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1.3 Piping and Instrumentation Diagram (P&ID)

The piping and instrumentation diagram (P&ID), also known as mechanical flow diagram (MFD), provides information needed by engineers to begin planning for the construction of the plant. The P&ID includes every mechanical aspect of the plant except the information given in Table 1.8. The general conventions used in drawing P&IDs are given in Table 1.9.

Table 1.8. Exclusions from Piping and Instrumentation Diagram


Operating Conditions T, P


Stream Flows


Equipment Locations


Pipe Routing

  1. Pipe Lengths
  2. Pipe Fittings


Supports, Structures, and Foundations

Table 1.9. Conventions in Constructing Piping and Instrumentation Diagrams

For Equipment—Show Every Piece Including

Spare Units

Parallel Units

Summary Details of Each Unit

For Piping—Include All Lines Including Drains and Sample Connections, and Specify

Size (Use Standard Sizes)

Schedule (Thickness)

Materials of Construction

Insulation (Thickness and Type)

For Instruments—Identify




Show Instrument Lines

For Utilities—Identify

Entrance Utilities

Exit Utilities

Exit to Waste Treatment Facilities

Each PFD will require many P&IDs to provide the necessary data. Figure 1.7 is a representative P&ID for the distillation section of the benzene process shown in Figure 1.5. The P&ID presented in Figure 1.7 provides information on the piping, and this is included as part of the diagram. As an alternative, each pipe can be numbered, and the specifics of every line can be provided in a separate table accompanying this diagram. When possible, the physical size of the larger-sized unit operations is reflected by the size of the symbol in the diagram.

Figure 1.7

Figure 1.7 Piping and Instrumentation Diagram for Benzene Distillation (adapted from Kauffman, D, “Flow Sheets and Diagrams,” AIChE Modular Instruction, Series G: Design of Equipment, series editor J. Beckman, AIChE, New York, 1986, vol 1, Chapter G.1.5, AIChE copyright © 1986 AIChE, all rights reserved)

Utility connections are identified by a numbered box in the P&ID. The number within the box identifies the specific utility. The key identifying the utility connections is shown in a table on the P&ID.

All process information that can be measured in the plant is shown on the P&ID by circular flags. This includes the information to be recorded and used in process control loops. The circular flags on the diagram indicate where the information is obtained in the process and identify the measurements taken and how the information is dealt with. Table 1.10 summarizes the conventions used to identify information related to instrumentation and control. Example 1.9 illustrates the interpretation of instrumentation and control symbols.

Table 1.10. Conventions Used for Identifying Instrumentation on P&IDs (ISA standard ISA-S5-1, [4])

Location of Instrumentation


Instrument Located in Plant


Instrument Located on Front of Panel in Control Room


Instrument Located on Back of Panel in Control Room

Meanings of Identification Letters xyy_circle.jpg

First Letter (X)

Second or Third Letter (Y)





Burner Flame





Density or Specific Gravity







Hand (Manually Initiated)








Time or Time Schedule

Control Station



Light or Low


Moisture or Humidity

Middle or Intermediate




Pressure or Vacuum



Quantity or Event


Radioactivity or Ratio

Record or print


Speed or Frequency







Valve, Damper, or Louver





Relay or Compute




Identification of Instrument Connections


The details of the other control loops in Figures 1.5 and 1.7 are left to problems at the end of this chapter. It is worth mentioning that in virtually all cases of process control in chemical processes, the final control element is a valve. Thus, all control logic is based on the effect that a change in a given flowrate has on a given variable. The key to understanding the control logic is to identify which flowrate is being manipulated to control which variable. Once this has been done, it is a relatively simple matter to see in which direction the valve should change in order to make the desired change in the control variable. The response time of the system and type of control action used—for example, proportional, integral, or differential—are left to the instrument engineers and are not covered in this text.

The P&ID is the last stage of process design and serves as a guide for those who will be responsible for the final design and construction. Based on this diagram,

  1. Mechanical engineers and civil engineers will design and install pieces of equipment.
  2. Instrument engineers will specify, install, and check control systems.
  3. Piping engineers will develop plant layout and elevation drawings.
  4. Project engineers will develop plant and construction schedules.

Before final acceptance, the P&IDs serve as a checklist against which each item in the plant is checked.

The P&ID is also used to train operators. Once the plant is built and is operational, there are limits to what operators can do. About all that can be done to correct or alter performance of the plant is to open, close, or change the position of a valve. Part of the training would pose situations and require the operators to be able to describe what specific valve should be changed, how it should be changed, and what to observe in order to monitor the effects of the change. Plant simulators (similar to flight simulators) are sometimes involved in operator training. These programs are sophisticated, real-time process simulators that show a trainee operator how quickly changes in controlled variables propagate through the process. It is also possible for such programs to display scenarios of process upsets so that operators can get training in recognizing and correcting such situations. These types of programs are very useful and cost-effective in initial operator training. However, the use of P&IDs is still very important in this regard.

The P&ID is particularly important for the development of start-up procedures where the plant is not under the influence of the installed process control systems. An example of a start-up procedure is given in Example 1.10.

These last three sections have followed the development of a process from a simple BFD through the PFD and finally to the P&ID. Each step showed additional information. This can be seen by following the progress of the distillation unit as it moves through the three diagrams described.

  1. Block Flow Diagram (BFD) (see Figure 1.1): The column was shown as a part of one of the three process blocks.
  2. Process Flow Diagram (PFD) (see Figure 1.5): The column was shown as the following set of individual equipment: a tower, condenser, reflux drum, reboiler, reflux pumps, and associated process controls.
  3. Piping and Instrumentation Diagram (P&ID) (see Figure 1.7): The column was shown as a comprehensive diagram that includes additional details such as pipe sizes, utility streams, sample taps, numerous indicators, and so on. It is the only unit operation on the diagram.

The value of these diagrams does not end with the start-up of the plant. The design values on the diagram are changed to represent the actual values determined under normal operating conditions. These conditions form a “base case” and are used to compare operations throughout the life of the plant.

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