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.
