DE4117796A1 - Superheated steam temp. regulation - uses post-injection enthalpy to allow controlled spraying of water into superheater even on saturation curve and in wet steam region - Google Patents

Abstract

Satd. steam from a separator (O) passes through two parallel superheaters with elements (e.g. 2,6,11) connected together in series by mixer tubes (4,9) into which water is injected (14,15). Cascade control in normal operation uses the enthalpy of the steam after controlled injection as an auxiliary parameter, determined from measurements of pre-injection temp., quantity of water added, steam outlet mass flow, and pressure measured at locations as close as possible to the points of injection. ADVANTAGE - Enthalpy used normally as auxiliary control parameter becomes main one during start-up and in low-load operation.

Description

The invention relates to a method according to the preamble of claim 1.

To protect the superheater of steam systems from excessive temperatures depends on the temperature level reached the injection started. To do this Injection control valves each with one from the heater and flow of the superheater and the temperature before and connected to the control circuit acted upon by the superheater.

Cascade control with temperature is usually used for this according to superheater as control variable and temperature according to Injection used as an auxiliary control variable (Thiel, H. J .: Power plant control technology, VEB German publishing house for basic material industry, Leipzig, 1969, p. 178 f).

It is known in the start-up and low-load operation of a steam boiler the slave controller alone as a starting controller and the auxiliary control variable Temperature after injection as a controlled variable to use. For this, a lower setpoint is specified, the time-dependent value from the sum of the measured pressure dependent saturation temperature and a constant or selectable Temperature surcharge to take into account a minimum necessary overheating is formed (D 2 27 768).

The technological working area of this injection lies thus in the area of the superheated steam. This is always then observed when the heating span of the to be regulated Superheater is smaller than the difference between the possible ones Limit temperature after the superheater stage and the saturation temperature.

If the superheater's heating span exceeds this working range in the area of superheated steam, that would be Compliance with the limit temperature after the superheater level only possible if the steam is injected into the Wet steam area is cooled.  

In the literature, reference is made to the fact that the temperature setpoint but do not reach the value of the saturation temperature may, because otherwise uncontrolled and ineffective in the Wet steam is injected (Heger, H. et al .: Better fully automatic Driving KW blocks; Energy and Automation Control technology in the power plant, April 1988 edition, pp. 24-27). Conventional control procedures then fail. This is also true to a known method according to DE 31 21 442, in which depending on a specified steam temperature setpoint an injection medium for temperature change in quantity regulated in the steam is injected by taking from the measured Temperatures and pressures in front of and behind the injection point the steam enthalpies are determined and by means of these sizes and the enthalpy of the injection medium and the Amount of incoming steam is the setpoint for the quantity of injection medium is determined.

The use of the temperature before and especially after injection namely presupposes that always overheated steam flows.

The invention is based on the technical problem, a method for steam temperature control with such a command variable to create a controlled injection too enabled when reaching the saturation line and in the wet steam area. This is done by a cascade control in normal operation and the use of a starting controller in the starting and Low load operation assumed.

According to the invention, the enthalpy after injection is during of normal control operation as an auxiliary control variable and during the Start-up and low-load operation when e.g. B. in known How the slave controller is the sole starting controller, as a controlled variable used. The setpoint is set depending on the process, d. H. it will be corrected. The actual value of the enthalpy after injection, the measured values become steam temperature before injection, amount of injection water, steam mass flow at the outlet of the steam line and pressure of one of the injection points  nearest measuring point according to the relationship

determined in the

h₂ the enthalpy after injection,
h₁ the enthalpy before injection,
h _w the enthalpy of the injection water (determined from measurements of the pressure and temperature of the injection water or fixed value specification),
the steam mass flow at the outlet of a steam train,
_E the amount of water injected and
f _E mean a factor

which is the quotient of the product of steam mass flow () and number of superheater trains per steam train, on the Outlet of the steam mass flow () is measured, and the Difference between steam mass flow () and the sum of the Injection water quantities of the injection point under consideration subsequent injections. This factor enables it, with the help of those present at the outlet of the steam train Measurement technology and the measured steam mass flow Determine the mass flow of steam after the injection point. He is determined for different load points and either as Constant or as an approximation function, e.g. B. linear for Load range specified.

