Level Measurement & Control


Why are rate of change alarms used?

March 2022 Level Measurement & Control

Rate of change alarms are used to predict an event that is abnormal behaviour for a system or process, with the objective of taking preventive action in order to avoid system or process failure or damage. If a process value suddenly increases at a rate beyond the norm, then it indicates something unusual has changed in the system or process. This is different to an absolute (low or high) alarm.

Rate of change can be an increasing rate or decreasing rate. Using rate of change alarms also means you need to understand your process dynamics very well. You are taking expert knowledge and implementing it to safeguard your plant in the absence of the expert.

Rate of change alarms differ from normal (level) alarms in that they respond to a specified rate of signal change rather than to the crossing of an absolute threshold in the measured variable. A good example is a process variable that may or may not have a fixed alarm point but we need to predict when the trajectory of the process is going to harm the smooth running of the process or to prevent catastrophic failure and damage.

We also know that physical process parameters often over-shoot the desired limits, hence the use of process controllers using PID to manage this. However, in failure mode a process can still cause damage if the controller cannot respond to manage the process. Rate of change alarms will kick in only when the normal rate of change is exceeded. This pre-supposes that the normal rate of change is determined for a given process or system, often by an expert with intimate knowledge of the system.

Here are some examples of rate of change alarms and how an effective system design responds to them:

1. A reservoir that is emptying too quickly may need to trigger additional pumps into action to refill the reservoir. By predicting the low level before it happens, corrective action is taken before the low-level alarm triggers.

2. A sudden increase in the levels of a contaminant may need to trigger servicing of the system. The high level of contaminants can be predicted before they can cause damage, for example, gearbox oil contamination initiating a shorter service interval alert to pre-empt maintenance.

3. A rapid rate of temperature increase may indicate imminent damage or catastrophic failure before normal alarm limits are reached. Corrective action is taken before alarm limits are reached, possibly preventing motor winding damage.

Often the domain of control and safety software, rate of change alarms can be implemented in hardware. This has the advantage of adding an additional layer of protection to systems or processes. The Omniterm TTT from Omniflex, with its rate of change alarms option, provides just such an additional layer of plant protection.


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