MRE 510 ppt... 2 Hydraulic Turbines.pptx

1. Micro-Hydropower
MRE 510

2. Hydraulic Turbines
• Hydraulic Turbine – converts the potential energy of water
into mechanical energy which in turn is converted into
electrical energy by a an electric generator.
• Classification of hydraulic turbines according to:
(i) Head & quantity of water available
- Impulse turbine requires high head & low flow
- Reaction turbine requires low head & high flow
(ii) Name of originator – Pelton, Francis, Kaplan etc.
(iii) Action of water on moving blades
- Impulse turbine – Pelton
- Reaction turbine – Francis, Kaplan & Propeller

3. Hydraulic Turbines
(iii) Direction of flow of water :
Tangential – Pelton; Axial flow – Kaplan; Mixed (radial & axial)
– Francis turbine
(iv) Disposition of turbine shaft – vertical or horizontal.
Modern turbine practice Pelton turbines usually have
horizontal shafts whereas the rest especially the large units
have vertical shafts
(v) According to specific speed - Specific speed is defined as
the speed of a geometrically similar turbine that would
develop 1 brake horsepower under the head of 1 m. All
geometrically similar turbines (irrespective of their sizes) will
have the same specific speed when operating under the same
conditions of head & flow.

4. Hydraulic Turbines
• Specific speed, Ns ,can be calculated from the
following relationship; N=
𝑁𝑠×𝐻5 4
√𝑃
Where, N = normal working speed in r.p.m
Po = Power output of turbine (kW)
H = net or effective head in m
• Generally turbines with low specific speeds work
under a high head & low discharge condition,
while high specific speed turbines work under
low head & high discharge conditions.

5. Hydraulic Turbines
Specific speeds for various types of runners

6. Hydraulic Turbines
Description of various type of turbines:
(1) Impulse turbines
Pelton wheel – Most important impulse turbine. It is a tangential flow
impulse turbine. All available P.E from a high head source is converted
into K.E before the jet strikes the buckets. Pressure all over the wheel
is constant & equal to atmospheric, so that energy transfer occurs due
to purely impulse action.
(2) Reaction turbines – The runner utilizes both P.E & K.E. The water
which acts on the runner blades is under a pressure above
atmospheric & the runner passages are always completely filled with
water.
(i) Francis turbine
(ii) Propeller & Kaplan turbines

7. Hydraulic Turbines
• Differences between Francis and Kaplan
Turbines.
• Comparison of efficiencies between propeller
(fixed blades) and Kaplan turbines.

8. Hydraulic Turbines
• Runaway speed – the maximum speed, governor being
disengaged, at which a turbine would run when there
is no external load but operating under design head
and discharge.
• All rotating parts including the rotor of alternator
should be deigned for centrifugal stresses caused by
this maximum speed.
• Practical values of runaway speeds for various turbines
with respect to their rated speed N are as follows;
Pelton wheel: 1.8 t0 1.9 N
Francis turbine (mixed flow): 2.0 to 2.2 N
Kaplan turbine (axial flow): 2.5 to 3.0 N

9. Hydraulic Turbines
• Draft tube – In the case of mixed & axial flow turbines a large
proportion of the energy is associated with water as it leaves the
runner.
• This exit energy varies from 4 to 25 % for mixed flow turbines &
from 20 to 50 % of the total head for axial flow turbines. As this
energy cannot be used in the runner, it becomes necessary to find a
way to recover this energy. Thus the draft tube is fixed at the runner
outlet.
• The draft tube is an integral part of the mixed & axial flow turbines.
• The draft tube serves the following purposes;
(i) It allows the turbine to be set above the tail-race water level,
without loss of head, & also facilitate inspection and maintenance
(ii) It regains, by diffusion action, the major portion of the Kinetic
energy delivered to it from the runner.

