CONTROL SYSTEMS FOR BOILERS

Abstract: A system for controlling the operation of once-through boilers utilises variable throttle pressure over a very wide load operating range. The system utilises a valving arrangement permitting a turbine valve (22) to be opened to a predeter­ mined open position, typically 70 percent of its fully open posi­ tion, as soon as possible during a system loading process. The turbine valve (22) is maintained in this predetermined open po­ sition while the flow of steam from the boiler is being varied by a control valve (40) and the output pressure of the boiler is vary­ ing accordingly. The turbine valve (22) is then allowed to open further from the predetermined open position when the system reaches full design pressure so as to vary the flow of steam from the boiler while the output pressure thereof is maintained substantially constant. In this manner, the boiler is operated at a variable output pressure over a very wide load operating range.

Description: CONTROL SYSTEMS FOR BOILERS The present invention relates to control systems for boilers. In a variable pressure boiler system, the throttle pressure varies with the load. In its ideal form, the throttle valves on the turbine are left wide open and the throttle pressure varies directly with the load. Such variable pressure operation is desirable since it can increase the efficiency of the turbine. However, the primary incentive for variable pressure operation is that it can increase the number of times that the turbine can be loaded and unloaded. This is because, with variable pressure operation, the change in the first stage steam exit temperature in the turbine is relatively minor, thus minimising thermal stress in the metal comprising the turbine. In contrast, in a constant pressure type of operation, the first stage steam exit temperature is load dependent. This can result in a greater change in temperature for the turbine which, in turn, can cause excessive metal fatigue. Because of the foregoing, it has become desirable to develop a control system for the operation of a once-through boiler so that a variable throttle pressure type of operation can be utilised over a very wide load range. According to the present invention there is provided a control system for a boiler, the system being characterised by first valve means operable during a first phase of operation of the system, second valve means in fluidic communication with the first valve means and operable to open to a predetermined position during the first phase of operation of the system, and third valve means in fluidic communication with the second valve means and operable to vary the flow of steam from the boiler in response to the load imposed on the system during a second phase of operation of the system. A preferred embodiment of the present invention described below solves or at least alleviates the aforementioned problems associated with the prior art by providing a boiler control system in which variable throttle pressure operation can be introduced at as low a load as possible and can be utilised for most of the load operating range. This is accomplished by opening a turbine valve to approximately 70 percent of its fully open position as soon as possible as the system is being loaded, utilising a flash tank while this is occurring unti the load demand exceeds minimum feedwater flow requirements, and then allowing the system to assume the variable throttle pressure mode of operation as the load is increased until throttle pressure approximates a designed operating pressure, at which time the turbine valve is regulated to meet load requirements. In essence, the preferred system provides for variable pressure operation from approximately 20 percent to 75 percent of load and also provides for smooth transition from low load operation to the variable pressure mode of operation, and from the variable pressure mode of operation to a full pressure mode of operation. In addition, while in the variable pressure mode of operation, a control coordinator is provided to monitor and correct steam flow, firing rate and feedwater flow. In this manner, the system can automatically adjust and compensate for deviations in these parameters from that which is desired. The preferred control system thus permits variable throttle pressure operation of a once-through boiler, enables a once-through boiler to be operated in a variable pressure mode of operation over a wide load range, and provides a smooth transition in a once-through boiler from a low load type of operation to a variable pressure mode of operation and from the variable pressure mode of operation to a full pressure mode of operation. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a system embodying the present invention; Figure 2 is a graph of percentage pressure or valve opening versus percentage load for the system and illustrates flash tank pressure, furnace pressure, turbine valve position and throttle pressure; and Figure 3 is a schematic diagram which illustrates the overall system. Figure 1 is a schematic drawing of a system 10 embodying the present invention. The system 10 comprises primarily a furnace 12 whose output is connected to an input to a primary superheater 14, a flash tank 16, a secondary superheater 18 whose output is connected to an input to a turbine 20 via a turbine valve 22, a generator 23, and a condenser 24. The condenser 24 is connected to an input to the furnace 12 via a low pressure heater 26, deaerator 28, a boiler feed pump 30, and a high pressure heater 32. The primary superheater 14 is connected to an input to the flash tank 16 via a valve 34 and the flash tank 16 is connected to the secondary superheater 18 via a valve 36. A pair of valves 38 and 40 are conected in parallel across the input to the valve 34 and an output of the valve 36. A valve 42 is provided between the flash tank 16 and the condenser 24 and controls the flow of water from the flash tank 16 to the condenser. A superheated steam attemperator valve 44 is provided between the output of the secondary superheater 18 and the flash tank 16. The principle of operation of the system is shown in Figure 2. In Figure 2, percentage pressure or valve opening is plotted versus percentage load, and flash tank pressure, furnace pressure, turbine valve position and throttle pressure are illustrated. The objective is to obtain variable throttle pressure at as low a load as possible, provide a smooth transition from low load operation to once-through operation, and incorporate the capabilities of a control coordinator 50, shown in Figure 3, during a variable throttle pressure phase of operation. This is accomplished by opening the turbine valve 22 as soon as possible, by operating the flash tank 16 until it is dry, and by using the valves 38 and 40 between the primary superheater 14 and the secondary superheater 18 as throttle valves, as wil hereinafter be described. A unique feature of this control strategy is that throttle pressure is not directly controlled, except at minimum pressure, but is permitted to float to whatever level is required for the desired load. Thus, variable pressure operation is achieved over a very substantial portion of the load range. In order to accomplish the foregoing, the system is organised in accordance with a schematic illustrated in Figure 3. In Figure 3, an incoming control signal is applied to a unit load demand development function 52, an output of which is directed to the control coordinator 50 and to a turbine valve program 54, a steam flow modifier 56, a firing rate modifier 58, a feedwater modifier 60, and controls for the valve 36. The turbine valve program 54 controls the operation of the turbine valve 22, the steam flow modifier 56 controls the operation of the valves 38 and 40, the firing rate modifier 50 controls the fuel and air mixture in the system, and the feedwater modifier 60 regulates the flow of feedwater throughout the system. A pressure transmitter 62 is connected to both the control coordinator 50 and the control valve 34, and an electrical transmitter 64, a feedwater temperature transmitter 66 and a superheater temperature transmitter 68 are also connected as inputs to the control coordinator 50 which, in turn, regulates the steam flow modifier 56, the firing rate modifier 58 and the feedwater modifier 60 by means of control signals generated therein. The control system has basically three modes of operation: low load operation, once-through variable pressure operation, and full pressure operation. Low load operation occurs when the boiler feedwater flow is limited to a minimum flow rate. Once-through variable pressure operation occurs when the feedwater flow rate exceeds its minimum flow rate and continues until throttle pressure reaches a full design pressure, i.e. furnace pressure. Full pressure operation occurs when the throttle pressure has reached full design pressure and continues until full load is achieved. During the low load phase of operation, i.e. between 0 and approximately 25 percent load, the throttle pressure is maintained constant and the turbine valve 22 is rapidly opened to approximately 70 percent of its fully open position, as shown in Figure 2. In this mode of operation, the valves 38 and 40 are closed and the valves 34 and 36, along with the turbine valve 22, are opened. The valve 34 controls the furnace pressure, whereas the valve 36 controls the throttle pressure. The valve 42 is also opened and regulates the water level in the flash tank 16. During this phase of operation, all flow from the furnace 12 is directed to the flash tank 16 and starts as water and, as firing is increased, becomes steam. The flash tank 16 acts as a steam and water separator and directs the water to the condenser 24 and the steam to the turbine 20. By the time steam flow to the turbine 20 equals the minimum feedwater flow rate of approximately 25 percent, the flash tank 16 has dried up. At that time, the valve 40 opens and the valves 34 and 36 start closing, stopping the flow to the flash tank 16. This occurs at approximately 25 percent of load and starts the next phase of operation, i.e. the variable throttle pressure phase or once-through variable pressure phase of operation. In the once-through variable pressure phase or variable throttle pressure phase of operation, the turbine valve 22 is maintained at approximately 