introduction
In chemical processes, key process parameters are crucial for achieving satisfactory product quality. The effectiveness of controlling these parameters directly determines various product indicators. Stability reports, which statistically analyze key process indicators, are beneficial for technical personnel in analysis and management. They provide process engineers with firsthand data to improve their operations and skills.
Introduction to the stability rate
The calculation of the stationarity rate involves the total number of points, statistical frequency, total number of statistical points, number of points exceeding the tolerance, and number of points removed. The calculation method for the stationarity rate and the meaning of each parameter in the formula are explained below:
Total points = (number of control points for stability rate × statistical time period) / statistical frequency (1)
Total statistical points = Total points - Total points removed; (2)
Total number of out-of-tolerance points = Total number of points exceeding the stability rate control point process index - Total number of rejected points (3)
Process operation stability rate = [(total statistical points - total number of deviation points) / total statistical points] × 100% (4)
The stability rate index is statistically analyzed once every 10 minutes. Due to fluctuations in the supply of raw materials outside the production area or during plant start-up and shutdown, some control points do not require stability rate statistics temporarily; these points are excluded from the stability rate calculation. The number of out-of-tolerance points is the number of points whose measured values are outside the required upper and lower limits. The stability rate of the control point can be easily calculated using the above formula. Then, according to the requirements of the process workshop, the data is archived by shift, day, and week.
Program Flowchart
To achieve the goal of collecting key process parameters every 10 minutes and determining whether these indicators are within the required range, the program design concept is shown in Figure 1:
Figure 1. Flowchart of the stability rate calculation procedure
Configuration implementation in CS3000
The process of calculating the stability rate generally includes sampling, determining whether the data meets the requirements, data recording and storage, flowchart design, report design, and printing. These processes will be described in detail below.
3.1 Sampling and Judgment Process
The above flowchart is implemented using the SFC function block _SFCSW (as shown in Figure 2) in the CS3000 system, and the timer block TM (as shown in Figure 2) is used as the sampling period. The following points should be noted in the program:
When running the program for the first time, the tag number, lower limit index, upper limit index, number of out-of-pocket points, number of eliminated points, and number of statistical points should be initialized.
When running the program for the first time, the settings of 1402SF001-TM should be initialized to 600S (i.e., the sampling period).
At the shift handover time, the number of points exceeding the acceptable limit for this shift is removed, and the total number of points is reset to zero.
An exclusion function has been added to the program, which can exclude a point from the stability rate statistics in the event of parking or external anomalies, and issue operator guidance information to the operator.
Figure 2 Sampling program configuration
The required control range for the same control point may differ depending on the product grade change and production load variations. To meet this requirement, 1402SF001-SEL (as shown in Figure 3) was added. This allows for the automatic correction of the lower and upper limits of the corresponding indicators for the control point based on the production grade and load selected by the process engineer.
Figure 3 shows the program configuration for switching grades and changing production load.
3.2 Data Recording and Storage
A multidimensional array called TAGNAME is defined in _SFCSW to store the data required to generate the report. TAGNAME[1,I] stores the instrument tag number being counted, TAGNAME[2,I] stores the lower limit of the indicator, TAGNAME[3,I] stores the upper limit of the indicator, TAGNAME[4,I] stores the number of points to be removed, TAGNAME[5,I] stores the number of points exceeding the tolerance among the removed points, TAGNAME[6,I] stores the number of points exceeding the tolerance, and TAGNAME[7,I] stores the number of sampling points.
The BYPS array is defined in _SFCSW to store the removal status corresponding to each tag number. It sends information to the operator when bypassing and activating the device to remind the operator.
To ensure long-term data storage and facilitate historical retrieval, historical trends were configured for TAGNAME[4,I], TAGNAME[5,I], TAGNAME[6,I], and TAGNAME[7,I]. Given that the stationarity rate program collects process data every 10 minutes, the trend period should not be set too short; a setting of 1 minute, 2 minutes, or 5 minutes is recommended. This effectively saves storage space and avoids waste.
3.3 Flowchart Screen Design
To facilitate process engineers' timely understanding of key production data for the current shift and whether it falls within the required range, a DCS screen (as shown in Figure 4) was created within the DCS Graphics. This screen is primarily used for viewing indicator data, the number of rejected points, the number of out-of-tolerance points, the number of statistical points, the shift stability rate, and for checking the rejection and activation status of key point stability rates.
3.4 Report Design and Printing
The YOKOGAW CENTUM CS3000 reporting function works by first collecting process data through trend acquisition and creating a trend data file. Then, it uses OPC to transfer the necessary data to Excel. Finally, it generates an Excel spreadsheet document based on a created report template and can schedule printouts according to user requirements. CENTUM CS3000 reports also feature a history management function, effectively managing output report files. Users can review, reprint, and delete reports.
Figure 4 shows the flowchart of the stability rate.
Because this program records data as trend data, the data in the report data items uses the Tend format. First, extract the total number of points, the number of eliminated points, and the number of out-of-range points corresponding to the corresponding position number at the start and end of the shift. Calculate the difference between these values to obtain the statistical number of points, eliminated points, and out-of-range points for this shift. Then, calculate the control stability rate for this shift using the stability rate calculation formula. Daily and weekly stability rate reports can be obtained using the same method. It is important to note that after the report design is completed, the designed report must be loaded in the ReportPackage, and the printer must be configured for the report in HISSETUP.
Based on the actual needs of process engineers, the scheduler settings in the HIS are configured to automatically print shift reports, daily reports, and weekly reports for the morning, afternoon, and evening shifts. This allows process engineers to receive the shift's stability rate report before handing over their shift, facilitating management.
4. Conclusion
The aforementioned method for calculating the stationarity rate can be easily implemented in the CS3000. Furthermore, the test results are excellent, completely replacing manual recording and calculation by operators, effectively reducing their workload and significantly improving the accuracy and reliability of the stationarity rate data.
