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