EP0639253B1 - Forced-flow steam generator - Google Patents

Abstract

A forced flow steam generator with an evaporator heating surface (4) has a control device for the furnace piloted by a reference value L allocated to the steam generator output and a control device (6) for the mass flow M of the supply water into the evaporator heating surface (4). To prevent any overshoot of the specific enthalpy at the outlet of the evaporator heating surface (4), a device (8) is superimposed on the supply water regulating device (6) which is used as a reference Ms for the mass flow of the supply water to form the quantity Q(L1)/(hsA(L2) - hiE). Here, hiE is the specific enthalpy at the inlet of the evaporator heating surface (4), Q(L1) is the value of the flow of heat into the evaporator heating surface (4) taken at a first power figure L1 from a function generator (10 to 14) and hsA(L2) is the reference derived with a second power figure L2 from the function generator for the specific enthalpy at the outlet from the evaporator heating surface (4). L1 is a first power figure which is delayed in relation to the reference L allocated to the steam generator output and L2 is a second power figure which is delayed in relation to the first power figure L1.

Description

The invention relates to a once-through steam generator with an evaporator heating surface and with a device upstream of the evaporator heating surface for adjusting the feed water mass flow Ṁ into the evaporator heating surface and with a control device associated with this device, the controlled variable of which is the feed water mass flow Ṁ and whose setpoint Ṁ _S for the feed water mass flow depends on one of the Steam generator power assigned setpoint L is performed.

Such a once-through steam generator is known from "VGB Kraftwerkstechnik 65", Issue 1, January 1985, page 29, Figure 6. In this known forced flow steam generator, the setpoint for the feedwater mass flow is guided from the setpoint of the steam generator output or from a setpoint assigned to the steam generator output via a delay element in order to synchronize the heat flow into the evaporator heating surface with the feedwater mass flow. No other measures are provided for this synchronization.

It has been found that in this known forced-flow steam generator, an overshoot of the specific enthalpy at the outlet of the evaporator heating surface cannot be avoided when the steam generator output changes as a result of load changes. Such overshoot can not only reduce the life of the once-through steam generator, but can also make it more difficult to regulate the temperature of the live steam emitted by the once-through steam generator.

The object of the invention is to substantially reduce or even avoid this disadvantageous overshoot of the specific enthalpy at the outlet of the evaporator heating surface.

• als Sollwert Ṁ_s für den Speisewassermassenstrom zugeordnet ist, und daß dieser Vorrichtung als Eingangsgrößen der Istwert h_iE der spezifischen Enthalpie am Eingang der Verdampferheizfläche und der der Dampferzeugerleistung zugeordnete Sollwert L zuführbar sind,
• wobei Q̇(L1) der mit einem ersten Leistungswert L1 aus einem Funktionsgeber nach einer fest vorgebbaren Funktion entnommene Wert für den Wärmestrom in die Verdampferheizfläche,
• wobei h_sA(L2) der mit einem zweiten Leistungswert L2 aus dem Funktionsgeber nach einer fest vorgebbaren Funktion entnommene Sollwert für die spezifische Enthalpie am Austritt der Verdampferheizfläche,
• wobei der erste Leistungswert L1 ein über ein erstes Verzögerungsglied gegenüber dem der Dampferzeugerleistung zugeordneten Sollwert L verzögerter Leistungswert und wobei der zweite Leistungswert L2 ein gegenüber dem ersten Leistungswert L1 durch ein zweites Verzögerungsglied verzögerter Leistungswert ist.

To achieve this object, a once-through steam generator of the type mentioned at the outset is characterized in that the control device is provided with a device for forming the size $$\dot{\text{Q}}{\text{(L1) / (h}}_{\text{sA}}{\text{(L2) - h}}_{\text{iE}}\text{)}$$

• is assigned as the target value Ṁ _s for the feed water mass flow, and that this device can be supplied with the actual values h _{iE of} the specific enthalpy at the input of the evaporator heating surface and the target value L assigned to the steam generator output as input variables,
• where Q̇ (L1) is the value for the heat flow into the evaporator heating surface taken from a function generator according to a predefinable function with a first power value L1,
• where h _sA (L2) is the setpoint for the specific enthalpy at the outlet of the evaporator _{heating surface,} which is taken from the function generator with a second power value L2 according to a predefinable function,
• wherein the first power value L1 is a power value delayed by a first delay element compared to the target value L assigned to the steam generator power, and wherein the second power value L2 is a power value delayed by a second delay element compared to the first power value L1.

