JPS6238081Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention uses the density signal of the water in the reference leg of the boiler as an input signal related to the temperature when calculating the drum level, thereby making it possible to output an accurate drum level signal. It relates to a level calculation circuit.

Accurately measuring the boiler drum level (the difference between the standard water level and the actual water level in the drum) is extremely important for boiler operation. For example, if the drum level is not measured accurately, normal hot water may flow into pipes through which overpressure steam should pass, damaging the turbine. That is, the drum level is still considered to be within tolerance and the plant's alarm system is not activated.

FIG. 1 is a block diagram showing a conventional boiler drum level measurement system. In the same figure, 1
is a boiler. 2 is a pressure detector that detects the steam pressure within the boiler. 3 is a differential pressure detector that detects the differential pressure between the pressure within the boiler 1 and the reference pressure within the reference leg 4. Reference numerals 5 and 6 indicate function generators that generate functions having characteristics as shown in FIG. 7
is an adder that receives the outputs of the differential pressure detector 3 and the function generators 5 and 6. 8 is a divider that divides the output of the adder by the output of the function generator 6. The output of the divider becomes the drum level signal. The operation of the circuit configured as described above is summarized as follows.

In FIG. 1, the drum level is assumed to be l. The difference between the reference water level (hereinafter abbreviated as NWL) and the upper tap is Hu ₁ The difference from the lower tap is Hl r _s is the steam density in the drum, rω is the drum water density, and r _a is the water density in the standard leg. . Under the above definition, the differential pressure ΔP between the upper tap and the lower tap is expressed by the following equation.

ΔP= _ra (Hu+Hl)−{r _s (Hu−l)+rω(Hl+l)} =( _ra −r _s )・(Hu+Hl)−(rω−r _s )・Hl−(rω−r _s )l (1) When equation (1) is solved for the drum level l, l becomes as shown in the following equation.

l=(ra _{- rs} )(Hu+Hl)-(rω- _rs )·Hl-ΔP/(rω- _rs ) (2) By the way, the densities _ra , _rs , and rω are functions of pressure. Therefore, the function generator 5 generates a signal related to (r _a −r _s ), and the function generator 6 generates a signal related to (rω−r
_s ). The function generators 5 and 6 can use, for example, a broken line function as shown in FIG. That is, the function generators 5 and 6 can be realized by polygonal function generators. The output of the function generator 5 is multiplied by (Hu+Hl) and input to the adder 7. On the other hand, the output of the function generator 6 is multiplied by -Hl and input to the adder 7. Furthermore, the output Δ of the differential pressure detector 3
P is multiplied by -1 and input to the adder 7. From the above, the output of the adder 7 corresponds to ( _ra - _rs )(Hu+Hl)-(rω- _rs )·Hl-ΔP. This output is divided by a signal related to (rω- _rs ) by a subsequent divider 8. This divided output becomes the drum level signal shown by equation (2).

In the process of calculating the drum level signal described above, the reference leg water density r _a is kept constant. _rs ,
Since rω is stable as a function of pressure due to the saturated density, there is no particular problem. However, r _a
is influenced by the outside air temperature or the furnace temperature, so it is actually a quantity that varies with temperature. Therefore, r _a
The drum level signal shown by equation (2), which is calculated assuming a constant value, includes error factors due to temperature changes. Therefore, accurate measurements cannot be made.

The present invention was developed in view of these points, and it is possible to output an accurate drum level signal by correcting the density signal in the reference leg of the boiler with a temperature-related signal when calculating the drum level. This is a drum level calculation circuit that can perform the following functions. Hereinafter, the present invention will be described in detail with reference to the drawings.

Figure 2 shows the boiler internal pressure P and the reference leg water density r.
It is a figure showing the relationship with _a . As mentioned above, since density is a function of pressure, a characteristic curve as shown in FIG. 2 is obtained. Normally, the temperature change of the water in the reference leg is about 100°C±50°C, and during this period, the curve of r _a with respect to the pressure change can be considered to be almost a parallel shift, as shown in the figure. In the same figure, f ₁ is at 100℃, f ₂ is at 120℃, f ₃
are the respective characteristic curves at 80°C. In this way, each curve is in a relationship of parallel movement, so one of these curves is used as the reference curve, and r _a at any temperature is the deviation _e from the reference curve. It can be calculated by adding e. That is, r in equation (2)
A more accurate drum level signal can be obtained by adding a correction e due to the temperature change of r _a to _a . Letting the drum level at this time be ^l1 , ^l1 is expressed by the following equation.

