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What is NPSH

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What is NPSH

  1. 1. • When altitude increases, pressure decreases, and water may boil at temperature below 100°C. • On every 27mmHg of atmospheric pressure increase, the boiling point of water will decrease 1°C.
  2. 2. Water vaporizing is related to the temperature and also pressure. T (°C) Ps (ata) Ps (kPa) 0 0.0062 0.6113 5 0.0089 0.8726 10 0.0125 1.2281 15 0.0174 1.7056 20 0.0238 2.3388 25 0.0323 3.1690 30 0.0432 4.2455 40 0.0752 7.3814 80 0.4829 47.3730 100 1.0332 101.3200 120 2.0245 160 6.302 Note: 1 ata = 10,0 m H2O 1 kPa = 101.97 mm H2O
  3. 3. The low pressure at the suction side of a pump can encounter the fluid to start boiling with • reduced efficiency • cavitation • damage of the pump as a result. Boiling starts when the pressure in the liquid is reduced to the vapor pressure of the fluid at the actual temperature.
  4. 4. • NPSHr : Required net positive suction head to avoid pump performance drop caused by cavitation. • NPSHa : Available net positive suction head at reference level of impeller inlet calculated by that total suction head minus absolute vapor pressure of the liquid. • When NPSHR becomes larger than NPSHA, cavitation occurs. The cavitation affects badly to pump and pipings as shown before. By this, the engineer should know how much margin before the liquid becomes vapor.
  5. 5. Suction Lift - 5m Losses 1m Reference Line Atm. Press. 1 atm = 10.3 m H2O NPSHa = 10.3 – 0.3 – 1 – 5 = 4 m Vapor Pressure 0.3m @ 25°C
  6. 6. Suction Lift - 1m Losses 1m Reference Line Atm. Press. 1 atm = 10.3 m H2O NPSHa = 10.3 – 4.8 – 1 – 1 = 3.5 m Vapor Pressure 4.8m @ 80°C
  7. 7. Reference Line Height of water Surface 3 m Pressure Tank 2 bar = 20.4m H2O Losses 2 m NPSHa = 20.4 + 3 – 4.8 – 2 = 16.6m Vapor Pressure @ 80°C 4.8m
  8. 8. Height of water Surface 8m Losses 3m NPSHa = 8 +21.47 – 21.47 – 3 = 5 m Pressure acts On liquid surface 21.47m Vapor Pressure 21.47m
  9. 9. • NPSHa = hsv: Available Suction Head (m) • Ps: Pressure acting on suction water level (kgf/m2abs) *absolute press. = gauge pressure + atm press • Pv: Absolute saturated vapor pressure of fluid under operating temperature (kgf/cm2abs) • ɣ: Weight of fluid per unit volume under operating temperature (kgf/m2) • hs: Height from suction water level up to reference level of impeller (m) • ᶉVs2/2g: Total head loss in suction line (m) NPSHa = hsv = Ps Pv ɣ ɣ– ± hs – ᶉV2 2g

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