In response to the letter on ISA symbol standard S5.1 in the June issue, there are a number of practical issues related to the design process that will affect the way that fieldbus and multi-functional instruments are represented on P&IDs. These relate to the short project programs within the design contractors. P&IDs are the fundamental documents generated by the process designers. From these are generated equipment lists and schedules that are an essential input to the procurement process.
They must be generated very early and very quickly if delivery dates are to be met, and once generated they should not need to be changed simply to reflect subtleties of detailed implementation. These two factors mean that P&IDs will be produced long before issues, such as suppliers and types and models of instruments, are known.
In reality, at that time you will not know if multi-function instruments are to be used, and you certainly won’t know the topology of any fieldbus implementation. By the time you do have that detail some months later there would not be much internal support for a wholesale update of the P&IDs!
P&IDs should not try to represent the minute detail of C&I implementation, but should convey clearly the process related information, such as measurement and control points and interfaces to control and safety systems, as, for instance, needed for HAZOPS.
Similarly, detail of complex loops should be avoided, as these are subject to ongoing modification. It is better to simply refer on the P&ID to another related drawing.
In fact, implementation detail, such as fieldbus topology and the precise nature of complex loops, is best generated by the actual control systems and associated tools.
Similarly, the next level of instrument detail, including issues relating to multi-function devices, can be kept in the data base, which is far easier to develop and maintain than a P&ID drawing.
As for actual tag numbering, with the power and flexibility of modern control systems, it seems pointless to tag signals with second order attributes, such as R’ for recording or I’ for indication. Keep it simple, just indicating the prime variable with say a T’ for analogue signals, S’ for digitals, with the usual Ax’ for alarms.
When it comes to tags associated with electrical equipment (motors, actuators etc.), make the item’s equipment number the prime part of the tag. For example, for a motor on pump P103A, the running status signal would be P103A-XR, and so on.
In conclusion, ISA needs to take a new approach to S5.1 issues which is different to the days of hard-wired panels. As leaders ISA needs to set standards for the future. If they don’t, events will pass them by, but in a less organised manner.