The processing of the actual value of the specific enthalpy at the entrance of the evaporator heating surface enables it to be used of the heat flow flowing into the evaporator heating surface for determining the setpoint for the feed water mass flow, so that the feed water mass flow supplied to the evaporator heating surface can be largely adapted to the heat flow supplied to the evaporator heating surface. This enables targeted control of the specific enthalpy at the outlet of the evaporator heating surface.

ein Differenzierglied aufweist, das mit seinem Eingang auf den zweiten Leistungswert L2 am Ausgang des zweiten Verzögerungsglieds oder auf den Istwert eines hinter der Verdampferheizfläche gemessenen Drucks geschaltet ist und das den Wert der als Sollwert Ṁ_s gebildeten Größe bei einem Ansteigen des zweiten Leistungswerts L2 am Ausgang des zweiten Verzögerungsglieds bzw. des Istwerts des hinter der Verdampferheizfläche gemessenen Drucks um einen Korrekturwert vorübergehend verringert und bei einem Absinken dieses zweiten Leistungswerts L2 bzw. des Istwerts des hinter der Verdampferheizfläche gemessenen Drucks um einen Korrekturwert vorübergehend erhöht. Dadurch findet die Energiespeicherung in den Metallmassen der Verdampferheizfläche Berücksichtigung, so daß eine noch bessere Anpassung des der Verdampferheizfläche zugeführten Speisewassermassenstroms an den dieser Verdampferheizfläche zugeführten Wärmestrom erfolgt.An advantageous development is directed to the device for forming the size $$\dot{\text{Q}}{\text{(L1) / (h}}_{\text{sA}}{\text{(L2) - h}}_{\text{iE}}\text{) =}{\dot{\text{M}}}_{\text{s}}$$

has a differentiating element which has its input connected to the second power value L2 at the output of the second delay element or to the actual value of a pressure measured behind the evaporator heating surface and which has the value of the variable formed as the setpoint value Ṁ _s when the second power value L2 rises at the output of the second delay element or the actual value of the pressure measured behind the evaporator heating surface is temporarily reduced by a correction value and is temporarily increased by a correction value when this second power value L2 or the actual value of the pressure measured behind the evaporator heating surface falls. As a result, the energy storage in the metal masses of the evaporator heating surface is taken into account, so that an even better adaptation of the feed water mass flow supplied to the evaporator heating surface takes place to the heat flow supplied to this evaporator heating surface.

ein Funktionsglied mit Differenzierverhalten aufweist, das mit seinem Eingang auf den Istwert h_iE der spezifischen Enthalpie am Eingang der Verdampferheizfläche geschaltet ist und das den Wert der als Sollwert Ṁ_s gebildeten Größe bei einem Ansteigen des Istwerts h_iE der spezifischen Enthalpie am Eingang der Verdampferheizfläche um einen Korrekturwert vorübergehend verringert und bei einem Absinken dieses Istwerts h_iE um einen Korrekturwert vorübergehend erhöht. Dadurch ist berücksichtigt, daß die Auswirkungen von Massenstrom- und Temperaturänderungen des in die Verdampferheizfläche eintretenden Speisewassers in der Verdampferheizfläche nicht synchron verlaufen.Another advantageous embodiment is characterized in that the device for forming the size $$\dot{\text{Q}}{\text{(L1) / (h}}_{\text{sA}}{\text{(L2) - h}}_{\text{iE}}\text{) =}{\dot{\text{M}}}_{\text{s}}$$

comprises a functional element having a differentiating behavior _iE h with its input to the actual value of the specific enthalpy is connected at the input of the evaporator, and the value of the desired value _S M formed size with an increase of the actual value h _iE of the specific enthalpy at the entrance of the evaporator to a correction value temporarily reduced, and a lowering of this actual value h _iE temporarily increased by a correction value. This takes into account that the effects of changes in mass flow and temperature of the feed water entering the evaporator heating surface do not run synchronously in the evaporator heating surface.