70 percent of its fully open position by the turbine valve program 54. During this phase of operation, steam flow control is regulated by the valve 40 and this valve, in essence, acts as a remote throttle valve. In this phase of operation, the feedwater flow is given the responsibility of controlling steam temperature, whereas the firing rate controls the load, and throttle pressure is permittted to float to whatever value is necessary to satisfy the load requirements. The control coordinator 50 assumes an important function in this phase of operation since it produces error or correction signals to the steam flow modifier 56, the firing rate modifier 58 and the feedwater modifier 60. These error or correction signals are as follows: a megawatt error minus a furnace pressure error control signal which is directed to the steam flow modifier 56, a megawatt error plus a furnace pressure error control signal which is directed to the firing rate modifier 58, and a superheat temperature error plus a feedwater temperature control signal which is directed to the feedwater modifier' 60. In this manner, the feedwater flow can be adjusted to maintain steam temperature while the steam flow and the firing rate can be corrected to maintain proper furnace pressure and megawatts. When the valve 40 approaches its fully open position, the valve 38 starts opening and the turbine valve 22 is permitted to start opening further from its 70 percent open position. This commences the next phase of operation, i.e. the full pressure phase of operation. In the full pressure phase of operation, the throttle pressure reaches full design pressure and the turbine valve 22 is allowed to open still further to maintain the load as is necessary. In this mode of operation, the steam flow is controlled by the turbine valve 22 rather than by the valve 40, and the combined capacity of the valves 38 and 40 is sufficient to provide the steam flow required by the turbine valve 22 is it opens further from its 70 percent open position. It should be noted that with respect to non-variable pressure once-through boiler systems, as in the prior art, this is the normal mode of operation once above the minimum feedwater flow rate, and thus, a variable pressure phase is never introduced therein. The foregoing control system produces a number of benefits. For example, by using variable pressure the first stage steam temperature can be closely controlled, which permits rapid loading of the turbine without creating excessive thermal stress. In addition, the foregoing system provides for quickly achieving variable pressure operation, turbine metal temperature matching, and a smooth transition to once-through operation. Further, the control coordinator regulates and controls the overall operation of the system in the variable pressure phase of operation and adjusts the system components to compensate for various operational deviations.

Claims: CLAIMS 1. A control system for a boiler, the system being characterised by first valve means (34, 36) operable during a first phase of operation of the system, second valve means (22) in fluidic communication with the first valve means and operable to open to a predetermined position during the first phase of operation of the system, and third valve means (40) in fluidic communication with the second valve means and operable to vary the flow of steam from the boiler in response to the load imposed on the system during a second phase of operation of the system. 2. A control system according to claim 1, wherein the second valve means (22) is maintained in said predetermined position during the second phase of operation of the system and the output pressure of the boiler is allowed to vary during the second phase of operation. 3. A control system according to claim 1 or claim 2, including a fourth valve means (38) operable during a third phase of operation of the system and in fluidic communication with the second valve means (22) permitting the second valve means (22) to open further from said predetermined position to vary the flow of steam from the boiler during the third phase of operation of the system. 4. A control system according to claim 3, wherein the input pressure of the boiler is maintained substantially constant at full design pressure during the third phase of operation of the system. 5. A control system according to any one of the preceding claims, including means (62, 64, 66, 68) for sensing certain system parameters and means for modifying the operation of the system in response to said system parameters. 6. A control system according to claim 5, including means (50) for processing said system parameters and producing signals in response thereto to modify the operation of the system. 7. A control system according to any one of the preceding claims, wherein the second phase of operation of the system commences when feedwater demand in the system exceeds minimum feedwater requirements of the boiler and terminates when the system has reached full design pressure.

Inventors: RUSSELL, THOMAS D., WALKER, ROBERT R.
Applicants: THE BABCOCK & WILCOX COMPANY
Application year: 1982
Main classification: F22B35
IPC codes: F01K13, F01K3, F22B35