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PUMP
 QUIZ

 

 

  
 
 
 
 
 
2.11. Hydraulic & Mechanical Performance 'B'
Qs11

In the absence of any thrust balancing configuration, an end suction pump is subjected to an axial thrust because the suction eye in the front portion of the impeller is exposed to the suction pressure while the entire back of the impeller is subjected to delivery pressure. To balance axial thrust in an end suction pump, balancing holes or pump out vanes are employed. Which of the following statement about the axial thrust in an end suction pump is not true ? 

 Answers
  • Change in the direction of flow at the impeller eye and hence change in momentum contributes to the axial thrust
  • Thrust balancing holes cause some amount of flow disturbance in the impeller eye. Additionally, flow through the balancing holes causes a drop in pump efficiency
  • Pump-out vanes at the back of the impeller keep solid particles away from the stuffing box, in addition to balancing axial thrust
  • By enlarging the impeller eye area one can reduce the axial thrust although the possibility of suction recirculation at part flow increases
Qs12 Axial thrust in a double suction impeller is, theoretically, in balance. However, in practice, this balance is not often achieved and the pump is always provided with a thrust bearing at the non-drive end to take care of any residual axial thrust.
Which of the following is not. a contributor to axial thrust for a pump with a double suction impeller?
 Answers
  • A 900 bend in the suction pipe, close to the suction flange of a double suction split-case pump and parallel to the axis of the pump shaft
  • Unequal impeller eye areas on two sides of a double suction impeller
  • Use of staggered vane impeller to reduce pressure pulsation contributes to axial imbalance
  • Unequal leakage through the two wear rings can cause axial thrust imbalance
Qs13

A steel plant has received offers for cooling water pump from four manufacturers. The pump is required to be run over a wide range of flows. On scrutiny, all four pumps are found to operate at or very close to B.E.P and their efficiencies are identical. The installation has flooded suction (NPSHa = 10 m).

Parameters
Pump A
Pump B
Pump C
Pump D
Type of Pump Split Case End Suction Split Case End Suction
Rated Q 1800m3/hr 1800m3/hr 1800m3/hr 1800m3/hr
Rated Head 30m 30m 30m 30m
Speed 1480 r.p.m 1480 r.p.m 980 r.p.m 980 r.p.m
NPSH (3% head drop) 6m 8.8m 3.5m 5.1m

Which of the four options is likely to offer the widest operating range? The allowable operating range is a function of pump specific speed & suction specific speed (pump B is lowest in price).

 Answers
  • Pump A
  • Pump B
  • Pump C
  • Pump D
Qs14

The difference between the power input to the pump shaft and energy imparted to the liquid by the impeller represent the power losses within the pump due to internal recirculation, fluid friction, losses in bearings, packing, etc. Most of these power losses are converted into thermal energy & transferred to the pumped liquid. The temperature rise of the liquid at a particular flow is a function of head at that flow, efficiency & specific heat of the liquid. Generally, the temperature rise of the liquid must be restricted to 80C (150F) to prevent flashing. This is accomplished by providing a minimum flow by pass line, automatic leak-off valve etc. Of the four pumps listed below, which will require the largest "minimum flow" to restrict temperature rise to 80C (150F)?

Refer to table given below-

Parameters
Pump A
Pump B
Pump C
Pump D
Flow 800m3/hr 800m3/hr 1600m3/hr 1600m3/hr
Head 100 m 130 m 50 m 65 m
Speed 1480r.p.m 1480r.p.m 1480r.p.m 1480r.p.m
Efficiency 85% 81% 86% 85%
BkW 256kW 350kW 254kW 334kW
Shut-off-power 90kW 200kW 150kW 105kW

 

 Answers
  • Pump A
  • Pump B
  • Pump C
  • Pump D
Qs15

Minimum continuous flow for stable operation is governed by the size of the pump and parameters such as pump specific speed, level of suction energy, etc. Of the design parameters listed below which one does not, directly influence minimum continuous flow for stable operation?

Answers
  • Impeller eye diameter
  • Volute throat area
  • Suction volute geometry
  • No of impeller vanes
Qs16 Cavitation is the formation and collapse of vapor bubbles when the liquid pressure inside a pump falls below the vapor pressure. Which of the following statements is not true for cavitation in centrifugal pumps? Answers
  • Primary cause of cavitation is the lack of available NPSH
  • Cavitation can also occur in the discharge side of the impellers
  • Release of absorbed air in a liquid stream is an important cause of cavitation
  • Cavitation damage is caused by the implosion of vapor bubbles on metal surface
Qs17 Reduction of cavitation damage of an existing pumping installation can sometimes be achieved by taking several actions individually or in combination. Which of the following steps is unlikely to reduce cavitation damage? Answers
  • Using a more cavitation resistant impeller material such as CA15 or Nickel aluminum bronze
  • Using a special impeller with larger eye diameter
  • Under-filing the impeller vanes at outlet
  • Cutting back the impeller vanes at the inlet to improve NPSH
Qs18

The following are four pumps for the same duty. Which pump is expected to be the noisiest when operated at 30% of the rated flow?

Parameters
Pump A
Pump B
Pump C
Pump D
Flow 1800m3/hr 1800m3/hr 1800m3/hr 1800m3/hr
Head 35 m 35 m 35 m 35 m
Speed 1480r.p.m 980r.p.m 1480r.p.m 980r.p.m
Type of Pump Split case Split case End Suction End Suction

 

 Answers
  • Pump A
  • Pump B
  • Pump C
  • Pump D
Qs19

A steel plant is looking for a cold well pump for an installation where the available
  NPSH is 8.0 m. The pump duty is
  Q = 1390 m3 /hr.
  H = 46 m.
  NPSHa = 5.0 m
The plant has received four offers as follows:
A) A split-case pump at 1480 rpm
B) A split-case pump at 980 rpm
C) An end suction pump at 1480 rpm
D) An end suction pump at 980 rpm
Which pump is likely to offer the plant maximum margin between NPSHa and NPSHr, assuming that all four operate at b.e.p for rated duty.

 

 Answers
  • Pump A
  • Pump B
  • Pump C
  • Pump D
Qs20

NPSH available is an installation parameter and depends on suction lift/suction head, head losses in the suction piping, vapor pressure at pumping temperature etc. Which of the following factors does not influence available NPSH?

i) Velocity of the fluid in the suction pipe
ii) Elevation above the mean sea level of the pumping installation
iii) Speed of the pump to be used
iv) Temperature of the liquid being pumped :

 Answers
  • A
  • B
  • C
  • D