l ¹ = (r _a - r _s + ω) (Hu + Hl) - r ω - r _s ) Hl - ΔP/(r ω - r _s ) (3) Fig. 3 is a configuration block diagram showing an embodiment of the present invention. It is. Components that are the same as those in FIG. 1 are designated with the same numbers. In the figure, 9 is a temperature detector that detects the internal water temperature at the upper part of the reference leg 4, and 10 is a temperature detector that detects the internal water temperature at the lower part. 11
is an averaging circuit that receives and averages the outputs of both temperature detectors. Now the output of temperature sensor 9
T ₁ and the output of the temperature detector 10 are assumed to be T ₂ . The averaging circuit 11 calculates (T ₁ +T ₂ )/2. After that, a deviation signal e related to (T ₁ +T ₂ )/2 is output. As explained in FIG. 2, if the averaging circuit 11 is adjusted so that the deviation signal e becomes zero when it is on the reference curve, all that is left is to adjust the deviation signal according to the temperature of the water in the reference leg. e is output.

12 is the output e of the averaging circuit and the function generator 5
This is a second adder that receives the output of . The output of the adder will be related to e+(r _{a -rs} ). This output is multiplied by (Hu+Hl) and input to the first adder 7. The adder 7 inputs the input, the differential pressure detector 3
and the output of the function generator 6 and add them. From the above, the output of the adder 7 corresponds to (ra _{- r s} +e) (Hu + Hl) - (rω-r _s )·Hl-ΔP. This output is divided by a subsequent divider 8. The output l ¹ of the divider 8 is as shown in equation (3). According to the above operation, the temperature-based correction signal e is added to the normal density signal r _a .
Therefore, it is possible to always calculate the correct drum level regardless of the temperature.

As described above, according to the present invention, since the arithmetic circuit has a correction signal based on the reference leg temperature, even if a change occurs in the reference leg temperature, it does not immediately result in a measurement error as in the conventional case. In a typical drum boiler, the span of the drum level is 500 mm, and if the reference leg temperature deviates by 50°C from the design value, the error in the level signal will be about 30 to 50 mm. Particularly, such a deviation occurs during maximum load operation when the temperature of the furnace is high or when the furnace is stopped when the fire is out, and is often a problem even now, so eliminating this problem would be of great benefit.

In addition, in FIG. 3, the temperature of the upper and lower parts of the reference leg 4 was determined, and the result was added and divided by 2 to determine the temperature of the reference leg. However, the method for measuring the temperature of the reference leg is not limited to this, and other methods may be used. In addition, when the average density of water in the drum is obtained as a reference, the function generator 5 simply calculates the vapor density r in the drum.
Enter only the function of _s , and adder 12 calculates (ra − _{r s} )
Alternatively, the calculation may be performed.

As explained above in detail, according to the present invention, an accurate drum level signal can be output by correcting the density signal in the reference leg of the boiler with a temperature-related signal when calculating the drum level. A drum level calculation circuit can be realized. Thereby, a more stable boiler automatic control device can be constructed.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional boiler drum level measurement system. Figure 2 shows the pressure P
It is a figure which shows the relationship between and the standard leg water density r _a .
FIG. 3 is a block diagram showing an embodiment of the present invention. 1...Boiler, 2...Pressure detector, 3...Differential pressure detector, 4...Reference leg, 5, 6...Function generator,
7, 12... Adder, 8... Divider, 9, 10...
...Temperature detector, 11...Averaging circuit.

Claims (1)

[Scope of utility model registration request] Input the steam pressure signal in the boiler and calculate the density difference signal between the boiler's standard leg water density ra and the drum steam density rs.
A first function generator that outputs ra-rs, and a second function generator that receives a steam pressure signal in the boiler and outputs a density difference signal rω-rs between drum water density rω and drum internal steam density rs. and a second adder for adding the signal e related to the average temperature of the water in the reference leg and the density difference signal ra-rs from said first function generator, and the pressure related to the drum level in the boiler. and the reference pressure in the reference leg, a signal related to the differential pressure ΔP between
a first adder that adds the signal from the adder and the signal from the second function generator; and the output signal of the first adder is divided by the signal from the second function generator. A drum level calculation circuit consisting of a divider and a divider.