2.4.3 Cylinder efficiency
There are two commonly used cylinder efficiency definitions for HP and IP turbines. The first is the one described above and is known as the internal efficiency. The second includes the effect of the pressure drop through the stop and governor valves on the HP turbine and intercept, stop and governor valves on the IP turbine and is known as the external efficiency (Fig 1.18).
It can be seen that, though the pressure drop across the valves is at constant enthalpy, the change in entropy affects the moving blade-exit isentropic enthalpy so that:
For HP and IP steam turbines, the kinetic energy effects at inlet and exhaust are small and usually ignored. The kinetic energy at inlet to the LP turbine is also negligible but at the exhaust it is not. As a result LP efficiencies are derived by considering either:
- The total exhaust conditions — which includes the steam kinetic energy and is measurable with a correctly aligned pitot.
- The static exhaust conditions — which represents conditions when the kinetic energy has been reduced to zero and is measurable with pressure gauge.
Both exhaust conditions are displayed in Fig 1.19 from which the following LP efficiencies can be obtained:
TT/TS efficiencies are usually obtained from heat rate tests (see Section 3 of this chapter). The exhaust total enthalpy is derived from the measured power output and the static isentropic enthalpy is normally taken at condenser pressure. TT/TT efficiencies are commonly derived from wetness probe (traverse) tests. The isentropic exhaust enthalpy in this case is at the blade exit plane total pressure. Traverse tests measure the wetness and other properties of the steam along the height of a moving blade. The test is restricted to the measurement across LP turbine stages where there is sufficient distance between stages to permit the free passage of the probe.