Post on 15-Apr-2017
What is NPSH?By Yudi Setiawan
Net Positive Suction Head
WATER WILL BECOMES VAPOR IN ATMOSPHERIC PRESSURE AT 0 ALTITUDE, WHEN TEMPERATURE REACH 100°C.
Boiling point ~ Altitude
• 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.
Vapor Pressure
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 6.302Note:1 ata = 10,0 m H2O1 kPa = 101.97 mm H2O
Pump engineer always have to consider NPSHr and NPSHa
The low pressure at the suction side of a pump can encounter the fluid to start boiling with• reduced efficiency• cavitation• damageof 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.
Pressure Drops on The Impeller
Cavitation and Implosion
Impeller Damaged Caused by Cavitation
NPSH Term• 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.
NPSHa should be > than NPSHrBy this, the engineer should know how much
margin before the liquid becomes vapor.
NPSH3 = NPSHr when the total head of pump 1st stage drops 3%
Calculate the NPSHa @ 25°C
Suction Lift- 5m
Losses 1mReferenceLine
Atm.Press.
1 atm =10.3 m H2O
NPSHa= 10.3 – 0.3 – 1 – 5= 4 m
Vapor Pressure 0.3m @ 25°C
Calculate the NPSHa @ 80°C
Suction Lift- 1m
Losses 1mReferenceLine
Atm.Press.
1 atm =10.3 m H2O
NPSHa= 10.3 – 4.8 – 1 – 1 = 3.5 m
Vapor Pressure 4.8m @ 80°C
Calculate the NPSHa @ 25°C Positive Suction & Pressurized Tank
ReferenceLine
Height of water Surface
3 m
Pressure Tank2 bar = 20.4m H2O
Losses2 m
NPSHa= 20.4 + 3 – 4.8 – 2= 16.6m
Vapor Pressure @ 80°C 4.8m
Calculate the NPSHa @ 120°C Suction Tank in which Saturated Vapor Pressure Acts
Height of waterSurface 8m
Losses3m
NPSHa= 8 +21.47 – 21.47 – 3= 5 m
Pressure actsOn liquid surface
21.47m
Vapor Pressure21.47m
Basic Formulation to Calculate NPSHa
• 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 Pvɣ ɣ– ± hs – ᶉV2
2g
GOOD LUCK!!
Thank you