and cakes by providing smaller dry capillaries to suck up the water (27). Sectors of a vacuum filter have been made of microporous ceramic, which conducts the moisture from the cake into the sector and removes the water on the inside by vacuum. Pore size is sufficiently small that the difference in pressure during vacuum is insufficient to displace water from the sector material, thus allowing a smaller vacuum pump to be effective (126). 

The suction head hs decreases and the discharge head hd increases with increasing liquid flow rate because of the increasing value of the friction head loss terms hfs and hfd. Thus the total head Ah which the pump is required to impart to the flowing liquid increases with the liquid pumping rate. 

The impeller is the working part of a centrifugal pump. The function of the impeller is to increase the velocity or kinetic energy of the liquid. The liquid flows into the impeller, and leaves the impeller, at the same pressure. The black dot shown at the top of the impeller in Fig. 23.6 is called the vane tip. The pressure at the vane tip is the same as the pump s suction pressure. However, as the high-velocity liquid escapes from the impeller and flows into the volute, its velocity decreases. The volute (which is also called the diffuser) is shaped like a cone. It widens out in the manner illustrated in Fig. 23.7. As the liquid flows into the wider section of the volute, its velocity is reduced, and the lost velocity is converted—well, not into pressure, but into feet of head. 

Let us try again. Suppose we decrease the temperature of the drum, shown in Fig. 25.1, from 140 to 110°F. This will also cool the liquid flowing into the suction of the pump by 30°F. Will this colder liquid then provide more NPSH to the pump, by suppressing the flashing of the liquid flowing into the pump s impeller ... 

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