CN101135438A - Method for correcting water level of steam drum based on temperature and pressure - Google Patents

Abstract

This invention relates to a correctional method of water level in the steam drum based on the temperature and pressure that belongs to the heat process control field including the following steps: the temperature difference DeltaT is linear to the water supply flow increment in small range, DeltaQ=-kDeltaT, theDeltaQ is used as the feedforward value in the main circuit of the cascade control of the water level in the steam drum, to eliminate the adverse effect of 'false water level' on the water supply control. When the load increases,DeltaQ>0, and the steam drum working condition is nearly stable, the correctional value is 0; when the load decreases, the controlled water supply is suppressed, DeltaQ<0; when the water flow increases, the temperature T of the steam drum decreases and the water supply flow increment DeltaQ<0; when the water supply flow is nearly constant, the correctional value is 0; when the water supply flow decreases, the controlled supply flow increases, DeltaQ>0. This method introduces the effect of steam drum pressure and water temperature on the steam bubble in the steam drum water to correct the water level control, which suppresses the overshoot phenomenon and ensures the steady operation of the boiler.

Description

The method of steam water-level being revised based on temperature, pressure

Technical field

The invention belongs to the thermal technology and control the field, specially refer to a kind of method of steam water-level being revised based on temperature, pressure.

Background technology

Drum is the device that boiler steam separates, and there is a large amount of steam water interfaces in drum inside.For boiler-steam dome, when load raise, steam flow increased, and caused water vapor pressure reduction in the drum, the also corresponding reduction of the boiling point of water, and the boiling aggravation, underwater steam bubble volume increases, thereby causes water level to raise suddenly; On the other hand, when feedwater flow increased, a part of heat was drawn in the feedwater that temperature is lower from original saturated vapor aqueous mixtures, make underwater steam bubble volume reducing, thereby caused water level to descend.This at steam flow or feedwater flow variation initial stage, the phenomenon people that steam water-level changes in the opposite direction are referred to as " false water level ".Actual water level in the drum is vapour, water mixed layer, and humidity is along the flex point face of height distribution curve.Mica water level meter, electric contact point fluviograph and differential pressure water level meter all are the quality water levels of measuring at present, and water level control also is to remove to control the real time execution that has actual water level in the drum really with imaginary, in fact non-existent " quality water level ".At present, the control of three momentums is adopted in boiler drum level control mostly, because the existence of " false water level " phenomenon is arranged, brought adverse influence for the control of drum liquid level, can make automatic control system overshoot occur and cause the situation of vibration, can make drum lack of water or full water accident occur if deal with improperly, influence the normal stable operation of boiler.Consider the influence of drum pressure and drum water temperature, introduce drum water temperature and drum pressure The Water Level Control of Steam is revised steam bubble in the drum water.

From control, " false water level " constituted the characteristic of a non-minimum phase.By operation commissioning experience and theory analysis, find that drum pressure and water temperature and stove water vapor content have specific relation to water level control.This kind relation is introduced drum level control system, and the dynamic quality of the control system that helps flooding reduces the generation of over control.Based on the influence of drum pressure and drum water temperature, introduce a kind of method that drum water temperature and drum pressure are revised The Water Level Control of Steam, can the flood dynamic quality of control system of this method to steam bubble in the drum water.

Drum water temperature and drum pressure that this modification method is introduced, its measuring point all is that boiler is controlled existing important measuring point, analog signals or digital quantity signal all than being easier to introduce control system, are therefore realized than being easier in actual applications.

