Back to the Basics: Alarm Investigation

Safety standards often require that, the analyzer must produce an alarm whenever the solvent concentration exceeds 60% LFL or some level that leaves a margin of safety. All analyzer systems should have provisions that prevent alarm levels without enough of a safety margin.

All alarms and deviations from expected operation are opportunities to determine if a fault exists in the process or analyzer system.

Back to the Basics: Rate-of-rise alarm

An alarm which activates when the rate of increase in solvent concentration exceeds the rate alarm level, can give early warning when a severe upset condition exists that is likely to eventually result in a very high solvent concentration. 

Because the alarm triggers on a change in reading per second, and not a reading level, the rate alarm can activate at the very beginning of the analyzer's response time rather than several seconds or more later when the reading has finally climbed above the warning or danger alarm levels. 

Back to the Basics: Routine Testing of Alarms

A positive test for correct operation of critical alarms is essential. 

At a minimum this should be performed as part of a regular yearly audit that includes verification of the alarm relays, alarm annunciators, fault relays and all corrective actions, including:

  • shut-down 
  • emergency-stop 
  • dampers 
  • off-coat 
  • burners 
  • horns 
  • lights

Back to the Basics: Two Alarm Levels

Two alarm levels provide a more complete system less likely to cause unwanted shut-downs. 

Back to the Basics: Operator Training & Visibility of Alarms

Regardless of the amount of automation in the process, it is critical to train all personnel, including operators and maintenance workers, on the exact meaning of the analyzer readings and alarms.

Following any shut-down, the process should not re-start until the alarms have been manually reset and cleared, and the cause of the alarm understood and corrected.

The alarm and reading of the analyzer should also be easily and quickly visible by the operator, so that prompt and clear actions can be taken in response to a dangerous condition. 

Happy Holidays


Warmest thoughts and best wishes for a wonderful holiday and a very Happy New Year from all of us at Control Instruments Corp.


Wishing you the gifts of the season - Peace, Joy, & Hope.


Back to the Basics: Alarm Actions

An analyzer system should be reliable. At minimum alarm actions include an annunciator to notify the operator with a loud, unique sounding, horn or buzzer at the "warning" level, and relays to initiate a shut-down at the “danger" level.

The shut-down should include (as appropriate):

  1. Stopping solvent input (stop conveyer, stop coater)
  2. Heating (which slows vaporization and possibly prevents a source of ignition)
  3. Increasing ventilation to a maximum


Back to the Basics: Accuracy Requirements for Transient Conditions

For transient conditions the response time of the analyzer is critical.

Upsets can cause the solvent concentration to reach 100% LFL within seconds. In the US, NFPA-86 indicates that five seconds’ response time could be required of the analyzer in order to make an effective alarm. Experience has shown that process upsets can produce solvent increases of 10% LFL per second or more.

The time lag in an analyzer’s response causes “dynamic error,” which is the instantaneous difference between the actual solvent concentration and the analyzer reading.

Back to the Basics: Accuracy Requirements for Steady-State Conditions

Common practice is to assume complete accuracy in the LFL values from whatever authority is recognized at the time. But, as we have seen, one can reasonably assign an uncertainty of 10% LFL to the initial LFL values, as indicated by the precision of the published LFL values and the amount of agreement between the various competent authorities. Temperature effects can account for perhaps an additional 10% LFL, if one chooses to use the lesser amount of correction and in a particular case, the greater amount of correction is appropriate.

Back to the Basics: Accuracy and Response Time Requirements

Accuracy and response time are closely related. The purpose of the analyzer system is to produce an alarm before the solvent concentration can increase to an unsafe level. This implies that the alarm is given in sufficient time to take effective corrective action. 

There are two cases to be considered: 

  1. the steady-state (time invariant)
  2. the transient (dynamic) 

This week let's look at the steady-state: