JP2017170390A - Chemical feed control method and chemical feed control apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a chemical feed control method and chemical feed control apparatus for realizing a suitable chemical feed amount in accordance with water quality change.SOLUTION: A chemical feed control method comprises: detecting water quality in feed water by a water quality sensor; determining a chemical feed concentration based on the detection result; and performing chemical feed to the feed water by a chemical feeder. In the method, sensor values from the water quality sensor are sampled at intervals of a first sampling period; a predicted sensor value is calculated by predicting a next sensor value from a present sensor value and a previous sensor value; if the predicted sensor value is less than a first prescribed value, the chemical feed concentration is set to a first chemical feed concentration; if the predicted sensor value is equal to or greater than a second prescribed value, the chemical feed concentration is set to a second chemical feed concentration; if the predicted sensor value is equal to or greater than the first prescribed value and less than the second prescribed value, the chemical feed concentration is set in accordance with the predicted sensor value; and then the chemical feed is performed.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a chemical injection control method and chemical injection control device for controlling the chemical injection amount of a chemical in an aqueous system such as boiler feed water.

  In the boiler water supply system, raw water is treated with a softener to remove hardness components, and then water is supplied to the boiler. The hardness component concentration in the water supply from the water softener is measured by a hardness sensor, and a chemical such as a scale inhibitor is added to the water supply according to the measurement result (Patent Document 1).

  When the measurement conditions such as the fluid flow velocity and the water temperature of the measurement object are large, the water quality measurement result by the sensor is not stable, so the chemical injection amount is determined based on the average value of the water quality in a certain period. However, in such a method for determining the amount of chemical injection, there is a time lag before the chemical concentration is reflected in the water quality, and the chemical concentration must be maintained when the chemical concentration must be kept above a certain level. The shortage period was likely to occur. In order to prevent a shortage period of chemical concentration, it is conceivable to add an excessive amount of medicine in consideration of the fluctuation range of water quality, but there is a possibility that the medicine gels in water.

JP 2003-329208 A

  An object of this invention is to provide the chemical injection control method and chemical injection control apparatus which can be set as the appropriate chemical injection amount according to a water quality change.

  The summary of the chemical injection control method of the present invention is as follows.

[1] In a chemical injection control method in which a water quality sensor detects water quality in water supply, determines a chemical injection concentration based on the detection result, and uses the chemical injection device to inject water into the water supply, Sampling is performed at one sampling period, the next sensor value is predicted from the current sensor value and the previous sensor value, and the predicted sensor value is calculated. If the predicted sensor value is less than the first predetermined value, the drug injection concentration is set to the first medicine. When the predicted sensor value is equal to or higher than the second predetermined value, the chemical concentration is set as the second chemical concentration, and when the predicted sensor value is equal to or higher than the first predetermined value and lower than the second predetermined value, A chemical injection control method, wherein chemical injection is performed as a chemical injection concentration corresponding to a predicted sensor value.

[2] In [1], when the current sensor value is greater than the previous sensor value, the difference between the current sensor value and the previous sensor value is added to the current sensor value to calculate a predicted sensor value. Control method.

[3] A chemical injection control method according to [2], wherein when the current sensor value is smaller than the previous sensor value, the predicted sensor value is made the same as the current sensor value.

[4] In [1], when the predicted sensor value is greater than or equal to the first predetermined value and less than the second predetermined value, a value obtained by multiplying a chemical injection concentration corresponding to the predicted sensor value by a predetermined magnification or a predetermined amount is added. A chemical injection control method characterized by performing chemical injection based on a value.

[5] In [1], when a first difference between the current sensor value and the previous sensor value and a second difference between the previous sensor value and the previous sensor value are obtained, and the first difference is equal to or greater than the second difference, A value obtained by adding the difference between the first difference and the second difference to the first difference is set as a prediction difference. When the first difference is smaller than the second difference, the first difference is set as a prediction difference. A chemical injection control method, wherein a predicted sensor value is calculated by adding the predicted difference to a sensor value.

