Enhanced and Advanced Alarm Methods
|Publication Date:||6 September 2012|
The following is a list of items not discussed in this technical report. In general they are left out of this report, not because they are unimportant, but because they are already covered by another ISA-18 technical report or otherwise assumed to be part of basic alarm management.
Alarm rationalization and basic alarm design
This technical report assumes that alarm rationalization and basic alarm design, as discussed in ISA-18.2, are being performed. Rationalization, also discussed in dTR2, is performed at an appropriate level, basic to advanced, as determined by the alarm philosophy (see dTR1) of the site. Basic alarm design is discussed in dTR3. Together these activities include such techniques as properly chosen alarm setpoints, deadband selection, use of on and off delays, and proper range specification.
Process variable calculations
Most process variable calculations (analog and logical) are performed in the control system and considered as part of the basic alarm design process. 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), 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 sensors
d) high-speed counters and accumulators, often needed in discrete manufacturing applications, which accumulate and aggregate within the control system before applying basic alarming
e) 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 running, 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 Clause 6.
Operating displays for basic alarming strategies
Often it is important for operating display design and alarm design to go hand in hand. This may be for basic or enhanced/advanced alarm strategies. This technical report deals with some human interface design issues but only as related to the enhanced/advanced alarming strategies presented.
Audible indication strategies
The use of different audible alarm indications (sounds) to distinguish operating consoles for multiple console control rooms is considered to be a human interface strategy, rather than an enhanced alarm strategy. For the purposes of this technical report , the use of specialized audible sounds is not covered here.
As discussed in 11.7 of ISA-18.2, alarm shelving is an important recommended function and can be used in a number of ways. It is also discussed in dTR1. Its use is not considered advanced or enhanced alarming.
As discussed in Clause 15 of ISA-18.2, maintenance is an important topic to address in alarm management at any site, and it can be addressed to varying degrees of sophistication, including communication support between members of the operating team. Such strategies, though important, are not considered to be enhanced/advanced alarming for the purposes of this technical report.
Model-based predictive alarming/alerting
ISA-18.2, Clause 12, briefly discusses model-based predictive alarming. It is not included in this technical report at this time. Very often, application of these techniques does not result in explicit operator notifications; and if there are notifications, they are often alerts and not alarms due to either the lack of immediate urgency or the inability of the system to treat them as alarms. As this technology develops and comes more into common use, such techniques may be included in a future version of this technical 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 overall control strategies are advanced, the alarming may not be. Basic alarming on an advanced control system is not in the scope of this technical 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 in TR5.