Introduction
A P&ID or a Piping and Instrumentation Diagram is one of the fundamental drawings produced and used in process and piping engineering.
It provides a simple 2D representation of the process system, showing piping, valves, equipment, and control and instrumentation. P&IDs are a staple in the engineering world, found in multiple industries such as; oil and gas, chemical, pharmaceutical, power and energy and food and beverage. At the same time they are used throughout the majority of a project lifecycle, from front end engineering design, through to construction and commissioning, and operation and maintenance.
Understanding how to read and interpret a P&ID is a fundamental skill for piping designers, engineers and plant operators.
What information does a P&ID show?
A P&ID does not show an exact layout with dimensions or physical routing. Instead, it focuses on:
- functional interconnecting relationships
- process connections
- control philosophy
- system operation
A typical P&ID will contain certain key elements, which are typically shown by standard symbology.
1. Equipment
All major process equipment such as:
- Vessels & Tanks
- Pumps
- Heat Exchangers
- Compressors
- Powder Mixer
- Column
- Receiver
Each of these items will be assigned their own individual unique tag, allowing them to be identified consistently across documents, schedules, models and drawings.
For example, for a vessel: V101
Or for a pump: P-001
2. Piping
Connecting the equipment together is the pipework, represented by lines. Primary lines (thicker line weight) are used for main process piping and headers, whereas secondary lines (thinner line weight) are used to show drains, by-passes and vents. The flow direction of the service is shown by arrows.
Each line is given a line number tag, which consists of:
- line size (usually DN or NPS)
- pipe specification code
- service code (identifying what fluid it is conveying)
- line number (starting from 001)
- insulation requirements (if applicable)
A typical line number tag, for a DN100 (4″) carbon steel line conveying chilled water that is insulated, might look like the following:
DN100 – CS01 – CW – 001 – IC
3. Valves
Regulating and directing the flow of the service fluid are the valves, located at key locations along the pipework.
Common valves that appear in a P&ID are:
- gate valves
- globe valves
- ball valves
- check valves
- control valves
These valves can either be manual or automated, and the two are distinguished by the valve tag they are given. For example:
A manual valve tag might be assigned: HV003
Whereas an automated valve would have the tag: CV003
4. Instrumentation
Instrumentation is used to monitor and control the process conditions. Typical instrumentation shown on a P&ID include:
- pressure indicator
- pressure transmitter
- temperature indicator
- temperature transmitter
- flow meters
- flow indicator
- level switches
- control loops
These are also given tags, such as: PT01
The importance of P&IDs
P&IDs are essential because they provide a common refernece for engineers in understanding how a process system works. They are employed not only in piping design, but are used across the board in:
- HAZOP studies
- safety reviews
- maintenance planniing
- operator training
- plant modiifications and upgrades
- construction
Without an accurate or up to date P&ID, understanding of an existing process and communication between different engineering disciplines becomes significantly more difficult.
Learning to Read P&IDs
Therefore, it is essential for beginners to learn how to read a P&ID. Though it may initially appear complex to the untrained eye, the best approach to take is to:
- Learn common symbols
- Learn tag abbreviations
- Follow process flow paths
- Study real examples
- Practice interpreting process systems
Over time, reading a P&ID will become easier. If in any doubt, always ask.
Common P&ID Standards
As P&ID symbology can vary slightly across industries and companies, a P&ID should be accompanied with a front sheet or document outlining the standard symbology used in it. These are based on recognised standards, including:
- ISO standards
- ISA standards
- company own standards
- project-specific standards
Standardisation is helpful to ensure drawings are consistent and therefore better understood across organisations and project teams.
Conclusion
P&IDs are the one of the most important engineering drawings. They are the skeleton of the hold process which holds everything together.
For anyone entering into piping design or process engineering, learning how to read and understand a P&ID is a foundational skill.
As Piping Design Hub develops, future articles will explore in more depth:
- common and standard P&ID symbols
- line numbering systems
- control valve basics
- tagging
- practical drawing examples