[6] In [2], the sensor value by the water quality sensor is sampled at a second sampling period shorter than the first sampling period, and the current sensor value at the first sampling period is smaller than the previous sensor value, and the current time If the sensor value in the second sampling period has increased immediately before the sensor value, the increase is added to the current sensor value to calculate a predicted sensor value, and the current sensor value in the first sampling period is calculated. When the sensor value in the second sampling period is smaller than the previous sensor value and immediately before the current sensor value, the predicted sensor value is made the same as the current sensor value. Method.

[7] In [1], the sensor value from the water quality sensor is sampled at a second sampling period shorter than the first sampling period, the previous sensor value is sampled at the first sampling period, and the current sensor value is sampled. A chemical injection control method characterized in that the maximum sensor value in the second sampling period is set as a predicted sensor value.

[8] A chemical injection control method according to any one of [1] to [7], wherein a chemical pH adjusting agent for steam condensate treatment, a pH adjusting agent for boilers, or an oxygen scavenger is injected.

[9] A chemical injection control device that acquires the water quality in the water supply detected by the water quality sensor, determines the chemical injection concentration based on the detection result, and controls the chemical injection device that injects the chemical into the water supply. The sensor value by the sensor is sampled at the first sampling period, the next sensor value is predicted from the current sensor value and the previous sensor value, the predicted sensor value is calculated, and if the predicted sensor value is less than the first predetermined value, The injection concentration is the first chemical injection concentration, and when the predicted sensor value is greater than or equal to a second predetermined value, the chemical injection concentration is the second chemical injection concentration, the predicted sensor value is greater than or equal to the first predetermined value and the second predetermined concentration. When it is less than the value, the chemical injection control device controls the chemical injection device as a chemical injection concentration corresponding to the predicted sensor value.

  According to the present invention,

It is a block diagram of the chemical injection control system which concerns on this Embodiment. It is a graph which shows an example of the calibration curve which shows the relationship between a sensor value and a required chemical injection density | concentration. It is a graph which shows the chemical injection density | concentration change of Example 1. FIG. It is a graph which shows the chemical injection density | concentration change of Example 2. FIG. It is a graph which shows the chemical injection density | concentration change of Example 3. It is a graph which shows the chemical injection density | concentration change of Example 4. It is a graph which shows the chemical injection density | concentration change of Example 5. It is a graph which shows the chemical injection density | concentration change of Example 6. It is a graph which shows the chemical injection density | concentration change of Example 7. It is a graph which shows the chemical injection density | concentration change of Example 8.

  Hereinafter, embodiments will be described with reference to the drawings.

  FIG. 1 shows a water supply system in which water supplied from the water treatment device 1 is introduced into a water supply tank 3 via a pipe 2 and supplied to the use point 4 from the water supply tank 3. The water quality of the water supplied from the water treatment device 1 is detected by the water quality sensor 5, the flow rate of the water supply is detected by the flow meter 8, the detected value is input to the control device 6, and the chemical injection pump 7 is operated according to the detected value. To control the amount of chemicals injected into the water supply tank 3. In addition, the chemical injection location may be downstream from the water supply sampling point of the water quality sensor 5, and may be other than the water supply tank 3.

  The water quality sensor 5 is not limited to using an electrical conductivity meter, pH meter, dissolved oxygen meter, ORP (oxidation-reduction potential) meter, hardness meter, thermometer, or the like. There are no particular restrictions on the type of chemicals to be poured, but there are some types of chemicals where it is desirable to always maintain the chemical concentration in the feed water above the required level, such as steam condensate treatment pH adjusters, boiler pH adjusters, and oxygen scavengers. Is preferred.

For example, when a chemical injection pump 7 having a maximum flow rate of 10 m ³ / h and a water injection system of 3000 g / h and a stroke ratio of 1: 100 is used, the minimum chemical concentration that can be set is 3 g / m ³ and the maximum chemical concentration is 300 g / h. the m ^3.

  The measurement frequency of the water quality by the water quality sensor 5 is not particularly limited, and may be once every 30 seconds to 1 day, and preferably about once every 1 to 30 minutes.

