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1.11. Hydraulic Principles
Qs1 Euler's equation He = U2Vu2/g assumes infinite no of vanes and radial entry into impeller eye, (Vu1 = 0).According to this equation, Euler's head depends on:  Answers
  • Impeller type, speed and outlet diameter
  • Impeller outlet vane angle, speed and outlet diameter
  • Impeller inlet and outlet diameter, speed and flow
  • None of the above
Qs2

For the pumping system shown in the figure below, calculate the total head and the power absorbed by the pump. Power absorbed is given by,
BkW = Q(m3/hr.) x H(m) x specific gravity/(3.67 x efficiency(%))
Assume pump efficiency = 80%

pq12

 Answers
  • 30 m and 46 kW
  • 32 m and 49 kW
  • 35 m and 53.6 kW
  • 30 kW and 4.6 kW
Qs3 A centrifugal pump generates a pressure of 100 psi (a head of 231 ft = 100 psi x 2.31) when handling clean water of specific gravity 1.0. What pressures and head will this pump generate when pumping brine with sp. gr. 1.2 and kerosene with sp. gr. 0.8
Answers
  • Brine (head = 192.5 ft, pressure = 100 psi) & kerosene (head = 288.7 ft, pressure = 100 psi)
  • Brine (head = 231 ft, pressure = 83.3 psi) & kerosene (head = 231 ft, pressure = 100 psi)
  • Brine (head = 231 ft, pressure = 120 psi) & kerosene (head = 231 ft, pressure = 80 psi)
  • Brine (head = 192.5 ft, pressure = 120 psi) & kerosene (head = 208.7 ft, pressure = 80 psi)
Qs4

The affinity laws of centrifugal pumps suggest that pump capacity varies directly with the impeller diameter, pump head as the square of the diameter and power as the cube of the diameter (assuming the efficiency is unchanged). The law holds good for impeller trims limited by the pump specific speed, impeller type and inlet diameter of the impeller. Assuming that the affinity laws hold good for the impeller trim shown below, what will be the capacity, head and efficiency for the current operating point?

pq122

 Answers
  • Q = 450 m3/hr., H = 81 m, efficiency = 82% and power = 121 kW
  • Q = 405 3/hr., H = 72.9 m, efficiency = 73.8% and power = 109 kW
  • Q = 450 3/hr., H= 72.9 m, efficiency = 82% and power = 109 kW
  • Q = 405 3/hr., H = 81 m, efficiency= 82% and power = 109 kW
Qs5

A pump manufacturer has to test an external fire fighting pump (for ship) of the following specifications:
Q = 2400 3/hr
H = 150 m
Efficiency = 86%
Speed = 1800 rpm
Medium - Sea water (sp. gr. 1.03)
Driver - Main Engine through power take-off and gear box
The manufacturer can test pump at his works, using one of the test motors at six pole speed (1000 rpm synchronous). What will be the rated duty condition of the pump at 1000 rpm at the test bed and what should be the rating of the test motor. Test bed uses clean cold water (sp. gr. = 1.0) for testing.

 Answers
  • Q = 1333 3/hr., H = 83.3 m, motor = 400 kW, 6 pole
  • Q = 1333 m3/hr., H = 46.3 m, motor = 215 kW, 6 pole
  • Q = 740 m3/hr., H = 83.3 m, motor = 215 kW, 6 pole
  • Q = 1200 3/hr., H = 37.5 m, motor = 160 kW, 6 pole
Qs6

The specific speed of an impeller is expressed as N.xQ0.5/H0.75, where N = speed, Q = flow in USgpm and H = head/stage in ft. Following are four impellers for double suction split-case pumps with their rated duties. What are their specific speeds (US units)?