For start-up and low-load operation, an upper temperature limit ϑ₁, a lower temperature limit ϑ₂, a dead time and delay time t _K are used for the temperature after the superheater and an enthalpy surcharge Δh₁ and a minimum distance Δh _min to the saturation enthalpy for the setpoint management of the enthalpy after injection in the superheated steam area ′ ', A setpoint correction element Δh₂ as well as an enthalpy correction k₂ and in the wet steam area a preset steam content value x _x , a setpoint correction element Δx and a steam content corrector k₁ by means of experimental or theoretical process analysis, and an intermediate setpoint X _z is reached when the upper temperature limit ϑ₁ or the lower temperature limit ϑ₂ is reached approached a permissible rate of change. The intermediate setpoint X _z is in the hot steam area when starting from the upper temperature limit ϑ₁ according to the relationship

X _z = h ′ ′ + Δh₁-Δh₂, (2)

when starting from the lower temperature limit ϑ₂ after the relationship

X _z = h ′ ′ + Δh _min + Δh₂ (3)

and in the wet steam area according to the relationship

X _z = h ′ + (x _x + Δx) (h ′ ′ - h ′) (4)

(where h ′ means the enthalpy of the boiling water) educated.

In a further embodiment of the invention, the intermediate setpoint for the superheated steam zone is corrected for the first time when the upper temperature limit is reached according to relationship (2). If this temperature limit is exceeded or again reached, it continues in time cycles which correspond to the dead time and delay time t _K until the difference between the enthalpy surcharge and the setpoint correction element Δh₂ is equal to or less than the minimum distance Δh _min . Thereafter, time cycles for correcting the intermediate setpoint for the wet steam region according to the relationship (4) take effect. When the lower temperature limit is reached, an analog reversal process begins and the intermediate setpoint is determined according to relationships (4) and / or (5).

Before the start of the cycle, the setpoint correction element Δh₂ for the superheated steam area is set in general and the setpoint correction element Δx for the wet steam area only when starting from the upper temperature limit ϑ₁ zero and the setpoint correction element Δh₂ by adding the enthalpy corrector k₂ with the current value and the setpoint correction element Δx for each time cycle t _K when starting from the upper temperature limit ϑ₁ by addition and when starting from the lower temperature limit ϑ₂ by subtracting the steam content corrector k₁ to / from the current value newly formed.

The invention ensures controlled injections in normal operation with cascade control and in start-up controller operation too when entering the wet steam area. The possibility of Determining the actual value of the enthalpy after injection by The basis is the patented linking of the measurable sizes for the selection of this enthalpy value as an auxiliary or main control variable. This has the advantage of also and low-load operation a continuous, d. H. for the hot and wet steam area equally applicable correctable setpoint through enthalpy surcharges or steam content values to enable the cascade control to take effect again becomes.

The process is explained in more detail using the drawing. Fig. 1 shows an example of an exported from two parallel-connected steam superheater strands strand with an infinite number of superheater stages and injections, as well as the normally present Betriebsmeßeinrichtungen, as far as they are relevant to the explanation of the determination of the enthalpy of the steam-cooled. Fig. 2, the linkage is of the steam temperature after the superheater with the preset value for the steam enthalpy by injection as a control variable in Anfahrreglerbetrieb again.

The dry saturated steam flows from the separation vessel 0 , with which the evaporation end point is fixed, through the lines 1 ; 3 ; 5 ; 7 ; 8 ; 10 ; 12 to the steam line outlet 13 . In the superheaters 2 ; 6 ; 11 , the temperature of the steam increases. The respective superheater outlet temperature is with the injections 14 ; 15 in the mixing tubes 4 ; 9 regulated. To determine the enthalpy of the boiling water h 'and the enthalpy of the dry saturated steam h''( Fig. 2), the pressure measuring points p₀ or p _n are used, their selection being made according to the position of the injection in the steam train. The enthalpy of the steam after injection h₂ in lines 5 ; 10 is newly determined for each time cycle of the controller from the measuring points steam temperature before injection ϑ₁ or ϑ _n-1 , _{amount of} injection _water m _w1 or m _wn , steam _{mass flow} at the outlet of the steam train _n and the pressure in the superheater system p₀ or p _n .

