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. 

Back to the Basics: Analyzer Response Time

Most analyzer specifications clearly indicate the time needed for the analyzer reading to reach 63% (or 90%) of a final reading, in response to a sudden increase in concentration. 

These times are based on the response of the analyzer alone, and do not take into account the following that may be present in the complete analyzer system as installed: 

Sample Transport Time:

  • sample tubing 
  • filtration

Alarm System: 

Back to the Basics: Response time - Sample Transport

The speed at which the sample is drawn from the process and reaches the detector is critical. 

It is known that in many cases the response time of the entire sensing system must be not more than a few seconds. The following add crucial seconds & should be avoided:

Back to the Basics:Continuous monitoring

Authorities require that an analyzer makes “continuous” measurements. 

Portable devices used for occasional checks are not recognized. Sequential sampling systems, which take samples from several locations in the dryer, and multiplex the sample stream to a single analyzer, are not recognized. 

A reasonable definition of a “continuous” analyzer is that it can detect a sudden increase in solvent concentration in time to make an effective alarm, at any time during the dryer’s operation.

Back to the Basics: Effects of Desensitizing Substances

The desensitizing effects of several substances common to industrial processes are well known. 

Back to the Basics: Effects of Condensation

As for the sampling system, the sensor, and all its components in contact with the sample, must be heated to prevent condensation. 

The analyzer must be heated to at least the highest flash point of all the solvents, with perhaps a 10°C margin, so that the solvent mixture produced in the heated process does not condense in the analyzer tubing and detector, causing a significant, possibly a complete, loss of reading. 

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