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2.11. Hydraulic & Mechanical Performance 'A'
Qs1 Losses in a centrifugal pump can be classified, for convenience of analysis, into the following categories:-
1) Impeller hydraulic losses
2) Mechanical losses
3) Leakage losses
4) Disk friction losses
5) Casing hydraulic losses
The following is a list of locations within the pump. Please match the location with the type of losses.

Location Type of Losses
A) Impeller shroud faces
1) Impeller hydraulic losses
B) Suction volute
2) Mechanical losses
C) Impeller - wear ring gap
3) Leakage loses
D) Gland Packing
4) Disk friction losses
E) Vane passage
5) Casing hydraulic losses
Answers
answer 1 answer 2 answer 3 answer 4
A - 1
A - 1
A - 4
A - 4
B - 3
B - 5
B - 5
B - 1
C - 5
C - 3
C - 3
C - 5
D - 2
D - 2
D - 2
D - 2
E - 4
E - 4
E - 1
E - 3
Qs2 The method followed in the field for a quick estimation of the minimum flow through the pump, to limit temperature rise to 80C (150F), is to calculate the power at B.E.P and consider 10% of the power (HP) in US gpm as the minimum flow (thermal) through the pump. For example, if the power absorbed is 100 HP (75 kW), the minimum flow will be estimated as 10 US gpm (2.3 m3/hr)

A multi-stage ring section pump is being used for the following duty:-
Q = 108 m3/hr.
Temperature - 900C
H = 500 m.
Specific gravity - 0.956
N = 2950 rpm
Medium - Demineralized water
Pump Efficiency - 72 %
The user needs to plan for a minimum flow recirculation arrangement to prevent temperature rise beyond 80C. As a first approximation, what minimum flow should be considered for the recirculation arrangement? 		Answers
  • 19.5 m3/hr
  • 6.0 m3/hr
  • 10.8 m3/hr
  • 4.4 m3/hr
Qs3

For API 610 process pumps, the governing design philosophy is a very high level of mechanical reliability. This requires adopting design features which can lead to sub-optimal pump efficiency. The following is a list of special design features of API process pumps. Please identify one feature that definitely does not compromise pump efficiency.

 

 Answers
  • Double volute casing construction
  • Large shaft to reduce shaft deflection
  • Wear ring clearances as per API standards
  • Large mechanical seals
  • Centre line mounted casing
Qs4 Disk friction drag is a major component of pump losses in low specific speed pumps. The following are the duties of five split-case pumps - all pumps are single stage with double suction impellers. Pump D uses a double volute casing while all others have single volute casings:
Pump A - 8x8-12
Pump B - 6x8-14
Pump C - 6x8-17
Pump D - 6x8-21
Q = 450 m3/hr.
 Q = 450 m3/hr.
 Q = 450 m3/hr.
 Q = 450 m3/hr.
H = 22 m.
 H = 32 m.
 H = 55 m.
 H = 90 m.
N = 1460 rpm
 N = 1480 rpm
 N = 1480 rpm
 N = 1480 rpm

Which pump is likely to have the highest disk friction loss as a proportion of the power input?

 Answers
  • A
  • B
  • C
  • D
Qs5

Leakage loss in a single stage centrifugal pump with shrouded impellers takes place through the clearances between the impeller and casing wear rings. Leakage loss is a function of the pump specific speed. Lower efficiency of low specific speed centrifugal pumps is due, mainly, to high leakage and disk friction losses. Which of the following statements regarding leakage losses is definitely not true?

 

Answers
  • Drop in efficiency due to increased leakage losses can be corrected by restoring wear ring clearances
  • Use of compatible impeller and wear ring materials with adequate anti-galling properties allow smaller wear ring clearances and reduce leakage losses
  • Leakage loss depends on the pressure differential between the suction & delivery side of wear ring and on wear ring clearance. Width of the wear ring does not have any effect on the leakage losses
  • Increased shaft stiffness reduces the shaft deflection at the impeller and allows lower clearance between impeller & wear ring and this reduces leakage losses.
Qs6

Power balance diagram and specific speed vs. efficiency charts suggest that for a required combination of capacity and head, a pump of optimum efficiency can be obtained if the operating speed is selected such that the pump specific speed is close to 2500 US units. It is to be noted that suction condition (NPSHa, limiting suction specific speed, etc.) often determine the pump operating speed & does not allow using the optimum specific speed of 2500 US units mentioned above. A refinery is considering installation of cooling water pumps of the following description:-

Type
Vertical Shaft Axially Split-case
Capacity
 2200 m3/hr
Head
 35m
Suction
 3.0 m positive head on the suction side
Supply System
 50 Hz

The refinery can choose operating speed of 3000 rpm, 1500rpm, 1000 rpm or 750 rpm. Which operating speed is likely to offer the best pump efficiency?

