Diploma Civil and Mechanical Engineering for Engineering Assistant · Chapter 4

Hydraulic Pumps and Turbines

What to remember

A pump converts mechanical energy into hydraulic energy; a turbine converts hydraulic energy into mechanical energy. Water power = ρgQH.
Centrifugal pumps give steady, high discharge at moderate head and need priming; reciprocating pumps are positive displacement, give high head with low discharge and Q = ALN/60 (single acting).
Pelton is an impulse turbine for high head; Francis is a reaction turbine for medium head; Kaplan is an axial-flow reaction turbine for low head.

Hydraulic machines and pump terms

Hydraulic machines use water to do work. Turbines take energy from flowing water. Pumps add energy to water to lift it or move it.
Head terms for a pump: suction head hs (centre of pump above the sump water level), delivery head hd (delivery outlet above the centre of the pump), static head Hs = hs + hd. Manometric head Hm = Hs + friction losses in suction and delivery pipes + delivery velocity head (the head actually delivered by the pump).
Water (hydraulic) power = ρgQHm (watts, with Q in m³/s and Hm in m). Shaft power is the input at the pump shaft. Overall efficiency = water power ÷ shaft power.
Worked example 1: Q = 0.05 m³/s and Hm = 20 m. Water power = 1000 × 9.81 × 0.05 × 20 = 9810 W = 9.81 kW. At 70% overall efficiency, shaft power = 9.81 ÷ 0.7 = 14.01 kW.
Pump classification: by working principle (centrifugal, reciprocating, rotary), by stages (single and multistage) and by action (single and double acting).

Centrifugal pumps

Working: the impeller rotates in a casing and throws the water outward by centrifugal force. This creates a low pressure at the eye, and atmospheric pressure pushes water up the suction pipe.
Main parts: impeller, casing (volute, vortex or with guide vanes/diffuser), suction pipe with a foot valve and strainer, delivery pipe with a delivery valve.
Priming means filling the suction pipe and casing with water so that no air remains. A centrifugal pump cannot lift water when full of air.
Starting: start with the delivery valve closed, because the power required is lowest at zero discharge, and open it slowly once the pump runs at full speed.
Volute casing has a gradually increasing area that converts part of the kinetic energy into pressure energy. Impeller vanes are usually backward curved.
Multistage pumps: pumps in series (several impellers on one shaft) raise the head; pumps in parallel raise the discharge.
Practical suction lift is limited to about 6 to 7 m; the theoretical limit at sea level is about 10.3 m of water.
Cavitation occurs when pressure at the inlet falls below the vapour pressure. It causes noise, vibration, pitting and loss of efficiency. It is prevented by keeping the suction head low and keeping a sufficient net positive suction head (NPSH).
Specific speed of a pump: Ns = N√Q ÷ Hm^(3/4). Low Ns means a radial-flow, high-head pump; high Ns means an axial-flow, low-head pump.
Affinity (similarity) laws for the same pump at a different speed: Q ∝ N; H ∝ N²; P ∝ N³. For geometrically similar pumps Q ∝ ND³, H ∝ N²D², P ∝ N³D⁵.
Worked example 2: if the speed of a pump doubles, Q becomes 2 times, H becomes 4 times and power becomes 8 times.

Reciprocating pumps and other devices

Reciprocating pump: a piston or plunger moves in a cylinder, with suction and delivery valves. It is positive displacement and can be self-priming.
Discharge: single acting Q = A·L·N/60; double acting Q = 2·A·L·N/60. Here A = piston area, L = stroke, N = rpm.
Worked example 3: d = 0.1 m, L = 0.2 m, N = 60 rpm (single acting). A = 0.007854 m². Q = 0.007854 × 0.2 × 60 ÷ 60 = 0.00157 m³/s.
Slip = theoretical discharge − actual discharge. Percentage slip = (slip ÷ theoretical) × 100. Coefficient of discharge Cd = actual ÷ theoretical. Example: theoretical 1.00 L/s and actual 0.95 L/s give 5% slip. Negative slip happens when the actual discharge exceeds the theoretical, when the suction pipe is long, the delivery pipe is short and the pump runs at high speed.
Air vessels on suction and delivery sides smooth the flow, reduce friction loss and allow a higher running speed. The delivery of a single-acting pump is not continuous; a double-acting pump is more uniform.
Hydraulic ram: pumps a part of the water to a higher level using the water hammer effect. It needs no external power.
Hydraulic press: works on Pascal's law, F₂/F₁ = A₂/A₁. Example 4: A₁ = 10 cm², A₂ = 500 cm², F₁ = 100 N gives F₂ = 5000 N, mechanical advantage = 50. Other devices: hydraulic jack, lift, crane, accumulator (stores energy at constant pressure) and intensifier (raises pressure).

