Take time for detailed instrumentation and control engineering

Engineering activities in general exist to provide safe, cost-effective, quality technical services to the user. Tasks specifically for instrumentation and control projects can be divided into two parts: front end and detailed services.

By N. E. (Bill) Battikha January 1, 1999

Engineering activities in general exist to provide safe, cost-effective, quality technical services to the user. Tasks specifically for instrumentation and control projects can be divided into two parts: front end and detailed services.

No detailed design should be started without the completion of front-end engineering. Front-end engineering provides for a common agreement about what a system is to provide and saves time, and usually money, later on by minimizing costly revisions and equipment changes. (This topic was covered in two earlier articles: “Applying Front-End Engineering to Instrumentation and Control Projects” (PE, June 1996, p 68, and September 1996, p 110, File 8550.)

This article reviews the basics of generating the minimum detailed engineering documentation required. First, the development of specifications is covered. Then, the preparation of drawings is examined. Because regulatory and code requirements vary from region-to-region, they are not considered here.


There are four common specifications typically prepared for instrumentation and control:

1. Instrument specification sheets

2. Control system specifications

3. Control panel (or control cabinet) specifications

4. Installation specifications.

Among the four, preparation of the instrument specification sheets is the most difficult and time consuming. Although some operations still prepare these specifications manually, many now use computer-based systems that can select the best instrument to fit the process conditions, then generate a specification sheet.

Such software packages also contain master specifications for control systems, control panels, installation activities, and other specifications. These tools save time and help produce consistent quality design.

Instrument specification sheets

The purpose of the instrument specification sheet (Fig. 1) is to list pertinent details for use by engineers and vendors. The information is also used by installation and maintenance personnel.

This specification sheet describes the instrument and provides a record of its function. The information should be uniform in content, presentation, and terminology. And, of course, the selection must consider all plant and process requirements and comply with any code requirements in effect at the site.

The most common specification sheets used in instrumentation and control are for:

– Flow measurement

– Level measurement

– Pressure measurement

– Temperature measurement

– Analyzers (including pH and conductivity)

– Control valves and regulators

– Pressure relief devices.

Typically, preparation of the instrument specification sheet involves several steps. If software is used, some of the procedure can be automated. First, the process data are completed, generally by a process or a mechanical engineer. Then, the best instrument for the job is chosen.

The specification sheet is completed to cover such points as type of enclosure, type of signal required, material in contact with the process, connection size, and the like. Vendors are selected, prices solicited, and finally an order is placed.

Control system specifications

The control system document outlines the parameters for the computer-based control system. It typically contains the requirements for code compliance, overview of the system, and detailed requirements.

The information generally begins with a master specification in a word processor document that can be tailored to the needs of each application. This document remains in use and is typically needed long after the system is up and running.

The content of a typical control system specification covers:

– Field conditions (including temperature, humidity, and environmental)

– Hardware requirements (such as cabinets, communications devices, inputs and outputs, controllers, and operator consoles)

– Software are requirements (including system configuration capabilities, graphics, alarms, trends, and reports)

– Service and support.

Control panel/cabinet specifications

The control panel document provides the guidelines for the design, construction, assembly, testing, and shipping of control panels and cabinets. As with the control system specification, the control panel specification generally originates with a master word processor specification to allow the requirements of each application to be easily customized.

A typical control panel specification is divided into sections covering design, construction, testing, and shipping. The document also should address certain details, such as nameplates, electrical and pneumatic requirements, and purging requirements, if necessary.

All electrically operated instruments, or electrical components incorporated in a panel or cabinet, must comply with the requirements of the current edition of the electrical code in effect at the site. All such equipment should be approved (by UL or CSA) and bear the approval label. ISA’s “Standards and Recommended Practices” also provide a valuable source of information and guidelines for instrumentation. (See More info box for contact and ordering information.)

Panel drawings may be generated with CAD tools, but the need for control panel specifications and drawings has diminished with the proliferation of computer-based control systems and the use of off-the-shelf cabinets. CAD drawings are still used, however, to show wiring and component locations in the cabinets.

Installation specifications

The installation specification provides the requirements for installing instruments, control systems, and their accessories. The contractor uses this document to estimate the cost of the installation. Once again, the information in the specification should be developed from a master specification document prepared in word-processor format to allow for convenient customization.

The installation specification marks the transition point between engineering and maintenance, who typically installs the equipment. The installation specification has many parts, each covering a section of the installation. Typically, these sections consist of an overview of the scope of the work.

It is followed by a description of how the instruments are to be mounted and installed, including the connections between the process and the instruments. The specification should also cover wiring and tubing requirements. Finally, checkout procedures should be defined to ensure that the control system as a whole is ready for operation.

All installation work should be based on the installation specification and reference documentation provided by the engineering phase. This reference documentation, which forms part of the contract, clearly identifies the scope of work, thereby minimizing misunderstandings, completion delays, and additional costs.


The most commonly prepared drawings for instrumentation and controls are logic diagrams, instrumentation index, loop diagrams, and interlock diagrams (or electrical schematics). Although many companies still design drawings manually before implementing them on a CAD system, some have moved to computer-based systems that produce a large portion of the design automatically. Such software packages save time and help produce a consistent design.