 Answers
  • 1500 rpm
  • 1000 rpm
  • 750 rpm
  • 3000 rpm
Qs7

Lower the specific speed of the pump, higher is the ratio of impeller outlet diameters (D2) to impeller inlet or eye diameter (D1). Lower the D2/D1 ratio, lower is the allowable range of operation to prevent recirculation, low flow cavitation, vibration & radial loading. Out of the three options listed below which pump will permit the widest range of head-capacity operation.

Pump A
Pump B
Pump C
Q = 2200 m3/hr.
 Q = 2200 m3/hr.
 Q = 2200 m3/hr.
H = 35 m.
H = 35 m.
 H = 35 m.
N = 1480 rpm
 N = 980 rpm
 N = 740 rpm
Type - Axially split-case
 Type - Axially split-case
 Type - Axially split-case

 

 Answers
  • Pump A
  • Pump B
  • Pump C
  • Insufficient Data
Qs8

NFPA 20 fire pumps are always supplied with packed glands. The reason for this is that packed gland pumps are not subject to sudden catastrophic failure due to stuffing box leakage like mechanical seal fitted pumps. However, between two identical fire pumps one fitted with mechanical seal & the other with packed gland, the former is likely to be slightly more efficient due to:

 

 Answers
  • Lower disk friction losses of mechanical seal fitted pump
  • Lower mechanical losses of mechanical seal fitted pump
  • Lower mechanical losses & leakage losses of mechanical seal fitted pump
  • Lower hydraulic losses of mechanical seal fitted pump
Qs9

The irrigation authority of a state has procured several mixed flow vertical wet-pit pumps for the following duty :-

Q = 6130 m3/hr.
H = 12.0 m.
>N = 740 r.p.m
Efficiency - 80 %
Power absorbed = 250 kW
Motor - 275 kW, 8 pole, 50 Hz

Pumps are used for lifting water from a river and discharging into a canal system. Pump supplier has stipulated that the pumps should be started with the delivery valve fully open. What is the main reason behind this starting requirement?

 Answers
  • High vibration in the closed valve and low flow region
  • High radial load at closed valve
  • Closed valve power is more than the rated output of the motor
  • Piping and valves are not designed for the high closed valve pressure
Qs10

An industrial unit has received an offer for a split-case pump for cooling water duty. The manufacturer has offered the following:
Type - Axially split-case pump

Performance at rated duty
Performance at B.E.P
Rated capacity - 2000 m3/hr.
Capacity - 1800 m3/hr.
Rated head - 32 m
 Head - 35 m
Pump efficiency at rated capacity - 84%
Effy = 86%
NPSHr - 6.5 m
 NPSHr - 5.3 m
Speed - 1480 r.p.m
 Speed - 1480 r.p.m

Available NPSH of the plant varies between 6 and 8.5 m depending on the water level in the suction sump. The flow requirement of the system varies due to varying cooling water requirement at different load points. However, the rated capacity of 2000 m3/hr. represents the maximum demand.

Customer is considering rejecting this offer and asking the manufacturer to make an offer at 1000 rpm. Which of the following is definitely not a reason for rejecting the offer?

Answers
  • The specific speed of the pump is 3750 US units. High specific speed pumps of this type cannot be operated over a wide range of duties.
  • The suction specific speed of the pump is 11000 US units. This is too high for a pump which will be operated at part load at times
  • Pump has been selected 11% to the right of B.E.P & NPSH margin is inadequate for this high suction energy pump
  • A pump selected closer to B.E.P at 980 rpm will have a specific speed of around 2500 & HIS chart suggests that efficiency of 90% may be possible. This will save energy & also make the pump more reliable in terms of off-peak operation and NPSH safety margin
  • Radial load of 1500 rpm pump working to the right of B.E.P will be much higher compared to a pump running at 980 rpm