JPH0781700B2 - Furnace pressure control method - Google Patents

Description

The present invention relates to a furnace pressure control system for a boiler, and more particularly to an MFT.
The present invention relates to a method of controlling the pressure inside a furnace by means of an induced draft fan when the internal pressure falls abnormally.

[Conventional technology]

The main fuel trip (hereinafter MFT) is used for conventional furnace draft control.
There is no circuit that prevents a drastic decrease of the furnace draft at the time of occurrence or during normal control and realizes an early recovery. Generally, closed loop control by proportional and integral calculation is the mainstream. FCB (quick load cutoff) compared to this conventional technology
Inducing a differential pressure fluctuation of the wind box in the furnace at the time of occurrence
An example of JP-A-61-29602 is to achieve stable combustion by restricting the moving blades and maintaining the ventilation to the wind box by a mixing fan (GMF).

[Problems to be solved by the invention]

Generally, the pressure inside the furnace of the boiler is controlled by the ID of the FDF outlet and the ID, which is controlled by the blades of the forced draft fan (hereinafter referred to as FDF) shown in Fig. 2.
It is controlled by both in-furnace control by F rotor blades. That is, the signal detected by the FDF outlet pressure transmitter 11 is added by the FDF outlet pressure setting signal obtained by the function generator (hereinafter referred to as FG) 12 from the boiler combustion master signal (hereinafter referred to as FDR) 31 and the subtractor 13. Combined proportional-plus-integral calculator (hereinafter referred to as PI)
14, the signal switching switch (hereinafter referred to as ASW) 15 to control the FDF moving blade 20. On the other hand, the signal detected by the furnace pressure transmitter 1 goes through the first-order lag (hereinafter LG) 2 to the furnace pressure signal setter (SG hereinafter) 4 and the subtractor 3, which is connected to PI5, adder ( Hereinafter referred to as AD) 6 and ASW 7 to control the IDF rotor blade 10. In addition, 8 and 16 are analog memories for manual operation, and 9 and 17 are selector stations.

During normal operation, the control on the FDF side increases the FDF31 according to the boiler master signal, and the FG12 also increases the FDF outlet pressure setting value 32 corresponding to the FRD, so the PI14 output signal 33 increases and the FDF increases.
IDF given to AD6 at the same time as the opening of rotor blade 20 increases
It acts as a leading signal of the moving blade 10 and acts in the direction of increasing the opening.

Therefore, the FDF outlet pressure increases, and eventually the furnace pressure also increases, but the IDF side always increases the opening of the IDF blades to attract the combustion gas so that the constant furnace pressure setting value specified by SG4 is maintained. By doing so, the pressure inside the furnace is controlled in a decreasing direction, so that the indentation action by the FDF and the attraction action by the IDF are always balanced and the pressure inside the furnace is kept constant. That is, the outlet of the FDF100 and the furnace 10 shown in the flue system diagram of Fig. 3 (a).
As shown in Fig. 3 (a), the internal heading of 1 and the system head curve at each point at the inlet of the IDF 102 are as shown in Fig. 3 (a). At the FDF outlet, air pressure is pushed in by the FDF and the pressure becomes the highest. The pressure decreases due to the action, and the pressure becomes lowest at the IDF inlet.

Now, when the system head at each point is the curve 202 in Fig. 3 (b), if the FRD31 in Fig. 2 increases, the system head curve becomes 201 indicated by the chain double-dashed line,
On the contrary, when FRD31 decreases, it becomes 203 shown by the one-dot chain line. In both cases, the indenting action by FDF and the inducing action by IDF show static characteristics such as seesaw motion and balance.

The above is the concept of normal furnace pressure control, but MFT may be generated or the FDF rotor blade may be narrowed down for some reason even though the control command 33 in Fig. 2 is increased. In each case, a phenomenon such as the system head curve 204 shown by a broken line in FIG. 3B occurs transiently and a phenomenon such as 205 occurs for a relatively long time.

