Symbol Standards – Instrumentation and Controls

The following should be agreed upon before much work is done on the P&IDs for a project:
• A standard for continuous modulating controls
• A standard for process safety and sequencing logic
• How to document P&ID special symbols
• The degree of details to be shown on the P&ID

Continuous Modulating Controls
Modulating controls indicate and control variables that can change continuously over a range of values. ISA Standard S5.1 (see Appendices) is the preferred standard.

Process Safety and Sequencing Logic
Variables for process safety and sequencing logic can normally assume only two states; a pump is either on or off; a temperature either is or is not too high; a burner either is or is not lit; a filter is or is not ready to be backwashed.

The logic symbol standard used most often is ISA Standard S5.2, (see Volume 2, Industry Codes and Practices). These symbols are most suitable for representing binary process logic, thus for documenting most safety systems. They do not easily represent sequencers such as drum programmers which have many output states.

In most cases the sequencing logic will be complex enough to require separate functional logic diagrams. The S5.1 symbols connect the individual instrument symbols to a box labeled with the name of the logic system. For example, a boiler P&ID may show a box labeled BURNER MANAGEMENT SYSTEM. There may be more than one logic box shown on the P&ID to represent different logic systems (see ISA S5.2, Appendix A, Figure 1).

The box should direct the reader to a logic document that is recoverable by future users of the P&ID. This logic document might be a drawing, such as that shown in S5.2, Appendix A, Figure 2. Note that this drawing is tied to Figure 1 by the instrument balloons on the interlock system in Figure 1 and adjacent to the logic in Figure 2.

Word descriptions can supplement or replace logic drawings such as that provided in S5.2, Appendix A, Section 3.1.

Figure 200-1 is a type of word description called a control philosophy. This is a very effective way to communicate complex or simple process control schemes.

Control Philosophy

Control philosophies (when used) are an integral part of the P&ID. They can be placed in an expanded note section of a P&ID or on separate P&IDs.

Simpler safety and sequencing logic can be shown entirely on the P&ID using the symbols of ISA Standard S5.1. For example, a low-level shutoff for a tank valve actuated by a level switch may be depicted without the need for a separate logic diagram.

Special Symbols
Any special symbols should follow the rules in ISA S5.1 and S5.2, and be defined on each drawing.

Degree of Detail
ISA S5.1 identifies three levels of detail, depending on user requirements, as follows:

• Simplified loop. See ISA S5.1, Section 6.12, Figure 1. Simplified symbolism and abbreviated identification identify the principal measurement and control functions. Process control diagrams often use simplified loops

• Conceptual loop. See ISA S5.1, Section 6.12, Figure 2. Functionally oriented symbolism and abbreviated identification show the control function but not the implementing hardware. Advanced process control diagrams and P&IDs intended primarily for the process operator normally use conceptual loops.
Detailed loops are frequently shown on additional drawings

• Detailed loop. See ISA S5.1, Section 6.12, Figure 3. Detailed symbolism and more complete identification show the type of hardware and kinds of signals.
Detailed loops are often needed by the plant control engineer and the design, control engineering and maintenance staffs. For operator training, the conceptual loops must frequently be shown on additional drawings