EP0065408B1

EP0065408B1 - Control systems for boilers

Info

Publication number: EP0065408B1
Application number: EP82302398A
Authority: EP
Inventors: Thomas D. Russell; Robert R. Walker
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1981-05-12
Filing date: 1982-05-11
Publication date: 1986-04-23
Also published as: AU8357282A; CA1211324A; ES8400580A1; JPS57198902A; ES512051A0; AU556280B2; KR830010338A; JPS6252122B2; EP0065408A2; EP0065408A3; MX152206A; KR870001505B1; DE3270729D1

Description

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 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.
U.S. Patent No. US-A-3 572 036 discloses a control system for a boiler having a flow path extending between a furnace and an output, the system comprising a first valve capable of diverting flow from the furnace out of the main path, a, second valve positioned at the output, a third valve and a fourth valve connected in parallel with one another in the flow path, and control means for the first to fourth valves, the control means being responsive to certain system parameters to cause the valves to operate differently during different phases of operation of the system. The control system of US-A-3 572 036 is concerned specifically with the initial start-up of the boiler (zero and half load) and the valves are regulated primarily by the control means detecting that different predetermined working fluid temperatures have been reached. Since US-A-3 572 036 is directed towards controlling the boiler over its initial or low load range during start-up, it is not directed to the problem of maximising the load range over which variable pressure operation can be carried out.
The present invention as disclosed hereinbelow is directed towards the provision of a control: system for a boiler which will allow a variable throttle pressure type of operation to be utilised" over a wide load range.
According to the present invention there is provided a control system for a boiler having a flow path extending between a furnace and an output, the system comprising:

a first valve capable of diverting flow from the furnace out of said flow path,
a second valve positioned at the output,
a third valve and a fourth valve connected in parallel with one another in the flow path, and
control means for the first to fourth valves, the control means being responsive to certain system parameters to cause the valves to operate differently during different phases of operation of the system,

(i) in the low load phase, to open the first valve, open the second valve to a predetermined position, and close the third and fourth valves,
(ii) in the variable pressure phase, to close the first valve, maintain the second valve in said predetermined position, open the third valve so that the third valve can vary the flow of steam from the boiler in response to the load imposed on the system, and maintain the fourth valve closed, and
(iii) in the full pressure phase, to maintain the first valve closed and the third valve open, and to open the fourth valve to permit the second valve to open further from said predetermined position to vary the flow of steam from the boiler.

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 the second valve (a turbine valve) to approximately 70 percent of its fully open position as soon as possible as the system is being loaded (low load phase or mode), utilising a flash tank while this is occurring until the load demand exceeds minimum feedwater flow requirements, and then allowing the system to assume the variable throttle pressure phase or 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 (full pressure phase or mode). 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 the full pressure mode of operation. In addition, while in the variable pressure mode of operation, a control coordinator monitors 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 connected 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 principal 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 will 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 direct 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 equal 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, act 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 permitted 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

1. A control system for a boiler having a flow path extending between a furnace (12) and an output, the system comprising:

a first valve (34) capable of diverting flow from the furnace (12) out of said flow path,

a second valve (22) positioned at the output,

a third valve (40) and a fourth valve (38) connected in parallel with one another in the flow path, and

control means (Figure 3) for the first to fourth valves, the control means being responsive to certain system parameters to cause the valves to operate differently during different phases of operation of the system,

characterised in that said phases of operation comprise a low load phase, a variable pressure phase and a full pressure phase, and in that the control means is operative:

(i) in the low load phase, to open the first valve (34), open the second valve (22) to a predetermined position, and close the third and fourth valves (40, 38),

(ii) in the variable pressure phase, to close the first valve (34), maintain the second valve (22) in said predetermined position, open the third valve (40) so that the third valve (40) can vary the flow of steam from the boiler in response to the load imposed on the system, and maintain the fourth valve (38) closed, and

(iii) in the full pressure phase, to maintain the first valve (34) closed and the third valve (40). open, and to open the fourth valve (38) to permit the second valve (22) to open further from said predetermined position to vary the flow of steam from the boiler.

2. A control system according to claim 1, which is operative to maintain the input pressure of the boiler substantially constant at full design pressure during the full pressure phase of operation of the system.

3. A control system according to claim 1 or claim 2, wherein the control means (Figure 3) is operative to cause the variable pressure phase of operation of the system to commence when feedwater demand in the system exceeds minimum feedwater requirements of the boiler and to terminate when the system has reached full design pressure.

4. A control system according to claim 1, claim 2 or claim 3, wherein the first valve (34) is capable of diverting flow out of the flow path and into a flash tank (16), wherein a fifth valve (36) is provided to enable steam from the flash tank (16) to flow back into the flow path, and wherein the control means (Figure 3) is operative to open the fifth valve (36) in the low load phase and close it in the other phases.

5. A control system according to any one of the preceding claims, wherein the third and fourth valves (40, 38) are positioned between primary and secondary superheaters (14, 18).