Hydraulic Turbines

Hydraulic turbines are mechanical devices that convert the kinetic energy and potential energy of a fluid (usually water) into rotational mechanical energy. This rotational energy is then typically used to drive an electric generator, producing electricity. They are the heart of hydroelectric power plants.

Classifications of Hydraulic Turbines

Hydraulic turbines can be classified based on several criteria, primarily related to the direction of water flow, the head under which they operate, and the type of energy they primarily utilize.

1. Based on the Direction of Flow

This classification describes how water moves through the turbine runner.

a) Radial Flow Turbines

In radial flow turbines, the water flows radially. This can be inwards or outwards.

• Inward Radial Flow Turbine: Water enters the runner from the outside and flows towards the center. The Francis turbine is a classic example of an inward radial flow turbine.
• Outward Radial Flow Turbine: Water enters the runner from the center and flows outwards. This type is less common in modern power generation.

b) Axial Flow Turbine

In axial flow turbines, the water flows parallel to the axis of the turbine shaft. The propeller turbine and the Kaplan turbine are examples of axial flow turbines. These are suitable for low head and high discharge conditions.

c) Tangential Flow Turbine

In tangential flow turbines, the water strikes the runner tangentially. The Pelton turbine is the most prominent example of this type. It is used for very high head conditions.

2. Based on the Head of Water

The operating head is the difference in water level between the source and the turbine, which determines the pressure and potential energy of the water.

a) High Head Turbines (Head > 250 m)

These turbines are designed to operate under very high water pressure. The Pelton turbine is the most suitable for high head applications.

b) Medium Head Turbines (Head between 60 m and 250 m)

These turbines operate under moderate water pressure. The Francis turbine is widely used for medium head applications.

c) Low Head Turbines (Head < 60 m)

These turbines are used where the water head is low, but the discharge is usually high. Kaplan and Propeller turbines are suitable for low head conditions.

3. Based on the Nature of Energy Utilized

This classification distinguishes between turbines that use only the kinetic energy of water and those that utilize both kinetic and pressure energy.

a) Impulse Turbines

In impulse turbines, the entire pressure energy of the water is converted into kinetic energy by a nozzle before it strikes the runner. The pressure of the water remains atmospheric throughout its passage through the runner. The Pelton turbine is a classic example of an impulse turbine.

b) Reaction Turbines

In reaction turbines, the water is released at a high velocity, and as it passes through the runner, a portion of the pressure energy is converted into kinetic energy, and another portion is converted into mechanical energy due to the reaction force. The pressure of the water changes as it flows through the runner. Francis turbines and Kaplan turbines are examples of reaction turbines.

Common Types of Hydraulic Turbines

While there are many variations, three main types of hydraulic turbines are commonly used:

a) Pelton Turbine

Principle: Operates on the impulse principle. High-pressure water is directed through a nozzle onto a series of buckets mounted on the periphery of a wheel. The jet of water strikes the buckets tangentially, imparting a force that causes the wheel to rotate. The kinetic energy of the jet is primarily responsible for the work done.

Construction: Consists of a nozzle, a spear (or needle) for flow control, a runner with buckets, and a casing.

Application: High head (> 250 m) and low discharge.

b) Francis Turbine

Principle: Operates on the reaction principle. Water enters the spiral casing (scroll case) and flows through guide vanes, which control the flow rate and direct the water onto the runner blades at an optimal angle. The water flows radially inwards and then axially, exerting a reaction force on the runner as its pressure and velocity change. It utilizes both the pressure and kinetic energy of the water.

Construction: Includes a scroll case, stay vanes, guide vanes, runner, and draft tube.

Application: Medium head (60 m to 250 m) and medium discharge.

c) Kaplan Turbine

Principle: Operates on the reaction principle, similar to the Francis turbine, but designed for low head conditions. It is essentially a propeller with adjustable blades. Water flows axially through the runner. The pitch of the blades can be adjusted to maintain high efficiency over a wide range of flow rates and heads.

Construction: Similar to Francis turbine but with a propeller-type runner whose blades are adjustable. It also has a draft tube.

Application: Low head (< 60 m) and high discharge.

Centrifugal Pumps

A centrifugal pump is a type of rotodynamic pump that uses a rotating impeller to increase the pressure and flow rate of a fluid. It converts rotational kinetic energy into hydrodynamic energy of fluid flow.

Classifications of Centrifugal Pumps

Centrifugal pumps can be classified based on several factors, including the design of the impeller, the direction of flow, the number of stages, and the casing design.

1. Based on Impeller Design

The impeller is the rotating component that imparts energy to the fluid.

a) Based on Flow Direction Through Impeller

• Radial Flow Impeller: Fluid enters axially at the center and is discharged radially outwards. This is the most common type.
• Axial Flow Impeller: Fluid flows parallel to the axis of rotation. These are more like propellers and are used for high flow, low head applications.
• Mixed Flow Impeller: Fluid enters axially and is discharged at an angle between radial and axial directions.

b) Based on Casing of Impeller

• Open Impeller: Vanes are attached to a central hub, but there is no shroud covering the vanes. Suitable for fluids with solids, but less efficient.
• Semi-Open Impeller: Vanes are attached to the hub on one side and have a shroud on the other.
• Closed Impeller: Vanes are sandwiched between two shrouds. This is the most common type, offering higher efficiency and better pressure development, but is susceptible to clogging with solids.