Feature	Centrifugal pump	Reciprocating pump
Action	Rotary, kinetic energy	Piston, positive displacement
Discharge	Steady, large	Pulsating, small
Head	Low to medium	High
Priming	Needed	Usually self-priming
Valves	One (delivery) and a foot valve	Suction and delivery valves in the cylinder
Moving parts and cost	Fewer, cheaper	More, costlier

Turbines: classification and general terms

By action: impulse (water at atmospheric pressure strikes the buckets; Pelton) and reaction (water pressure falls through the runner; Francis, Kaplan, propeller).
By head: high head (Pelton), medium head (Francis), low head (Kaplan and propeller).
By flow direction: tangential (Pelton), radial/inward and mixed (Francis), axial (Kaplan).
Specific speed of a turbine: Ns = N√P ÷ H^(5/4). It increases in the order Pelton < Francis < Kaplan.
Hydraulic power from a plant: P = η₀ × ρgQH. Worked example 5: Q = 2 m³/s, H = 50 m and η₀ = 0.9 give P = 0.9 × 1000 × 9.81 × 2 × 50 = 882,900 W ≈ 883 kW.
Unit quantities (for H = 1 m): unit speed Nu = N/√H; unit discharge Qu = Q/√H; unit power Pu = P/H^(3/2). Example: N = 300 rpm at H = 25 m gives Nu = 60.
Efficiencies: hydraulic (work done on the runner ÷ water energy supplied), mechanical (shaft output ÷ runner power) and overall (shaft output ÷ water power supplied).
Governing keeps speed constant when load changes by changing the flow: a spear (needle) valve in the Pelton; guide vanes in the Francis and Kaplan.
Runaway speed is the speed reached if the load is suddenly removed and the governor fails.
Draft tube: a gradually widening tube from the runner exit to the tail race in reaction turbines. It lets the runner be set above the tail water, regains kinetic head as pressure and keeps the runner exit below atmospheric pressure. Impulse turbines do not need one.
Penstock carries water from the forebay or reservoir to the turbine; a surge tank protects it from water hammer.

Pelton, Francis and Kaplan turbines

Feature	Pelton wheel	Francis turbine	Kaplan turbine
Type	Impulse	Reaction	Reaction
Head	High	Medium	Low
Discharge	Low	Medium	High
Flow	Tangential	Inward radial (mixed at exit)	Axial
Admission	Partial, by nozzle	Full, through guide vanes	Full
Control	Spear valve	Guide vanes	Guide vanes and runner blades
Draft tube	Not needed	Used	Used

Pelton wheel: a free jet from a nozzle strikes double hemispherical (split) buckets fixed around the rim of a wheel. The splitter divides the jet in two. The wheel is at atmospheric pressure and the casing only prevents splashing. A brake nozzle is used to stop the wheel quickly.
Jet velocity V₁ = Cv√(2gH). For best efficiency the bucket speed u = V₁/2. With a full 180° reversal of the jet the ideal hydraulic efficiency is 100%; if the bucket exit angle is φ (measured from the reversed direction, so φ = 0 for full reversal) it is (1 + cos φ)/2 at u = V₁/2. In practice the buckets turn the jet through about 165°, so the efficiency is slightly less.
Example 6: a jet speed of 40 m/s gives the best bucket speed 20 m/s.
Francis turbine: a spiral casing distributes water evenly through fixed and adjustable guide vanes (wicket gates) to the runner. The runner is always full of water. Water enters radially inward and leaves axially. It has high efficiency over a wide range of medium heads.
Kaplan turbine: an axial-flow turbine with an adjustable-blade runner that gives high efficiency even at partial loads. A propeller turbine is the same but with fixed blades. Both suit low head and large discharge, such as run-of-river plants.
Cavitation in turbines occurs at the runner exit or draft tube inlet when the pressure falls to the vapour pressure. It is checked by Thoma's cavitation factor and avoided by a proper setting height of the runner.
Hydel power in Andhra Pradesh is generated at dam-based stations on the Krishna, Godavari and Sileru river systems, such as Srisailam, Upper Sileru and Donkarayi, and the Polavaram project on the Godavari includes a hydroelectric component. Check the latest official release for current status.