This is directly due to the fact that the temperature inside the furnace drops sharply due to the rapid cutoff of fuel when MFT occurs, and the internal pressure of the furnace drops. The indirect cause is that the furnace pressure control method described above cannot be followed up. If the FDF rotor blade is narrowed down, the FDF rotor blade control command 33 in FIG. 2 is in the increasing direction, so the IDF rotor blade using this signal as the preceding signal is also in the increasing direction. Since the FDF blade is narrowed down due to malfunction, the FDF outlet pressure becomes low and the furnace pressure also becomes low. Therefore, by detecting this with the furnace pressure transmitter 1, the output deviation signal of the subtractor 3 decreases. Proceed to and become a negative value. Therefore, the output of PI5 also decreases, but the leading signal 33 is F
Since the output of AD6 does not immediately decrease as the RD31 increases, the IDF rotor blades are not narrowed down for a while, which causes the phenomenon shown in 205 of FIG. 3 (b).

Here, the boiler must perform furnace purging when MFT (main fuel trip) occurs, and must secure the minimum flow rate of air necessary for this, and the FDF and IDF blades are suitable for the puff performance of the boiler. It is controlled to a constant opening. When the FDF blade is narrowed down unexpectedly while the boiler load is increasing, the PI5 for controlling the furnace internal pressure shown in Fig. 2 executes control so as to recover the pressure drop in the furnace by narrowing down the IDF blade while delaying the integration operation. To do.

However, the drawback of the conventional furnace internal pressure control method described in FIG. 2 in the above two types of abnormal modes is that there is a delay in the integral operation of proportional-plus-integral control (hereinafter referred to as PI control). It has the drawback of being forced to undergo a sharp and large drop in.

In any case, if the furnace internal pressure exceeds the design allowable value of the furnace, etc., it will cause damage, and even if it does not exceed the design allowable value, repeated abnormal decrease will cause fatigue failure, so abnormal decrease of the furnace internal pressure is as much as possible. Must be avoided.

However, the above-mentioned conventional technique does not take into consideration the prevention of the decrease in the furnace when MFT occurs, and the recovery operation of the decrease in the pressure inside the furnace during normal operation, and there is a risk that the worst case, damage to the furnace wall and ducts will occur. Atsuta

In view of the above points, an object of the present invention is to provide a furnace internal pressure control system provided with means capable of preventing a rapid decrease in furnace internal pressure during MFT and the like and capable of performing a recovery operation during normal operation.

[Means for solving problems]

The above-mentioned problem is controlled by an added value signal obtained by adding a forced draft rotor blade for controlling the air volume of the forced draft fan, and a proportional integral value of the forced draft rotor blade control signal and a value based on the pressure in the furnace. In a furnace internal pressure control method that controls the pressure in the furnace by an induced draft fan blade that controls the air volume of the machine, a trip signal generating means for generating a main fuel trip signal at the time of main fuel trip, and the furnace internal pressure from a predetermined value. An internal pressure abnormal reduction control signal generating means for generating an internal pressure abnormal reduction control signal when the internal pressure abnormally decreases is provided, and the added value signal is switched to the main fuel trip control signal when the main fuel trip occurs, and when the internal pressure abnormally drops, the internal pressure is reduced. This can be solved by a furnace internal pressure control system that switches to an abnormal lowering control signal to control the induction fan moving blades.

[Action]

With the above configuration, the addition value signal obtained by adding the proportional-integral value of the value due to the pressure in the furnace and the pusher fan blade control signal is switched to the main fuel trip control signal for main fuel trip generation,
When the internal pressure abnormally drops, the control signal is switched to the internal pressure abnormally low control signal to control the induced draft fan blades.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 and 4 to 6.

Prior to the description of the embodiments, the concept of the basic control of the present invention will be described in a fourth section.
It will be described with reference to the drawings.