Further refinements are characterized in the subclaims.

The invention and its advantages are explained in more detail with the aid of exemplary embodiments.

Figure 1 shows schematically a once-through steam generator according to the invention.

FIGS. 2 and 3 show in a diagram the time course of the specific enthalpy at the outlet of the evaporator heating surface of the once-through steam generator according to FIG. 1.

In Figure 1, a feed water control is shown. The associated control of the furnace results from Figure 6 of the above-mentioned reference "VGB Kraftwerkstechnik 65".

The forced-flow steam generator according to FIG. 1 has a feed water preheating surface (economizer heating surface) 2, which is located in a gas train (not shown).

This feed water preheating surface 2 has a feed water pump 3 connected upstream and an evaporator heating surface 4 connected downstream. A measuring device 5 for measuring the feed water mass flow Ṁ _i (= time derivative of the mass) through the feed water line is arranged in the feed water line led from the feed water pump 3 to the feed water preheating heating surface 2. Furthermore, a measuring device 9 for measuring the actual value h _{iE of} the specific enthalpy of the feed water at the inlet of the evaporator heating surface 4 is provided at the entry of the evaporator heating surface 4 in the connecting line between the feed water preheating heating surface 2 and the evaporator heating surface 4.

A drive motor on the feed water pump 3 is assigned a very fast controller, namely a PI controller 6, at the input of which is the controlled variable the control deviation ΔṀ of the feed water mass flow Ṁ _i measured with the measuring device 5. The controller 6 is assigned a device 8 for forming the setpoint Ṁ _s for the feed water mass flow. This device 8 has, on the one hand, as input variables a setpoint L for the output of the once-through steam generator, which is output by a setpoint generator 7, and, on the other hand, the actual value h _{iE of} the specific enthalpy at the inlet of the evaporator heating surface 4 determined by the measuring device 9.

The setpoint value L of the power of the once-through steam generator, which changes over and over again during operation and which is fed directly to the fuel controller in the combustion control circuit (not shown), is also fed to the input of a first delay element 13 of the device 8. This delay element 13, which is of higher order, for example of 2nd order, gives a first signal or a delayed first power value L1 from. This first power value L1 is fed to the inputs of function transmitter units 10 and 11 of the function transmitter of the device 8. A value Ṁ (L1) for the feed water mass flow appears at the output of the function transmitter unit 10, and a value Δh (L1) for the difference between the specific enthalpy h _iA at the outlet of the evaporator _{heating surface} 4 and the specific enthalpy h _iE am appears at the output of the function transmitter unit 11 Entry of this evaporator heating surface 4. The values Ṁ and Δh as functions of L1 are stored in the function generator units 10 and 11, respectively. They are determined from stationary values for Ṁ and Δh, each of which was measured during stationary operation of the once-through steam generator and entered into the function generator units 10 and 11. Possible functions are shown in the boxes of units 10 and 11. According to this, an essentially proportionally increasing or decreasing function curve is provided in the range from 35% to 100% (= full load) of the load value L.

The output variables Ṁ (L1) and Δh (L1) of the function generator units 10 and 11 are multiplied together in a multiplication element 14 of the function generator of the device 8. The product value Q̇ (L1) obtained corresponds to the heat flow into the evaporator heating surface 4 at the power value L1. This quantity Q̇ (L1) is entered as a counter in a divider 15.

The difference between the nominal value h _sA (L2) of the specific enthalpy at the outlet of the evaporator _{heating surface} 4 and the actual value h _{iE of} the specific enthalpy at the inlet of the evaporator heating surface 4, which is formed with the aid of the measuring device 9, is used as the denominator in the dividing element 15 is measured, entered.

