In addition to utilizing the traditional "feedforward-feedback" control scheme, the most prominent feature of supercritical unit furnace pressure control is the design of several override protection circuits, ensuring safer operation.
1. Override Control During MFT Triggering
Once the boiler receives the MFT triggering signal, the furnace fire is typically extinguished within seconds. Once the fire is extinguished, the furnace temperature drops sharply. From the ideal gas state equation, pV=RT, we know that when the flue gas volume V in the furnace remains unchanged, the furnace pressure p will decrease as the furnace temperature T decreases, resulting in a significant negative pressure inside the furnace. Furthermore, combustion in a furnace is a rapid chemical process, and the flue gas after combustion includes not only primary and secondary air, but also CO and water vapor generated during combustion. When the boiler fire is extinguished, the CO and water vapor levels decrease significantly, significantly reducing the flue gas mass flow rate. If the induced draft fan guide vanes remain open at this time, a significant negative furnace pressure will inevitably result. If no action is taken, the boiler could explode. To prevent such accidents, the furnace pressure control system is designed with an override protection circuit to prevent boiler explosions.
When the MFT activates, the control system first forcibly closes the induced draft fan guide vanes by a certain angle to reduce the fan output and prevent a rapid drop in furnace pressure.
Currently, large units using distributed control systems have slightly different designs for the override protection circuits, which can be roughly divided into two categories: Under all load conditions, the induced draft fan guide vane angle is fixedly closed by approximately 10% after MFT activation; After MFT activation, the amount of reduction in the induced draft fan guide vane angle varies with load.
The first approach assumes that when the boiler is operating at or near full load, the CO2 and water vapor produced by coal combustion will not increase the induced draft fan output by more than 10%. Therefore, when the MFT activates, the induced draft fan guide vane angle is initially closed by approximately 10% (the full opening range is 100%), then gradually released to ensure that the induced draft fan output is balanced with the air flow. This estimate is relatively rough, and when the MFT activates, the furnace pressure response time to normalize is relatively long. The boiler input command signal (BID) is transmitted through the function block to form the curve shown in Figure 2. This curve shows that as the boiler input command increases, the function block output decreases.
When the unit is operating normally, MFT = 0, and the function block output signal cannot be transmitted downstream, thus inactivating the override protection circuit.
When a main fuel trip occurs in the boiler, MFT = 1, and a negative signal corresponding to the boiler input command is sent to the ES| input of the adder block downstream of the A and B induced draft fan M/A control stations. Therefore, regardless of whether the A and B induced draft fan M/A control stations are in automatic or manual mode, this signal will quickly close the guide vanes of both induced draft fans by one degree, preventing the risk of low furnace pressure after MFT activation. Because this signal has a higher control priority than the automatic and manual signals, it is called an override control signal.
The signal that overrides the induced draft fan guide vane by one degree can only be maintained for a short period of time before gradually recovering. This time is set by the "delayed on" function block [TD]ON in Figure 2. After the function block's ON time expires, the override signal, which closed the ID fan guide vanes by one angle, returns to zero, releasing the guide vanes and resuming normal furnace pressure control mode. Alternatively, the MFT triggers a shutdown of the boiler, enabling natural ventilation.
The override protection also includes a correction loop for the number of ID fans in operation. Constants 1.0 and 2.0 are output via the switch T and the multiplication block [. When both ID fans are operating, the multiplication block's × correction factor is 1.0; when only one ID fan is operating, it is 2.0.
2. ID Fan Guide Vane Directional Lockout Protection
When the ID control is in automatic mode, if the furnace pressure is too low, the guide vanes of both ID fans are at their maximum position, or both ID fans are near the nozzle oscillation zone, the ID fan control command is prohibited from increasing further (locked up) until the "low furnace pressure" logic condition disappears. If the furnace pressure is too high, the ID fan control command is prohibited from decreasing further (locked down).
When the surge lockout function detects that the induced draft fan is about to surge, the directional lockout function also prevents the induced draft fan guide vane command from increasing (locking out the increase), thereby preventing the fan from surging.
Whether the induced draft fan M/A control station is in manual or automatic mode, the induced draft fan guide vane directional lockout protection circuit activates when the furnace pressure deviation is excessive.
MFT override protection, furnace negative pressure protection, and directional lockout protection are all designed downstream of the induced draft fan M/A control station to prevent operators from inadvertently making erroneous operations that could further deteriorate the furnace pressure, ensuring safe boiler operation.