The following relationship applies to injection 4 in superheater train 1

in which

h₅ the enthalpy after injection,
h₃ the enthalpy before the injection, determined from ϑ _1.1 and p₀,
h _{w is} the enthalpy of the injection water, determined from pressure and temperature measurements in the injection water line (not shown) or specified as a fixed value,
_n the steam mass flow at the outlet of a steam train,
_w1.1 the _{quantity of} injection _water and
f _{E1.1 mean} a factor
which is the quotient of the product of steam _{mass flow n} and the factor 2 (number of superheater branches per steam branch, at the outlet of which the steam _{mass flow} is measured) and the difference between steam _{mass flow n} and the sum of the injection _{water quantities} of the injections following the injection _{point under} consideration ( _w2.1 to _wn.1 ).

In the steam enthalpy diagram acc. Fig. 2 is the setpoint control through the curve h _nE (enthalpy after injection) in the superheated steam area above the saturation enthalpy h '' and within the wet steam area between the saturation enthalpy h '' and the enthalpy of the boiling water h 'over time t depending on the course the curve steam temperature after superheater ϑ _nÜH in relation to the determined or specified lower and upper temperature limit ϑ₁; ϑ₂ marked.

At the time t 1, the criteria for starting the injection are met, and the lower setpoint X _{su is} approached in a known manner, the time-dependent value of which is dependent on the sum of the pressure p₀ or p _n dependent saturation enthalpy h '' and a constant or selectable Enthalpy surcharge Δh₁ to take into account the at least necessary superheating of the cooled steam (time t₂). As soon as the temperature after the superheater reaches the upper temperature limit ϑ₁ (time t₃), the intermediate setpoint X _{z1 is formed} according to the relationship

X _z = h ′ ′ + Δh₁-Δh₂ (2)

With

Δh₂ = Δh₂ + k₂ (2a)

and the boundary condition

X _z h ′ ′ - Δh _min , (2b)

being set to zero for the formation of X _z1 Δh₂.

Here mean

X _{z is} the intermediate setpoint,
h ′ ′ the pressure-dependent enthalpy of the saturated steam,
Δh₁ an enthalpy surcharge,
Δh₂ a setpoint correction element,
k₂ an enthalpy corrector,
Δh _min the minimum enthalpy value to be set for control in the superheated steam area.

When X _z1 (time t₄) is reached, a delay is activated which keeps the setpoint constant for the time t _K , where t _K corresponds to the dead time and delay time of the controlled system. After t _K (time t₅) the superheater outlet temperature is checked again.

If their value is still greater than ϑ₁, the intermediate setpoint X _{z is} newly formed according to (2) and (2a). However, if condition (2b) is no longer met (time t₇), the setpoint is generated according to the relationship

X _z = h ′ + (x _x -Δx) (h ′ ′ - h ′) (4)

and the initial condition Δx = 0,
in which

X _{z is} the intermediate setpoint for the enthalpy control of the steam in the wet steam area,
h ′ the pressure-dependent enthalpy of the boiling water,
x _x a preset steam content value,
Δx a setpoint correction element,
k₁ a steam content corrector,
h '' mean the pressure-dependent enthalpy of the saturated steam.

The intermediate _setpoint X _z3 is also approached with the enthalpy change rate and reached at time t₈. After t _K (time t₉), the temperature is checked again after the superheater. If its value is still greater than ϑ₁, the setpoint X _{z is} newly formed according to relationship (3), the setpoint correction element Δx being determined according to the following relationship

Δx = Δx + k₁ (4a)

At the time t₁₂ the temperature falls below superheater the lower temperature limit ϑ₂.

The intermediate _setpoint X _z5 is further developed according to relationship (4), wherein

Δx = Δx-k₁ (4b)

is. This setpoint is reached at time t₁₃. After the dead time and delay time t _K , the temperature after the superheater is still lower than the lower temperature limit Δ₂, so that the intermediate setpoint X _{z is formed} as long as (4) and (4b) as the boundary condition

x _x + Δx 1

is satisfied.

