JPS6023707A - Method of controlling pressure of deaerator - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention is applicable to thermal power plants and nuclear power plants, in which deaerators can be degassed by restricting the flow rate of condensate to a deaerator and introducing auxiliary steam when the load on a turbine suddenly decreases or is shut off. The present invention relates to a deaerator pressure control method for suppressing the amount of pressure drop in the deaerator.

[Background of the invention]

The deaerator of a thermal power or atomic couplant heats the condensate sent to it (like a condenser) using the extracted air from the turbine, turns it into saturated water, deaerates it, and stores it. By sending this degassed water to a steam stopper (hereinafter referred to as a boiler), oxidative corrosion of the boiler is minimized. When the plant is shut down for a short period of time, auxiliary steam is used to heat and maintain the pressure inside the deaerator slightly higher than atmospheric pressure, so that oxygen in the atmosphere dissolves in the water stored in the deaerator. Plans are often made to avoid increasing the dissolved oxygen concentration so that deaerated water with low dissolved oxygen can be sent to the boiler the next time the plant is started up.

On the other hand, in order to maintain the water level in the deaerator water storage tank at a constant level, a deaerator water level control device is installed to detect the water level in the deaerator (water storage tank) and adjust the flow rate of condensate to deaerator. One-element control that controls the boiler water level control valve, or the deaerator water level can be adjusted using the deaerator water level signal as well as the feed water flow rate (amount of water sent to the boiler) and condensate amount (amount of water flowing into the deaerator). Three-element control or the like is usually employed to control the valves. If the extraction air from the turbine is suddenly cut off due to a sudden decrease in the load on the turbine or a shutdown, the condensate flowing into the deaerator and the water stored in the deaerator will be heated and degassed. As a result, the pressure inside the deaerator dropped rapidly, the deaerator water level became unstable, and the net pushing pressure (hereinafter referred to as NP8H) of the boiler feed water pump was temporarily insufficient, causing the pump to There is a concern that cavitation may occur at the inlet.

Figure 1 shows the transient characteristics of the deaerator pressure, the effective NPSH of the boiler feed pump, and the necessary NP8H when the turbine load suddenly decreases or is shut off using the conventional deaerator pressure control method. (curve ■) suddenly decreases after the turbine load is suddenly reduced or shut off, and the effective NPSH of the boiler feed pump (curve ■) also decreases at a similar rate of decline. On the other hand, the required NPSH (curve 0) is the same as the conditions before the turbine load suddenly decreases or is shut off, due to the self-evaporation delay in the deaerator and the replacement time T, t of water in the deaerator downcomer pipe. Since the supply water temperature is the same, it will not drop for a certain period of time after the turbine load suddenly decreases or after shutoff. As a result, if the deaerator pressure drop is large, or if it takes a long time to replace the water in the deaerator downcomer pipe, the effective NPSH will be smaller than the required NP8H, and water will be supplied at the boiler feed pump inlet. flashes F and causes cavitation.

A solution to this problem is to detect a sudden load change or a load shedding phenomenon, and use that signal to limit the opening of the deaerator water level control valve, thereby suppressing the flow rate of condensate flowing into the deaerator. Means have already been proposed to reduce the rate of pressure drop within the deaerator. This will be explained using a diagram.

Figure 2 shows a diagram of the piping and instrumentation system mainly consisting of a deaerator circuit, which shows the conventional technology. The same system diagram showing an example is shown.

This will be explained in detail with reference to FIG. Steam generated in the boiler 1 is sent to a turbine 3 through a main steam pipe 2 to do work, and a generator 4 generates electricity. The steam that has done work is condensed in a condenser 5, passed through a condensate pipe 6, pressurized by a condensate pump 7, and sent to a deaerator 11. The condensate pipe 6 is often provided with a condensate flow meter 8, a deaerator water level control valve 9, a low-pressure heater IO, a gland condenser, an air extractor, etc., but these are omitted in this figure. The deaerator 11 is composed of a deaeration chamber 11a for heating and deaeration of condensate, and a water storage tank llb for storing deaeration of saturated water. The water sent to the boiler feed water pump 13 through the deaerator downcomer pipe 12 is pressurized here, and is sent to the boiler l through the high pressure heater 15 provided in the water supply pipe 14.
It is sent as a water supply to The water level in the deaerator 11 is controlled by sending an output signal from a water level controller 19 attached to the water storage tank 11b to the deaerator water level control valve 9 through a signal line 20 to keep the water level constant. Although this figure shows an example of one-element control based only on the deaerator water level, it is also possible to perform three-element control in consideration of the condensate flow rate and the feed water flow rate signal. Deaerator 1
1 receives the bleed air from the turbine through the bleed air pipe 16 during normal operation, and heats and degasses the condensate. The degassed gases are sent to the atmosphere or to the condenser 5 and discharged outside the system.

When the plant is stopped for a short period of time, the deaerator 11 is steam-sealed and heated steam is sent through the auxiliary steam pipe 17 and kept under pressure in order to prevent the dissolved oxygen concentration of the water stored in the deaerator from increasing. At this time, the pressure in the deaerator 11 is determined by passing the output signal of the deaerator pressure regulator 21 installed in the deaerator chamber 11a through the signal line 22, and then using the deaerator pressure control valve 18 installed in the auxiliary steam pipe 17. The pressure is controlled to maintain the specified pressure. The above is the same as the prior art shown in FIG.
In the example shown in the figure, in which the pressure drop rate inside the deaerator is reduced when the load suddenly decreases or is cut off, if the bleed air from the turbine is cut off when the load is suddenly reduced or cut off, The command signal is sent to the selector 24 through the condensate flow rate setter 23, and the selector 24 sends a valve opening signal to the deaerator water level control valve 9, which forcibly limits the valve opening and controls the condensate flow. By controlling the flow rate, the deaerator pressure drop can be suppressed. As a result, the amount of condensate flowing into the deaerator is reduced, and the amount of flushed water stored in the deaerator is similarly reduced, so as a natural result, the amount of pressure drop within the deaerator is suppressed. Figure 4 shows the suppressing effect.The rate of pressure drop inside the deaerator is suppressed compared to the conventional technology shown in Figure 1, and when the water replacement in the deaerator downcomer pipe is completed, T.
Effective NPSH of boiler feed pump (curve ■)〉
The required NPSH (curve O) can be secured.

