EP0639253A1

EP0639253A1 - Forced-flow steam generator.

Info

Publication number: EP0639253A1
Application number: EP93908800A
Authority: EP
Inventors: Axel Butterlin; Hermann Doerr; Joachim Franke
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1992-05-04
Filing date: 1993-04-21
Publication date: 1995-02-22
Anticipated expiration: 2013-04-21
Also published as: US5529021A; EP0639253B1; JP2563099B2; DK0639253T3; CN1044404C; KR100251011B1; CN1086299A; KR950701420A; WO1993022599A1; DE59304751D1; JPH07502803A

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

Forced steam generator

The invention relates to a once-through steam generator with an evaporator heating surface and with a device connected upstream of the evaporator heating surface to set the feed water mass flow M 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 M. and whose setpoint Ms "for the feed water mass flow is guided as a function of a setpoint L assigned to the steam generator output.

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, for synchronizing the heat flow into the evaporator heating surface with the feed water mass flow, the set value for the feed water mass flow is guided from the set value of the steam generator output or from a set value assigned to the steam generator output via a delay element. No other measures are provided for this synchronization.

It has been found that in this known continuous 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 an overshoot can not only reduce the lifespan of the continuous steam generator, but also make it more difficult to regulate the temperature of the live steam emitted by the continuous steam generator. The object of the invention is to substantially reduce or even completely avoid this disadvantageous overshoot of the specific enthalpy at the outlet of the evaporator heating surface.

To achieve this object, a forced-flow steam generator of the type mentioned at the outset is characterized in accordance with the invention in that the control device is provided with a device for forming the size

Ö (Ll) / (h _sft (L2) - h _iE )

is assigned as setpoint M for the feed water mass flow, and that this device can be supplied with the actual values h. ^ of the specific enthalpy at the entrance to the evaporator heating surface and the setpoint L assigned to the steam generator output, with Q (L1) being the input value a first power value L1 from a function generator for a heat flow into the evaporator heating surface taken from a function which can be predetermined, where h1 (L2) is the setpoint for the specific value taken from the function generator with a second power value L2 after a function which can be predetermined Enthalpy at the outlet of the evaporator heating surface, the first power value L1 being a power value delayed by a first delay element compared to the setpoint value L assigned to the steam generator power, and the second power value L2 being a power value delayed by a second delay element compared to the first power value L1.

Processing the actual value of the specific enthalpy at the inlet of the evaporator heating surface enables drawing the heat flow flowing into the evaporator heating surface to determine 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 guidance of the specific enthalpy at the outlet of the evaporator heating surface.

An advantageous development is directed to the device for forming the size

Q (Ll) / (h _sft (L2) - h. _E ) = M _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 Ms q ³ when the increase the second power value L2 at the output of the second delay element or the actual value of the behind

Evaporator heating surface measured pressure temporarily reduced by a correction value and 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 decreases. 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.

Another advantageous embodiment is characterized in that the device for forming the size

Q (Ll) / (h _sA (L2) - h. _E ) = M _s has a functional element with differentiating behavior which, with its input, corresponds to the actual value h- _{E of} the specific

Enthalpy is switched at the input of the evaporator heating surface and this temporarily reduces the value of the variable formed as the setpoint Ms g ³ when the actual value h .. - the specific enthalpy at the input of the evaporator heating surface by a correction value and when this actual value h decreases. _p 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 in the evaporator heating surface ^{* are} not synchronous.

Further configurations 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.

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 FIG. 6 of the above-mentioned literature reference "VGB Kraftwerkstechnik 65".

The forced flow steam generator according to Figure 1 has a feed water preheating surface (economizer heating surface) 2, which is located in a gas train, not shown. In terms of flow, this feed water preheating surface 2 is preceded by a feed water pump 3 and an evaporator heating surface 4. A measuring device 5 for measuring the feed water mass flow M (= 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- _{F 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, specifically a PI controller 6, at the input of which the control deviation ^ as a controlled variable. M of the feed water mass flow M measured with the measuring device 5. The controller 6 is assigned a device 8 for forming the setpoint M 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- _{F 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 time and again during operation and which is fed directly to the fuel controller in the (not shown) firing control loop, 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 worth Ll. 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 M (L1) for the feed water mass flow appears at the output of the function transmitter unit 10, and a value Δh (Ll) for the difference from the specific enthalpy h appears at the output of the function transmitter unit 11. _fl at the outlet of the evaporator heating surface 4 and the specific enthalpy h. _F at the entrance to this evaporator heating surface 4. The values M and Δh as functions of L1 are stored in the function transmitter units 10 and 11, respectively. They are determined from stationary values for M and Δh, which were measured during steady-state operation of the once-through steam generator and input 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 M (L1) and Δh (Ll) 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 Q (L1) obtained corresponds to the heat flow into the evaporator heating surface 4 at the power value Ll. This quantity Q (L1) is entered as a counter in a divider 15.

