ISA TR18.2.3

General applicability

This technical report addresses alarm systems for facilities in the process industries to improve safety, quality, and productivity.

In the design stage, the alarm attributes are specified and designed based on the requirements found in the alarm philosophy and determined by rationalization (e.g., alarm priority and setpoint). There are three areas of design: basic alarm design, HMI design, and design of advanced alarming techniques. This technical report addresses considerations for the basic alarm design. Note that HMI design detailed requirements and recommendations can be found in ISA-18.2 Clause 11 and ANSI/ISA-101.01-2015, Human Machine Interfaces for Process Automation Systems. Guidance on advanced alarming techniques is provided in TR4.

Basic alarm design versus advanced alarm

Basic alarm design (TR3) versus enhanced and advanced alarm (TR4)

This technical report, TR3, addresses basic alarm design, while TR4 focuses on advanced alarm design—enhanced and advanced alarm methods. The purpose of this subclause is to provide clarity as to what methods and techniques are treated as basic alarm design versus those that are treated as advanced alarm design.

Basic alarm design

Basic alarm design covers the design and specification of individual alarm attributes and the selection of appropriate alarm types. Attributes can include deadband, priority, and on/off delay.

Process variable (PV) calculations, analog and logical, are considered part of the control system and basic alarm design. PV calculations can be complex yet have basic alarming functionality. This includes such techniques as:

a) common alarms, e.g., a common high-temperature alarm coming from multiple temperature transmitters on a tank or a common toxic-gas alarm coming from multiple gas detectors;

b) numeric calculations within the control system used with basic alarming, such as rate calculations (producing rate-of-change alarms if not already present), statistical calculations (producing statistical alarms, such as alarming on standard deviations, etc.), and other complex calculations;

c) simple to complex models used to estimate process values online, often referred to as virtual, soft, inferential sensors or equivalent terms; high-speed counters and accumulators, often needed in discrete manufacturing applications, which accumulate and aggregate within the control system before applying basic alarming;

d) logic calculations within the control system to create an alarm only when it is a valid alarm (e.g., a calculation including a logical AND of low pressure and the associated pump run status to create a logical PV that is alarmed).

NOTE If the alarm is created by the control system, and logic is added to conditionally suppress it, this falls under the definition of advanced alarming and is discussed in TR4.

Enhanced and advanced alarm methods

The enhanced and advanced alarm methods in TR4 include ways to modify alarm attributes dynamically for one or more process variables. Different from basic alarm design, advanced alarm design often has a larger scope (applies to multiple variables at once) and ensures that alarm attributes track the process (are relevant for different operating states). Additional layers of logic, programming, modeling, or a combination of these beyond basic alarming methods, are used to modify alarm behavior to ensure alarms are relevant when presented to the operator. These additional layers may be used to improve operator guidance to better meet the performance objectives of an alarm system. The techniques outlined in TR4 include the following examples:

a) advanced alarm methods provide ways to modify alarm behavior through such techniques as designed suppression and state-based alarming;

b) enhanced alarm methods provide ways to augment alarms with better operator guidance, including identification of causes, and redirecting the alarm to the location of the desired responder.

NOTE TR6 is also a relevant reference regarding modifying alarm behavior as batch processes typically require such functionality.