  For example, when the water supply system shown in FIG. 1 is a boiler water supply system, a softener (water softener) is used for the water treatment apparatus 1, and the use point 4 is a boiler (such as a small reflux boiler). The feed water flow rate from the softener 1 and the quality of the water in the feed water are detected at a frequency of once every 5 minutes by the flow sensor 8 and the water quality sensor 5 provided in the pipe 2, and the discharge amount of the chemical injection pump 7 is based on the detection result. By controlling, the amount of chemical solution added is controlled. For example, when the water quality sensor 5 is a pH meter, the chemical injection pump 7 discharges a solution of a boiler pH adjuster, and when the water quality sensor 5 is a hardness sensor, the chemical injection pump 7 discharges a solution of a scale inhibitor.

  The control device 6 predicts a sensor value to be measured next time from a plurality of latest sensor values by the water quality sensor 5, obtains a chemical injection concentration from the predicted sensor value, and supplies the detected chemical injection concentration and the water supply detected by the flow meter 8. The chemical injection amount is calculated from the flow rate, and the chemical injection amount of the chemical injection pump 7 is controlled.

For example, the control device 6 substitutes the predicted sensor value into a calibration curve as shown in FIG. In the example shown in FIG. 2, when the sensor value S is S _min or less, the chemical injection concentration C is C _min, and when the sensor value S is S _max or more, the chemical injection concentration C is C _max . When the sensor value S is included in the range of S _{min to} S _max , the minimum chemical injection concentration C _min and the maximum chemical injection concentration C _max are directly proportional.

  Hereinafter, a plurality of chemical injection control methods (basic control, first to fourth application control) including calculation of a predicted sensor value and chemical injection concentration will be described.

[Basic control]
Using a sensor value S _n of the measurement of current by the water quality sensor 5 (measurement time T _n), the sensor value S _n-1 at the previous measurement (measurement time T _n-1), the next measurement (measurement time The predicted sensor value ES _{n + 1} at T _{n + 1} ) is calculated from the following equation.
When S _n −S _n−1 ≧ 0: Predicted sensor value ES _{n + 1} = S _n + (S _n −S _n−1 )
When S _n −S _n−1 <0: Predicted sensor value ES _{n + 1} = S _n

That is, if the time T sensor values from _n-1 toward T _n is unchanged or has decreased, the sensor value at time T _{n + 1} is expected to remain unchanged from the sensor value at time T _n. On the other hand, if the sensor value increases, during the period from the time T _n to T _{n + 1,} the sensor value is expected to increase at the same increase rate as between time T _n-1 to T _n.

When the predicted sensor value ES _{n + 1} at the next measurement (measurement time T _{n + 1} ) is calculated, the current chemical injection concentration C _n is calculated from the following equation.
In the case of ES _{n + 1} <S _min : Chemical injection concentration C _n = C _min
When S _min ≦ ES _{n + 1} ≦ S _max : Chemical injection concentration C _n = C _min + (ES _{n + 1} −S _min ) × (C _max −C _min ) / (S _max −S _min )
When ES _{n + 1} > S _max : Chemical injection concentration C _n = C _max

That is, the predicted sensor value is of less than _{S min,} the dosing concentration _{C n} minimize chemical feed concentration _{C min.} When the predicted sensor value is included in the range of S _{min to} S _max , the minimum chemical injection concentration C _min and the maximum chemical injection concentration C _max are directly proportional. If the predicted sensor value is greater than _{S max,} the dosing concentration _{C n} and the maximum chemical feed concentration _{C max.}

In this basic control, when the sensor value tends to increase, the predicted sensor value ES _{n + 1} is obtained by predicting that the sensor value will increase at the same increase rate. On the other hand, when the sensor value tends to decrease, the predicted sensor value ES _{n + 1} is obtained by predicting that the sensor value will not change. Therefore, the predicted sensor value ES _{n + 1} is likely to be slightly larger than the sensor value S _{n + 1} that is actually measured next time (at the measurement time T _{n + 1} ), and the chemical concentration in the water supply is easily maintained above the required amount.