Summary of the invention

The technical issues that need to address of the present invention are at " false water level " phenomenon that occurs in the boiler water level control, to bring adverse effect for the control of drum liquid level, if deal with improperly, lack of water or full water accident can appear in drum, in order to overcome this drawback, just must revise The Water Level Control of Steam.The purpose of this invention is to provide a kind of method of steam water-level being revised based on temperature, pressure, can the flood dynamic quality of control system of this method.For realizing the object of the invention, be to take following technical scheme to carry out, a kind of method of steam water-level being revised based on temperature, pressure, it is characterized in that this method is carried out according to the following steps: 1. do to give a definition earlier, the difference of corresponding saturated water temperature Tp and actual water temperature T is as temperature difference T under the drum pressure, there is complicated corresponding relation between it and the feedwater flow increment Delta Q, in very little scope, its simplification is defined as linear relationship, Δ Q=-k Δ T, k is a proportionality coefficient; Δ Q eliminates " false water level " to giving the adverse effect of water management as the feedforward amount of major loop in the control of steam water-level tandem.2. when the load rising, when steam flow increases, produce the virtual height water level, drum pressure descends, and Tp descends, and than the decline of drum water temperature rapidly, Tp＜T, Δ T＜0, the control confluent increases Δ Q〉0, eliminate the harmful effect of virtual height water level with this; 3. the drum operating mode is when stablize, Δ T=0, and the increment Delta that feeds water Q=0, correction is 0; 4. when the load reduction, when steam flow reduces, produce empty low water level, drum pressure rises rapidly, and Tp rises, Tp〉T, Δ T〉0, control feedwater increment reduces Δ Q＜0, eliminates the harmful effect of empty low water level with this; 5. when discharge increased, the drum water temperature T descended, and steam bubble reduces, and produces empty low water level, and Tp is constant substantially, Tp〉T, Δ T〉0, the harmful effect of empty low water level is eliminated with this in feedwater increment Delta Q＜0; 6. when feedwater flow when stablize, drum pressure descends, near saturated, actual water temperature T=Tp, Δ T=0, the increment Delta that feeds water Q=0, correction is 0; 7. when feedwater flow reduced, the drum water temperature T rose, and steam bubble increases, and produced the virtual height water level, Tp＜T, and Δ T＜0, the control confluent increases Δ Q〉0, eliminate the harmful effect of virtual height water level with this.

Beneficial effect of the present invention is, control at boiler drum level on the basis of three momentums control, introduce the influence of drum pressure and drum water temperature to steam bubble in the drum water, thereby The Water Level Control of Steam is revised, can effectively improve the dynamic quality of boiler drum level control, reduce the generation of over control, guarantee the normal stable operation of boiler.

Description of drawings

SEA LEVEL VARIATION when Fig. 1 is the steam disturbance;

Fig. 2 is the The Water Level Control of Steam logic diagram through revising;

Fig. 3 is the drum level control system schematic diagram through revising.

The flow chart that Fig. 4 changes for feedwater flow

The specific embodiment

With reference to Fig. 1, the SEA LEVEL VARIATION the during disturbance of expression steam, under the constant situation that feeds water, Load lifting, if cause steam flow to increase the influence of not considering other factors, then straight line descends steam water-level because steam flow is greater than feedwater flow.Shown in Fig. 1 curve c; Quantity of steam increases the initial stage, because of increasing, steam bubble volume in the water causes the temporary transient rising earlier of water level, because the steam bubble volume can only increase to the drum pressure level, so this rising is limited, after a period of time, it is stable that drum pressure just reaches, water level also no longer rises, as curve a, curve b is both resultant effects, i.e. the actual effect of steam water-level variation.

With reference to Fig. 2, the The Water Level Control of Steam logic diagram of expression through revising, Fig. 3 is the control system schematic diagram, symbol is among Fig. 2, P: drum pressure; T: drum water temperature; D: main steam flow; H: steam water-level; W: feedwater flow T _SAT: saturation steam table; Kz: executing agency's characteristic; K: proportionality coefficient; PID1: PID master controller; PID2: PID submaster controller SP1, PV1, FFW1, FFW2 are respectively setting value, measured value, the feedforward 1 of master controller and feedover 2; SP2, PV2 are respectively the setting value and the measured value of submaster controller.Symbol is among Fig. 3, W: feedwater flow; D: steam flow; H: drum level measurement value; H ₀: the steam water-level setting value; γ _H, γ _w, γ _D, γ _p, γ _T: the characteristic of water level, feedwater flow, steam flow, drum pressure and drum temperature measurement transmitter; α _D, α _w: the diverting coefficient of steam flow, feedwater flow; K _z: executing agency's characteristic; K _f: device characteristics; W _T1(s): main PID transfer function; W _T2(s): secondary PID transfer function; W _h(s): the transfer function of SEA LEVEL VARIATION under the feedwater flow disturbance $W_h\left(S\right)=\frac{\mathrm{\&epsiv;}}{S(T_{1}S+1)}$ W _D(s): the transfer function of SEA LEVEL VARIATION under the steam flow disturbance $W_D\text{(S)=}\frac{K}{(1+T_{2}S)}-\frac{\mathrm{\&epsiv;}}{S}$

By operation commissioning experience and theory analysis to water level control, we have found that drum pressure and water temperature and stove water vapor content have specific relation.At first do to give a definition: the saturation water temperature that drum pressure is corresponding down is T _p, drum water actual temperature T, the poor Δ T=T of two temperature _p-T.We think that there are complicated corresponding relation in temperature difference and feedwater flow increment Delta Q, but under the situation of the normal operation of boiler, variation of temperature is less, and we are reduced to linear relationship with it in more among a small circle.So the feedwater flow increment that is formed by temperature difference is Δ Q=-k Δ T, k is a proportionality coefficient, need determine that by debugging negative sign represents that Δ T is opposite with Δ Q action direction in debug process.

