We've discussed the 6 essentials of meeting NFPA 86, this week let's take a closer look at the second monitoring requirment: Speed of Response.
5 second System Response Time
The caution note found in section 220.127.116.11 warns operators of the need for fast response, stating that in many cases the system “shall be capable of detecting and responding to process upset conditions to initiate reduction of the vapor concentration before the concentration exceeds 50 percent of the LFL.” Annex E of NFPA 86 adds “It cannot be emphasized too strongly that the solvent vapor concentration measurement system is to have a very fast response time...A response time of as little as 5 seconds might be required”.
System response time is the sum of the sample delivery time plus the sensor response time. This further emphasizes the importance of short sample delivery lines and no sequential sampling. Annex A states “This response time requires the controller to be located close to the sampling point...and precludes the use of one controller sequentially sampling multiple points.”
Of the sensor choices listed in Annex E, catalytic sensors have the longest response times. The other sensors have fast responses, but care must be taken to select a sensor that can accurately measure all solvents present in the sample stream.