[First application control]
The method for obtaining the predicted sensor value ES _{n + 1} is the same as in the basic control. The chemical injection concentration C _n is calculated from the following formula.
In the case of ES _{n + 1} <S _min : Chemical injection concentration C _n = C _min
When S _min ≦ ES _{n + 1} ≦ S _max and S _n > S _n−1 : Chemical injection concentration C _n = C _min + (ES _{n + 1} −S _min ) × (C _max −C _min ) / (S _max −S _min ) + X1, X1 are positive real numbers S _min ≦ ES _{n + 1} ≦ S _max and S _n ≦ S _n−1 : Chemical injection concentration C _n = C _min + (ES _{n + 1} −S _min ) × (C _max −C _min ) / (S _max −S _min )
When ES _{n + 1} > S _max : Chemical injection concentration C _n = C _max

In the first applied control, when the predicted sensor value is included in the range of S _{min to} S _max and the sensor value tends to increase, it is obtained in direct proportion between the minimum chemical injection concentration C _min to the maximum chemical injection concentration C _max. by adding a constant X1 to the value, a predetermined amount raised the dosing concentration C _n than the basic control. Thereby, it becomes easy to keep the chemical concentration in the water supply more than necessary.

In the case of S _min ≦ ES _{n + 1} ≦ S _max and S _n > S _n−1 , the chemical injection concentration C _n may be increased by a certain factor as in the following equation.
Dosing concentration _{C n = C min + (ES} n + 1 -S min) × (C max -C min) × X2 / (S max -S min), X2 is a real number larger than 1

[Second application control]
In a second application control, sensor value _{S n} in the current measurement (measurement time _{T n),} in addition to the sensor value _{S n-1} at the previous measurement (measurement time _{T n-1),} measurement of the time before last (Measurement The predicted sensor value ES _{n + 1} is calculated using the sensor value Sn _-2 at time _Tn-2 ).

First, the sensor value _{S n-2} and the sensor value _{S n-1} difference between _{ΔS (n-2_n-1)} , calculates a difference ΔS _{(n-1_n)} between the sensor value _{S n-1} and the sensor value _{S n} .
ΔS _{(n−2_n−1)} = S _n−1 −S _n−2
ΔS _{(n−1_n)} = S _n −S _n−1

Difference _{ΔS (n-2_n-1)} , using ΔS _{(n-1_n),} calculates predicted sensor values _{ES n + 1} and the prediction difference [Delta] S between the sensor values _{S n} to _{(n_n + 1)} from the following equation.
When ΔS _{(n−1_n)} −ΔS _{(n−2_n−1)} ≧ 0: Prediction difference ΔS _{(n_n + 1)} = ΔS _{(n−1_n)} + (ΔS _{(n−1_n)} −ΔS _{(n−2_n−1)} )
If ΔS _{(n−1_n)} −ΔS _{(n−2_n−1)} <0: Prediction difference ΔS _{(n_n + 1)} = ΔS _{(n−1_n)}

Using the prediction difference ΔS _{(n_n + 1)} , the prediction sensor value ES _{n + 1} is obtained from the following equation. In addition, the method of calculating the chemical injection concentration C _n from the predicted sensor value ES _{n + 1} is the same as the basic control.
When the prediction difference ΔS _{(n_n + 1)} ≧ 0: Predicted sensor value ES _{n + 1} = S _n + prediction difference ΔS _{(n_n + 1)}
Prediction difference ΔS _{(n_n + 1)} <0: Predicted sensor value ES _{n + 1} = S _n

In the second applied control, when the difference in sensor value tends to increase, the difference in sensor value from time T _n to T _{n + 1} is also predicted to increase in the same manner. On the other hand, if the difference between the sensor value tends to decrease, the difference of the sensor values from the time T _n toward T _{n + 1} is predicted from the time T _n-1 to be the same as the difference between the sensor value toward T _n. Therefore, the predicted sensor value ES _{n + 1} is easily calculated to be slightly larger than the sensor value S _{n + 1} that is actually measured next time (at the measurement time T _{n + 1} ), and the chemical concentration in the water supply is kept above the required amount. Can do.

