pump.pdf

Search Custom Search

Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

45Recommend

NPSH - Net Positive Suction Head

A definition and an introduction to Net Positive Suction Head - NPSH

Sponsored Links

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.

To characterize the potential for boiling and cavitation, the difference between the total head on the suction side of the pump - close to the impeller, and the liquid vapor pressure at the actual temperature, can be used.

Suction Head

Based on the Energy Equation - the suction head in the fluid close to the impeller can be expressed as the sum of the static and the velocity head:

hs = ps / γ + vs2 / 2 g (1)

where

hs = suction head close to the impeller

ps = static pressure in the fluid close to the impeller

γ = specific weight of the fluid

vs = velocity of fluid

g = acceleration of gravity

Liquids Vapor Head

The liquids vapor head at the actual temperature can be expressed as:

hv = pv / γ (2)

where

hv = vapor head

pv = vapor pressure

Ads by Google Pump Pressure Centrifugal Pumps Vertical Pump Pump Curves

Swimming Pool Pumps Swimming Pool Pumps & Equipment Brands: Sta-Rite, Onga, Nocchi www.proline.com.sg

Flow Meters, Australia/NZ Magflow, Vortex, Coriolis, VA, PD, Annubar, Ultrasonic, Thermal www.cmctechnologies.com.au

Slip On Flange Manufacture slip on flanges in A105, 304, 316, 316L, 321, etc. www.hongyechina.com

of 7NPSH - Net Positive Suction Head

12/25/2011http://www.engineeringtoolbox.com/npsh-net-positive-suction-head-d_634.html

Note! The vapor pressure in a fluid depends on temperature. Water, our most common fluid, starts boiling at 20 oC if

the absolute pressure in the fluid is 2.3 kN/m2. For an absolute pressure of 47.5 kN/m2, the water starts boiling at 80 oC. At an absolute pressure of 101.3 kN/m2 (normal atmosphere), the boiling starts at 100 oC.

Net Positive Suction Head - NPSH

The Net Positive Suction Head - NPSH - can be expressed as the difference between the Suction Head and the Liquids Vapor Head and expressed like

NPSH = hs - hv (3)

or, by combining (1) and (2)

NPSH = ps / γ + vs2 / 2 g - pv / γ (3b)

Available NPSH - NPSHa

The Net Positive Suction Head made available the suction system for the pump is often named NPSHa. The NPSHa can be determined during design and construction, or determined experimentally from the actual physical system.

The available NPSHa can be calculated with the Energy Equation. For a common application - where the pump lifts a fluid from an open tank at one level to an other, the energy or head at the surface of the tank is the same as the energy or head before the pump impeller and can be expressed as:

h0 = hs + hl (4)

where

h0 = head at surface

hs = head before the impeller

hl = head loss from the surface to impeller - major and minor loss in the suction pipe

In an open tank the head at surface can be expressed as:

h0 = p0 / γ = patm / γ (4b)

For a closed pressurized tank the absolute static pressure inside the tank must be used.

The head before the impeller can be expressed as:

hs = ps / γ + vs2 / 2 g + he (4c)

where

he = elevation from surface to pump - positive if pump is above the tank, negative if the pump is below the tank

Transforming (4) with (4b) and (4c):

patm / γ = ps / γ + vs2 / 2 g + he + hl (4d)

The head available before the impeller can be expressed as:

ps / γ + vs2 / 2 g = patm / γ - he - hl (4e)

or as the available NPSHa:

NPSHa = patm / γ - he - hl - pv / γ (4f)

Available NPSHa - the Pump is above the Tank

If the pump is positioned above the tank, the elevation - he - is positive and the NPSHa decreases when the elevation of the pump increases.

At some level the NPSHa will be reduced to zero and the fluid starts to evaporate.

Available NPSHa - the Pump is below the Tank

If the pump is positioned below the tank, the elevation - he - is negative and the NPSHa increases when the elevation of the pump decreases (lowering the pump).

It's always possible to increase the NPSHa by lowering the pump (as long as the major and minor head loss due to a longer pipe don't increase it more). This is important and it is common to lower the pump when pumping fluids close to evaporation temperature.



Required NPSH - NPSHr

The NPSHr, called as the Net Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump.

The required NPSHr for a particular pump is in general determined experimentally by the pump manufacturer and a part of the documentation of the pump.

The available NPSHa of the system should always exceeded the required NPSHr of the pump to avoid vaporization and cavitation of the impellers eye. The available NPSHa should in general be significant higher than the required NPSHr to avoid that head loss in the suction pipe and in the pump casing, local velocity accelerations and pressure decreases, start boiling the fluid on the impeller surface.

Note that the required NPSHr increases with the square capacity.

Pumps with double-suction impellers has lower NPSHr than pumps with single-suction impellers. A pump with a double-suction impeller is considered hydraulically balanced but is susceptible to an uneven flow on both sides with improper pipe-work.

Example - Pumping Water from an Open Tank

When increasing the the elevation for a pump located above a tank, the fluid will start to evaporate at a maximum level for the actual temperature.

At the maximum elevation NPSHa is zero. The maximum elevation can therefore be expressed by (4f):

NPSHa = patm / γ - he - hl - pv / γ = 0

For optimal theoretical conditions we neglect the major and minor head loss. The elevation head can then be expressed as:

he = patm / γ - pv / γ (5)

The maximum elevation or suction head for an open tank depends on the atmospheric pressure - which in general can be regarded as constant, and the vapor pressure of the fluid - which in general vary with temperature, especially for water.

The absolute vapor pressure of water at temperature 20 oC is 2.3 kN/m2. The maximum theoretical elevation height is therefore:

he = (101.33 kN/m2) / (9.80 kN/m3) - (2.3 kN/m2) / (9.80 kN/m3)

= 10.1 m

Due to the head loss in the suction pipe and the local conditions inside the pump - the theoretical maximum elevation is significantly decreased.

The maximum theoretical elevation of a pump above an open water tank at different temperatures can be found from the table below.

Suction Head as Affected by Temperature

TemperatureVapor Pressure

Max. elevation

(oC) (oF) (kN/m2) (m) (ft)

0 32 0.6 10.3 33.8

5 41 0.9 10.2 33.5

10 50 1.2 10.2 33.5

15 59 1.7 10.2 33.5

20 68 2.3 10.1 33.1

25 77 3.2 10.0 32.8

30 86 4.3 9.9 32.5

35 95 5.6 9.8 32.2

40 104 7.7 9.5 31.2

45 113 9.6 9.4 30.8

50 122 12.5 9.1 29.9

55 131 15.7 8.7 28.5

60 140 20 8.3 27.2

65 149 25 7.8 25.6

70 158 32.1 7.1 23.3

75 167 38.6 6.4 21

80 176 47.5 5.5 18

85 185 57.8 4.4 14.4

90 194 70 3.2 10.5



Search


Max. elevation

(oC) (oF) (kN/m2) (m) (ft)

95 203 84.5 1.7 5.6

100 212 101.33 0.0 0

Pumping Hydrocarbons

Be aware that the NPSH specification provided by the manufacturer in general is for use with cold water. For hydrocarbons these values must be lowered to account for the vapor release properties of complex organic liquids.

Note that the head developed by a pump is independent of the liquid, and that the performance curves for water from the manufacturer can be used for Newtonian liquids like gasoline, diesel or similar. Be aware that required power depends on liquid density and must be adjusted.

Sponsored Links

Search the Engineering ToolBox

Custom Search

Related Topics

Pumps - Piping systems and pumps - centrifugal pumps, displacement pumps - cavitation, viscosity, head and pressure, power consumption and more

•

Related Documents

Centrifugal Pumps - An introduction to Centrifugal Pumps •Classifications of Pumps - Selecting between Centrifugal Pumps and Positive Displacement Pumps •Condensate Pumping - High temperatures and danger of impeller cavitation is the major challenge of condensate pumping

•

Equation of Mechanical Energy - The equation of mechanical energy in terms of Energy per Unit Mass, Energy per Unit Volume and Energy per Unit Weight involving head

•

Pump Affinity Laws - Turbo machines affinity laws are used to calculate volume capacity, head or power consumption in centrifugal pumps when changing speed - rpm - or wheel diameters

•

Pumps - Specific Suction Speed - Specific Suction Speed may be used to determine what general pump design to use for maximum efficiency

•

Static Pressure and Pressure Head in Fluids - Static pressure and pressure head •

Engineering ToolBox - SketchUp Edition - Online 3D modeling!