The difference between the nominal value h _ή (L2) of the specific enthalpy at the outlet of the evaporator heating surface 4 and the actual value h- _{F of} the specific enthalpy at the inlet of the evaporator heating surface 4, which is formed by a summing element 19, is used as the denominator is measured using the measuring device 9, entered. A setpoint h (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 first 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 "(L2) as a function of L2 are stored in the third function generator unit 12; they are determined from values for h ", which were respectively obtained during a steady-state operation of the continuous steam generator and were entered into the third function generator unit 12. A possible function is shown in the box of the 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,

The output of the divider 15 can be the setpoint

M _s = Q (Ll) / (h _sA (L2) -h _iE ) = Δh (Ll) x M (Ll) / (h _sA (L2) - h- _E ) for the feed water mass flow for those in a summing member 23, the formation of the control deviation .DELTA.M supplied to the controller 6 of the actual value measured by the device 5 for the feed water mass flow into the feed water preheating heating surface 2 can be taken.

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 switched negatively to a summing element 18. This summing element 18 corrects the value for the heat flow Q (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 - on a device 30 for measuring the actual value of the pressure p. are located 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). Between the input of the differentiating element 17 and such a device 30 for measuring the actual value of the pressure p. can also be connected to a function generator, for example, as the output signal that the measured pressure p. outputs the corresponding saturated steam temperature to the differentiating member 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- _{E 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 the delay element 13 and L2 at the output of the delay element 16 are thus also constant; they have the same value as the setpoint L.

In this steady-state operation in the steady state of the once-through steam generator, h- _E corresponds to the stationary value for the specific enthalpy at the entrance to the evaporator heating surface 4, and the value M output 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 4. The product formed in the multiplier 14

Δh (Ll) x M (L1) = Λh (L) x M (L) corresponds to a stationary value for the heat flow into the evaporator heating surface 4.

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

The specific enthalpy h changes simply because a mass flow requires a finite period of time to flow through the evaporator heating surface 4. _ft at the outlet of Ver¬ dampferheizfläche 4 with a change of the heat flow in this evaporator 4 with a further delay Ver¬, which is taken into account by the second delay element 16 of the apparatus. 8

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

The differentiator 17 on the one hand reduces the setpoint value M for the feed water flow by a corresponding correction value as long as the power value L2 increases in time and the heating of the metal masses of the evaporator heating surface 4 reduces the heat flow which is in the mass flow in the evaporator heating surface 4 arrives, reduced. The Differentiator 17, on the other hand, increases the setpoint M 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 enters the mass flow in the evaporator heating surface 4.

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

The differentiator 24 on the one hand reduces the setpoint Ms for the feed water mass flow into the once-through steam generator by a correction value as long as the actual value h- _{E of} the specific enthalpy at the inlet of the

Evaporator heating surface 4 increases. On the other hand, the differentiator 24 increases the desired value M by a correction value as long as the actual value h- _E 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 standardization 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 differentiating behavior.

The course shown in Figure 2 (curves I to IV) of the four specific enthalpies h. _fl in kJ / kg at the outlet of the evaporator heating surface 4 as a function of the time t was determined 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% within 200 seconds. The same applies to that shown in Figure 3 time course (curves I to IV) of the four specific enthalpies h. _ft in kJ / kg, which are based on a ramp-like change in the setpoint value L of the power of the forced flow 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 M (L1) of the function generator unit 10 is the uncorrected setpoint M for the controller 6. Curves II apply in the event that differentiators 17 and 24 are not present in the circuit according to FIG. 1, while curves III apply to the circuit corresponding to FIG. 1, but without differentiator 24. 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 there is an overshoot of the specific enthalpy h- _A at the outlet of the evaporator heating surface 4 Avoid as much 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 _fl (L2) output at the output of the third function transmitter unit 12 and negatively with the actual value h- _{A of} the specific enthalpy at the outlet of the evaporator heating surface 4. This actual value h- _A 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 setpoint Ms "of the feed water flow in the Forced-flow steam generator when the measured actual value h- _{ft of} the specific enthalpy at the outlet of the evaporator heating surface 4 as a result of external interference, such as fluctuations in the calorific value of the fuel supplied to the continuous-flow steam generator or changes in the fire situation in the combustion chamber of the continuous-flow steam generator, from the setpoint h _ft (L2) deviates for the specific enthalpy at the outlet of the evaporator heating surface 4, which is emitted by the third function transmitter unit 12.