A setpoint h _sA (L2) is taken from a third function generator unit 12 of the function generator of the device 8. The input value of the function generator unit 12 arises at the output of a second delay element 16, in particular a delay element of the 1st order, the input variable of which is the first power value L1 at the output of the first delay element 13. Accordingly, the input value of the third function generator unit 12 is a second power value L2, which is delayed compared to the first power value L1. The values h _sA (L2) as a function of L2 are stored in the third function generator unit 12; they are determined from values for h _sA , which were respectively obtained during a steady-state operation of the _{once-through steam generator} and entered into the third function generator unit 12. A possible function is shown in the box of unit 12. According to this, an essentially linearly decreasing function curve is provided in the range from 35% to 100% (= full load) of the load value L.

für den Speisewassermassenstrom für die in einem Summierglied 23 stattfindende Bildung der dem Regler 6 zugeführten Regelabweichung ΔṀ des mit der Vorrichtung 5 gemessenen Istwerts für den Speisewassermassenstrom in die Speisewasservorwärmheizfläche 2 entnommen werden.The output of the divider 15 can be the setpoint $${\dot{\text{M}}}_{\text{s}}\text{=}\dot{\text{Q}}{\text{(L1) / (h}}_{\text{sA}}{\text{(L2) -h}}_{\text{iE}}\text{) = Δh (L1) ×}\dot{\text{M}}{\text{(L1) / (h}}_{\text{sA}}{\text{(L2) - h}}_{\text{iE}}\text{)}$$

for the feed water mass flow for the formation in a summing member 23 of the control deviation 6 supplied to the controller 6 of the actual value measured with the device 5 for the feed water mass flow into the feed water preheating heating surface 2.

At the output of the second delay element 16 there can advantageously be the input of a differentiating element 17, the output of which is connected negatively to a summing element 18. This summing element 18 corrects the value for the heat flow Q̇ (L1) into the evaporator heating surface 4 by the output signal of the differentiating element 17. The input of the differentiating element 17 can also - as in FIG. 1 only indicated by dashed lines - are located on a device 30 for measuring the actual value of the pressure p _i behind the evaporator heating surface 4 (for example also behind a superheater heating surface of the forced-flow steam generator connected downstream in terms of flow). A function generator can also be connected between the input of the differentiating element 17 and such a device 30 for measuring the actual value of the pressure p _i , which, for example, outputs the saturated steam temperature corresponding to the measured pressure p _i to the differentiating element 17.

A further differentiating element 24 can advantageously be provided as a functional element with differentiating behavior. This differentiating element 24 has, as an input variable, the actual value h _{iE of} the specific enthalpy at the inlet of the evaporator heating surface 4, determined with the measuring device 9. The output of the differentiating element 24 is also connected negatively to the summing element 18.

In normal steady-state load operation, the once-through steam generator is in a steady state and the setpoint L for the steam generator output is constant. The power values L1 at the output of delay element 13 and L2 at the output of delay element 16 are thus also constant; they have the same value as the setpoint L.

In this steady state operation of the continuous _{steam generator} , h _iE corresponds to the stationary value for the specific enthalpy at the entrance to the evaporator heating surface 4, and the value Ṁ _s given by the device 8 corresponds to the stationary setpoint for the feed water flow into the feed water preheating heating surface 2 and thus into the evaporator heating surface 4th

The product Δh (L1) × Ṁ (L1) = Δh (L) × Ṁ (L) formed in the multiplication element 14 corresponds to a stationary value for the heat flow into the evaporator heating surface 4.

When the setpoint L for the steam generator output at the setpoint generator 7 changes, a new stationary value Q̇ (L) for the heat flow into the evaporator heating surface 4 is only delayed because the firing of the once-through steam generator follows a change in the setpoint L of the steam generator output only with a delay. This is taken into account by the first delay element 13 of the device 8 (synchronization).

Already because a mass flow needs a finite period of time to flow through the evaporator _{heating surface} 4, the specific enthalpy h _iA at the outlet of the evaporator _{heating surface} 4 changes with a further delay when the heat flow into this evaporator _{heating surface} 4 changes, which is taken into account by the second delay element 16 of the device 8 is.

Taking into account the specific enthalpy h _iE measured at the entry into the evaporator heating surface 4 when forming the setpoint value Ṁ _s for the feed water mass flow takes into account in particular the temporal behavior of the heating of the feed water outside the once-through steam generator.

