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WorksheetsUnit 3.2 – Pumps
Total questions: 47
Worksheet time: 24mins
Pumps that impart velocity to fluid and convert it to pressure are called:
Positive displacement pumps
Dynamic pumps
Reciprocating pumps
Gear pumps
Single-stage centrifugal pumps are preferred for:
High head, low flow
Most general chemical industry service
Slurries only
Vacuum service
Positive displacement pumps are normally used when:
High head at low flow rate is required
Large flow at low head is needed
Liquids are near boiling
System curve is steep
For high head at large flow rate, which pump is typically selected?
Single-stage centrifugal
Multi-stage centrifugal
Diaphragm
Gear
Centrifugal pumps convert:
Thermal energy to kinetic
Rotational kinetic energy to hydrodynamic energy
Pressure to electricity
Potential to nuclear
The reverse of a centrifugal pump is:
Vacuum pump
Water turbine
Air compressor
Diaphragm pump
The pump head HH in feet can be calculated from pressure and specific gravity as:
H=2.31P/SG
H=0.433P×SG
H=P×SG
H=1.0P/SG
Velocity head developed by an impeller is given by:
V^2 / (2g)
2g/V
gV^2
V / 2g
Relation between impeller speed (N), diameter (D) and velocity head:
V=ND/229
V=229/(ND)
V=ND/144
V=N^2 D
Pump capacity Q (gpm) from pipe cross-section and velocity is approximated by:
Q=449AV
Q=2.31AV
Q=3960AV
Q=144AV
Break horsepower (BHP) is:
Liquid power delivered
Actual power to pump shaft
Theoretical hydraulic power
Motor efficiency
Hydraulic horsepower (WHP) is:
Pump shaft power
Liquid power delivered by the pump
BHP minus electrical losses
Motor brake torque
Formula for BHP is:
BHP=Q×TDH×SG/3960×η
BHP=Q×TDH/2g
BHP=3960×Q×TDH
BHP=η×Q×TDH
Pump efficiency η is defined as:
BHP / WHP
WHP / BHP
TDH / Q
Q / TDH
Total dynamic head is:
Total discharge head + total suction head
Total discharge head − total suction head
Static head only
Suction lift only
Static suction head is the distance from:
Pump discharge to liquid free level
Pump centerline to free liquid level
Bottom of vessel to pump
Motor base to tank bottom
When source is below pump centerline, the suction condition is termed:
Positive suction
Suction lift
Static discharge
Downflow
Lummus rule for frictional loss (non-boiling liquids, 0–250 gpm) is approx.:
1 psi per 100 ft
6 psi per 100 ft
10 psi per 100 ft
0.1 psi per 10 ft
Lummus rule for boiling liquids in same range:
1 psi/100 ft
6 psi/100 ft
0.5 psi/100 ft
10 psi/100 ft
Cavitation occurs when:
Pump speed is too low
Pressure falls below liquid vapor pressure
Efficiency is >100%
Discharge head is negative
NPSH available is:
Pump manufacturer data
Pressure at pump suction above vapor pressure, expressed as liquid head
Static discharge minus suction
Motor torque
Recommended minimum NPSH for capacities up to 100 m³/h is:
1 m
3 m
6 m
10 m
For capacities above 100 m³/h, NPSH should exceed:
2 m
6 m
12 m
1 m
Equation for NPSH available is:
P/ρg + H − P2/ρg − Pv/ρg
Pv/ρg + H
H − P/ρg
P2/ρg + Pv/ρg
According to pump affinity laws, capacity varies directly with:
N²
N³
N × D
D²
Head varies proportionally to:
N
N² × D²
N³
D
Power varies proportionally to:
N × D
N² × D²
N³ × D³
N²
If speed increases 20%, flow rate increases by about:
10%
20%
40%
60%
If impeller diameter is reduced 10%, head decreases by roughly:
10%
19% (since H ∝ D²)
30%
5%
Primary criteria for pump selection are:
Flow rate and head
Pipe schedule and color
Temperature alone
Material cost only
Additional considerations include:
Corrosion and presence of solids
Pump brand
Electrical frequency
Paint type
Total energy needed for pumping includes overcoming:
Pipe friction only
Friction, fittings, equipment losses, elevation, and pressure difference
Motor slip
Sonic velocity
If the energy equation yields negative W, it indicates:
A turbine can recover energy
A pump is required
Cavitation
Flow reversal
The primary purpose of a pump shaft seal is to:
Increase speed
Contain liquid and prevent leakage/ingress
Reduce impeller diameter
Control noise
Packed glands are best used for:
Flammable fluids
Non-toxic, non-corrosive fluids
Radioactive slurries
Superheated steam
Lantern rings are used to:
Reduce motor power
Lubricate and cool packing
Support bearings
Adjust impeller diameter
Mechanical seal faces are kept in contact by:
Magnetic coupling
A spring
Lantern ring
Pressure head
Outside mechanical seals are easier to:
Lubricate
Maintain
Pressurize
Cool
Double mechanical seals are selected when:
Leakage to atmosphere must be prevented
Pumping water
Flow rate is low
Efficiency is >80%
Seal-less (canned) pumps are used when:
Leakage must be absolutely zero
Speed must be variable
Suction lift is high
Efficiency is unimportant
A pump characteristic curve plots:
Flow rate vs motor speed
Head vs flow rate
Power vs impeller diameter
NPSH vs temperature
On this curve, efficiency typically:
Rises to a maximum then falls
Falls steadily
Remains constant
Oscillates randomly
The system curve represents:
Pump efficiency vs speed
Total pressure head vs liquid flow rate of the piping system
NPSH vs cavitation
Power vs density
Operating point of a pump is found where:
Pump and system curves intersect
Head equals zero
Efficiency is maximum only
TDH equals NPSH
When using a control valve, the system curve should be plotted for:
Valve fully open and partially open
Only fully open
Only closed
Random positions
A pump delivers 500 gpm against 80 ft TDH with SG = 1.0 and efficiency 70%. Required BHP ≈ ?
(500×80)/(3960×0.7)≈14.5hp
8 hp
20 hp
40 hp
NPSH available is 5 m but manufacturer requires 6 m. Expect:
Normal operation
Cavitation risk
Higher efficiency
Lower TDH
