Flowsheet Symbols and P&I Diagrams

The purpose of this section is to help the reader establish a uniform means of depicting and identifying all classes of instruments, instrumentation systems, and functions used for measurement, monitoring, and control. It is done by presenting a designation system of graphic symbols and identification codes.*

It must be noted that a significant part of this section has been extracted from the revision work of the ISA** SP5.1 subcommittee, and much of it has been based on draft working documents being utilized at the time of this writing, documents with which one of the authors has been actively involved. Other portions of this section, dealing with certain symbols, graphics, and practical tips, are based on the authors’ experience in industry and are not part of the SP5.1 subcommittee’s proposed forthcoming revision.

A disclaimer to any future ISA standards documents is hereby stated: The reader is cautioned that the draft ISA document that provided much of the information in this section has not been approved as of the time of this writing. It cannot be presumed to reflect the position of ISA or any other committee, society, or group. The intent is to pass along to the reader the best and latest thinking on this subject at this point in time, although many items are contentious and are ultimately subject to change in the continuously evolving fields of digital control systems and digital data buses.

Another view of flowsheet and piping and instrument diagram (P&ID) symbols and diagrams covered in this section is in terms of practical aspects and practices used by

instrumentation and control practitioners in the engineering, procurement, and construction (EPC) industry.

SCOPE**

General

The procedural needs of various users are different, and these differences are recognized, when they are consistent with the objectives of this standard, by providing alternative symbol and identification methods.

A limited number of examples are provided later that illustrate (with the emphasis on digital systems/loops) how to accomplish the following:

a) Design an identification system and construct an identification number

b) Use graphic symbols to construct the following:

1) Schematic diagrams of instrument devices and functions in monitoring and control loops

2) Schematic and ladder diagrams of electrical circuits

c) Add information and simplify diagrams

Examples of symbol applications are generally shown as applied in the oil and chemical processing industries as in the original version of this standard, but the principles shown are applicable to most other industries.

Specific applications are to be addressed in greater detail and will be forthcoming in the planned S5.1 (now ANSI/ISA-5.01.01) series of Technical Reports dedicated to the various processing, generating, and manufacturing industries. These will include processes such as continuous and batch chemical, oil, and metal refining, pulp and paper, water and waste treatment, power generation and distribution, and discrete

parts manufacturing.

Application to Industries

The proposed revised ISA S5.1 (now ANSI/ISA-5.01.01)

standard will be suitable for use in the above-mentioned process industries and in discrete parts manufacturing where the use of control system schematic and functional diagramming is required to describe the relationship with processing equipment and the functionality of measurement and control equipment.

Certain fields, such as astronomy, navigation, and medicine, use very specialized instruments that are different from conventional industrial process instruments. No specific effort was made to have the ISA standard meet the requirements of those fields. However, it is expected that, in certain areas such as control functional diagrams, they will prove applicable for such specialized fields.

* Used with permission of the Instrument, Systems and Automation Society.

**Formerly called the Instrument Society of America.

Application to Work Activities

The proposed revised ISA S5.1 (now ANSI/ISA-5.01.01) standard will be suitable for use whenever reference to measurement and control instrumentation, control device functions, or software applications functions is required for the purposes of symbolization and identification. Such references may be required for the following uses as well as others:

a) Design sketches

b) Teaching examples

c) Technical papers, literature, and discussions

d) Instrumentation system diagrams, loop diagrams, logic

diagrams, and functional diagrams

e) Functional descriptions

f) Conceptual drawings: process flow diagrams (PFDs) and utility flow diagrams (UFDs)

g) Construction drawings: engineering flow diagrams (EFDs), mechanical flow diagrams (MFDs), piping and instrument diagrams (P&IDs), and system flow

diagrams (SFDs)

h) Specifications, purchase orders, manifests, and other lists

i) Identification and tag numbering of instruments and control functions

j) Installation, operating, and maintenance instructions, drawings, and records The standard is intended to provide sufficient information to enable anyone with a reasonable amount of process and instrumentation knowledge to understand the methods of measurement and process control.

It is not necessary to possess the detailed knowledge of a specialist in instrumentation and control systems to understand the standard.

Application to Classes of Instrumentation and to Instrument Functions

The symbolism and identification methods provided in the standard are applicable to all classes and types of measurement and control instruments and functions.

The methods can be used for, but are not limited to, describing and identifying the following:

a) Discrete (individual) instruments and their functions

b) Shared display and control functions

c) Distributed control functions

d) Computer control functions

e) Programmable logic controller display and control functions

f) Application software display and control functions

Extent of Loop and Functional Identification

The ISA S5.1 standard (now ANSI/ISA-5.01.01) provides identification codes and methods for the alphanumeric identification of monitoring and controlling loops, instruments, and functions. The user is free to apply additional identification by serial, equipment, unit, area, or plant number or any other additional means required for the unique identification of a loop, instrument, or function.

A unique function identification number shall identify each instrument, its inherent functions, and each configurable function that requires or allows a user-assigned, unique microprocessor or computer address required by a loop.

Extent of Symbolization

The standard provides symbol sets for the graphic depiction of limited or total functionality for instruments and other devices, entire monitor/control loops, or control circuits. The amount of detail to be shown by the use of symbols depends on the purpose and audience for which the document is being prepared.

A sufficient number of symbols should be used to show the functionality of the instrumentation and control loop being depicted. However, it is not considered necessary to provide a symbol for each instrument device and each function

within a loop.

Additional construction, fabrication, installation, and operation details of an instrument are better described in a suitable specification, data sheet, drawing, sketch, or other

document intended for individuals who require such details.

Inclusion of the New S5.1 Standard (now ANSI/ISA-5.01.01) in User/Owner Documents

This is a new concept in ISA standards at this point in time. Mandatory use of the standard is required by users/owners based on the following statements.

When the latest issue of the standard is included in user/owner’s engineering and/or design guidelines or standards by reference and

a) “Without exception,” then the standard in its entirety shall be mandatory.

b) “With exceptions,” then the parts of the standard:

1) “Excepted to” shall be fully described and detailed.

2) “Not excepted to” shall be mandatory.

When a previous issue of the standard is included by reference with or without exception in user/owner’s engineering and design guidelines or standards, that standard in part or in its entirety shall be mandatory until such time as the user/owner’s guidelines or standards are revised. When the new issue is used as a guide in the preparation of user/owner’s guidelines or standards, symbols and letter and symbol meanings different from those in the standard shall be fully described and detailed.

Symbols and the meanings of letters and symbols from previous issues of the S5.1 standard (now ANSI/ISA-5.01.01) that are different from those contained in this new issue may continue to be used, provided that they are fully described and detailed.