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Net Positive Suction Head calculations

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Net Positive Suction Head  calculations

Net Positive Suction Head calculations

As we have mentioned in the previous article, there are couple considerations we`ll explain it including examples let`s  see.

Required NPSH :

Net Positive Suction Head calculations

we have reviewed the NPSH, available, suction,vaporizing pressure, formulas and conditions, but still couple questions:

what about oil pumping?

what the two different levels pumping?

what well pumping?

what about the liquid density ?

all these answers and more details you`ll find in the next coming up lines and examples .

Net Positive Suction Head calculations

NPSHr; been specified by manufacturer and experimentally, besides the NPSHa should be higher than NPSHr, to avoid the cavitation in the suction pipe,casing, velocity acceleration will decrease the pressure. so fluid will start boiling on the impeller surface.

double suction impeller pump better than single suction; NPSHr lower and balanced.

but double suction impeller susceptible for uneven liquid flow for both ends.

example:

Net Positive Suction Head calculations

if we lift the pump above an open water tank should note couple things.

the NPSHa will be zero at the maximum elevation so the liquid will start evaporating at its actual temperature.

NPSHa = patm / γ – he – hl – pv / γ

          = 0

then he = patm / γ – pv / γ                             

well about the suction pipes loss we can drop it assuming it constant.

as we noticed the elevation head will depend on the atmospheric pressure(constant) and, the vaporization pressure  the last depends on the temperature.

so for the water :

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

    = 10.1 m

we can see the different elevations for the pump according to the liquid temperature:

(oC)

(oF)

(kN/m2, kPa)

(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

 

Pumping Hydrocarbons: will be specified by manufacturer for use with cold water as in the next table,

 

Fluid

Temperature
(oC)

abs Vapor Pressure
(kPa)

Ethanol

20

5.9

65

58.2

Methyl Acetate

20

22.8

55

93.9

NPSH and Liquids with Dissolved Gas:

NPSH calculations might be modified if there is significant amount of dissolved gas in a liquid. The gas saturation pressure is often much higher than a liquid’s vapor pressure.

see you in the next design articles

to be continue


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