[Third application control]
The sensor value S _{n — m} is _obtained by sampling the sensor value a plurality of times from time T _n−1 to time T _n . Sensor values from the water sensor 5 S _n, than the first sampling period for sampling the S _n-1, etc., towards the second sampling period for sampling the sensor value S _{n_m} is short. Sensor values S _n even if was reduced from the sensor value S _n-1, the time T _n-1 from the time T sensor values sampled at multiple timings closest to time T _n among sampled between _n S _{When n_m (last)} has increased from the sensor value _{Sn_m (last) -1} , the predicted sensor value ES _{n + 1} is calculated based on the slope (increase).

When the sensor value S _n has increased from the sensor value S _n−1 , the predicted sensor value ES _{n + 1} is the same as in the basic control regardless of the magnitude relationship between S _{n_m (last)} and S _{n_m (last) −1.} Is calculated. Specifically, the predicted sensor value ES _{n + 1} is calculated from the following equation. The method for calculating the chemical injection concentration C _n from the predicted sensor value ES _{n + 1} is the same as the basic control.
When S _n −S _n−1 ≧ 0: Predicted sensor value ES _{n + 1} = S _n + (S _n −S _n−1 )
When S _n −S _n−1 <0 and _{Sn_m (last)} −S _{n_m (last) −1} ≧ 0: Predicted sensor value ES _{n + 1} = S _n + (S _{n_m (last)} −S _{n_m (last) − 1} )
When S _n −S _n−1 <0 and S _{n_m (last)} −S _{n_m (last) −1} <0: Predicted sensor value ES _{n + 1} = S _n

In the third applied control, even _if the sensor value is sampled finely with a short sampling period between the time T _n-1 and the time T _n and the sensor value _Sn is decreased from the sensor value _Sn-1 , If the sensor value has increased at a timing close to T _n , the sensor value is considered to be increasing and the predicted sensor value ES _{n + 1} is obtained. Therefore, the frequency of calculating the predicted sensor value ES _{n + 1} that is larger than that in the basic control is increased, and the chemical concentration in the water supply can be easily maintained above the necessary amount.

In the third applied control, when an expected fluctuation range of the sensor value is defined from the average value of the sensor values within a certain period, the sensor value _{Sn_m (last)} is not within this fluctuation range. Data processing that is not adopted as the sensor value may be performed.

[Fourth application control]
Similarly to the third applied control, the sensor value is sampled a plurality of times from time T _n−1 to time T _n to obtain the sensor value S _{n_m,} and the maximum value S _{n_m (max)} among the plurality of sampling values is obtained. The predicted sensor value is ES _{n + 1} . The method for calculating the chemical injection concentration C _n from the predicted sensor value ES _{n + 1} is the same as the basic control.
Predicted sensor value ES _{n + 1} = S _{n_m (max)}

  In the fourth applied control, even if data processing is performed in which an assumed sensor value fluctuation range is defined from an average value of sensor values within a certain period, and sensor values not within the fluctuation range are not adopted. Good.

[Example 1]
Table 1 shows an example of control by basic control.

  In Table 1, the hardness component concentration (sensor value S) in the water supply is measured by the sensor every 5 minutes for 70 minutes from time 1: 00 to 2:10, and the next sensor value (predicted sensor value) according to the result. ) Is calculated to determine the chemical concentration of the scale inhibitor. The hardness component concentration in the feed water was almost constant after the increase.

  The sensor value S in Table 1 is the measured value of the sensor at the time, and the unit (%) is a value calculated from (sensor measured value / sensor measurement range upper limit value) × 100. For example, when a conductivity meter measuring 0 to 2000 mS / m indicates 200 mS / m, it is expressed as (200/2000) × 100 = 10%. It is obvious that these sensor values and units vary depending on the type of detector employed. The predicted sensor value is calculated using the sensor value at the time and the past sensor value. For example, the predicted sensor value at time 1:05 is the sensor value at time 1:00 and the time 1:05. Is used to predict the sensor value at time 1:10.

