The steam temperature control system enablessteamThe temperature (superheated steam temperature and reheated steam temperature) is kept within the rated value or allowable deviation rangeautomatic control system。Boiler operation has strict requirements on steam temperature, and it is generally not allowed to deviate from ± 5 ℃ of rated steam temperature.Excessive steam temperature will affect the safety of superheater, reheater and steam turbine;Too low steam temperature will reduce the thermal efficiency of the power plant and increase the exhaust steam humidity of the turbine, thus affecting the safety of the turbine.[1]
Chinese name
Steam temperature control system
Foreign name
steam temperature
Discipline
Thermal control system
Application Object
Boiler steam
influence factor
Constant steam load, boiler feed water temperature
There are many factors that affect the change of steam temperature, such as desuperheating water flow, steam load, boiler feed water temperature, combustion conditions, flue gas temperature, furnace heat load, etc. To sum up, the main ones are the disturbance at steam water side and the disturbance at flue gas side.Regardless of the disturbance of any factor, the step response curve of steam temperature has a certain time delay and inertia, and is finally balanced at a new value.
Superheated steam temperature control
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Steam temperature regulation involves the structure of the boiler, including surface type desuperheating, spray desuperheating, swing burner, etc., or a combination of several methods, in which spray desuperheating is widely used (see boiler steam temperature regulation).Superheated steam temperature control systemIt mainly includes: cascade steam temperature control system;Steam temperature control system with leading differential signal;Sectional steam temperature control system.
Cascade steam temperature control
Figure 1
withSpray desuperheaterThe rear steam temperature is used as an auxiliary regulated quantity, which is a steam temperature control system composed of main regulator and auxiliary regulator.The principle of the system is shown in Figure 1.Under the disturbance of desuperheating water volume, the steam temperature θ a before conduction is reflected ahead of the main steam temperature θ 0.Therefore, using θ a signal to form a cascade steam temperature control system can improve the control quality of steam temperature.As long as the steam temperature θ a before guide changes, the auxiliary regulator will change the desuperheating water volume to maintain the steam temperature θ a at the inlet of the rear superheater within a certain range for rough adjustment, while the steam temperature θ 0 at the outlet of the superheater plays a correction role through the main regulator.After adjustment, the steam temperature θ a before guide may be stable at a value different from the original value, while the main steam temperature is equal to the given value.
In the cascade steam temperature control system, since the task of the main regulator and the main loop is to maintain the main steam temperature θ 0 constant, the regulator with integral function (PI or PID) is generally selected.As for the task of sub regulator and sub circuit is to quickly eliminate disturbance, it is generally optionalProportional regulator(P) Or scaleDifferential regulator(PD), or proportionalIntegrating regulator(PI) (See analog control system).
Steam temperature control of leading differential signal
The double loop steam temperature control system is composed of differential of steam temperature before guide as supplementary signal.It is the deformation of cascade steam temperature control system, and its principle is shown in Figure 2.Since the differential signal θ a 'of steam temperature θ a before guide is introduced into the system, it is equivalent to improving the dynamic characteristics of the regulating object.When the water injection volume is disturbed, the steam temperature θ a before pilot makes the regulator act in advance, which can effectively reduce the dynamic deviation of the main steam temperature θ 0.In the dynamic process, the regulator acts according to the differential θ a 'of the steam temperature θ a before pilot and the main steam temperature θ 0. In the static state, θ a no longer changes, and the output of the differentiator disappears (that is, θ is equal to zero). At this time, the main steam temperature θ 0 must return to the given value.
Figure 2
Sectional steam temperature control
The whole superheater is divided into several sections, and a desuperheater is set between each section to control the steam temperature of each section respectively, so as to make the main steam temperature equal to the given value of the steam temperature control system.Figure 3 is the schematic diagram of the two section steam temperature control system. The control scheme of each section adopts the double loop steam temperature control system with leading differential signal.Generally speaking, the control quality of the segmented control system is better than that of the single-stage control system.[2]
Figure 3
Reheat steam temperature control
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The control of reheat steam temperature depends on the design and structure of the boiler. Generally, steam steam heater, flue gas baffle, flue gas recirculation, swing burner and other different methods are used (see boiler steam temperature regulation).Generally, the flue gas side is used as the main regulating means, and the spray desuperheating can only participate in the regulation or serve as the protection means for overtemperature when the steam temperature exceeds the limit value.In order to overcome the lag and inertia of the controlled object and better maintain the reheated steam temperature, air flow or steam flow can be introduced as the leading signal of the reheated steam temperature.The principle of the control system is shown in Figure 4.Air flow V (or steam flow D) is a leading signal of reheated steam temperature. When it changes, the amount of flue gas to the reheater or the amount of flue gas recirculation changes in proportion to the air flow.In this way, as long as the parameters of the function converter are set, the reheat steam temperature can be basically unchanged.When the reheat steam temperature deviates from the specified value, it shall be corrected by the reheat steam temperature regulator.When the reheated steam temperature exceeds the temperature, the water spray regulator will open the water spray regulating valve through the actuator to limit the further increase of steam temperature.In general, the water spray regulating system does not participate in temperature regulation.
Figure 4
Technical difficulties
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Steam temperature is an important parameter for safe, efficient and economical operation of thermal power units. Therefore, the requirements for steam temperature control are quite strict. If the steam temperature is too high, the superheater and turbine HP cylinder will be damaged due to excessive thermal stress. If the steam temperature is too low, the thermal efficiency of the unit will be reduced and economic operation will be affected.However, steam temperature control has always been a major problem in thermal control, mainly in the following aspects:
1) High control accuracy (± 5 ℃) is required;
2) Large system lag;
3) There are many interference factors, including: changes in feed water temperature, desuperheating water disturbance, load disturbance, combustion disturbance, changes in air coal ratio, etc;
4) Uncertainty of object characteristics: the object characteristics of superheater vary greatly under different loads and combustion conditions;
5) The control means are single. At present, spray desuperheating is the main control means.
The steam temperature control schemes adopted at present mainly include cascade control, leading differential, phase compensation, sectional control, temperature difference control, etc., but the operation effect is not ideal.Based on the above factors, although many control experts at home and abroad have done a lot of research in this area and put forward many new and advanced control schemes, the problem of steam temperature control in thermal power plants has not been completely solved due to difficulties in engineering implementation and few applications.
