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ow pressure at the suction side of a pump can encounter the fluid to start boilinith

reduced efficiency

cavitation

damage

f the pump as a result. Boiling starts when the pressure in the liquid is reduced

he vapor pressure of the fluid at the actual temperature.

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

Suction Head

ased on the Energy Equation - the suction head in the fluid close to the impelan be expressed as the sum of the staticand the velocity head:

s = ps / + vs2 / 2 g (1)

where

s = suction headclose to the impeller

s = static pressure in the fluid close to the impeller

= specific weightof the fluid

s = velocity of fluid

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= acceleration of gravity

iquids Vapor Head

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

v= pv / (2)

where

v= vapor head

v= vapor pressure

ote! The vapor pressure in a fluid depends on temperature. Water, our mostommon fluid, starts boiling at 20oCif the absolute pressure in the fluid is 2.3N/m2. For an absolute pressure of47.5 N/m2, the water starts boiling at 80oC. An absolute pressure of101.3 kN/m2 (normal atmosphere), the boiling startst 100oC.

Net Positive Suction Head - NPSH

he Net Positive Suction Head - NPSH - can be expressed as the differenceetween the Suction Head and the Liquids Vapor Head and expressed like

NPSH = hs - hv (3)

r, by combining (1) and (2)

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

Available NPSH - NPSHa

he Net Positive Suction Head made available the suction system for the pumpften named NPSHa. The NPSHa can be determined during design andonstruction, or determined experimentally from the actual physical system.

he available NPSHa can be calculated with the Energy Equation. For a commopplication - where the pump lifts a fluid from an open tank at one level to anther, the energy or head at the surface of the tank is the same as the energy o

ead before the pump impeller and can be expressed as:

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0= hs + hl (4)

where

0= headat surface

s = headbefore the impeller

= headloss from the surface to impeller - major and minor loss in the suctionipe

n an open tank the head at surface can be expressed as:0= p0 / = patm / (4b)

or a closed pressurized tank the absolute static pressure inside the tank must sed.

he head before the impeller can be expressed as:

s = ps / + vs2 / 2 g + he (4c)

where

e = elevation from surface to pump - positive if pump is above the tank, negativthe pump is below the tank

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

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

he head available before the impeller can be expressed as:

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

r as the available NPSHa:

NPSHa = patm / - he - hl - pv / (4f)vailable NPSHa - the Pump is above the Tank

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

t some level the NPSHa will be reduced to zero and the fluid starts to evaporat

vailable NPSHa - the Pump is below the Tank

the pump is positioned below the tank, the elevation - he - is negative and the

PSHa increases when the elevation of the pump decreases (lowering the pumps always possible to increase the NPSHa by lowering the pump (as long as

he major and minor head loss due to a longer pipe don't increase it more). Thismportant and it is common to lower the pump when pumping fluids close tovaporation temperature.

Required NPSH - NPSHr

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

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he required NPSHr for a particular pump is in general determined experimentay the pump manufacturerand a part of the documentation of the pump.

he available NPSHa of the system should always exceeded the requiredPSHrof the pump to avoid vaporization and cavitation of the impellers eye. Thvailable NPSHa should in general be significant higher than the requiredPSHr to avoid that head loss in the suction pipe and in the pump casing, local

elocity accelerations and pressure decreases, start boiling the fluid on thempeller surface.

ote that the required NPSHr increases with the square capacity.

umps with double-suction impellers has lower NPSHrthan pumps with single-uction impellers. A pump with a double-suction impeller is consideredydraulically balanced but is susceptible to an uneven flow on both sides with

mproper pipe-work.

Example - Pumping Water from an Open TankWhen increasing the the elevation for a pump located above a tank, the fluid wiltart to evaporate at a maximum level for the actual temperature.

t the maximum elevation NPSHa is zero. The maximum elevation can thereforee expressed by (4f):

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

or optimal theoretical conditions we neglect the major and minor head loss. Thevation head can then be expressed as:

e = patm / - pv / (5)

he maximum elevation or suction head for an open tank depends onhe atmospheric pressure - which in general can be regarded as constant, and tapor pressure of the fluid - which in general vary with temperature, especiallyorwater.

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

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

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= 10.1 m

ue to the head loss in the suction pipe and the local conditions inside the pumphe theoretical maximum elevation is significantly decreased.

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

Suction Head as Affected by Temperature

Temperature Vapor Pressure Max. elevation

(oC) (o F) (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

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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

95 203 84.5 1.7 5.6

100 212 101.33 0.0 0

Pumping Hydrocarbons

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

ote that the head developed by a pump is independent of the liquid, and that therformance curves for water from the manufacturer can be used for Newtonianquids like gasoline, diesel or similar. Be aware that required power depends onquid density and must be adjusted.

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uction Specific Speed may be used to determine what pump geometry - radialmixed flow or axial - to use for maximum efficiency and prevent cavitation. Sucti

pecific Speed is commonly used as a basis for estimating the safe operatingange of a pump.

uction Specific Speed is dimensionless and are expressed as

Nss = q1/2 / NPSHr

3/4 (1)

where

Nss = Suction Specific Speed

= pump shaft rotational speed (rpm)

= flow rate capacity (m3/h, l/s, m3/min, US gpm, British gpm) atBest Efficiencyoint BEP

NPSHr=pump NPSH required at best efficiency point(m, ft)

pecific Suction Speed can be compared with Specific Speed but instead of usihe total head of the pump, the available Net Positive Suction Head N.P.S.H.a issed.

s a rule of thumb the Specific Suction Speed should be below8500(calculateith US gpm) to avoid cavitation.

Online Specific Suction Speed Pump Calculator

he calculator below can used to calculate pump specific speed:Top of Form

1750

- pump shaft rotational speed (rpm)500

q - flow rate (m3/h, l/s, m3/min, US gpm, British gpm)20

NPSHr - required Net Positive Suction Head(m, ft)Bottom of Form

ote! When comparing pumps and their documentation be aware of the unitssed.

uction Specific Speed - Ss - is useful in determining the required operatingonditions on the suction side of the pump. For geometrically similar pumpshe Ss have the same value.

mpirical studies indicates that higher Suction Specific Speed is, the narrower ishe safe operating range from its Best Efficiency Point (BEP). Pump modelsith Ssd (US units) less than 11000have more stable operation and are more

eliable.

Convert between Imperial units (gpm) and Metric units (m3/h,

/s)

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Nss (US gpm) = 1.63 Nss (metric l/s) = 0.86 Nss (metric m3/h)

Nss (Metric l/s) = 0.614 Nss (US gpm)

Nss (Metric l/s) = 0.67 Nss (British gpm)

Example - Specific Suction Speed

he available Net Suction Head - NPSHa - of a pump is determined to be 20 ft.With a rotational speed of1750 rpm and a flow rate of500 US gpm the Specific

uction Speed can be calculated as

Nss = (1750 rpm) (500gpm)1/2 / (20 ft)3/4

= 4138

Well below the limit 8500 to avoid cavitation.

Double Suction Type Pumps

or a double suction pump the flow at the inlet is divided by two. Using a double

uction pump is one way of meeting system NPSH and obtaining a higher headSponsored Lin