The required chemical injection concentration (mg / L) in Table 1 is the chemical injection concentration obtained from the sensor value S actually measured next time with reference to the calibration curve of FIG. For example, the required chemical injection concentration at time 1:05 is obtained by applying the sensor value S at time 1:10 to the calibration curve in FIG. The S _min of the calibration curve in FIG. 2 was 5%, the minimum chemical injection concentration C _min was 10, S _max was 95%, and the maximum chemical injection concentration C _max was 300.

The chemical injection concentration C _n (mg / L) in Table 1 is the chemical injection concentration obtained from the predicted sensor value at that time with reference to the calibration curve of FIG. For example, the chemical injection concentration C _n at time 1:05 is obtained by applying the predicted sensor value at time 1:05 to the calibration curve in FIG.

  The difference in Table 1 is the difference between the sensor value at the time and the sensor value of the previous measurement. For example, the difference at time 1:05 is the difference between the sensor value at time 1:05 and the sensor value at time 1:00.

Figure 3 is a graph showing the transition of necessary chemical feed concentration in Table 1 and chemical feeding, chemical dosing concentration C _n.

[Example 2]
The chemical injection control was performed in the same manner as in Example 1 except that the hardness component concentration in the feed water to be controlled by the basic control decreased after increasing and became substantially constant thereafter. The control result shown in Table 2, shows a transition of necessary chemical feed concentration and chemical feeding, chemical dosing concentration C _n in FIG.

[Example 3]
The chemical injection control was performed in the same manner as in Example 1 except that the hardness component concentration in the feed water to be controlled by the basic control became substantially constant after the decrease. The control result shown in Table 3, Figure 5 shows the transition of necessary chemical feed concentration and chemical feeding, chemical dosing concentration C _n.

[Example 4]
The chemical injection control was performed in the same manner as in Example 1 except that the hardness component concentration in the feed water to be controlled by the basic control increased after decreasing and became substantially constant thereafter. The control result shown in Table 4 shows the transition of necessary chemical feed concentration and chemical feeding, chemical dosing concentration C _n in FIG. 6.

[Example 5]
The chemical injection control by the second applied control was performed on the water supply that became almost constant after the hardness component concentration increased. The control result is shown in Table 5 shows the transition of necessary chemical feed concentration and chemical feeding, chemical dosing concentration C _n in FIG. The difference ΔS _{(n−1_n) in} Table 5 is the difference between the sensor value at the time and the sensor value of the previous measurement. The prediction difference ΔS _{(n_n + 1)} is the difference between the sensor value at the time and the prediction sensor value, and the prediction sensor value is obtained by adding the sensor value at the time to the prediction difference ΔS _{(n_n + 1)} .

[Example 6]
The chemical injection control was performed in the same manner as in Example 5 except that the hardness component concentration in the water supply controlled by the second application control decreased after increasing and then became substantially constant. The control result shown in Table 6, Figure 8 shows the evolution of required chemical feed concentration and chemical feeding, chemical dosing concentration C _n.

[Example 7]
The chemical injection control was performed in the same manner as in Example 5 except that the hardness component concentration in the feed water targeted for chemical injection control by the second applied control became substantially constant after the decrease. The control result shown in Table 7, shows the transition of necessary chemical feed concentration and chemical feeding, chemical dosing concentration C _n in FIG.

[Example 8]
The chemical injection control was performed in the same manner as in Example 5 except that the hardness component concentration in the feed water to be controlled by the second applied control increased after decreasing and became substantially constant thereafter. The control result is shown in Table 8, Figure 10 shows the transition of necessary chemical feed concentration and chemical feeding, chemical dosing concentration C _n.

From the results of the chemical injection control in Examples 1 to 8, the chemical injection concentration C _n based on the predicted sensor value changes in the same manner as the required chemical injection concentration and is less likely to fall below the required amount (necessary chemical injection concentration). confirmed.

  The above embodiment is an example of the present invention, and the present invention may be other than the above.