2. Based on Casing Type

The casing surrounds the impeller and collects the fluid discharged by it, converting some of the kinetic energy into pressure energy.

a) Volute Casing

The casing has a gradually increasing cross-sectional area towards the discharge pipe. This design helps to efficiently convert the velocity head of the fluid into pressure head.

b) Diffuser Casing

This type uses stationary guide vanes (diffuser vanes) arranged around the impeller periphery. These vanes form expanding passages that slow down the fluid and convert its kinetic energy into pressure energy more efficiently than a simple volute, especially at the design point.

3. Based on Number of Stages

The number of impellers determines the total head the pump can generate.

a) Single-Stage Pump

Has only one impeller. Used for low to medium head applications.

b) Multi-Stage Pump

Has two or more impellers mounted on the same shaft. The discharge from one impeller is fed to the suction of the next. This arrangement allows for the generation of very high heads.

4. Based on Suction Type

This refers to how the fluid enters the impeller.

a) Single-Suction Pump

The fluid enters the impeller from one side only.

b) Double-Suction Pump

The fluid enters the impeller from both sides simultaneously. This balances the axial thrust and allows for higher flow rates.

Principles of Centrifugal Pumps

The operation of a centrifugal pump is based on Bernoulli's principle and the principle of centrifugal force.

1. Fluid Entry: The fluid enters the pump at the eye of the impeller. 2. Impeller Action: The rotating impeller vanes exert a centrifugal force on the fluid, pushing it radially outwards. As the fluid moves from the eye to the periphery of the impeller, its velocity and pressure increase. 3. Casing Action: The fluid leaves the impeller at high velocity and enters the casing. The casing is designed (either as a volute or with diffusers) to gradually slow down the fluid. According to Bernoulli's principle, as the velocity of the fluid decreases, its pressure increases. 4. Discharge: The high-pressure fluid is then discharged from the pump outlet.

The head developed by a centrifugal pump is primarily due to the centrifugal force imparted by the rotating impeller. The theoretical head developed by the impeller is given by:

H_th = (v₂² - v₁²) / (2g)

Where:

• v₂ is the absolute velocity of the fluid at the impeller outlet.
• v₁ is the absolute velocity of the fluid at the impeller inlet (eye).
• g is the acceleration due to gravity.

In reality, the actual head developed is less than the theoretical head due to various losses (friction, shock, leakage, etc.).

Performance of Centrifugal Pumps

The performance of a centrifugal pump is typically represented by characteristic curves, which plot various parameters against the flow rate (discharge).

Performance Curves

These curves are generated by testing the pump at a constant speed and varying the discharge by adjusting the discharge valve. The main curves are:

1. Head-Capacity (H-Q) Curve

This curve plots the head (H) developed by the pump against the flow rate (Q) or discharge. For most centrifugal pumps, the head decreases as the flow rate increases. The shape of the H-Q curve can be:

• Stable: Head continuously decreases with increasing discharge. This is the most desirable type, allowing the pump to operate efficiently at different points.
• Unstable (or Surging): The head may rise initially with increasing discharge, then fall, and then rise again. This can lead to instability and damage.
• Drooping: A stable curve where the head at shut-off (Q=0) is maximum.
• Rising: Head increases with discharge, which is rare and undesirable as it can lead to motor overload.

2. Power-Capacity (P-Q) Curve

This curve plots the power input to the pump shaft (brake horsepower) against the flow rate (Q). The power consumed generally increases with increasing flow rate. The power at shut-off is usually non-zero due to internal recirculation and friction.

3. Efficiency-Capacity (η-Q) Curve

This curve plots the efficiency (η) of the pump against the flow rate (Q). Efficiency is defined as the ratio of hydraulic power output to the mechanical power input. There is a specific flow rate at which the pump operates at its maximum efficiency. This is known as the Best Efficiency Point (BEP).

4. Net Positive Suction Head (NPSH) Curve

This curve plots the required NPSH (NPSH_R) against the flow rate (Q). NPSH_R is the minimum pressure head required at the pump suction to prevent cavitation. Cavitation occurs when the pressure in the fluid drops below its vapor pressure, causing vapor bubbles to form and then collapse, leading to noise, vibration, and damage to the impeller.

Factors Affecting Pump Performance

• Speed of Rotation: Increasing the speed increases head, discharge, and power (approximately H ∝ N², Q ∝ N, P ∝ N³).
• Impeller Diameter: Increasing the diameter increases head, discharge, and power.
• Fluid Properties: Viscosity, density, and presence of solids affect performance. Higher viscosity increases power consumption and reduces efficiency.
• System Resistance: The characteristics of the piping system (length, diameter, fittings, elevation difference) determine the operating point on the pump's H-Q curve.