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:

Back to the Basics: Time to Alarm

Each drying process has a unique rate of solvent increase in the normal and upset conditions. The potential maximum rate of solvent concentration increase should be estimated, so that the analyzer has time to generate an alarm, and the method of corrective action has time to reduce the solvent concentration, before a flammable or explosive limit is reached in the dryer. 

Some of the factors include: 

Back to the Basics: Total Analyzer System Response Time

The response time of the analyzer system, including all components in the final installation, is so critical that it should be given careful attention. 

One useful method of testing the system time response of the analyzer is to inject test gas directly into the end of the probe inside the process, and to obtain the time that it takes for the alarm to sound. The test gas concentration should be equivalent to at least 10% LFL above the high alarm point. 

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