When load raises, when steam flow increases, can produce the water level of virtual height.This moment, drum pressure descended, corresponding T _pAlso descend, and T changes than the pressure variation slowly, so T _p＜T,, Δ T＜0, the feedwater increment Delta Q that forms by temperature difference like this〉0, with the feedforward amount of this feedwater increment, eliminate " false water level " to giving the adverse effect of water management with this as major loop in the control of steam water-level tandem.When operating mode in the drum near stable the time, drum water actual temperature equals the temperature of saturation water under the drum pressure, Δ T=0, feedwater increment Delta Q=0, as seen this modification method is just made correction to the influence of " false water level " when working conditions change, and this correcting action has just disappeared when stable operation.In like manner, when load reduces the steam flow minimizing, can produce empty low water level.This moment, drum pressure rose, T _pRise T _pT, and Δ T〉0, the empty low adverse effect of water level to bringing to water management eliminated with this in feedwater increment Delta Q＜0 that is formed by temperature difference.

When feedwater flow increased, the temperature T of drum water descended, and the drum in the water reduces at this moment, formed empty low water level.This moment, the pressure of drum changed not quite T _pSubstantially constant, T _pT, and Δ T〉0, the empty low adverse effect of water level to bringing to water management eliminated with this in feedwater increment Delta Q＜0 that is formed by temperature difference.When operating mode in the drum near stable the time, drum pressure descends gradually near saturation pressure, drum water actual temperature equals the temperature of saturation water under the drum pressure, Δ T=0, feedwater increment Delta Q=0, correcting action disappears.In like manner, when feedwater flow reduces, have opposite situation and take place.

According to above analysis, we introduce drum pressure and steam water-level, can draw as Δ T〉0 the time, feedwater increment Delta Q＜0; Otherwise, when Δ T＜0, feedwater increment Delta Q〉0.Can make correction to the automatic control logic of steam water-level in view of the above.

With reference to Fig. 4, the flow chart that the expression feedwater flow changes, on behalf of feedwater flow, Δ W change among the figure, and Δ T is a variations in temperature, and Δ Q is the feedwater increment.

Claims (1)

1. method of steam water-level being revised based on temperature, pressure, it is characterized in that, this method is carried out according to the following steps: 1. do earlier to give a definition, the difference of corresponding saturated water temperature Tp and actual water temperature T is as temperature difference T under the drum pressure, existing complicated corresponding relation is simplified and is defined as linear relationship between it and the feedwater flow increment Delta Q, Δ Q=-k Δ T, k is a proportionality coefficient; 2. when the load rising, when steam flow increases, produce the virtual height water level, Tp descends, Tp＜T, and Δ T＜0, the increase of control confluent, the harmful effect of virtual height water level is eliminated with this in Δ Q＞0; 3. the drum operating mode is when stablize, Δ T=0, and the increment Delta that feeds water Q=0, correction is 0; 4. when the load reduction, when steam flow reduces, produce empty low water level, Tp rises, Tp＞T, and Δ T＞0, control feedwater increment reduces Δ Q＜0, eliminates the harmful effect of empty low water level with this; 5. when discharge increased, the drum water temperature T descended, and steam bubble reduces, and produces empty low water level, and Tp is constant substantially, Tp＞T, and Δ T＞0, the harmful effect of empty low water level is eliminated with this in feedwater increment Delta Q＜0; 6. when feedwater flow when stablize, drum pressure descends, near saturated, actual water temperature T=Tp, Δ T=0, the increment Delta that feeds water Q=0, correction is 0; 7. when feedwater flow reduced, the drum water temperature T rose, and steam bubble increases, and produced the virtual height water level, Tp＜T, and Δ T＜0, the control confluent increases, and the harmful effect of virtual height water level is eliminated with this in Δ Q＞0.

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