Power Plant Engineering

GAS TURBINE POWER PLANT 289

(d) High Lube Oil Temperature. High lube oil temperature in the lubrication system is a dan-
gerous signal as it is an indication of low lube oil supply or failure in bearings, gears, etc. This protection
is generally accomplished through the use of a temperature-sensing device immersed in the lube oil. The
sensing device may be a thermal switch, which triggers an alarm or shut down the turbine.

(e) Excessive Vibration. A slight increase in vibration is a cause of warning. Protection against
vibration is accomplished by stalling one or moves vibration pick-ups. The output signal is fed to a
monitoring device, which may shut down the turbine if the vibration increases a certain value.

(2) Modulating Controls
The purpose of modulating control is to sense an impending malfunction or a condition, which
could adversely affect turbine life and make some modification to the operating condition of the turbine
in order to alleviate the undesired conditions. An example of this control may be maximum turbine inlet
temperature and maximum speed. The modulating control is more complex and more costly than the
shutdown control. But it offers and advantage in allowing a turbine or turbine driven plant to continue
operating when normally a shut down occurs. There are some conditions such as high vibrations and
low lube oil pressure which cannot be taken care by corrective measures as in this case shut down of the
turbine is essential.

Modulating controls for maximum turbine inlet temperature and maximum speed are slight modi-
fications for over temperature and overspend controls.

9.8 Gas Turbine Efficiency

Gas turbines may operate either on a closed or on an open cycle. The majority of gas turbines

currently in use operate on the open cycle in which the working fluid, after completing the cycle is

exhausted to the atmosphere. The air fuel ratio used in these gas turbines is approximately 60:1.

The ideal cycle for gas turbine is Brayton Cycle or Joule Cycle. This cycle is of the closed type

using a perfect gas with constant specific heats as a working fluid. This cycle is a constant pressure cycle

and is shown in Fig. 9.24. On P-V diagram and in Fig. 9.25 on T-φ diagram. This cycle consists of the

following processes:

The cold air at 3 is fed to the inlet of the compressor where it is compressed along 3-4 and then

fed to the combustion chamber where it is heated at constant pressure along 4-1. The hot air enters the

turbine at 1 and expands adiabatically along 1-2 and is then cooled at constant pressure along 2-3.

Pressure, P

Volum e, V

32

4 1

Adiabatic 1

2

3

4

Constant

pressure

Entropy φ

Te m p. T

Fig. 9.24 Fig. 9.25