The differentiator 17 on the one hand reduces the setpoint value Ṁ _s for the feed water flow by a corresponding correction value as long as the power value L2 rises over time and the heating of the metal masses of the evaporator heating surface 4 reduces the heat flow that gets into the mass flow in the evaporator heating surface 4. The Differentiator 17, on the other hand, increases the desired value Ṁ _s by a corresponding correction value as long as the power value L2 drops in time and the cooling of the metal masses of the evaporator heating surface 4 increases the heat flow that reaches the mass flow in the evaporator heating surface 4.

The output of the differentiating element 17 can also be switched positively to the other summing element 19, possibly via a normalizing element.

The differentiator 24 reduces the setpoint Ṁ _s for the feed water mass flow into the once-through steam generator by a correction value as long as the actual value h _{iE of} the specific enthalpy at the entrance to the evaporator heating surface 4 increases. On the other hand, the differentiator 24 increases the target value Ṁ _s by a correction value as long as the actual value h _iE falls in time. The output of the differentiating element 24 can also be connected to the summing element 19 in a positive manner - possibly via a normalization element.

The differentiating element 24 can be a pure functional element with differentiating behavior. However, it can also include additional computing elements that modify the differentiation behavior.

The course shown in FIG. 2 (curves I to IV) of the four specific enthalpies h _iA in kJ / kg at the outlet of the evaporator _{heating surface} 4 as a function of time t was for a _once-through steam generator with a ramp-shaped change in the setpoint L for the output of this steam generator from 50% to 100% determined within 200 seconds. The same applies to that shown in Figure 3 Time course (curves I to IV) of the four specific enthalpies h _iA in kJ / kg, which are based on a ramp-shaped change in the setpoint value L of the power of the _{once-through steam generator} from 100% to 50% within 200 seconds.

The curves I in FIGS. 2 and 3 apply in the event that the output value Ṁ (L1) of the function generator unit 10 is the uncorrected setpoint value Ṁ _s for the controller 6. The curves II apply in the event that the differentiators 17 and 24 are not present in the circuit according to FIG. 1, while the curves III apply to the circuit corresponding to FIG. 1, but without the differentiator 24. The curves IV apply to the circuit according to FIG 1. The diagrams according to FIGS. 2 and 3 show that the complete circuit according to FIG. 1 with the curves IV is the cheapest if it is important to avoid overshoot of the specific enthalpy h _iA at the outlet of the evaporator _{heating surface} 4 as far as possible.

In FIG. 1, an enthalpy correction controller 20 is also shown in broken lines, the input of which is connected to the output of a summing element 21. This summing element 21 is _{supplied with the} desired value h _sA (L2) output at the output of the third function generator unit 12 and negatively with the actual value h _{iA of} the specific enthalpy at the outlet of the evaporator _{heating surface} 4. This actual value h _iA is measured with a measuring device 22 located in the outlet line of the evaporator heating surface 4. The correction signal at the controller output is fed positively to the summing element 19 of the device 8.

This enthalpy correction controller 20 advantageously corrects the target value Ṁ _{s of} the feed water flow in the _{Forced-flow steam generator} , if the measured actual value h _{iA of} the specific enthalpy at the outlet of the evaporator _{heating surface} 4 due to external interference, such as fluctuations in the calorific value of the fuel supplied to the continuous-flow steam generator or changes in the fire position in the combustion chamber of the continuous-flow _{steam generator} , from the setpoint h _sA (L2) for the specific enthalpy am Exit of the evaporator heating surface 4 deviates, which is emitted by the third function generator unit 12.