10. Hydraulic Turbines
Selection of a turbine using the shape number (Kn )
The choice mainly depends on the site conditions, i.e. the
head and flow rate. A useful parameter that is used to choose
a suitable turbine is the shape type Kn.
Kn =
𝑛
𝑃𝑜
𝜌
(𝑔𝐻)
5
4
Where Kn is a dimensionless constant,
PO = Power output
ρ = Density of water
H = Operating head
g = acceleration due to gravity

11. Hydraulic Turbines
Turbine optimality ranges:
Parameter Pelton Francis Kaplan
(Kn) 0.015-0.055 0.055-0.37 0.37-0.75
Hmax (m) 1500 300 30
η 0.85 0.9 0.9

12. Turbine selection chart

13. Hydraulic Turbines
Efficiencies of turbines:
Important efficiencies of turbines are:
(i) Hydraulic efficiency, ηh
(ii)Mechanical efficiency, ηm
(iii)Volumetric efficiency, ηv
(iv)Overall efficiency, ηo

14. Hydraulic Turbines
(i) ηh =
𝑃𝑜𝑤𝑒𝑟 𝑑𝑒𝑣𝑒𝑙𝑜𝑝𝑒𝑑 𝑏𝑦 𝑟𝑢𝑛𝑛𝑒𝑟
𝑃𝑜𝑤𝑒𝑟 𝑑𝑒𝑣𝑒𝑙𝑜𝑝𝑒𝑑 𝑎𝑡 𝑖𝑛𝑙𝑒𝑡
(ii) ηm =
𝑃𝑜𝑤𝑒𝑟 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑎𝑡 𝑠ℎ𝑎𝑓𝑡 𝑜𝑓 𝑡𝑢𝑟𝑏𝑖𝑛𝑒
𝑃𝑜𝑤𝑒𝑟 𝑑𝑒𝑣𝑒𝑙𝑜𝑝𝑒𝑑 𝑑𝑒𝑣𝑒𝑙𝑜𝑝𝑒𝑑 𝑏𝑦 𝑟𝑢𝑛𝑛𝑒𝑟
(iii) ηv =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑠𝑡𝑟𝑖𝑘𝑖𝑛𝑔 𝑡ℎ𝑒 𝑟𝑢𝑛𝑛𝑒𝑟
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑡𝑜 𝑡𝑢𝑟𝑏𝑖𝑛𝑒
(iv) ηo =
𝑆ℎ𝑎𝑓𝑡 𝑝𝑜𝑤𝑒𝑟
𝑊𝑎𝑡𝑒𝑟 𝑝𝑜𝑤𝑒𝑟
=
𝑃
ρ𝑔𝐻𝑄

15. Hydraulic Turbines
Cavitation- Phenomenon which manifests itself in the
pitting of the metallic surfaces of turbine parts because of
formation of cavities. This occurs in any turbine part if
pressure drops below the evaporation pressure resulting
in the boiling of the liquid and formation of large number
of bubbles of vapour. When the bubbles are carried along
the stream to higher pressure zones they suddenly
collapse resulting in very high local pressure as high as
7000 atmospheres. Formation of cavities & high pressure
are repeated many thousand times a second. This causes
pitting on the metallic surface of runner blades.

16. Hydraulic Turbines
Methods to avoid cavitation:
(i) Runner/turbine may be kept under water, or avoid
cavitation zone without keeping turbine under water by
using a runner of low specific speed
(ii)Cavitation free runner may be designed to fulfil the
given conditions with extensive research.
(iii) Select materials which resist cavitation effect.
(iv) Polishing surfaces and that is why cast steel runners
and blades are coated with stainless steel.
(v) Select runner of proper specific speed for a given
head.

17. Hydraulic Turbines
Performance of Hydraulic turbines:
Governing of Hydraulic turbines:

18. Minimum technical flow of turbines
Table below gives the minimum technical flow for
different types of turbines as a percentage of the
design flow.
Turbine type Qmin (% of Qdesign)
Francis 50
Semi Kaplan 30
Kaplan 15
Pelton 10
Turgo 20
Propeller 75

19. Variation of head with flow
Depending on the river flow and the flow admitted to the
turbines, the head can vary significantly. Fig. below shows an
example of turbine efficiency as function of flow.

20. Variation of head with flow
In medium and high head schemes the head can be
considered constant, because variations in the
upper or lower surface levels are small compared
with the head. In low head schemes, when the flow
increases over the value of the rated flow of the
water surface level, both in the intake and in the
tailrace, may increase but at different rates, so that
the head can potentially increase or decrease.

21. Hydraulic Turbines
Firm energy: Firm energy is defined as the power
that can be delivered by a specific plant during a
certain period of the day with at least 90 –95%
certainty. A run-of-river scheme has a low firm
energy capacity. A hydropower plant with storage
does, however, have considerable capacity for firm
energy.
NB: If a small hydro scheme has been developed as
the single supply to an isolated area, the firm
energy is extremely important.