However, as shown in Figure 4, the boiler feed water pump NPS
In many cases, the most severe point on H occurs some time after the completion of water replacement in the deaerator downcomer pipe, and at this point the effective NP
8H is required.If the NP8H is smaller, there is a concern that the water supply will swell at the boiler feed water pump inlet and cavitation will occur.

In addition, as a method to solve this problem, a method has already been proposed in which auxiliary steam is forcibly introduced into the deaerator by detecting a sudden load reduction or cutoff, and using that signal to forcefully introduce auxiliary steam into the deaerator. Since the condensate and auxiliary steam flowing into the deaerator are heated and degassed, the amount less than the internal pressure of the deaerator is heated and degassed by the flash steam of the water stored in the deaerator. Therefore, the rate of decrease in the deaerator pressure is greatly suppressed, which is very effective from the viewpoint of instability of the deaerator water level due to flushing of the deaerator storage water. However, as shown in Figure 5, the NPSH of the boiler feed pump is not very effective, and an effective NPSH is required after the water replacement in the deaerator downpipe is completed, which is smaller than NP8H. There is concern that flash may occur and cavitation may occur.

[Purpose of the invention]

The present invention provides a deaerator pressure control method that solves the above problems and allows normal operation to continue even when the turbine load is suddenly reduced or the turbine bleed air is suddenly shut off. It is in.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. A sudden decrease or cutoff of load (detected and determined when the rate and amount of change in turbine output exceeds a certain value) is detected and sent to the deaerator pressure regulator 21. An appropriate timer 26. is provided. Here, Thailand-r-
26 is adjusted to match the time when the load suddenly decreases or the flash becomes intense even after shutoff, and the deaerator pressure regulator 21
is increased stepwise to the deaerator internal pressure at that point. Furthermore, the output signal of the deaerator pressure regulating needle 21 is sent to the deaerator pressure regulating valve 18 through the signal 2 in 22, the valve is suddenly opened, and auxiliary steam is introduced into the deaerator. The cent of the deaerator pressure regulator 21 is set to a cascade system in which the deaerator internal pressure is increased stepwise to the current deaerator internal pressure and then gradually lowered to the normal set pressure at a certain rate of change.

Additionally, in addition to the example shown in Figure 3, restrictions on condensate flow to the deaerator will be applied at the point when water supply to the boiler is no longer required (herein referred to as OT/H water supply command) when the load suddenly decreases or is cut off. ) detects the OT/H water supply command and limits the condensate flow rate based on the command signal. The OT/H water supply command will be explained using the case of a drum boiler as an example. If the load suddenly decreases or the fuel is suddenly throttled when the engine is shut off,
Air bubbles in the furnace wall tubes collapse due to a decrease in furnace heat due to fuel throttling and a transient increase in pressure, resulting in a drop in drum level. To prevent this, sudden load reduction,
The machine detects the phenomenon of interruption, and according to the signal, a certain water supply command is given. Next, once the drum level has recovered and conditions are stable, the water supply command is canceled. The cancellation condition for this water supply command is the OT/H water supply command. In other words, as explained in Fig. 6, when the load suddenly decreases or is cut off,
A certain water supply command is given by detecting that condition, but an OT/H water supply command that occurs after a certain period of time is detected,
The command signal is sent to the selector 24 through the condensate flow rate setting device 25, and the selector 24 sends a valve opening signal to the deaerator water level control valve 9, which forcibly limits the valve opening and controls the condensate flow rate. Do the following.

〔Effect of the invention〕

According to the present invention, as shown in FIG. 7, it is possible to secure a sufficiently large effective head for the boiler feed water pump NPSH.

[Brief explanation of the drawing]

Figure 1 shows the deaerator pressure and boiler feed pump NP8H transient characteristics diagram when the load suddenly decreases or is cut off according to the prior art.
The figure shows a piping and instrumentation system diagram based on the prior art deaerator circuit, Figure 3 shows a system diagram of a conventional improvement example, and Figure 4 shows the effect of the example shown in Figure 3. Deaerator pressure and boiler feed water pump NPSH transient characteristic diagram. Figure 5 shows the steam pressure and Gay 2 feed water pump NPSH transient characteristic diagram when auxiliary steam is introduced to the deaerator under the conditions of sudden load reduction or cutoff. 24...Selector, 25...Condensate flow rate setter,
2- γ 1IiU 2nd m 3rd (21 4th (2) fr=+t through M/r ′iI Figure FNIfil!zl!/r qit Figure 70 ia+i production interim

Claims (1)

[Claims] 1. The exhaust gas of the turbine is condensed in a condenser, the condensed water is extracted with a condensate pump and sent to a deaerator, and during normal operation, the air extracted from the turbine/deaerator is used. In a method in which water is heated, degassed, stored, and then sent to a boiler using a boiler water pump, when the load on the turbine suddenly decreases or is cut off, that condition is detected and the signal is used to adjust the pressure in the deaerator. The pressure control valve of the deaerator is connected to the deaerator pressure control valve through a meter, and an appropriate time delay device is installed on the signal line to force the pressure control valve of the deaerator to operate at a time when the pressure drop rate of the deaerator is large. A deaerator pressure control method characterized by: