Net Positive Suction Head (NPSH) Safety margin against occurrence of Cavitation
UNIT -6 Sec. 12.2.3
WHAT IS CAVITATION? • It is a condition inside a turbomachine where the local pressure drops to the vapor pressure of the liquid; and • As a result vapor-filled cavities are formed • It severely damages components of the machine and reduces its performance
Water vaporizes at lower temperature at lower pressure. For example, at 1 atm (10.33 m) water vaporizes at 100 C, but it will vaporize at about 30 C at 0.04 atm (0.44m Head)
How cavitation occurs Pressure at pump inlet = Patm - Hs ρ g. When Pinlet < Potm the water may boil at lower temperature creating vapor bubbles. As the bubbles travels into regions of high pressure in blades they suddenly collapse generating extremely high local pressure.
Effect of Cavitation • Damage of blade/casing through pitting, erosion and fatigue • Noise • Vibration • Lowering of head and discharge • Lowering of efficiency
Effects of cavitation • Damage of blade/casing through pitting, erosion and fatigue • Loss in capacity. • Loss head (pressure) developed • Efficiency loss. • Noise • Vibration
NPSH • Static head at pump inlet Pi = Patm – ρ g Hs • The suction head, Hs includes suction lift ZS , pipe losses, hs and the velocity head v2/2g Hs = Zs +hs + v2/2g • When cavitation starts Pi=
Figure 12.10 – Cavitation bubble distribution in an impeller region. (Courtesy of Sulzer Pumps Ltd.)
Cavitation • Sometimes the pressure at the inlet of a pump drops below the vapor pressure of the liquid. So vapor bubbles formed are carried in the pump • The bubbles suddenly collapse when the pressure increases above its vapor pressure • If the collapse takes place on any pump surface pitting occurs due to high local pressure gradient. • Cavitation also causes rapid decrease in the performance of the pump • To avoid this a minimum head of fluid is maintained at the pump inlet. It is called Net Positive Suction Head (NPSH)
Cavitation Number (Thoma coefficient) • Besides Net positive suction head (NPSH), another parameter (Cavitation number) is used for cavitation design • Cavitation number (Thoma Coefficient, σ) is the ratio of NPSH and the total pump head (H). σ = NPSH/H • For two similar pumps head coefficients are same Ch1= Ch2= gH/(w2D2) • So by substituting NPSH for H we can get NPSH1 N1 = NPSH 2 N 2
2
D1 D2
2
Problem • Cavitation tests were performed on a pump giving the following results: Q = 0.05 cu.m/s; H= 37 m; barometric pressure 760 mm of mercury; ambient temperature 25 C. Cavitation began when the total head at pump inlet was 4 m. Calculate the value of Thoma cavitation coefficient and the NPSH. • What would be the maximum height of this pump above water level if it is to operate at the same point on its characteristic in the ambient conditions of a barometric pressure 640 mm of Mercury and a temperature of 10 C • [Ans 0.162, 6.086 m. 2.55 m. Douglas 25.11]
Problem • For the system shown in Fig P12.6 water at 20 C flows through the pump at a rate of 50 L/s. The allowable NPSH provided by the manufacturer at that flow is 3 m. By using the vapor pressure from the table determine the height z above the water surface that the pump can be located to operate without cavitating. Include all losses in the suction pipe of .1 m dia. [Potter 12.6]
Figure P12.6
Figure 12.26 – Cavitation considerations: (a) schematic; (b) representative cavitation number curve. (Courtesy of Voith Siemens Hydro Generation, Inc.)
