Piping & Instrumentation Diagram, P&ID

An Introduction

Sample P&IDSample P&ID

Piping and Instrumentation Diagrams (P&IDs) use specific symbols to show the connectivity of equipment, sensors, and valves in a control system. These symbols can represent actuators, sensors, and controllers and may be apparent in most, if not all, system diagrams. P&IDs provide more detail than a process flow diagram with the exception of the parameters, i.e. temperature, pressure, and flow values. “Process equipment, valves, instruments and pipe lines are tagged with unique identification codes, set up according to their size, material fluid contents, method of connection (screwed, flanged, etc.) and the status (Valves – Normally Closed, Normally Open).” These two diagrams can be used to connect the parameters with the control system to develop a complete working process. The standard notation, varying from letters to figures, is important for engineers to understand because it a common language used for discussing plants in the industrial world.

P&IDs can be created by hand or computer. Common programs, for both PC and Mac, that create P&IDs include Microsoft Visio (PC) and OmniGraffle (Mac). As with other P&IDs, these programs do not show the actual size and position of the equipment, sensors and valves, but rather provide a relative positions. These programs are beneficial to produce clean and neat P&IDs that can be stored and viewed electronically. See below for P&ID templates for these programs.

This section covers four main types of nomenclature. The first section describes the use of lines to describe process connectivity. The second section describes letters used to identify control devices in a process. The third section describes actuators, which are devices that directly control the process. The final section describes the sensors/transmitters that measure parameters in a system.

A List of P&ID items

  • Instrumentation and designations
  • Mechanical equipment with names and numbers
  • All valves and their identifications
  • Process piping, sizes and identification
  • Miscellanea – vents, drains, special fittings,sampling lines, reducers, increasers and swagers
  • Permanent start-up and flush lines
  • Flow directions
  • Interconnections references
  • Control inputs and outputs, interlocks
  • Interfaces for class changes
  • Computer control system input
  • Identification of components and subsystems delivered by others

A good P&ID should include:

  • Instrumentation and designations
  • Mechanical equipment with names and numbers
  • All valves and their identifications
  • Process piping, sizes and identification
  • Miscellaneous – vents, drains, special fittings, sampling lines, reducers, increasers and swagers
  • Permanent start-up and flush lines
  • Flow directions
  • Interconnections references
  • Control inputs and outputs, interlocks
  • Interfaces for class changes
  • Seismic category
  • Quality level
  • Annunciation inputs
  • Computer control system input
  • Vendor and contractor interfaces
  • Identification of components and subsystems delivered by others
  • Intended physical sequence of the equipment
  • Equipment rating or capacity

A P&ID should not include:

  • Instrument root valves
  • control relays
  • manual switches
  • primary instrument tubing and valves
  • pressure temperature and flow data
  • elbow, tees and similar standard fittings
  • extensive explanatory notes

Line Symbols

Line symbols are used to describe connectivity between different units in a controlled system. The table describes the most common lines.

Line_types

 

 

 

 

 

 

 

 

 

In Table table above, the “main process” refers to a pipe carrying a chemical. “Insulated” is straightforward, showing that the pipe has insulation. “Trace heated” shows that the pipe has wiring wrapped around it to keep the contents heated. “Lagged” indicates on a P&ID that the pipe is wrapped in a cloth or fiberglass wrap as an alternative to painting to improve the appearance of the pipe. The last column in the Table shows pipes that are controlled by a controller. “Electrical impulse” shows that the manner in which information is sent from the controller to the the pipe is by an electrical signal, whereas “pneumatic impulse” indicates information sent by a gas.

In addition to line symbols, there are also line labels that are short codes that convey further properties of that line. These short codes consist of: diameter of pipe, service, material, and insulation. The diameter of the pipe is presented in inches. The service is what is being carried in the pipe, and is usually the major component in the stream. The material tells you what the that section of pipe is made out of. Examples are CS for carbon steel or SS for stainless steel. Finally a ‘Y’ designates a line with insulation and an ‘N’ designates one without it. Examples of line short codes on a P&ID are found in the figure below.

PID_-_line_labeling

 

 

 

 

 

 

 

This is useful for providing you more practical information on a given pipe segment.

For example in stream 39 in Figure above, the pipe has a 4″ diameter, services/carries the chemical denoted ‘N’, is made of carbon steel, and has no insulation.

Instrumentation identification letters

The following letters are used to describe the control devices involved in a process. Each device is labeled with two letters. The first letter describes the parameter the device is intended to control. The second letter describes the type of control device.

First Letter Measurement
A Analysis
B Burner, Combustion
C User’s Choice (usually Conductivity)
D User’s Choice (usually Density)
E Voltage
F Flow
G User’s Choice
H Hand
I Current
J Power
K Time, Time Schedule
L Level
M User’s Choice
N User’s Choice (usually Torque)
O User’s Choice
P Pressure
Q Quantity
R Radiation
S Speed, Frequency
T Temperature
U User’s Choice (usually Alarm Output)
V Vibration, Mechanical Analysis
W Weight, Force
X User’s Choice (usually on-off valve as XV)
Y Event, State, Presence
Z Position, Dimension
Second Letter Type of Control Device
A Alarm
C Control
I Indicator
T Transmit
V Valve

For example, the symbol “FI,” is a “Flow indicator.”

Valve Symbols

The following symbols are used to represent valves and valve actuators in a chemical engineering process. Actuators are the mechanisms that activate process control equipment.

Valve Symbols

Valves

 

 

 

 

 

 

 

 

Valve Actuator Symbols

Actuators