From- PPE/Black & Veatch
Ciculated by- CC/PE, KTPS
TURBINE STEAM FLOW CONTROL : The steam flow to a power powe r plant steam turbine is controlled by the main steam control valves. These valves regulate the steam flow to the high pressure turbine that also controls the steam flow to remaining turbine sections. The operation of these valves in conjunction with pressure variation in the steam gen erator provides for four methods of controlling steam turbine generator output: 1. 2. 3. 4.
Thro Thrott ttli ling ng cont contro roll Govern Governing ing cont control rol (Exi (Existi sting ng at KTPS KTPS)) Vari Variab able le-pr -pres essu sure re contro controll Hybrid Hybrid variabl variable-pr e-press essure ure-gov -govern erning ing control control
These four methods of control and comparisons of the heat rates for each mode are presented in the following section. The control methods are also referred to as admission modes. Throttling control :
Throttling control is the simultaneous operation of all main steam co ntrol valves at the same time. The main steam pressure is typically held constant at rated conditions and the control valves are all opened the same amount. The steam turbine output increases as the valves are opened and full load is reached when the valves are wide open. This type of operation minimizes the mechanical loadings on the steam turbine control stage as a result of the reduced pressure to the stage and the equal loading on all stage sections. At partial loads, throttling operation is the least efficient of control modes, b ecause of the throttling process that reduces the energy available in the steam expansion process. Depending on the turbine design, d esign, this mode of operation may also be referred to as full arc admission because of the steam admission to all portions of the control stage. The term typically is used in conjunction with the start up of a turbine. During the start up of some units, the control valves are wide open. Steam is initially admitted to the turbine by throttling the steam flow by use of bypass valves internal to the main steam stop valves. The flow control method is used up to 15% to 25% load. Above this load, the main steam control valves are used to control the steam flow and main steam stop valves are wide open. GOVERNING CONTROL: (Existing at KTPS)
Governing control is the sequential operation of the main steam control valves. Governing control varies the output of the steam turbine by increasing or decreasing the arc of admission of steam flow to the turbine control (first) stage. Each control valve feeds a particular section of the control co ntrol stage, and the amount of arc in use is determined by the number of valves open. The valves are opened in a particular order that is determined by the allowable stresses on the con trol stage. Typically, a minimum of 25% of the control valves are opened o pened simultaneously when initially starting a unit to minimize stresses on the control stage. When these valves are fully open, the remaining valves are
From- PPE/Black & Veatch
Ciculated by- CC/PE, KTPS opened in series. This control mode is also referred to as partial arc admission. Rated throttle conditions are used throughout the load range to the extent allowed by the steam generator. This control mode is more efficient than throttling control because the throttling process loss is minimized by reducing the number of control valves throttling at any one time. VARIABLE PRESSURE CONTROL:
Variable pressure control is steam flow control by varying the steam ge nerator pressure with the main steam control valves in a fixed position. In this control mode, the main steam control valves typically are wide open. The steam flow to the turbine is controlled by the pressure of the main steam from the steam generator. Main steam pressure is controlled by the steam generator firing rate. The main advantage of variable pressure operation is that the turbine first-stage temperature remains relatively constant across the load range which shortens startup times and increases turbine rotor life. The disadvantages of variable pressure operation are thermodynamic pe rformance and limited load response capability. The lower main steam pressures of this mode result in less available energy than in the governing mode, but more than in the throttling mode. This reduced efficiency still exists on an overall cycle basis even when the reduced powers requirements of the boiler feed pump (due to reduced discharge pressure) are included. The response time is limited to the rate the steam generator firing can be increased since the main steam pressure is controlled by the firing rate. In the other modes of control, steam turbine load can be increased more rapidly by opening the closed main steam control valves and utilizing pressure and thermal energy stored in the steam generator. HYBRID VARIABLE-PRESSURE GOVERNING CONTROL:
The hybrid variable-pressure-governing control mode uses the low-load hightemperature, no throttling loss advantages of variable-pressure operation and the thermodynamic and load response advantages of governing control. In addition, hybrid operation provides improvements in part load c ontrol stage efficiency. At low loads, some of the main steam control valves are wide open and steam flow is controlled by variable pressure operation of the steam generator. Increasing main steam pressure increases steam turbine load until the steam pressure reaches rated conditions. Load is increased further by maintaining the rated main steam pressure and sequentially opening the remaining main steam control valve as in the governing control. HEAT RATE COMPARISONS:
A comparison of the steam cycle overall heat rates for the four control modes is shown in figure. The throttling control has the worst overall heat rate as a result of throttling losses. The variable-pressure control mode has a slightly better heat rate than throttling control, but still has poor performance because of the low steam pressures. The governing control has the best heat rate at higher loads because of the high steam pressures and minimized throttling losses resulting from throttling with only one valve at a time. The hybrid control mode takes advantage of the governing high load performance while reducing
From- PPE/Black & Veatch
Ciculated by- CC/PE, KTPS throttling losses and boiler feed pump power at low load with the use of variable pressure operation. This hybrid mode results in the most efficient operation while extending unit life by maintaining high temperature at low loads, reducing cycle effects. Governing heat rates are typically plotted as a “locus of valve points”, that is, as a smooth curve passing through the heat rate points where any valves open are wide open. The governing heat rate is shown by the solid line in figure. The actual heat rate curve is represented by a valve loop that incorporates the throttling losses associated with a valve throttling between full closed and full open. This actual heat rate is represented by the dashed line. As can be seen from the heat rate curve, the more control valves, the smaller the valve loop, the greater the possibility of operating without throttling, and the better the heat rate.
