Back to the Basics: Fault Detection

Analyzers designed to industry standards incorporate malfunction indicators and relay outputs for most potential detectable faults, such as: 

  • low sample flow 
  • downscale reading
  • electrical fault in sensor wire 

It is essential to connect these to the control system so that the operator is notified and the process reverts to a safe condition during an analyzer malfunction. 



Back to the Basics: Hard-Wired Relays for Danger and Malfunction

While programmable controls and computers provide a great degree of convenience, unless these controls can be shown to conform to strict standards for the use of programmable devices in safety systems, it is preferable to use hard-wired connections for the critical alarm and analyzer malfunction signals. 

Back to the Basics: Interlocks Between Calibration and Control System

Where an analyzer’s signal controls the ventilation rate, it is particularly important to prevent a false control system during the “zero” portion of calibration, where zero gas is injected to the analyzer, forcing the analyzer’s signal to 0% LFL. 

This would force the control system to reduce ventilation to an unsafe level, since the analyzer, for the duration of the zero calibration, reports a false low solvent concentration.

Back to the Basics: Limit to Adjustment of Calibration

In normal uses, most analyzers will require adjustment to their sensitivity to make up for a gradual reduction in sensitivity over time. 

The normal amount of adjustment of the signal should be well understood by the maintenance personnel so that problems other than loss of sensitivity are not corrected by the improper use of the "span" adjustment. 

For Example:

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: 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. 

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.