181

Water ingress into the turbine lube oil is a common problem. Water presence could lead to bacterial and fungal contamination in the oil systems. This contamination appears as a yellow or black material similar to grease. This growth occurs in sediment in the oil system. It is very difficult to remove it from the system. Hence the following precautions are required to be taken:

v  Minimizing the water content in the oil by using an oil purification system. The concentration of water in the oil should be maintained at less than 0.05%.

v  Removing the sludge from the sumps of the oil system regularly.

v  If bacterial or fungal growth occurs, a correct amount of biocide should be added to kill it.

182

What is the function of a turbine gland sealing system?

The two functions of the turbine glands and seals are:

1. To prevent or reduce steam leakage between the rotating and stationary components of the turbines, if the steam pressure is higher than atmospheric.

2. To prevent or reduce air ingress between the rotating and stationary components of the turbines, if the steam pressure is less than atmospheric. The last few stages in the low pressure (LP) turbines are normally under vacuum.

A power loss is associated with steam leakage or air ingress. Thus, the design of glands and seals is optimized to reduce any leakage.

Steam turbines use labyrinth glands to restrict steam and air leakage.

183

Explain the working principle of a labyrinth seal?

Labyrinth seal consists of a ring having a series of machined fins.

The fins form a number of fine annular restrictions. An expansion chamber follows each restriction.

When the steam enters a restriction, the velocity increases and the pressure decreases (conversion of pressure energy into kinetic energy—the first law of thermodynamics).

When the steam enters the expansion chamber, the kinetic energy is converted by turbulence into heat. The pressure is not recovered.

The pressure is progressively reduced when the steam goes through successive restrictions.

The finned ring and the shaft are usually stepped to enhance the conversion of energy.

184

What are the types of Labyrinth seals?

Few types of labyrinth seals are:

a.    Plain labyrinth seal

b.    Stepped labyrinth seal

c.    Double stepped labyrinth seal, and

d.    Vernier labyrinth seal

Few other types of labyrinth seals are:

a.    Axial radial labyrinth seal, and

b.    Spring back labyrinth seal

184

The gland rings are normally made of four or more segments. The gland sealing system supplies steam to seal the turbine shaft glands under all operating conditions. It also extracts leak off steam from the glands.

185

Explain the steam turbine gland sealing system in detail?

The gland sealing system is normally divided into two parts.

One part supplies steam to the glands of the high-pressure (HP) and intermediate-pressure (IP) turbines. The second supplies steam to the glands of the LP turbine.

This is done to accommodate the range of temperatures experienced throughout the turbine.

The gland sealing system has two modes of operation.

The first supplies steam at the outlet conditions of the superheater. This is known as live steam. It is used during start-up, shutdown, and when the unit is operating at low loads.

The second mode of operation involves taking steam from the HP and IP turbine and using it to seal the glands of the LP turbine during normal power operation.

The use of steam from the HP and IP turbines rather than live steam results in increased efficiency.

The changeover from one source of steam to the other is entirely automatic.

A desuperheater is used to lower the temperature of the steam supplied to the glands.

An HP desuperheater controls the temperature of the steam supplied to the glands of HP and IP turbines.

An LP desuperheater controls the temperature of the steam supplied to the glands of the LP turbines.

The glands are normally divided into sections. After each section, the steam is fed back to an appropriate stage in the turbine or to a gland steam condenser.

Thus, energy is returned to the cycle to improve the efficiency.

The HP leak-off steam is normally connected to the IP turbine. Its pressure is maintained at the IP exhaust pressure.

The steam pressure at the packing leak-off point is normally maintained slightly above atmosphere. The steam taken from the leak-off is normally used to seal the gland of the LP turbine.

Since the steam is moving outward in the gland, it prevents air ingress into the turbine and condenser.

At higher loads, the glands of the HP/IP turbines are self-sealing. Excess steam from these glands enters the LP desuperheater to seal the glands of the LP turbines.

Two strainers are used to prevent impurities from entering the glands. The first is used for the HP/IP system. The second is used for the LP system. These strainers are installed after the desuperheater.

186

Explain the concept of reversibility in a thermodynamic process?

Mr. Sadi Carnot introduced the concept of reversibility and laid the foundations for the second law of thermodynamics.

A reversible process, also called an ideal process, can reverse itself exactly by following the same path it took in the first place. Thus, it restores to the system or the surroundings the same heat and work previously exchanged.

In reality, there are no ideal (reversible) processes. i.e. all real processes are irreversible.

However, the degree of irreversibility varies between processes.

There are many sources of irreversibility in nature. The most important ones are friction, heat transfer, throttling, and mixing.

Mechanical friction is one in which mechanical work is dissipated into a heating effect. One example would be a shaft rotating in a bearing. It is not possible to add the same heat to the bearing to cause rotation of the shaft.

An example of fluid friction is when the fluid expands through the turbine, undergoing internal friction. This friction results in the dissipation of part of its energy into heating itself at the expense of useful work. The fluid then does less work and exhausts at a higher temperature.

The more irreversible the process, the more heating effect and the less the work.

Heat transfer in any form cannot reverse itself.

Heat transfer causes a loss of availability because no work is done between the high and low-temperature bodies.

187

What are external and internal irreversibility?

External irreversibility are those that occur across the boundaries of the system. The primary source of external irreversibility in power systems is heat transfer both at the high and low-temperature ends.

Internal irreversibility are those that occur within the boundaries of the system. The primary source of internal irreversibility in power systems is fluid friction in rotary machines, such as turbines, compressors, and pumps.

188

What do you mean by the term ‘cogeneration’?

Cogeneration is the simultaneous generation of electricity and steam (or heat) in a power plant.

Cogeneration is recommended for process industries where steam is also required in addition to electric power, as it can produce electricity more cheaply and more conveniently than a utility.

A Cogeneration plant ensures that the total energy needs (heat and electricity) of the industry are met from within.

189

What are the two main categories of cogeneration cycle?

The two main categories of cogeneration are (1) the topping cycle and (2) the bottoming cycle.

The Topping Cycle:

In this cycle, the primary heat source is used to generate high-enthalpy steam and electricity.

Depending on process requirements, process steam at low enthalpy is taken from any of the following:

v  Extracted from the turbine at an intermediate stage (like feed water heating).

v  Taken from the turbine exhaust. The turbine in this case is called a back-pressure turbine.

Process steam requirements vary widely, between 0.5 and 40 bar.

The Bottoming Cycle:

In this cycle, the primary heat (high enthalpy) is used directly for process requirements [e.g., for a high-temperature cement kiln/ furnace].

The low-enthalpy waste heat is then used to generate electricity at low efficiency.

This cycle has lower combined efficiency than the topping cycle. Thus, it is not very common. Only the topping cycle can provide true savings in primary energy.

190

How is steam energy converted into mechanical work in a steam turbine?

In a steam turbine, high-enthalpy (high pressure and temperature) steam is expanded in the nozzles (stationary blades), where the kinetic energy is increased at the expense of pressure energy (increase in velocity due to decrease in pressure).

The kinetic energy (high velocity) is converted into mechanical energy (rotation of a shaft - increase of torque or speed) by impulse and reaction principles.