Engineering ToolBox - SketchUp Edition - enabled for use with the amazing, fun and free Google SketchUp

Translate the ToolBox

Arabic - Chinese (Simplified) - Chinese (Traditional) - Dutch - French - German - Italian - Japanese - Korean - Portuguese - Russian - Spanish - - Select Your own language . .

About the ToolBox

We appreciate any comments and tips on how to make The Engineering ToolBox a better information source. Please contact us by email

[email protected] •

if You find any faults, inaccuracies, or otherwise unacceptable information.

The content in The Engineering ToolBox is copyrighted but can be used with NO WARRANTY or LIABILITY. Important information should always be double checked with alternative sources. All applicable national and local regulations and practices concerning this aspects must be strictly followed and adhered to.

Advertise in the ToolBox

If you want to promote your products or services in the Engineering ToolBox - please use Google Adwords.

Home

• Acoustics

• Air Psychrometrics

• Basics

• Combustion

• Drawing Tools

• Dynamics

• Economics

• Electrical

• Environment

• Fluid Mechanics

• Gas and Compressed Air

Hydrostatic Testing Units Industrial hydrostatic pipe & tubing testing units www.gardnerdenverproducts.com

Hydraulic pump repair Oilgear, HPM, Denison, Rexroth Vickers, HUGE Exchange Bank www.hsi-usa.com

Hydraulic Flow Meter Hydraulic system check and control, cylinder movement measurement... www.awflowmeters.com/industr



• HVAC Systems

• Hydraulics and Pneumatics

• Insulation

• Material Properties

• Mathematics

• Mechanics

• Miscellaneous

• Physiology

• Piping Systems - Codes and Standards

- Corrosion

- Design Strategies

- Dimensions

- Fluid Flow and Pressure Drop

- Heat Loss and Insulation

- Pressure Ratings

- Temperature Expansion

- Valve Standards

• Process Control

• Pumps

• Standards Organizations

• Steam and Condensate

• Thermodynamics

• Water Systems

Convert Units

Temperature

0

oC

oF

Convert!

Length

1

m

km

in

ft

yards

miles

nautical miles

Convert!

Volume

1

m3

liters

in3

ft3

us gal

Convert!

Velocity

1

m/s

km/h

ft/min

ft/s

mph

knots

Convert!

Pressure

1

Pa (N/m2)

bar

mm H2O

kg/cm2

psi

inches H2O

Convert!

Flow

1

m3/s

m3/h

US gpm

cfm

Convert!

Free Industry Magazines

Ads by Google

Pump Vibration

Pump Volute

AR Pump



GPS World

E&P (Hart's E&P)

Industrial Laser Solutions for Manufacturing

Engineering Standards

Sponsored Links

Free Industry Magazines

GPS World



E&P (Hart's E&P)

Industrial Laser Solutions for Manufacturing

Engineering Standards



Search Custom Search

Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

45Recommend

NPSH - Net Positive Suction Head

A definition and an introduction to Net Positive Suction Head - NPSH

Sponsored Links

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.

To characterize the potential for boiling and cavitation, the difference between the total head on the suction side of the pump - close to the impeller, and the liquid vapor pressure at the actual temperature, can be used.

Suction Head

Based on the Energy Equation - the suction head in the fluid close to the impeller can be expressed as the sum of the static and the velocity head:

hs = ps / γ + vs2 / 2 g (1)

where

hs = suction head close to the impeller

ps = static pressure in the fluid close to the impeller

γ = specific weight of the fluid

vs = velocity of fluid

g = acceleration of gravity

Liquids Vapor Head

The liquids vapor head at the actual temperature can be expressed as:

hv = pv / γ (2)

where

hv = vapor head

pv = vapor pressure

Ads by Google Pump Pressure Centrifugal Pumps Vertical Pump Pump Curves

Swimming Pool Pumps Swimming Pool Pumps & Equipment Brands: Sta-Rite, Onga, Nocchi www.proline.com.sg

Flow Meters, Australia/NZ Magflow, Vortex, Coriolis, VA, PD, Annubar, Ultrasonic, Thermal www.cmctechnologies.com.au

Slip On Flange Manufacture slip on flanges in A105, 304, 316, 316L, 321, etc. www.hongyechina.com

Page 1 of 7NPSH - Net Positive Suction Head


Note! The vapor pressure in a fluid depends on temperature. Water, our most common fluid, starts boiling at 20 oC if the absolute pressure in the fluid is 2.3 kN/m2. For an absolute pressure of 47.5 kN/m2, the water starts

boiling at 80 oC. At an absolute pressure of 101.3 kN/m2 (normal atmosphere), the boiling starts at 100 oC.