Pump A Pump B Pump C Pump D
Impeller- 5HS12 Impeller- 5HS12 Impeller- 5HS12 Impeller- 5HS12
Q = 1110 US gpm Q = 1760 US gpm Q = 2200 US gpm Q = 3170 US gpm
H = 72 ft H = 180 ft H = 295 ft H = 295 ft
N = 1450 rpm N = 1480 rpm N = 1480 rpm N = 1480 rpm

 

 Answers
Answer 1 Answer 2
Pump A - 900 Pump A - 975
Pump B - 975 Pump B - 1954
Pump C - 1263 Pump C - 1263
Pump D - 1954 Pump D - 840
Answer 3 Answer 4
Pump A - 1954 Pump A -1263
Pump B - 1263 Pump B -1954
Pump C - 975 Pump C - 975
Pump D - 840 Pump D- 900
Qs7 Referring to Fraser-Sabini efficiency charts, the efficiencies of the four pumps listed in Qs no 6 are estimated to be as follows: Answers
Answer 1 Answer 2 Answer 3 Answer 4
Pump A- 84% Pump A- 83% Pump A- 82% Pump A- 81%
Pump B- 81% Pump B- 78% Pump B- 84% Pump B - 67%
Pump C- 80.5% Pump C- 84% Pump C- 83% Pump C - 85%
Pump D - 79% Pump D- 80% Pump D - 75% Pump D - 72%
Qs8 Suction specific speed is an indicator of the suction capability of a pump. It is also a measure of suction energy of the pump & its permissible range of operation. External fire pumps for ships (FiFi pumps) operate at a single duty point and the typical duties are as follows:-
Pump A
Pump B
Pump C
Pump D
Q= 2400 m3/hr Q= 1200 m3/hr Q= 600 m3/hr Q= 300 m3/hr
H= 150 m H= 140 m H= 140 m H= 140 m
N= 1800rpm N= 1800rpm N= 1800rpm N=2100rpm

The suction specific speed (Nss) is given by, NxQ0.5/(NPSHr)0.75 , where,
N = speed in rpm ,
Q = flow/eye in US gpm and
NPSHr is in ft.

What would be the expected NPSHr of pumps A, B, C & D if all of them are double suction split-case single stage pumps,considering that most commercially designed pumps achieve Nss = 9000 (US units)?

 Answers
Answer 1 Answer 2 Answer 3 Answer 4
Pump A - 17.2m Pump A - 10.8m Pump A - 6.4m Pump A - 4.3m
Pump B - 10.8m Pump B - 6.8m Pump B - 4.1m Pump B - 6.8m
Pump C - 6.8m Pump C - 4.3m Pump C - 2.5m Pump C - 10.8m
Pump D - 5.3m Pump D - 3.3m Pump D - 2.0m Pump D - 3.3m
Qs9 A ship-owner is considering installation of two diesel engine driven external fire pumps on the ship deck. Pump duty and suction conditions are as follows:-
a) Rated capacity of each pump - 600 m3/hr
b) Rated head - 140 m.
c) Static lift (minimum water level to pump center line) = 3.0 m.
d) Total losses in the pipe line (strainer, bend, straight pipe, etc.) = 0.5 m.
e) Vapor pressure = 0.6 m.
f) Atmospheric pressure = 10.3 m.
g) Available NPSH = 6.2 m (10.3 - 3.0 - 0.5 - 0.6 m)

The ship-owner wants to maintain a safety ratio of 1.2 (NPSHa/NPSHr) to prevent cavitation.
What is the maximum speed at which he can run
a) an end suction pump
b) a double suction pump 
(Consider that pumps operate at B.E.P at the rated duties and that they have been designed for Nss = 9000 US units.)

 Answers
Answer 1 Answer 2 Answer 3 Answer 4
End Suction- 2080 r.p.m End Suction- 2080 r.p.m End Suction- 1470 r.p.m End Suction- 1667 r.p.m
Double Suction- 2080 r.p.m Double Suction- 1470 r.p.m Double Suction- 2080 r.p.m Double Suction- 2370 r.p.m
Qs10

What is the NPSHa for the following system: <br>(Given, vapor pressure of water at pumping temperature is 0.5 m &amp; suction vessel is open to atmospheric pressure.)

pq123

 Answers
  • 6.8 m
  • 7.8 m
  • 8.8 m
  • 9.9 m