HMI as the name represents, it is the interface between the human technician and machine. Humans in the control room are informed via alarms in the control panel about important events and fault alarms.
Alarms can be displayed on an advertiser panel, individual indicators, a VDU screen or a programmable display device. Depending on their function, alarm displays can be colour coded. An example might be as follows:
| Alarms | Yellow |
|---|---|
| Pre-alarms | Orange |
| Shutdown | Red |
| Bypass | Red |
Alarm lists should be properly designed to ensure that high-priority alarms are readily identified, that low-priority alarms are not missed, and that the list is readable even in high-alarm or repeat-alarm periods.
In terms of which alarms need the most immediate user consideration, alarms should be prioritized. Allowing alarms to be attached to an alarm system without specific priority requirements can result in alarms that have the same implications being allocated to different priorities. A proposed warning breakdown by system group priorities is: 14% High Priority, 44% Medium Priority, 42% Low Priority.
Alarms should be presented within the operators field of view, and use consistent presentation style (color, flash rate, naming convention)
Each alarm should provide appropriate user data to be readily identified for the alarm situation, affected facility, required intervention, alarm frequency, alarm time and alarm status. It is important to avoid vague or misleading warning signals, e.g. when a boiler feed water pump has dropped the message should be “BFW Pump Tripped” and not “BFW Pump Low Pressure.”
One of the most ignored elements of the design of the alarm system is to align the detectors with the screens. Making the wrong decisions in meshing the 2 can have far-reaching implications and can hinder the user to reach the process’s problem area.
Another point to consider when meshing display alarms is how the alarm colors relate to other displays used.
The color coding used for the alarms should not conflict with any other part of the display system. Color coding inconsistencies cause delays in the processing of information as the technician must decipher the sense in which the color is used, e.g. “RED” for emergency vs. pump / fan / motor state, closed valve, etc.
The visual display device can be increased by audible warnings, which at the frequency of the signal should be considerably higher at a level than the ambient noise. Where multiple audible warnings exist, they should be designed to distinguish them readily from each other and emergency alarm systems.
In heavy operator workload cases, they should be designed to avoid disruption of the operator. Where both constant frequency and variable frequency signals (including pulsed or intermittent) are used, a higher level of risk or a more immediate need for action should be demonstrated later.