Exam traps

A pump adds energy to water; a turbine takes energy from water.
Start a centrifugal pump with the delivery valve closed, but a reciprocating pump with the delivery valve open.
A centrifugal pump must be primed. A reciprocating pump is usually self-priming.
Reciprocating pump Q = ALN/60 is for single acting; double acting is twice this.
Pelton is impulse (not reaction) and needs no draft tube. Francis and Kaplan are reaction and use a draft tube.
Highest head: Pelton. Lowest head with the largest discharge: Kaplan. Specific speed is also in the order Pelton < Francis < Kaplan.
Best bucket speed in a Pelton wheel is half of the jet speed, not equal to it.
Affinity laws: H ∝ N² and P ∝ N³. Do not take both proportional to N.

One-liners

1. Water power = ρgQH.
2. Pump overall efficiency = water power ÷ shaft power.
3. A centrifugal pump has to be primed before starting.
4. Cavitation damages blades and pump parts and is caused by low pressure.
5. Reciprocating pump discharge (single acting) = ALN/60.
6. An air vessel makes the flow smoother and reduces friction loss.
7. A Pelton wheel is a high-head impulse turbine with split buckets.
8. Best bucket speed of a Pelton wheel is half the jet speed.
9. A Francis turbine is a medium-head, inward radial-flow reaction turbine.
10. A Kaplan turbine is a low-head, axial-flow turbine with adjustable blades.
11. A draft tube regains kinetic head and allows the turbine to be set above the tail water.
12. A hydraulic press uses Pascal's law: F₁/A₁ = F₂/A₂.

Practice questions

A turbine converts
1. electrical energy into hydraulic energy
2. heat energy into hydraulic energy
3. mechanical energy into hydraulic energy
4. hydraulic energy into mechanical energy
Answer
D. hydraulic energy into mechanical energy
A turbine is driven by flowing water and produces shaft power.
Priming of a centrifugal pump means
1. increasing the speed
2. closing the delivery valve
3. lubricating the bearings
4. filling the suction pipe and casing with water
Answer
D. filling the suction pipe and casing with water
Priming removes air so the pump can create the necessary suction.
In a centrifugal pump the energy is imparted to the liquid by
1. a spring
2. the rotating impeller
3. a reciprocating piston
4. a gear wheel
Answer
B. the rotating impeller
The impeller throws the liquid outward by centrifugal force.
A centrifugal pump is generally started with the delivery valve
1. closed
2. removed
3. fully open
4. half open
Answer
A. closed
Power demand is lowest at zero discharge, so the motor starts under a light load.
Cavitation in a pump is caused by
1. an excess of lubricant
2. oil leakage
3. pressure falling to the vapour pressure
4. very high suction pressure
Answer
C. pressure falling to the vapour pressure
Vapour bubbles form in low-pressure regions and collapse with damaging effect.
A reciprocating pump is a
1. centrifugal pump
2. positive displacement pump
3. axial flow pump
4. jet pump
Answer
B. positive displacement pump
It displaces a fixed volume of liquid with each stroke.
The purpose of an air vessel in a reciprocating pump is to
1. increase the stroke length
2. increase suction lift
3. smooth out the flow and reduce friction head
4. reduce slip to zero
Answer
C. smooth out the flow and reduce friction head
The air cushion absorbs pulsations in the pipe.
Which turbine is an impulse turbine?
1. Propeller turbine
2. Kaplan turbine
3. Francis turbine
4. Pelton wheel
Answer
D. Pelton wheel
In a Pelton wheel the jet at atmospheric pressure strikes the buckets.
Which turbine is best suited to a very high head and small discharge?
1. Pelton wheel
2. Propeller turbine
3. Francis turbine
4. Kaplan turbine
Answer
A. Pelton wheel
The Pelton wheel works with high head and low discharge.
Which turbine has adjustable runner blades?
1. Francis turbine
2. Pelton wheel
3. Kaplan turbine
4. Turgo wheel
Answer
C. Kaplan turbine
The Kaplan runner blades can change their angle with load.
The part used to regulate the flow in a Pelton wheel is the
1. draft tube
2. guide vane
3. volute
4. spear valve (needle)
Answer
D. spear valve (needle)
The spear moves in the nozzle and changes the jet area.
The draft tube is used in
1. Pelton wheels only
2. reaction turbines
3. reciprocating pumps
4. hydraulic presses
Answer
B. reaction turbines
It connects the runner exit to the tail race of a reaction turbine.
A hydraulic press works on
1. Pascal's law
2. Newton's law of viscosity
3. Bernoulli's theorem
4. Archimedes' principle
Answer
A. Pascal's law
Pressure applied to an enclosed fluid is transmitted equally.
The hydraulic ram works on the principle of
1. water hammer
2. capillarity
3. centrifugal force
4. buoyancy
Answer
A. water hammer
The pressure surge from stopping a flowing column lifts part of the water.
The water power for Q = 0.05 m³/s and a manometric head of 20 m (g = 9.81 m/s²) is
1. 19.62 kW
2. 0.981 kW
3. 98.1 kW
4. 9.81 kW
Answer
D. 9.81 kW
P = ρgQH = 1000 × 9.81 × 0.05 × 20 = 9810 W.
A pump has a water power of 9.81 kW and an overall efficiency of 70%. The shaft power is about
1. 9.81 kW
2. 14 kW
3. 6.9 kW
4. 7 kW
Answer
B. 14 kW
Shaft power = 9.81 ÷ 0.7 = 14.01 kW.
The static head of a pump with a suction head of 4 m and a delivery head of 16 m is
1. 64 m
2. 12 m
3. 20 m
4. 4 m
Answer
C. 20 m
Static head = hs + hd = 4 + 16 = 20 m.
A single-acting reciprocating pump has a piston diameter of 0.1 m, a stroke of 0.2 m and runs at 60 rpm. The theoretical discharge is about
1. 0.00157 m³/s
2. 0.00314 m³/s
3. 0.000785 m³/s
4. 0.0157 m³/s
Answer
A. 0.00157 m³/s
Q = ALN/60 = 0.007854 × 0.2 × 60 ÷ 60 = 0.00157 m³/s.
If the same pump is double acting, the theoretical discharge is about
1. 0.00157 m³/s
2. 0.0063 m³/s
3. 0.00314 m³/s
4. 0.00079 m³/s
Answer
C. 0.00314 m³/s
The double-acting pump delivers twice as much.
The theoretical discharge of a pump is 1.00 L/s and the actual is 0.95 L/s. The slip is
1. 10%
2. 5%
3. 95%
4. 0.05%
Answer
B. 5%
Slip = (1.00 − 0.95) ÷ 1.00 × 100 = 5%.
The speed of a centrifugal pump is doubled. The head developed becomes
1. 8 times
2. 2 times
3. 16 times
4. 4 times
Answer
D. 4 times
H ∝ N².
If the speed of a pump is halved, the power required becomes
1. 1/8
2. 1/2
3. 1/16
4. 1/4
Answer
A. 1/8
P ∝ N³, so (1/2)³ = 1/8.
A pump discharges 0.2 m³/s at 1200 rpm. At 600 rpm the discharge becomes
1. 0.4 m³/s
2. 0.05 m³/s
3. 0.025 m³/s
4. 0.1 m³/s
Answer
D. 0.1 m³/s
Q ∝ N, so it halves.
A Pelton wheel jet has a velocity of 30 m/s. The best bucket speed is
1. 30 m/s
2. 15 m/s
3. 10 m/s
4. 60 m/s
