ISA TR18.2.4

Exclusions

The following is a list of items not discussed in this technicalreport. In general they are left out of this report, not becausethey are unimportant, but because they are already covered byanother ISA-18 technical report or otherwise assumed to be part ofbasic alarm management.

Alarm rationalization and basic alarmdesign

This technical report assumes that alarm rationalization andbasic alarm design, as discussed in ISA-18.2, are being performed.Rationalization, also discussed in dTR2, is performed at anappropriate level, basic to advanced, as determined by the alarmphilosophy (see dTR1) of the site. Basic alarm design is discussedin dTR3. Together these activities include such techniques asproperly chosen alarm setpoints, deadband selection, use of on andoff delays, and proper range specification.

Process variable calculations

Most process variable calculations (analog and logical) areperformed in the control system and considered as part of the basicalarm design process. PV calculations can be complex yet have basicalarming functionality. This includes such techniques as:

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

b) numeric calculations within the control system used withbasic alarming, such as rate calculations (producing rate-of-changealarms), statistical calculations (producing statistical alarms,such as alarming on standard deviations, etc.) and other complexcalculations

c) simple to complex models used to estimate process valuesonline, often referred to as virtual sensors

d) high-speed counters and accumulators, often needed indiscrete manufacturing applications, which accumulate and aggregatewithin the control system before applying basic alarming

e) logic calculations within the control system to create analarm only when it is a valid alarm, e.g., a calculation includinga logical AND of low pressure and the associated pump running, tocreate a logical PV that is alarmed

NOTE If the alarm is created by the control system, and logic isadded to conditionally suppress it, this falls under the definitionof advanced alarming and is discussed in Clause 6.

Operating displays for basic alarmingstrategies

Often it is important for operating display design and alarmdesign to go hand in hand. This may be for basic orenhanced/advanced alarm strategies. This technical report dealswith some human interface design issues but only as related to theenhanced/advanced alarming strategies presented.

Audible indication strategies

The use of different audible alarm indications (sounds) todistinguish operating consoles for multiple console control roomsis considered to be a human interface strategy, rather than anenhanced alarm strategy. For the purposes of this technical report, the use of specialized audible sounds is not covered here.

Alarm shelving

As discussed in 11.7 of ISA-18.2, alarm shelving is an importantrecommended function and can be used in a number of ways. It isalso discussed in dTR1. Its use is not considered advanced orenhanced alarming.

Maintenance strategies

As discussed in Clause 15 of ISA-18.2, maintenance is animportant topic to address in alarm management at any site, and itcan be addressed to varying degrees of sophistication, includingcommunication support between members of the operating team. Suchstrategies, though important, are not considered to beenhanced/advanced alarming for the purposes of this technicalreport.

Model-based predictive alarming/alerting

ISA-18.2, Clause 12, briefly discusses model-based predictivealarming. It is not included in this technical report at this time.Very often, application of these techniques does not result inexplicit operator notifications; and if there are notifications,they are often alerts and not alarms due to either the lack ofimmediate urgency or the inability of the system to treat them asalarms. As this technology develops and comes more into common use,such techniques may be included in a future version of thistechnical report.

Basic alarms on advanced control

Advanced control, such as model-based predictive control,usually needs some basic alarming to go with it. Though the overallcontrol strategies are advanced, the alarming may not be. Basicalarming on an advanced control system is not in the scope of thistechnical report.

Advanced alarm analysis techniques

There are many advanced alarm analysis techniques in use today,including relative time-based analysis and post-mortem,alarm-pattern recognition. Such techniques are discussed inTR5.

Edition:

12

Published:

09/06/2012

Number of Pages:

56

File Size:

1 file , 780 KB