P&ID or Piping and Instrumentation Diagram is one of the most important documents for any project. A P&ID is an engineering document developed by process engineers that shows the piping and other related items for process flow. P&ID provides a schematic illustration of the actual processes that are happening in any plant using various P&ID symbols. The graphical representation establishes the functional relationship of piping, instrumentation, and mechanical equipment. This is the most extensively used engineering document and used by all engineering disciplines like Process, piping, mechanical, civil, instrumentation.
Why use a P&ID?
P&ID (Also known as PEFS, Process Engineering Flow Scheme) is a fundamental engineering document that serves various purposes like
- Provides key piping and instrumentation items along with their proper arrangement.
- Serves as a basic document for operation, control, and shut down schemes.
- Provides a basis for maintenance and modification works.
- Gives the regulatory and plant safety requirement.
- Serves as a guide for start-up and operational data.
- Develop guidelines and standards for facility operation
- It is the basic training document to explain the process details to operation guys, field engineers, and maintenance professionals. The P&ID drawings help them to track the interconnection between the piping and instrumentation and equipment.
- Provides the design and construction sequence for the plants for systematic planning of activities.
- Serves as a basis for studying different mechanical and chemical steps to find the root cause if something goes wrong.
- It also provides basic information for initial project cost estimation.
- An important document for HAZOP, Model review, Process Safety Management, etc.
- Provide a common language for discussing plant operations
Limitations of P&ID
P&IDs being graphical schematic process representation have some limitations like
- They are not to the scale, similar to real models.
- They are not standardized documents so vary from company to company.
What should a P&ID include?
There is no exact code or standard that dictates what exactly should be included in the P&ID document. That is the reason P&IDs from different organizations varies slightly. Broadly all P&IDs normally include the following:
- All Mechanical equipment with equipment numbers and names.
- All valves with proper identification.
- Instrumentation details with designations.
- piping with line numbers, sizes, material specs, and other details.
- Fluid Flow directions.
- Miscellaneous items like drains, vents, special fittings, reducers, sampling lines, expansion joints, flexible hose connections, increasers, and swaggers.
- Piping and equipment interfaces with scope demarcation, class changes.
- Permanent start-up and flush lines.
- References for Interconnections.
- Interlocks, Control inputs, and outputs.
- Annunciation inputs
- a physical sequence of the piping items and equipment.
- Equipment rating or capacity; sometimes short design and dimensional details.
- Interfaces with vendors and contractors with scope.
- Computer control system input.
- Seismic category
- Quality level
- Details like equipment operating, standby, normally no flow, etc are included in some P&IDs.
- Notes related to two-phase flow, special pipe length requirements, etc.
- Piping slope requirements, Piping Insulation requirements.
Details excluded in a P&ID
The in-depth details are not included in the P&IDs. Various supporting documents are prepared for the detailed design and description of those items. Normally the following details are not included in a P&ID:
- Process Flow Diagram
- Pipe Route and length
- Elbow, tees, and similar standard fitting details.
- Pipe Support and Structural Details
- Pressure temperature and flow data.
- Manual switches and indicating lights
- Extensive explanatory notes
- Control relays
- Instrument root valves
- Primary instrument valves and tubing
- Equipment rating or capacities
- Equipment locations
Supporting Documents of P&ID
As Piping and Instrumentation Diagram is not a detailed document, various supporting documents are prepared to complete the overall details of the P&ID. Few of those documents are:
- PFD or Process Flow Diagram from which P&ID is generated.
- PMS or Piping Material Specification which provides material details of piping and related items.
- Equipment and Instrument Item datasheets specifying the required details about Equipment or instrument items.
Process engineers use various P&ID symbols while constructing P&ID. All those P&ID symbols are normally described at the start of the P&ID set. For the same design consultant, those symbols are normally constant. One should study those p&ID symbols to accurately read the P&IDs.
Instrumentation symbols in a P&ID are standardized as per ANSI/ISA’s S5.1 standard. This standard ensures a consistent, system independent means of communicating instrumentation, control, and automation intent by providing standardized Instrumentation Symbols and Identification so everyone understands.
Four graphical elements are defined in ISA S5.1 for instrumentation. Those are discrete instruments, shared control/display, computer function, and programmable logic controller. The standard also groups them into three location categories as the primary location, auxiliary location, and field mounted location category. Click here to check all the P&ID symbols that ANSI/ISA’s S5.1 standard provides.
BS 5070 and ISO 10628 also provide few guidelines and best practices for P&ID symbols. The following images (Fig.1 to Fig. 7) provides an example of the P&ID symbols that are normally used in a typical P&ID.
Please note that few organization use the words like P&ID Legends, P&ID Lead sheets or P&ID Legend drawings in place of P&ID Symbols.
How to Read P&ID
Reading or tracing a P&ID to understand the process or design requirements are quite easy if the P&ID symbols are properly understood. So it is always preferable to go through the P&ID symbols again and again those are provided in the initial 4-5 pages of the P&ID sets. Once that is ready open the P&ID and start reading it.
Here, we will learn how to read P&ID with a simple example.
Refer to Fig. 8 which shows a part of a P&ID. We will read part of the suction and discharge line of the pump P-1519.
- As can be seen clearly the suction line is coming from a different P&ID and entering into the bucket strainer S-1575.
- The line number, pipe size, material spec, etc of the line are provided clearly.
- The line number changed from the strainer outlet, one PSV connection is attached before the pump suction reducer.
- After reducer material spec is changing and then the line is connected to the pump suction flange.
- In a similar way pump discharge flange is connected to the discharge line, Like number, size, PMS, etc are mentioned.
- So in a similar way, we can easily read the P&IDs as per our requirements and extract data to use for our purpose.
- Related notes are provided wherever required. We have to refer those notes for knowing any other requirements.