Logic diagrams

Logic diagrams are needed to define discrete (on/off) controls. These controls cover all time-based and state-based logic used in process control, including PLC sequences and hard-wired trip systems.

If the logic is simple, a written description in the control system definition or a description on the P&ID is generally adequate. However, whenever intricate logic is used, logic diagrams (Fig. 2), typically drawn to conform with ANSI/ISA Standard S5.2, are required.

Instrument index

An instrument index lists all items of instrumentation for a specific project or for a particular plant. Its purpose is to act as a cross-reference document for each item of instrumentation and for all documents and drawings related to the particular item. An instrument index is typically generated and maintained on a PC using a database program. A computer-based approach facilitates updating and retrieving data.

The instrument index is normally presented in tabular form (Fig. 3), is generated at the start of a project, and stays active throughout the life of the facility. The following items are typically shown on an instrument index:

– Tag number

– Description

– P&ID number

– Line/equipment number

– Instrument specification sheet number

– Manufacturer’s drawing numbers

– Loop drawing number

– Interlock diagram number

– Location diagram number

– Miscellaneous notes.

Some users add other information they consider important, such as the equipment supplier and model number, installation details, purchase order number, and the like.

Loop diagrams

A loop diagram should be prepared for each instrument loop in the project that contains more than one instrument. The only instruments not requiring loop diagrams are interlock systems (these instruments are shown on the interlock diagrams) and local devices such as relief valves (an instrument index entry should suffice for these devices).

Loop diagrams are generated to show the detailed arrangement for instrumentation components in all loops. All pneumatic and electronic devices with the same loop number are generally shown on the same loop diagram. The content and format of loop diagrams should conform to ANSI/ISA Standard S5.4.

Interlock diagrams

Interlock diagrams (electrical schematics) show the detailed wiring arrangement for discrete (on/off) control. However, with the introduction and extensive use of programmable electronic systems to perform logic functions, the use of interlock diagrams has diminished over the years.

A good schematic always agrees with its corresponding logic diagram. It has a tag number, location notation, and service description for all devices on the diagram. In addition, all rungs are numbered sequentially.

Specifications and drawings can be time consuming and tedious, but they are essential documents in the detail design phase of instrumentation and control projects. All documentation should be prepared by competent personnel, whether they be plant employees or outside engineering contractors, and whenever possible they should make use of computer-based design tools that save money and ensure a quality final product. — Edited by Jeanine Katzel, Senior Editor, 847-390-2701, j.katzel@cahners.com

Share your questions and answers related to this article with fellow readers by using our new “Ask the Expert” feature on Plant Engineering Online: www.plantengineering.com.

Key concepts

Taking time to perform front-end and detailed engineering for instrumentation and control projects minimizes costly revisions and equipment changes in the future.

Four specifications are typically prepared: instrument, control system, control panel (or control cabinet), and installation.

Common drawings for instrumentation and control are frequently prepared with the help of computer-based systems.

Instrument specification – flow transmitter

Company name: Revision: 1

Plant location: Issued date: 18 AUG 96 Issued by: NB

Project name: Revised date: 6 JAN 97 Revised by: CJ

Project number:

Related drawings

P&ID: PD-23928981 Loop diagram: LD-1234

P&ID location: D6 Electrical schematic: ES-10923872

Equip./Line No.: 4-TES-191 Wiring diagram: WD-0191YU

Service Desc.: CHEM A Mech./Piping drawing: M-22G

Instrument location drawer: L-19217

Installation drawing: ID-1018


Supplier information

Supplier’s name: RST Manufacturer’s name: XYZ

PO Number: PO-10918 Model number: FTX2927

Serial number: 282716

Instrument specifications

Element type: Vortex shedding

Element material: 316 stainless steel

Connection type: 4-in.-150RF

Output type: 4-20 mA

Output local indication: In engineering units

Power: 2 wire – 24 Vdc, +/-

Separate alarm contacts: Not required

Approval authority: UL

Attached tag: Yes, embossed stainless steel

Enclosure type: NEMA 4

Mounting: Vendor’s standard

Remote electronics: No. One unit with sensor

Calculation sheet: Required from vendor

Options: Vendor to supply calibration sheet


Process data

Flowing material: Beer

Material state: Liquids, clean

Pipe material: 316 stainless steel

Pipe diameter/schedule: 4 in.

Process connection: 150RF

Flow range: 200 USGPM

Alarm values: N/A

Required accuracy: 2.0% of full scale

Upstream operation pressure: 0 to 150 psig

Operating temperature: 0 to 100 F

Maximum pressure drop: Medium

Specific gravity: 1.0

Viscosity: 1 cp


More info

“Standards and Recommended Practices” and other publications and standards are available from ISA (the international society for measurement and control), 67 Alexander Dr., P.O. Box 12277, Research Triangle Park, NC 27709; 919-549-8411; fax: 919-549-8288; e-mail: info@isa.org; online: www.isa.org.

Questions about the technical content of this article may be directed to the author by phone at 905-305-0370, by fax at 905-305-9574, or e-mail at 104656.3603@compuserve.com

Previously published materials on this subject cited at the opening of this article are available on the Plant Engineering web site: www.plantengineering. com. Articles also may be purchased by calling 847-390-2692.