Many fires and explosions occur in facilities which thought they had adequate protection against such hazards. Selecting a sensor which responds accurately to the combustible vapors found in process applications is essential. But so is the speed of response.
NFPA 86 requires the use of continuous analyzers to measure the potential flammability of vapor concentrations in any oven or dryer zone exceeding 25% LFL, “It can not be emphasized too strongly that the solvent vapor concentration measurement system is to have a very fast response time so that corrective action can be taken in response to upsets such as excessive introduction of solvent into the oven…A response time of as little as 5 seconds might be required in some cases" (NFPA 86).
The speed at which a sample is drawn from the process and reaches the analyzer's detector is critical. Long sample lines, lines with small radius bends or in-line sample conditioning filters, can induce dangerous delays.
Keeping this in mind, let’s compare the installation of a flammability analyzer and catalytic sensor in a process application:
Response Time in Catalytic Sensors
By their very nature catalytic bead sensors are relatively slow devices. Since a hot surface itself can act as a source of ignition, catalytic-bead sensors are shielded by a sintered-metal cap or a series of fine-mesh wire screens. These protective flame arrestors slow the ability of a flammable gas to reach the catalytic sensing element. Although some product specifications may give a response time based upon the diffusion rate of a light gas such as methane, process solvent vapors such as toluene or xylene diffuse much more slowly.
That slow response alone may eliminate a catalytic sensor from consideration as a process monitoring device. They are generally well suited to area-monitoring applications where response times of ten to twenty seconds are acceptable.
Response Time in Flammability Analyzers
A flammability analyzer on the other hand has a less than 1-second response time. It can be mounted directly on the process duct without heat trace sample lines, pumps, or blowers, eliminating the unpredictability of moving parts and shortening the sample path significantly. This radically reduces sample delivery time, while ultimately accelerating response time.
An analyzer’s T90 time is pretty meaningless if it isn’t designed and installed correctly. A time lag from sample tubing and sample filters can take too many seconds. So consider only the “time to alarm” as the analyzer is actually installed. Of course, a slow analyzer, like a catalytic sensor, never gets any better even if it is installed perfectly.