Guidelines for Process Alarm Systems: Alarm System Management

This article is about Guidelines for Process Alarm Systems of Alarm Management System.

Alarm System Management: Guidelines for Process Alarm Systems

Alarms are an important part of the process control system and are used to alert the operator of any abnormal conditions that may arise. Alarm systems are designed to provide rapid access to the tools needed by the operator to perform corrective action and provide a comprehensive historical record of the information needed to assess such abnormal conditions. This article will provide guidelines for the design of process alarm systems, outlining the configuration of the alarms, visual and audible alarm indication, alarm acknowledgment, alarm inhibition and alarm shelving, and alarm printing.

Alarm Priorities and Configuration

Process Alarms shall be assigned to one of four priority levels depending upon the criticality of the alarm. The alarm priorities shall be assigned using a consistent alarm philosophy and shall accurately convey the seriousness of the situation which is being alarmed. The overall distribution of alarms by priority should adhere to the industry best practices as follows:

Alarm Priority Percentage of total alarms configured
Priority 3 80%
Priority 2 15%
Priority 1 5%

Four alarm levels shall be used as a minimum: HH (high high), H (high), L (low), and LL (low low).

These levels may be used in association with any category. All automatic trip setpoints shall be pre-alarmed, including trip setpoints implemented in auxiliary systems such as ESD systems. All process alarms shall be configured with an alarm deadband to prevent excessive re-alarming of a tag when the measurement is fluctuating near to the alarm setpoint.

Visual Alarm Indication

Process Alarms – Alarms shall be invisible on the operator graphics, appearing only while an alarm is active. All alarms shall be displayed with a small square or rectangular shape on process graphic associated with the tag. The color of the shape shall be configured to correlate to the alarm priority. The color code for priority two and three alarms shall be determined on a project-by-project basis. The system shall display unacknowledged alarms with a blinking background color. Blinking shall cease when the alarm is acknowledged; however, the background color indicating an alarm is present shall remain until the alarm condition is cleared. The system shall display alarms which are unacknowledged and have returned to normal with a visibly distinct appearance from unacknowledged, active alarms.

Overall Indication. All process displays shall have an overall process alarm status indicator. The indicator shall convey whether alarms are active, the highest priority active alarm and whether there are any unacknowledged alarms in the process area to which the display is associated.

Audible Alarm Annunciation

Alarms shall be annunciated only on the workstation(s) or console configured for those alarms. Distinct audible tones shall be used to distinguish between the three different priority process alarms. A fourth tone shall be used to indicate system alarms. Audible tone decibel levels shall be loud enough to be heard over normal control room background noise. Audible alarm tone shall be adjustable by engineering with proper access credentials. The audible alarm signal for an operator console shall continue until either a “horn silence” is initiated at the operator console or an active alarm is “selected” (on either alarm summary or other displays).

Alarm Acknowledgment
Alarms may be acknowledged only at consoles configured for those alarms. It shall be possible for an operator to acknowledge any alarm configured at a workstation by no more than two actions. An alarm shall be acknowledgeable only if it is shown on an active display.

Alarm Inhibit and Alarm Shelving

Alarms shall be grouped by equipment and by process unit to enable inhibiting / disabling and re-enabling alarms by group when the equipment of process unit is taken in and out of service. Inhibiting of alarms shall be logged in the system with the time and date that the alarm was inhibited. The system shall be supplied with the capability to move an active alarm off of the current alarm display into a separate page. This function is sometimes referred to as ‘Alarm Shelving’. The purpose is to remove alarms which are planned to be active for extended periods of time out of the current alarm display so that the operators may focus on only the most critical alarms. The system shall be supplied with the capability to produce a list of alarms which are inhibited and / or shelved for both display and print-out.

Alarm Printing
Printing of alarms at the time of the alarm or event shall not be implemented. Capabilities shall be provided on all systems to store alarms in an on-line database and to produce a report of alarms and/or events during user-defined time periods which can be printed at the time the report is generated.

System Alarms

The system shall be configured to activate system alarms for failures to the following as a minimum: failed modules, communication errors, power supply failures, cabinet fan failure, cabinet high temperature, smoke or incipient fire detection, diagnostic error detections and messages. System alarms shall initiate both audible and visual annunciation at the operator console to which the failed equipment is associated.

A “System Alarm Summary” display showing all active system alarms shall be provided. System alarms shall be configured to have a distinct horn tone and visual indication at the console to distinguish system alarms from process alarms.

Alarm systems are an important part of the process control system and are used to alert the operator of any abnormal conditions that may arise. Alarms must be configured to provide the operator with the information and tools needed to respond to the alarm quickly and effectively. This article provided guidelines for the design of process alarm systems, outlining the configuration of the alarms, visual and audible alarm indication, alarm acknowledgment, alarm inhibition and alarm shelving, and alarm printing.

Logging of Operator and Engineering Actions

In order to ensure that process control systems are properly maintained and monitored, it is important to log operator and engineering actions. This log should track user name, time of change, and an abbreviated text of the change. It should also be divided into “operation” and “engineering”.

Operator actions that should be logged include changes to controller mode, setpoint, output, acknowledgement of active alarms, toggle of alarms between inhibit and enable, changes to alarm limits, activating soft-bypasses of ESD points, and responses to operator prompts.

Engineering actions that should be logged include changes to tuning parameters, download or modification of tag or module configuration, modification to software used by the PCS, forcing members of redundant pairs on or off primary status, placing devices on-line or off-line, placing tags on-scan or off-scan, and responses to engineer prompts.

Alarm and Events History

Process control systems should also store all process alarms, system alarms, sequence of events messages, and operator, engineering or maintenance actions for a minimum period of seven days. For systems which define the alarm retention time based on the number of messages, the system should store a minimum number of events. Alarm and event data should be stored using a First-in / First-out mechanism to prevent too many alarms from being stored on the system. The system should delete the oldest messages to enable storage of new messages once the minimum retention period or maximum number of alarms has been reached.

System Redundancy

Process control systems should also be designed with redundancy in mind. Redundancy is an important factor in ensuring that the process control system is able to continue running in the event of a failure. Redundancy can be achieved by using multiple computers, networks, or controllers in a system, as well as using redundant components such as power supplies or other hardware components.

Guidelines for Process Alarm Systems: Alarm System Management