The signal detected by the pressure transmitter 1 in the furnace goes through the first-order delay LG2 and then branches, and one of them is the first pressure signal setter SG4 in the furnace.
And subtractor 3 are added to each other, and the proportional-plus-integral calculator PI5 is connected to the second
The FDF moving blade setting signal FDFPOS33 explained in the figure is added by the adder AD6, the automatic / manual switching switch 7 is switched to the switching switch ASW58a, and the other is the second furnace internal pressure signal setting device 51 and the furnace internal pressure deviation calculation. It is matched by the subtractor 50 and the gain 60 is switched to the switch 58a. Switching switch 5
The signal that outputs 8a is output from the minimum opening signal setter SG42 during the main fuel trip, and is selected by the control signal during the main combustion trip with the primary delay LG43 and the switching switch 58b to control the induction fan blade 10. To do. Next, the operation of FIG. 4 will be described.

During normal operation, the induction fan blade 10 is switched so that the ASWs 58a and 58b are both selected for the b-side signal, and PI5 is used for proportional-integral control.

If the main fuel trip occurs here, the induced draft fan blade 10
Since the ASW58b selects the signal on the a side, the minimum opening signal SG of the induced draft fan blade that corresponds to the minimum air flow rate during the main fuel trip
With an opening of 42, it is narrowed down rapidly due to the temporary delay characteristic of LG43, preventing a decrease in furnace pressure. Furnace pressure at this time,
Fig. 5 shows the behavior of the opening degree of the induced draft fan blades. In Fig. 5, the internal pressure of the furnace during normal operation and the blade opening of the induced draft fan are respectively
It is 81a, 82a, and if the PI control is continued when the main fuel trip occurs, it can be seen that it is difficult to prevent the decrease in the internal pressure of the furnace by fluctuating as 81b, 82b shown by the broken line. Therefore, when the main fuel trip occurs, the induction fan blade opening is rapidly narrowed down with a primary delay as 82c by switching to the minimum opening constant control by SG42 in Fig. 4, and as a result, the furnace pressure is shown at 81C. Thus, it is possible to prevent a sharp drop.

On the other hand, when the furnace internal pressure drops significantly due to some cause other than the main fuel trip, the ASW 58a in Fig. 4 is switched to the a side, and the proportional calculation consisting of the furnace internal pressure signal setter SG51, the furnace internal pressure deviation calculation subtractor 50 and the gain 60 is performed. Controls the induction draft rotor blades. In this case, the behaviors of the furnace internal pressure and the induced draft fan blade opening are as shown in Fig. 6. If PI control is continued, the induced draft fan blade opening 92a will be 9
1a is not able to be followed completely and the delay time is greatly delayed as shown by the broken line 92b, and the furnace internal pressure is greatly reduced as shown by the broken line 92b, but the recovery time is also long.However, switching to proportional control causes the control side to respond to fluctuations in the furnace internal pressure. Since the induction ventilator blades behave like 92C because they are not delayed, the internal pressure of the furnace does not fluctuate as 91C and the stability is faster.

Next, referring to FIG. 1, an embodiment of the present invention based on the concept of control described in FIG. 4 will be described with reference to FIG.

For some reason during normal operation, the internal pressure of the furnace dropped significantly,
This is a full scale 10 in comparison with the furnace pressure setting device SG4.
% If the value is lower than the setting of SG51, which is the furnace pressure for overriding, the deviation signal from the set value of SG51 becomes negative due to the subtractor 50, and the set value is set by the low value selector (hereinafter referred to as LS) 52. The internal pressure abnormality signal 73, which is compared with the output of SG53 of 0% and the deviation signal is selected, is given to the AD55 and added together with the PI control signal 71 and the main fuel trip control signal 72 which becomes a valid signal only at the time of main fuel trip. A lower value compared with the PI control signal 71, that is, the override control signal 74 is selected, and the IDF moving blade 10 is narrowed down through the ASW 58 to suppress the decrease in the furnace pressure and perform the recovery operation.

Here, the monitor relay (hereinafter referred to as MR) 54 has the set value SG51.
With the same setting as the above, the relay operates at a set value β or less, triggers the internal pressure abnormal drop detection signal 65, and sets the memory 63 to the OR gate 62, and the output 68 of the memory 63 operates and the ASW58
When the deviation between the override control signal 74 and the PI control signal 71 becomes less than the specified value α after the control is switched to the side a and the control becomes stable, the MR57 relay operates and triggers the signal 66, and the time-delay timer 64 causes the timer to operate for several seconds. Memory after counting and ensuring stable control
By resetting 63, the ASW 58 is switched to the b side and the PI control is restored.