Net Positive Suction Head - NPSH

The Net Positive Suction Head - NPSH - can be expressed as the difference between the Suction Head and the Liquids Vapor Head and expressed like

NPSH = hs - hv (3)

or, by combining (1) and (2)

NPSH = ps / γ + vs2 / 2 g - pv / γ (3b)

Available NPSH - NPSHa

The Net Positive Suction Head made available the suction system for the pump is often named NPSHa. The NPSHa can be determined during design and construction, or determined experimentally from the actual physical system.

The available NPSHa can be calculated with the Energy Equation. For a common application - where the pump lifts a fluid from an open tank at one level to an other, the energy or head at the surface of the tank is the same as the energy or head before the pump impeller and can be expressed as:

h0 = hs + hl (4)

where

h0 = head at surface

hs = head before the impeller

hl = head loss from the surface to impeller - major and minor loss in the suction pipe

In an open tank the head at surface can be expressed as:

h0 = p0 / γ = patm / γ (4b)

For a closed pressurized tank the absolute static pressure inside the tank must be used.

The head before the impeller can be expressed as:

hs = ps / γ + vs2 / 2 g + he (4c)

where

he = elevation from surface to pump - positive if pump is above the tank, negative if the pump is below the tank

Transforming (4) with (4b) and (4c):

patm / γ = ps / γ + vs2 / 2 g + he + hl (4d)

The head available before the impeller can be expressed as:

ps / γ + vs2 / 2 g = patm / γ - he - hl (4e)

or as the available NPSHa:

NPSHa = patm / γ - he - hl - pv / γ (4f)

Available NPSHa - the Pump is above the Tank

If the pump is positioned above the tank, the elevation - he - is positive and the NPSHa decreases when the elevation of the pump increases.

At some level the NPSHa will be reduced to zero and the fluid starts to evaporate.

Available NPSHa - the Pump is below the Tank

If the pump is positioned below the tank, the elevation - he - is negative and the NPSHa increases when the elevation of the pump decreases (lowering the pump).

It's always possible to increase the NPSHa by lowering the pump (as long as the major and minor head loss due to a longer pipe don't increase it more). This is important and it is common to lower the pump when pumping fluids close to evaporation temperature.



Required NPSH - NPSHr

The NPSHr, called as the Net Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump.

The required NPSHr for a particular pump is in general determined experimentally by the pump manufacturer and a part of the documentation of the pump.

The available NPSHa of the system should always exceeded the required NPSHr of the pump to avoid vaporization and cavitation of the impellers eye. The available NPSHa should in general be significant higher than the required NPSHr to avoid that head loss in the suction pipe and in the pump casing, local velocity accelerations and pressure decreases, start boiling the fluid on the impeller surface.

Note that the required NPSHr increases with the square capacity.

Pumps with double-suction impellers has lower NPSHr than pumps with single-suction impellers. A pump with a double-suction impeller is considered hydraulically balanced but is susceptible to an uneven flow on both sides with improper pipe-work.

Example - Pumping Water from an Open Tank

When increasing the the elevation for a pump located above a tank, the fluid will start to evaporate at a maximum level for the actual temperature.

At the maximum elevation NPSHa is zero. The maximum elevation can therefore be expressed by (4f):

NPSHa = patm / γ - he - hl - pv / γ = 0

For optimal theoretical conditions we neglect the major and minor head loss. The elevation head can then be expressed as:

he = patm / γ - pv / γ (5)

The maximum elevation or suction head for an open tank depends on the atmospheric pressure - which in general can be regarded as constant, and the vapor pressure of the fluid - which in general vary with temperature, especially for water.