Claims

1. Continuous flow steam generator with an evaporator heating surface (4) and with a device (3) upstream of the evaporator heating surface (4) for setting the feed water mass flow M into the evaporator heating surface (4) and with a control device assigned to this device (3) (6), the controlled variable of which is the feed water mass flow M and whose setpoint M for the feed water mass flow is guided as a function of a setpoint L assigned to the steam generator output, characterized in that the control device (6) has a device (8) for forming the variable

Q (L1) / ( _sft (L2) - h _iE )

is assigned as setpoint M for the feed water mass flow, and that this device (8) as input variables the actual value h. _E the specific enthalpy at the input of the evaporator heating surface (4) and the setpoint value L assigned to the steam generator output can be supplied, Q (L1) being the one with a first output value L1 from a function generator (10, 11, 12, 14) Value taken for the heat flow into the evaporator heating surface (4) according to a predefinable function, where h _fl (L2) is the one with a second power value L2 from the function generator (10, 11, 12, 14) according to a predefinable function Desired setpoint for the specific enthalpy at the outlet of the evaporator heating surface (4), the first power value L1 being a power value delayed via a first delay element (13) with respect to the setpoint L assigned to the steam generator power and wherein the second power value L2 is a power value delayed by a second delay element (16) compared to the first power value L1.

2. forced-flow steam generator according to claim 1, d a ¬ d u r c h g e k e n n e z e i c h n e t that the device (8) for forming the size

Q (Ll) / (h _sA (L2) - h _iE ) = _s

has a differentiating element (17) which is switched with its input to the second power value L2 at the output of the second delay element (16) or to the actual value of a pressure measured behind the evaporator heating surface (4) and which has the value of The quantity formed as the setpoint M is temporarily reduced by a correction value when the second power value L2 increases at the output of the second delay element (16) or the actual value of the pressure measured behind the evaporator heating surface (4) and when this second power value L2 drops or the actual value of the pressure measured behind the evaporator heating surface (4) temporarily increased by a correction value.

3. forced flow steam generator according to claim 1 or 2, d a d u r c h g e k e n n z e i c h n e t that the device (8) for forming the size

Q (Ll) / (h _sft (L2) - h _lE ) = M _s

has a functional element (24) with differentiating behavior which, with its input, corresponds to the actual value h. _E the specific enthalpy at the input of the evaporator heating surface (4) is switched and that is the value of the setpoint size formed temporarily increases by a correction value when the actual value h- _{E of} the specific enthalpy at the entrance to the evaporator heating surface (4) increases, and when this actual value decreases h. _E temporarily increased by a correction value.

4. forced-flow steam generator according to claim 1, 2 or 3, characterized in that an enthalpy correction controller (20) is provided, at the controller input the size (h _fl (L2) - h. _Fl ) is as Regelabwei¬ tion and at the controller output Correction value can be given, which is added to the difference (h _ft (L2) - h _iE ), where h. _{fl is} the actual value of the specific enthalpy at the outlet of the evaporator heating surface (4).

5. Continuous flow steam generator according to one of claims 1 to 4, characterized in that the function generator (10, 11, 12, 14) comprises a first and second function generator unit acted upon by the first power value L1 (10, 11), the output signals (M (L1 ), Δh (Ll)) are fed to a multiplication element (14).

6. forced-flow 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) acted upon by the second power value L2, the output signal (h »(L2)) of one Summing element (12) can be fed.

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 education fertil ^s size includes.

8. forced flow steam generator according to one of claims 1 to 7, d a d u r c h g e k e n n z e i c h n e t that a measuring device (5, 9) is provided for detecting the actual value of the specific enthalpy at the input and / or output of the evaporator heating surface (4).