22. Pumps used as turbine (PAT)
• Centrifugal pumps can be used as turbines
Advantages of using Pump as Turbine
1) Due to large number of standard pumps produced, standardized PAT
can be significantly less expensive than a specially designed turbine.
2)Delivery time is much less than for turbines
3)Spare parts & maintenance or repair services are much more readily
available for pumps than turbines.
4) Control of PAT is simple due to absence of blade pitch change
5) All the above factors contribute to cost saving.
NB: The use of combined pump-motor units is recommended for micro-
hydro schemes that are to be used only for the production of electricity,
and where the simplest installation possible is required.

23. Pumps used as turbine (PAT)
Limitations of PAT
1) Turbine speed is fixed to speed of generator,
thus reducing the range of flow rates when
matching the PAT performance to the site
conditions.
2) Limited choice of generators available for a
particular PAT.
3) No possibility of connecting mechanical loads
directly to the PAT.
4) It is difficult to characterize the turbine
performance

24. Pumps used as turbine (PAT)
Types of Pumps used as Turbines.
1) Radial flow pump (Ns =10 to 40)
2) Mixed flow pumps with outlet edge parallel to
machine axis (Ns =40 to 80)
3) Mixed flow with outlet edge inclined to machine
axis provided with volute chamber (Ns =80 to
100)
4) High specific speed axial flow pumps delivering
axially, provided with vanes (Ns =100 to 1000)

25. Pump used as turbine (PAT)
Turbine performance of a pump
Based on actual tests carried out the
characteristics indicate the following;
• Turbine best efficiency point is at higher flow
and head than pump best efficiency point.
• Turbine maximum efficiency tends to occur
over a wide range of capacity.

26. Application of Cross-flow & PAT

27. Pump used as turbine (PAT)
Suitable Site Conditions for PAT
What dictates the use of a pump as turbine is that it requires a
fixed flow rate and is therefore suitable for sites where there
is a sufficient supply of water throughout the year. Long term
water storage is not generally an option for a micro-hydro
scheme because of the high cost of constructing a reservoir.
Due to difficulty of site selection for PAT (Pump As Turbine), it
is recommended that the client should confirm its
performance to the designer or pump manufacturer in
advance, including the characteristics of the pump and its
induction motor to avoid that the characteristics of pump are
different by its manufacturer.

28. Pump used as turbine (PAT)
Selection of a PAT to match site conditions:
• Running conditions, in terms of head & flow, for
best efficiency as a turbine are very different
from the rated pump output, but the efficiency in
each case will be approximately the same.
• Friction & leakage losses within a centrifugal
pump result in reduction of head & flow from
theoretical maximum. When running as a turbine
the head & flow required will be greater than the
theoretical values in order to make up for the
losses.

29. Pump used as turbine (PAT)
• The following equations can be used to predict turbine head & flow;
QT = 1.1 x
𝑁𝑔
𝑁𝑚
x
𝑄𝑝
η𝑝
0
.
8
HT = 1.1 x
𝑁𝑔
𝑁𝑚
2 x
𝐻𝑝
η𝑝
1
.
2
Where 𝑁𝑚 = rated motor speed (rpm)
𝐻𝑝, 𝑄𝑝, η𝑝 are the head, flow rate, & efficiency of the pump at its maximum
efficiency operating point, which can be found from manufacturer’s data
sheet.
𝑁𝑔 = generator speed. For output at rated frequency f (50 or 60 Hz) the
generator speed is given approximately by:
𝑁𝑔 = 240 x
𝑓
𝑝
- 𝑁𝑚, where 𝑝 is number of poles (2, 4, 6….)
NB: These equations are only approximate & actual values may be as much as
± 20 % of predicted value for the best efficiency point

30. Pump used as turbine (PAT)
• It is therefore recommended that, after initial
selection, the pump is tested under turbine
conditions to find what output power will be
produced at the available head.
NB: Care must be taken to make sure that the induction
machine is not overloaded when running as a
generator.
• Electrical output of the generator is approximately:
Pg = ηmηp x 9.81QTHT (kW)

31. Pump used as turbine (PAT)
This value should not exceed 80 % of the rated
motor power. Often pump manufacturers supply
a range of motors for each pump size. The
largest of these motors is usually powerful
enough to run as a generator in conjunction
with the pump running as a turbine
• Laboratory test can be done using two pumps,
with one pump proving the head while the
other one operates as the turbine