151

Water can enter the turbine due to backflow of condensate/ feed water from feed heaters. This can occur upon a load rejection. In this case, the pressure inside the turbines drops to a lower value than the one in the feed heaters. Hence, Check valves (NRVs) are installed on the extraction lines to the feed heaters to prevent this reverse flow.

152

Write short notes on the history and nature of coal?

Coal is a product of nature and time, derived from the decay of ancient trees, bushes, ferns, mosses, vines, and other forms of plant life that flourished millions of years ago in humid, tropical climates.

Time, coupled with pressure, heat, and chemical and bacterial decay, has transformed the plant remains into coal.

ASTM D 388 establishes categories or ranks of coal on the basis of measurable properties that relate to its metamorphism or its degree of transformation while buried.

Coal is heterogeneous. Unlike natural gas or fuel oil, composition or quality of coal can vary significantly depending on the degree of metamorphosis, type of vegetation, location in seam, surrounding materials, etc.

153

What do you mean by a turbine trip?

A turbine trip closes the turbine steam valves and opens the generator circuit breaker.

154

Write short notes on Turbine Over speed trip?

The over speed trip is initiated when the governing system fails to limit the speed rise of the turbine shaft.

It is the final line of defense to prevent a catastrophic failure of the turbine.

Turbine over speed can occur following a load rejection (when the unit becomes disconnected from the grid). It can also occur when the unit is operating in the islanding (unsynchronized) mode. If the governing system fails, higher steam flow can enter the turbine, leading to over speed.

If the over speed becomes excessive (approaching 100 percent), the centrifugal forces acting on the rotating parts become extremely high. The blades will start to rupture and penetrate through the casing.

The manufacturer normally performs an over speed test at 120 percent of the speed. This speed is significantly lower than the design limit at which blade rupture could occur (180 to 200 percent over speed).

The over speed trip is normally set in the range from 110 to 112.5 percent speed.

155

What are the six categories of turbine instrumentation?

The six categories of turbine instrumentation are:

v Supervisory instrumentation

v Efficiency instrumentation

v Auxiliary system instrumentation

v Condition-monitoring instrumentation

v Instrumentation associated with protection and control equipment

v Instrumentation to provide post-incident records

156

What are supervisory instruments?

The supervisory instruments are required continuously to determine the condition of the main rotating and stationary components.

157

Explain the main functions of turbo supervisory instruments in detail.

The main functions of supervisory instruments include the following:

1. To ensure safe operation of the turbine generator within acceptable limits.

2. To provide advanced warning of deterioration in the performance of the turbine generator.

The parameters measured includes the following:

a. Rotor axial position: These measurements provide the relative axial movement of the rotor. They are used to ensure that clearance margins are maintained under all operating conditions.

b. Cylinder expansions: These measurements provide the relative radial movement between the rotors and the stators. They are used to ensure that radial clearance margins are maintained under all operating conditions.

c. Bearing pedestal vibrations: These measurements are taken at each bearing. They continuously monitor the dynamic behavior of the machine.

d. Shaft eccentricity: The radial excursion of the rotor relative to the stationary parts is measured on each rotor. This is done to indicate abnormal or unsafe conditions.

e. Shaft speed: The shaft speed is measured independently of the turbine governor. This measurement is used for operator reference. It is mainly used during run-up.

f. Steam valve positions: The position of each steam valve is measured. These measurements are used as a general reference for the operator. They are used to determine if the load can be increased or for diagnostic purposes.

g. Metal temperature measurement: The temperature of the turbines is measured during normal operation and transient states. The instruments are located in the high pressure (HP) and interceptor steam valve chests, and in the cylinders of the HP and intermediate-pressure (IP) turbines.

h. Thrust bearing wear: These measurements are taken to ensure that the wear of the thrust pads is within acceptable limits. If the wear is higher than the acceptable limit, the turbine generator rotor will move with respect to the stator. This could have disastrous consequences on the machine.

All of the above measured turbo supervisory parameters are displayed continuously for the operator to initiate appropriate action.

158

The turbine bearings must be lubricated to prevent damage that is caused by wear or increased temperatures.

159

It is necessary to lift the turbine generator shaft before starting to turn the shaft. The jacking oil system is used to perform this function.

160

What is a Babbitt metal?

Babbitt metal is an alloy of tin with some copper and antimony.

Babbitt metal is used as a lining material for bearings to reduce friction.