In addition to utilizing the traditional "feedforward-feedback" control scheme, the most prominent feature of supercritical unit furnace pressure control is the design of several override protection circuits, ensuring safer operation.
1. Override Control During MFT Triggering
Once the boiler receives the MFT triggering signal, the furnace fire is typically extinguished within seconds. Once the fire is extinguished, the furnace temperature drops sharply. From the ideal gas state equation, pV=RT, we know that when the flue gas volume V in the furnace remains unchanged, the furnace pressure p will decrease as the furnace temperature T decreases, resulting in a significant negative pressure inside the furnace. Furthermore, combustion in a furnace is a rapid chemical process, and the flue gas after combustion includes not only primary and secondary air, but also CO and water vapor generated during combustion. When the boiler fire is extinguished, the CO and water vapor levels decrease significantly, significantly reducing the flue gas mass flow rate. If the induced draft fan guide vanes remain open at this time, a significant negative furnace pressure will inevitably result. If no action is taken, the boiler could explode. To prevent such accidents, the furnace pressure control system is designed with an override protection circuit to prevent boiler explosions.
When the MFT activates, the control system first forcibly closes the induced draft fan guide vanes by a certain angle to reduce the fan output and prevent a rapid drop in furnace pressure.
Currently, large units using distributed control systems have slightly different designs for the override protection circuits, which can be roughly divided into two categories: Under all load conditions, the induced draft fan guide vane angle is fixedly closed by approximately 10% after MFT activation; After MFT activation, the amount of reduction in the induced draft fan guide vane angle varies with load.
The first approach assumes that when the boiler is operating at or near full load, the CO2 and water vapor produced by coal combustion will not increase the induced draft fan output by more than 10%. Therefore, when the MFT activates, the induced draft fan guide vane angle is initially closed by approximately 10% (the full opening range is 100%), then gradually released to ensure that the induced draft fan output is balanced with the air flow. This estimate is relatively rough, and when the MFT activates, the furnace pressure response time to normalize is relatively long. The boiler input command signal (BID) is transmitted through the function block to form the curve shown in Figure 2. This curve shows that as the boiler input command increases, the function block output decreases.
When the unit is operating normally, MFT = 0, and the function block output signal cannot be transmitted downstream, thus inactivating the override protection circuit.
When a main fuel trip occurs in the boiler, MFT = 1, and a negative signal corresponding to the boiler input command is sent to the ES| input of the adder block downstream of the A and B induced draft fan M/A control stations. Therefore, regardless of whether the A and B induced draft fan M/A control stations are in automatic or manual mode, this signal will quickly close the guide vanes of both induced draft fans by one degree, preventing the risk of low furnace pressure after MFT activation. Because this signal has a higher control priority than the automatic and manual signals, it is called an override control signal.
The signal that overrides the induced draft fan guide vane by one degree can only be maintained for a short period of time before gradually recovering. This time is set by the "delayed on" function block [TD]ON in Figure 2. After the function block's ON time expires, the override signal, which closed the ID fan guide vanes by one angle, returns to zero, releasing the guide vanes and resuming normal furnace pressure control mode. Alternatively, the MFT triggers a shutdown of the boiler, enabling natural ventilation.
The override protection also includes a correction loop for the number of ID fans in operation. Constants 1.0 and 2.0 are output via the switch T and the multiplication block [. When both ID fans are operating, the multiplication block's × correction factor is 1.0; when only one ID fan is operating, it is 2.0.
2. ID Fan Guide Vane Directional Lockout Protection
When the ID control is in automatic mode, if the furnace pressure is too low, the guide vanes of both ID fans are at their maximum position, or both ID fans are near the nozzle oscillation zone, the ID fan control command is prohibited from increasing further (locked up) until the "low furnace pressure" logic condition disappears. If the furnace pressure is too high, the ID fan control command is prohibited from decreasing further (locked down).
When the surge lockout function detects that the induced draft fan is about to surge, the directional lockout function also prevents the induced draft fan guide vane command from increasing (locking out the increase), thereby preventing the fan from surging.
Whether the induced draft fan M/A control station is in manual or automatic mode, the induced draft fan guide vane directional lockout protection circuit activates when the furnace pressure deviation is excessive.
MFT override protection, furnace negative pressure protection, and directional lockout protection are all designed downstream of the induced draft fan M/A control station to prevent operators from inadvertently making erroneous operations that could further deteriorate the furnace pressure, ensuring safe boiler operation.