Claims (8)

1. Forced once-through steam generator having an evaporator heating surface (4) and a device (3), connected upstream of the evaporator heating surface (4) in terms of flow, for setting the feed-water mass flow Ṁ into the evaporator heating surface (4), and having a control device (6) which is assigned to said device (3), whose control variable is the feed-water mass flow Ṁ and whose setpoint value Ṁ_s for the feed-water mass flow is controlled as a function of a setpoint value L assigned to the steam generator power, characterized in that a device (8) for deriving the variable $$\dot{\text{Q}}{\text{(L1) / (h}}_{\text{sA}}{\text{(L2) - h}}_{\text{iE}}\text{)}$$

   

   as setpoint value Ṁ_s for the feed-water mass flow is assigned to the control device (6), and in that the actual value h_iE of the specific enthalpy at the inlet of the evaporator heating surface (4) and the setpoint value L assigned to the steam generator power can be fed to said device (8) as input variables,

   where Q̇(L1) is the value for the heat flow into the evaporator heating surface (4), which value is derived, along with a first power value L1, from a function generator (10, 11, 12, 14) in accordance with a function which can be predetermined in a fixed manner,

   where h_sA(L2) is the setpoint value for the specific enthalpy at the outlet of the evaporator heating surface (4), which setpoint value is derived, together with a second power value L2, from the function generator (10, 11, 12, 14) in accordance with a function which can be predetermined in a fixed manner,

   where the first power value L1 is a power value which is delayed by means of a first delay element (13) with respect to the setpoint value L assigned to the steam generator power, and

   where the second power value L2 is a power value which is delayed by a second delay element (16) with respect to the first power value L1.
2. Forced once-through steam generator according to Claim 1, characterized in that the device (8) for deriving the variable $$\dot{\text{Q}}{\text{(L1) / (h}}_{\text{sA}}{\text{(L2) - h}}_{\text{iE}}\text{) =}{\dot{\text{M}}}_{\text{s}}$$

   

   has a differentiating element (17) whose input is connected to the second power value L2 at the output of the second delay element (16) or to the actual value of a pressure measured downstream of the evaporator heating surface (4) and which temporarily reduces by a correction value the value of the variable derived as setpoint value Ṁ_s if the second power value L2 at the output of the second delay element (16) or of the actual value of the pressure measured downstream of the evaporator heating surface (4) rises, and temporarily increases it by a correction value if said second power value L2 or the actual value of the pressure measured downstream of the evaporator heating surface (4) decreases.
3. Forced once-through steam generator according to Claim 1 or 2, characterized in that the device (8) for deriving the variable $$\dot{\text{Q}}{\text{(L1)/(h}}_{\text{sA}}{\text{(L2) - h}}_{\text{iE}}\text{) =}{\dot{\text{M}}}_{\text{s}}$$

   

   has a functional element (24) having a differentiating characteristic, whose input is connected to the actual value h_iE of the specific enthalpy at the inlet of the evaporator heating surface (4) and which temporarily reduces by a correction value the value of the variable derived as setpoint value Ṁ_s if the actual value h_iE of the specific enthalpy at the inlet of the evaporator heating surface (4) rises, and temporarily increases it by a correction value if said actual value h_iE decreases.
4. Forced once-through steam generator according to Claim 1, 2 or 3, characterized in that an enthalpy correction control (20) is provided to whose controller input the variable (h_sA(L2) - h_iA) is applied as control deviation and at whose controller output a correction value can be supplied which is added to the difference (h_sA(L2) - h_iE), where h_iA is the actual value of the specific enthalpy at the outlet of the evaporator heating surface (4).
5. Forced once-through steam generator according to one of Claims 1 to 4, characterized in that the function generator (10, 11, 12, 14) comprises a first and a second function generator unit (10, 11) to which the first power value L1 is applied and whose output signals (Ṁ(L1), Δh(L1) are fed to a multiplication element (14).
6. Forced once-through steam generator according to one of Claims 1 to 5, characterized in that the function generator (10, 11, 12, 14) comprises a third function generator unit (12) to which the second power value L2 is applied and whose output signal (h_sA(L2)) is fed to a summing element.
7. Forced once-through steam generator according to one of Claims 1 to 6, characterized in that the device (8) comprises a dividing element (15) for deriving the variable Ṁ_s.
8. Forced once-through steam generator according to one of Claims 1 to 7, characterized in that a measuring device (5, 9, 22) is provided for determining the actual value of the specific enthalpy at the inlet and/or at the outlet of the evaporator heating surface (4).

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