When the main fuel trip occurs, the main fuel trip generation signal 67 is triggered immediately, the ASW44 is switched to the side a, and the main fuel trip control signal 72 becomes a valid value from 0% of SG45.
Further, the main fuel trip occurrence signal 67 is a one-shot timer.
The memory 63 is set through the TDWO 61 and the OR gate 62, and the ASW 58 is switched to the a side as described above. At this time, the signal hold element HOLD40 holds the induced draft fan command 75 when the main fuel trip occurs for several seconds which is the set value of the one-shot timer TDW61, and subtracts this hold value from the value of the minimum opening setter SG42. The negative signal obtained by subtracting is LG4
3, ASW44 to AD55 same as the above, PI control signal 71 is overridden with abnormal internal pressure drop suppression signal 73, so that override signal 74 has a lower value than PI control signal 71.
The override signal 74 is selected by the LS 56, and the induction fan blade 10 is narrowed down through the ASW 58 to prevent the pressure inside the furnace from decreasing.

〔The invention's effect〕

According to the present invention, the addition value signal obtained by adding the proportional integral value of the value due to the pressure in the furnace to the forced draft fan blade control signal is switched to the main fuel trip control signal when the main fuel trip occurs,
When the internal pressure abnormally drops, the control signal is switched to the internal pressure abnormally low control signal to control the induced draft fan blades to prevent the internal pressure of the furnace from decreasing and to enable an early recovery operation.

[Brief description of drawings]

FIG. 1 is a block diagram of a furnace pressure control block equipped with an override control circuit according to an embodiment of the present invention, FIG. 2 is a conventional control block diagram of a furnace, and FIGS. 3 (a) and 3 (b) are smoke ducts. Fig. 4 is a diagram showing a system and system head curve, Fig. 4 is a conceptual control block diagram of the present invention, Fig. 5 is a diagram showing behavior of the furnace internal pressure and IDF moving blades at the time of MFT generation in Fig. 4, and Fig. 6 is a diagram. FIG. 5 is a diagram showing the behavior of the furnace internal pressure and the IDF moving blade after the occurrence of abnormal decrease in the furnace internal pressure in FIG. 4. 1 …… Furnace pressure transmitter, 10 …… IFD blade, 11 …… FDF outlet pressure transmitter, 20 …… FDF blade, 31 …… Boiler combustion master signal, 65 …… Internal pressure drop signal, 67… … MFT generation signal, 75 …… IDF rotor blade command.

Claims (1)

[Claims] Claim: What is claimed is: 1. A forced draft fan blade for controlling an air volume of a forced draft fan, and a proportional value based on a control signal of the forced draft fan blade, a detection signal of a pressure transmitter in the furnace, and a deviation signal of a set value of the pressure signal in the furnace. In the furnace internal pressure control method for controlling the internal pressure of the furnace by the induction fan blades for controlling the air volume of the induction fan, which is controlled by the added value signal obtained by adding the integrated value, in the main fuel trip during the main fuel trip Means for storing the induction fan blade opening and generating a main fuel trip control signal based on a deviation signal between the opening and the minimum induction fan blade opening signal sufficient to secure the minimum air flow rate during the main fuel trip And when the internal pressure of the furnace falls below a predetermined value when the internal pressure is abnormally lower than the preset value of the furnace internal pressure setter, the set value of the override internal furnace pressure setter and the internal pressure of the furnace generated for overriding A means for generating an internal pressure abnormal reduction suppression signal based on a deviation signal of the detector signal, and the addition value signal is used when the main fuel trip occurs, the main fuel trip control signal, the internal pressure abnormal reduction suppression signal and the already added value signal The signal with the three added values and the added value signal are compared and switched to the lower value.On the other hand, when the internal pressure abnormally decreases, the signal with the added value signal added to the internal pressure abnormal decrease suppression signal and the added value signal are compared. A furnace internal pressure control system characterized by controlling the induction fan moving blade by switching to a lower value.