Answer
B. 15 m/s
Best bucket speed u = V₁/2.
A turbine uses 2 m³/s at a 50 m head with an overall efficiency of 0.9 (g = 9.81 m/s²). The shaft power is about
1. 981 kW
2. 88.3 kW
3. 883 kW
4. 8830 kW
Answer
C. 883 kW
P = 0.9 × 1000 × 9.81 × 2 × 50 = 882,900 W.
A hydraulic press has piston areas of 10 cm² and 500 cm². A 100 N force on the small piston gives a force on the large piston of
1. 5000 N
2. 50 N
3. 500 N
4. 50,000 N
Answer
A. 5000 N
F₂ = 100 × 500 ÷ 10 = 5000 N.
The mechanical advantage of the above press is
1. 5
2. 50
3. 0.02
4. 500
Answer
B. 50
MA = A₂/A₁ = 500/10 = 50.
A turbine runs at 300 rpm under a 25 m head. The unit speed is
1. 7500 rpm
2. 300 rpm
3. 60 rpm
4. 12 rpm
Answer
C. 60 rpm
Nu = N/√H = 300/5 = 60.
A turbine has a hydraulic efficiency of 0.9 and a mechanical efficiency of 0.95. The overall efficiency is about
1. 1.85
2. 0.925
3. 0.95
4. 0.855
Answer
D. 0.855
Overall = 0.9 × 0.95 = 0.855.
A turbine receives water power of 100 kW and has an overall efficiency of 88%. The shaft output is
1. 12 kW
2. 112 kW
3. 88 kW
4. 8.8 kW
Answer
C. 88 kW
Output = 0.88 × 100 = 88 kW.
Which turbine is used for a low head and a very large discharge, as in run-of-river plants?
1. Pelton
2. Francis
3. Turgo
4. Kaplan
Answer
D. Kaplan
Axial flow Kaplan or propeller turbines suit low head and high flow.
In a Francis turbine, water flows through the runner in what manner?
1. Inward radial, leaving axially
2. Purely axial
3. Tangentially at atmospheric pressure
4. Outward radial
Answer
A. Inward radial, leaving axially
The guide vanes direct the flow inward and it leaves the runner along the axis.
Which of the following is NOT a feature of a Pelton wheel?
1. It works with a free jet
2. It has a draft tube
3. It is governed by a spear valve
4. It has split buckets
Answer
B. It has a draft tube
A Pelton wheel runs at atmospheric pressure, so a draft tube is not used.
The suction lift of a centrifugal pump is limited in practice mainly by
1. the shape of the motor
2. atmospheric pressure and vapour pressure
3. the size of the delivery valve
4. the colour of the pipe
Answer
B. atmospheric pressure and vapour pressure
Low suction pressure leads to cavitation, so lift is limited to about 6 to 7 m.
The type of pump best suited for a high head with a small discharge is the
1. centrifugal pump with low specific speed
2. propeller pump
3. reciprocating pump
4. axial flow pump
Answer
C. reciprocating pump
Reciprocating pumps produce a high head with a small flow.
Which statements are correct? 1. A centrifugal pump must be primed. 2. A reciprocating pump must always be primed before each start.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
A reciprocating pump is usually self-priming.
Which statements are correct? 1. A draft tube regains kinetic head. 2. A draft tube is used in a Pelton wheel.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
A Pelton wheel works in air at atmospheric pressure, so no draft tube is needed.
Which statements are correct? 1. A Kaplan turbine has adjustable blades. 2. A Pelton wheel is a reaction turbine.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
The Pelton wheel is an impulse turbine.
Which statements are correct? 1. Pumps in series increase the head. 2. Pumps in parallel increase the discharge.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are standard results for combining pumps.
Which statements are correct? 1. Specific speed of a Kaplan turbine is higher than that of a Pelton wheel. 2. A Kaplan turbine is used for high heads.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Kaplan turbines suit low heads.
Which statements are correct? 1. Negative slip can occur in a reciprocating pump. 2. A double-acting pump delivers a more uniform flow than a single-acting one.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both statements are correct.
Match the turbine with its flow type: (a) Pelton (b) Francis (c) Kaplan. Which pairing is correct?
1. (a) tangential, (b) axial, (c) inward radial
2. (a) axial, (b) tangential, (c) radial
3. (a) radial, (b) axial, (c) tangential
4. (a) tangential, (b) inward radial, (c) axial
Answer
D. (a) tangential, (b) inward radial, (c) axial
This is the standard classification of flow in the three turbines.
The overall efficiency of a pump is 60% and the manometric efficiency is 80%. Assuming no other factor, the combined mechanical and volumetric efficiency is
1. 48%
2. 75%
3. 140%
4. 20%
Answer
B. 75%
Overall = manometric × (mechanical and volumetric), so 60 ÷ 80 = 75%.
Two geometrically similar pumps have diameters in the ratio 1:2 and run at the same speed. The discharge ratio is
1. 1:2
2. 1:16
3. 1:4
4. 1:8
Answer
D. 1:8
Q ∝ D³ at equal N, so the ratio is 1:8.
Which hydraulic machine stores energy at constant pressure?
1. Hydraulic ram
2. Draft tube
3. Venturimeter
4. Hydraulic accumulator
Answer
D. Hydraulic accumulator
An accumulator holds pressurised fluid to supply bursts of power.

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