The absolute vapor pressure of water at temperature 20 oC is 2.3 kN/m2. The maximum theoretical elevation height is therefore:

he = (101.33 kN/m2) / (9.80 kN/m3) - (2.3 kN/m2) / (9.80 kN/m3)

= 10.1 m

Due to the head loss in the suction pipe and the local conditions inside the pump - the theoretical maximum elevation is significantly decreased.

The maximum theoretical elevation of a pump above an open water tank at different temperatures can be found from the table below.

Suction Head as Affected by Temperature


Max. elevation

(oC) (oF) (kN/m2) (m) (ft)

0 32 0.6 10.3 33.8

5 41 0.9 10.2 33.5

10 50 1.2 10.2 33.5

15 59 1.7 10.2 33.5

20 68 2.3 10.1 33.1

25 77 3.2 10.0 32.8

30 86 4.3 9.9 32.5

35 95 5.6 9.8 32.2

40 104 7.7 9.5 31.2

45 113 9.6 9.4 30.8

50 122 12.5 9.1 29.9

55 131 15.7 8.7 28.5

60 140 20 8.3 27.2

65 149 25 7.8 25.6

70 158 32.1 7.1 23.3

75 167 38.6 6.4 21

80 176 47.5 5.5 18

85 185 57.8 4.4 14.4



Search


Max. elevation

(oC) (oF) (kN/m2) (m) (ft)

90 194 70 3.2 10.5

95 203 84.5 1.7 5.6

100 212 101.33 0.0 0

Pumping Hydrocarbons

Be aware that the NPSH specification provided by the manufacturer in general is for use with cold water. For hydrocarbons these values must be lowered to account for the vapor release properties of complex organic liquids.

Note that the head developed by a pump is independent of the liquid, and that the performance curves for water from the manufacturer can be used for Newtonian liquids like gasoline, diesel or similar. Be aware that required power depends on liquid density and must be adjusted.

Sponsored Links

Search the Engineering ToolBox

Custom Search

Related Topics

Pumps - Piping systems and pumps - centrifugal pumps, displacement pumps - cavitation, viscosity, head and pressure, power consumption and more

•

Related Documents

Centrifugal Pumps - An introduction to Centrifugal Pumps •Classifications of Pumps - Selecting between Centrifugal Pumps and Positive Displacement Pumps •Condensate Pumping - High temperatures and danger of impeller cavitation is the major challenge of condensate pumping

•

Equation of Mechanical Energy - The equation of mechanical energy in terms of Energy per Unit Mass, Energy per Unit Volume and Energy per Unit Weight involving head

•

Pump Affinity Laws - Turbo machines affinity laws are used to calculate volume capacity, head or power consumption in centrifugal pumps when changing speed - rpm - or wheel diameters

•

Pumps - Specific Suction Speed - Specific Suction Speed may be used to determine what general pump design to use for maximum efficiency

•

Static Pressure and Pressure Head in Fluids - Static pressure and pressure head •

Engineering ToolBox - SketchUp Edition - Online 3D modeling!

Engineering ToolBox - SketchUp Edition - enabled for use with the amazing, fun and free Google SketchUp

Translate the ToolBox

Arabic - Chinese (Simplified) - Chinese (Traditional) - Dutch - French - German - Italian - Japanese - Korean - Portuguese - Russian - Spanish - - Select Your own language . .

About the ToolBox

We appreciate any comments and tips on how to make The Engineering ToolBox a better information source. Please contact us by email

[email protected] •

if You find any faults, inaccuracies, or otherwise unacceptable information.

The content in The Engineering ToolBox is copyrighted but can be used with NO WARRANTY or LIABILITY. Important information should always be double checked with alternative sources. All applicable national and local regulations and practices concerning this aspects must be strictly followed and adhered to.

Advertise in the ToolBox

If you want to promote your products or services in the Engineering ToolBox - please use Google Adwords.

Home

• Acoustics

• Air Psychrometrics

• Basics

• Combustion

• Drawing Tools

• Dynamics

• Economics

Hydrostatic Testing Units Industrial hydrostatic pipe & tubing testing units www.gardnerdenverproducts.com

Hydraulic pump repair Oilgear, HPM, Denison, Rexroth Vickers, HUGE Exchange Bank www.hsi-usa.com

Hydraulic Flow Meter Hydraulic system check and control, cylinder movement measurement... www.awflowmeters.com/industr



pump.pdf

Documents

Transcript of pump.pdf