Alarming 101

In a process alarm control panel situation, an alarm functions as a visual and audible indicator that a process has deviated from normal conditions. Generally speaking, these are classified into two broad categories; low level and high level alarms. The low level alarms simply function to let an operator know that the process is deviating from the intended steady-state operating set points. This could be something as simple as a low or high pressure reading at a pressure indicating transmitter [PIT] or as complex as the initial detection of a % LEL in a compressor building. Operations must then take action to get the process back under control. If a process deviates to a point where it can create a dangerous condition, a high level alarm is triggered. Typically a high level alarm also causes the entire process to be shut down. Inlet and outlet control valves will shut to contain the area, pumps and compressors will be stopped, etc. A high level alarm is essentially the same thing as a low level alarm, but the process has reached a critical level. An example would be a tank level reaching zero or the pressure in a pipe getting close to the Maximum Allowable Operating Pressure [MAOP/MAWP].

Alarm Signal Analysis

There are numerous ways a signal from a transmitter can be analyzed and processed. Typically, the alarm control panel in the area will contain a Programmable Logic Controller [PLC]. The PLC is where most of the signals analysis will occur. This typically means that the signal from the transmitters responsible for the alarm are de-bounced to filter out any electrical noise from the analog inputs. If the sensors are communicating with some industrial protocol such as I/O Link, Ethernet/IP or Profi-Net, then the signal de-bouncing routine is not required. After the signal has been processed, there typically is timer function in the logic that will only activate the alarm if all of the conditions have been met for a specified amount of time. Simultaneously, the PLC is usually networked with a Distributed Control System [DCS] and Supervisory Control and Data Acquisition [SCADA]. The alarms that can be acknowledged at the alarm control panel can also be seen and acknowledged in the control room via the DCS or remotely through SCADA.

Alarm Set Up

The final piece of alarm analysis is how they are set up in a plant. The alarm control panel in the area typically has an HMI where operations personnel can program the alarm set points as well as program the alarm. Typically the control panel will allow a variety of inputs ranging from analog I/O to more modern industrial protocols such as Ethernet/IP and Modbus/TCP. It is important to know that the alarm control panel will usually be integrated with the SCADA and DCS for the entire plant. This is done to provide operations with seamless capability to modify, create and acknowledge alarms without the requirement that an operator is physically present at the control panel.

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