In the above embodiment, the example in which the sensor value S and the chemical concentration C are directly proportional has been described. However, it may be inversely proportional. For example, when the sensor value S is equal to or less than S _min , the chemical injection concentration C is set to the maximum chemical injection concentration C _max, and when the sensor value S is equal to or higher than S _max , the chemical injection concentration C is set to the minimum chemical injection concentration C _min . When the sensor value S is included in the range of S _{min to} S _max , the maximum chemical injection concentration C _max and the minimum chemical injection concentration C _min are inversely proportional. Further, in the basic control and the first to fourth applied controls, the direction of the inequality sign is reversed.

1 Water Treatment Equipment 3 Tank 4 Use Point 5 Water Quality Sensor 7 Chemical Injection Pump 8 Flowmeter

Claims (9)

In the chemical injection control method of detecting the water quality in the water supply with the water quality sensor, determining the chemical injection concentration based on the detection result, and injecting the water into the water supply by the chemical injection device,
Sampling the sensor value by the water quality sensor at the first sampling period,
Calculate the predicted sensor value by predicting the next sensor value from the current sensor value and the previous sensor value,
When the predicted sensor value is less than the first predetermined value, the chemical injection concentration is the first chemical injection concentration, and when the predicted sensor value is greater than or equal to the second predetermined value, the chemical injection concentration is the second chemical injection concentration. A chemical injection control method, wherein when a sensor value is not less than the first predetermined value and less than the second predetermined value, chemical injection is performed as a chemical injection concentration corresponding to the predicted sensor value.   2. The chemical injection control method according to claim 1, wherein when the current sensor value is larger than the previous sensor value, a predicted sensor value is calculated by adding a difference between the current sensor value and the previous sensor value to the current sensor value.   3. The chemical injection control method according to claim 2, wherein when the current sensor value is smaller than the previous sensor value, the predicted sensor value is made the same as the current sensor value.   In Claim 1, when the predicted sensor value is not less than the first predetermined value and less than the second predetermined value, it is based on a value obtained by multiplying a chemical concentration according to the predicted sensor value by a predetermined magnification or a value added with a predetermined amount. A chemical injection control method characterized in that the chemical injection is performed. In claim 1, the first difference between the current sensor value and the previous sensor value and the second difference between the previous sensor value and the previous sensor value are obtained,
When the first difference is equal to or greater than the second difference, a value obtained by adding the difference between the first difference and the second difference to the first difference is set as a prediction difference.
When the first difference is smaller than the second difference, the first difference is set as a prediction difference,
A chemical injection control method characterized in that the predicted sensor value is calculated by adding the predicted difference to the sensor value this time. In Claim 2, the sensor value by the water quality sensor is sampled at a second sampling period shorter than the first sampling period,
If the current sensor value in the first sampling period is smaller than the previous sensor value and the sensor value in the second sampling period has increased immediately before the current sensor value, the increment is added to the current sensor value. To calculate the predicted sensor value,
When the current sensor value in the first sampling period is smaller than the previous sensor value and the sensor value in the second sampling period is decreasing immediately before the current sensor value, the predicted sensor value is set as the current sensor value. A chemical injection control method characterized by making the same. In Claim 1, the sensor value by the water quality sensor is sampled at a second sampling period shorter than the first sampling period,
A chemical injection control method characterized in that a maximum value of sensor values in the second sampling period during the sampling of the current sensor value after the previous sensor value is sampled in the first sampling period is set as a predicted sensor value. .   8. The chemical injection control method according to any one of claims 1 to 7, wherein a chemical pH adjusting agent for steam condensate treatment, a pH adjusting agent for boiler, or an oxygen scavenger is injected. A chemical injection control device that acquires the water quality in the water supply detected by the water quality sensor, determines the chemical injection concentration based on the detection result, and controls the chemical injection device that injects the chemical into the water supply,
Sampling the sensor value by the water quality sensor at the first sampling period,
Calculate the predicted sensor value by predicting the next sensor value from the current sensor value and the previous sensor value,
When the predicted sensor value is less than the first predetermined value, the chemical injection concentration is the first chemical injection concentration, and when the predicted sensor value is greater than or equal to the second predetermined value, the chemical injection concentration is the second chemical injection concentration. When the sensor value is greater than or equal to the first predetermined value and less than the second predetermined value, the medicinal injection device is controlled as the medicinal concentration according to the predicted sensor value.

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