STEAM TURBINE GOVERNING :
Nozzle Governing :
With nozzle governing a series of nozzle valves open is the sequence as the load increases this type of governing is most efficient and is used for impulse turbine.
Throttle Governing :
With throttle governing a single large control valve controls the load from 0% to 100% for large turbines two control valves operating in parallel replace a large single valve.
When steam is throttled the superheat increase and the turbine exhaust steam is drier reducing the turbine blade erosion but with the drier steam entering the condenser. The condenser losses increase throttling of steam through a valve is an isentropic (constant enthalpy) process and not heat is lost the so-called throttling losses occur in the condenser.
By-pass Governing (or) Overload Governing :
This system is used an impulse, as well as reactions turbines an extra set of control valves, admit steam to the space behind the cruets wheel or for a reaction turbine to an annular space behind the first 8 – 12 stages by-passing part of the turbine increases the turbine capacity (overload the turbines) but at a reduced efficiency the bypass – valves are smaller than the regular governor valves as too much bypass steam in steam at very high density and the blades may overheat.
Turbine Governors :
The two general types of governors used are the speed sensitive governor and the pressure sensitive governor. Speed sensitive governors are applied to all kinds of a turbine. Pressure sensitive governors are applied to back pressure and extraction turbines in connection with the speed sensitive governor.
Speed Governing :
The frequency of 60 Hz is used as the set point or balance between supply and demand of an electronic network any oversupply of energy will increase the frequency and an undersupply will decrease the frequency the supply. The supply must at any time be – equal to the demand in order to keep the frequency at exactly 60Hz.
The speed governor is a proportional – action controller each change in power causes a change in the turbine speed. The governor control is the opening of the control values as a function of this speed change because of the governor speed drop the frequency is not constant over the full range of load without external adjustment governor speed droop is the percent change in speed required for a load change equal to the rated capacity of the turbine it is the same as 0% proportional band for controllers.
- A) Mechanical
- B) Mechanical – Hydraulic
- C) Electro – Hydraulic
A turbine manufacturer may use all three types depending system for each type. It is, therefore, more used served system for each type. It is therefore impossible to describe more than a single example such as the standard mechanical hydro system applied to most person turbine