Piping and Instrumentation Diagram

This drawing is commonly referred to as the "Piping and Instrumentation Diagram" Its objective is to indicate all process and service lines, instruments and controls, equipment and data necessary for the design groups. The process flow diagram is the primary source of information for developing the P&ID.The P&ID should define piping, equipment and instrumentation well enough to cost estimation and for subsequent design, construction, operation and modification of the process. Material balance data, flow rates, temperatures, pressures and piping fitting details are not shown and purely mechanical piping details such as elbows, joints and unions are inappropriate to P&ID's.

Piping and Instrumentation Diagram (P&ID) Layout

The layout of the P&ID should resemble, as far as practicable, that of the process flow diagram. The process relationship of equipment should correspond exactly. Often it is useful to draw equipment in proportion vertically, but to reduce horizontal dimensions to save space and allow room for flow lines between equipment. Crowding information is a common drafting fault - it is desirable to space generously, as more often than not, revisions add information. On an elevational P&ID, a base line indicating grade or first-floor level can be shown. Critical elevations are noted.For revision purposes, a P&ID is best made on a drawing sheet having a grid system - this is a sheet having letters along one border and numbers along the adjacent border. Thus, references such as "A6", "B5", etc., can be given to an area where a change has been made.

Flow Lines On Piping and Instrumentation Diagram's

All flow lines and interconnections should be shown on P&IDs. Every line should show direction of flow and be labeled to show the area of project, conveyed fluid, line size, piping material or specification code number (company code) and number of the line.

Interconnecting Piping and Instrumentation Diagram (P&ID)

This drawing shows process and service lines between buildings and units, etc. and serves to link the P&IDs for the individual processes, units or buildings. Like any P&ID, the drawing is not to scale. It resembles the layout of the site plan, which enables line sizes and branching points from headers to be established and assists in planning pipe ways.

A P&ID shows all of piping including the physical sequence of branches, reducers, valves, equipment, instrumentation and control interlocks.

The P&ID are used to operate the process system.

A Piping and Instrumentation Diagram (P&ID) should include:

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• 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

This figure depict a very small and simplified Piping and Instrumentation Diagram:

Piping and Instrumentation Diagram

A Piping and Instrumentation Diagram (P&ID) should not include:

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

Fig. 1 shows the basic types of lines, fitting symbols, and valve symbols used in flow diagrams. Instrumentation Drawing Symbols.

Fig. 1: Basic Piping & Instrumentation Symbols

Figs. 2 and 3 show the symbols used to denote common process equipment. Again, these are not exhaustive lists, but they contain the majority of symbol types for equipment. Separators and towers, in particular, can have a wide range of internal devices, and this results in many variations of equipment symbols. Similarly, there are different, and yet common, ways of indicating the same types of heat exchangers.

Fig. 2: Process Vessels

Fig. 3: Towers and Heat Exchangers

Fig. 4 shows some of the typical symbols used for pumps, compressors, and the devices which are used to drive them. Often, the diagram will show the type of driver associated with a pump or compressor, particularly if the driver is itself part of the overall plant process, such as a steam turbine. If a pump or compressor is shown without a driver, then the conventional understanding is that it is driven by a motor.

Fig. 4: Pumps, Compressors and Drivers

Here are some general guidelines:

- check valve on pump discharge line
- block valves on the discharge and suction side of pumps
- drain valve on pump suction
- bypass around flow control valves
- double block and bleed where necessary
- line numbering including line size and mat'l of construction
- show set pressure, in- and outlet diameters on PRVs
- legend sheets
- lines continueing to the next drawing should be at the same height, show drawing nr.
- main process lines should be thicker than utility lines
- Install flowmeters upstream of a flow control valve.
- If your drawing shows elevation, the flowmeter should ideally be installed on the vertical portion of the pipe where flow is going upward.
- Install temperature gauge at the outlet of a heat exchanger.

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