Because alarms are important devices in industrial process systems, it is important to realize that you cannot run out and buy the first one you see. There are different alarms for different needs. Knowing your system requirements and heeding the advice given in the following article will help ensure that you find the alarm system that best suits the needs of your application.
1. Fit Your ConditionsThere are basically two types to choose from: process/temperature alarms or equipment alarms. The former are activated when process parameters are exceeded; the latter identify equipment malfunctions that can affect the process by disrupting normal instrument hardware or software operation.
2. Link to a Separate CircuitIf you decide to install a process or temperature unit, make sure it is linked to a separate circuit from the main temperature controller. Including the alarm within the temperature controller would not provide fail-safe operation in the event of a low temperature reading caused by, for example, shorting of the thermocouple leads.
3. Know How Many You NeedWhen purchasing a process alarm, you need to decide how many alarms are required per unit. Some instruments provide as many as four alarms; however, it is worth bearing in mind that HI and LO alarms only provide annunciation, while HI-HI and LO-LO alarms provide both annunciation and shutdown interlocks.
4. Detect ChangesIf you want the process alarm to detect changes in temperature and flow, then you should consider a deviation alarm. This type of alarm is activated when preset deviation values are exceeded for a user-defined time period. The alarm relay remains activated until the deviation decreases below the limit.
5. Consider CustomizedA host of equipment alarms is available, and it is worth considering investing in a customized system to avoid serious malfunctions. All equipment alarms, like sensor-break and loop-break alarms, generate display and communications error messages and can be connected to fixed relay alarm outputs that default to safe conditions.
6. Choose the Right ModeIt is important to choose the right equipment alarm mode to enable the process engineer to select the most appropriate alarm response for the conditions. Alarm modes define how an alarm is detected and executed.
7. Identify Critical ChangesLatched alarms remain in the alarm state until an operator intervenes to disable the “threat.” Standard non-latched alarms reset when the measured value moves back into the “safe zone.” While both are equally effective, consider your process and whether having the operator aware of and responsive to process deviations is an absolute necessity. If it is, be sure to specify latched alarms.
8. Build in Delays and StopsAlarm delay and inhibitor modes can be useful where less critical system malfunctions are concerned. Alarm delays are used to delay alarms for a short time to avoid nuisance trips. Inhibitor modes suppress the alarm when maintenance activity is taking place or during operations not involving the alarm equipment.
9. Go LogicalWhen alarm modes are combined using a logical function within the instrument, the output then is attached to a single relay output. This enables alarm strategies that can eliminate multiple external alarm-collection hardware.
10. Select Fail-Safe AlarmsFor an equipment alarm, combining the output relay’s energize or de-energize feature offers a greater degree of flexibility when it comes to selecting an alarm strategy that matches specific plant requirements. An alarm that is configured such that its coil is energized and its normally open (NO) contacts are closed when the alarm is off is considered fail-safe because the external circuit through the alarm relay is complete only if there is no alarm and the instrument is working properly.
In conclusion, with a little planning, controls users can create an alarm strategy that helps ensure safe and effective operation of their processes.