Also, the history files gets deleted every three days. What could be the reason?
You just need to change the history settings on the trend tag, it's all detailed in the Citect help files (copy and paste below)
No. Files
The number of history files stored on your hard disk (for this tag) (4 characters maximum). The maximum number of files you can specify per trend tag is 999. Performance and storage will be severely impacted by having a large number of history files per trend.
By default, 2 history files are stored on your hard disk.
Period
The period of the history file, in hh:mm:ss (32 characters maximum). Alternatively, you can:
•Specify a weekly period by entering the day of the week on which to start the history file, for example Monday, Tuesday, Wednesday, etc.
•Specify a monthly period by entering the day of the month on which to start the history file, for example 1st, 2nd, 3rd, 4th, 5th, etc.
•Specify a yearly period by entering the day and the month on which to start the history file, for example 1st January, 25th February, etc. The day and month needs to be separated by a space.
If you do not specify a period, the period defaults to Sunday (weekly).
When deciding on a period setting, be aware that the performance of a trend viewer (be it the existing CitectSCADA or Process Analyst) may be impacted by the size of a trend file. This is particularly true when displaying event-based trend data.
Note: If you edit this property in an existing project, delete or move the associated trend files before you run the new runtime system. (For location of the trend files, see File Name.)
No. Files
The number of history files stored on your hard disk (for this tag) (4 characters maximum). The maximum number of files you can specify per trend tag is 999. Performance and storage will be severely impacted by having a large number of history files per trend.
If you do not specify the number of files, 2 history files are stored on your hard disk.
Note: If you edit this property in an existing project, delete or move the associated trend files - before you run the new runtime system. (For location of the trend files, see the File Name.)
Time
The time of day to synchronize the beginning of the history file, in hh:mm:ss (32 characters maximum). If you do not specify a time, the file is synchronized at 0:00:00 (i.e. midnight). The time needs to be specified in Greenwich Mean Time, not the local time zone.
Note: If you edit this property in an existing project, delete the associated trend files - before you run the new runtime system. (For location of the trend files, see the File Name.)