The intermediate setpoint is then created

X _z = h ′ ′ + Δh _min + Δh₂ (3)

with the initial condition Δh₂ = 0 and for further time cycles with Δh₂ according to (2a).

Claims (3)

1. A method for steam temperature control on superheaters of steam systems by injecting water via injection control valves with a normal control mode, in which a cascade control is effective, and a start-up and low-load mode, in which a start-up controller is effective, characterized in that the enthalpy after injection during the Normal control operation as an auxiliary control variable and during start-up and low-load operation, if z. B. in a known manner, the slave controller is the sole starting controller, is used as a control variable, the setpoint being set depending on the process and corrected, and that the actual value of the enthalpy after injection from the measured values steam temperature before injection, quantity of injection water, steam mass flow at the outlet of the steam train and pressure the measuring point closest to the injection point according to the relationship is determined, in which the enthalpy after injection,
h₁ the enthalpy before injection,
h _w the enthalpy of the injection water (determined from measurements of the pressure and temperature of the injection water or fixed value specification),
the steam mass flow at the outlet of a steam train,
_E the amount of water injected and
f _E mean a factor that is the quotient of the product of the steam mass flow () and the number of superheater strands per steam train, at the outlet of which the steam mass flow () is measured, and the difference between the steam mass flow () and the sum of the injection water quantities following the injection point under consideration Injections is and which was determined for different load points and either as a constant or as an approximate function, for. B. linear, is specified for the load range. 2. The method according to claim 1, characterized in that for the start-up and low-load operation for the temperature after superheater an upper temperature limit (ϑ₁), a lower temperature limit (ϑ₂), a dead and delay time (t _K ) and for the setpoint management of Enthalpy after injection in the superheated steam area, an enthalpy surcharge (Δh₁) and a minimum distance (Δh _min ) to the saturation enthalpy (h ′ ′), a setpoint correction element (Δh₂) and an enthalpy corrector (k₂) and in the wet steam area a set steam content value (x _x ), a setpoint correction element (Δx ) and a steam content corrector (k₁) can be determined by means of experimental or theoretical process analysis, and that when the upper temperature limit (ϑ₁) or the lower temperature limit (ϑ₂) is reached, an intermediate setpoint (X _z ) is approached with a permissible rate of change (), the intermediate setpoint ( X _z ) in the superheated steam area when starting from the upper temperature raturgrenze (θ₁) according to the relationship X _z = h '' + Δh₁-Δh₂, (2) when starting from the lower limit temperature (θ₂) to the BeziehungX _z = h '' + .DELTA.h _min + Δh₂ (3) and in the wet steam region according to the relationship X _z = h ′ + (x _x + Δx) (h ′ ′ - h ′) (4) (where h ′ means the enthalpy of the boiling water). 3. The method according to claim 1 and 2, characterized in that the correction of the intermediate setpoint (X _z ) for the superheated steam area according to the relationship (2) for the first time when the upper temperature limit (ϑ₁) is reached and when this temperature limit is exceeded or reached again (ϑ₁ ) in time cycles that correspond to the dead time and delay time (t _K ), until the difference between the enthalpy surcharge (Δh₁) and the setpoint correction element (Δh₂) is equal to or less than the minimum distance (Δh _min ), and then time cycles (t _K ) to correct the intermediate setpoint (X _z ) for the wet steam region according to the relationship (4) and that an analog reversal cycle begins when the lower temperature limit (ϑ₂) is reached and the intermediate setpoint (X _z ) according to the relationships (3) or (4) is determined, the setpoint correction element (Δh₂) for the superheated steam area in general and the setpoint correction element (Δx ) for the wet steam area only when starting from the upper temperature limit (ϑ₁) set to zero and at every time cycle (t _K ) the setpoint correction element (Δh₂) by adding the enthalpy corrector (k₂) with the current value and the setpoint correction element (Δx) when starting from the upper temperature limit (ϑ₁) by addition and when starting from the lower temperature limit (ϑ₂) by subtracting the steam content corrector (k₁) to / from the current value.