LEL measurement of NCG

N

Thread Starter

Neil W. W

I presently have one supplier that can provide a method of measurement by introducing a sample to a flame of known temperature, the sample causes a controlled explosion. The resulting increase in temperature is proportional to measure of combustables, thereby an indication of LEL. This approach is proven but relatively expensive to install. Electrochemical and Infarred technologies have been discounted since they apparently only measure constituents of the gas, not the entire mixture. Suggestions?
 
Chemistry can tell the LEL based on molecular constituants of the mixture. A gas analyzer(mass spectrometer) is designed for providing quantitative proportion of the constituants. Then you have the tools to solve the problem. Safety and money are NOT synonymous words.
 
> Chemistry can tell the LEL based on molecular constituants of the mixture. A gas analyzer(mass spectrometer) is designed for providing quantitative proportion of the constituants. > Then you have the tools to solve the problem. > Safety and money are NOT synonymous words. Agreed. My intent in posting this request is to find alternative suppliers, perhaps with the same or an alternate approach, thereby creating a competitive commercial environment without sacrifice to safety. Thank you for your input. NWW
 
I think that you need to supply some more information regarding your application. In my experience, ambient LELs are commonly measured by catalytic bead point sensors. There are some solid state sensors available but I don't think that they are as commonly used. Infrared point sensors are a new breed of sensors that have become available recently. Another LEL sensor type that has become available is the IR open-path analyzer. Electrochemicals are more commonly used to measure specific toxics or low level specific hydrocarbon levels.

> Electrochemical and Infrared technologies have been discounted since they
> apparently only measure constituents of the gas, not the entire mixture.

I don't understand this statement in regards to commonly available LEL detectors. Please explain some more. For some manufacturers of LEL detectors, check out General Monitors, Teledyne, Gas Tech, & MSA for catalytic bead types. General Monitors and Drager for point infrared. Spectrex, Drager, MSA, and General Monitors for open path analyzers. Bill Mostia ======================================================= William(Bill) L. Mostia, Jr. PE Independent I &E Consultant WLM Engineering Co. P.O. Box 1129 Kemah, TX 77565 [email protected] 281-334-3169
 
B
I apologize for the lateness of the reply, it's been quite a while since I've been on this site.

Neil, I believe that the type of sensor you are referring to is a flame ionization device (sometimes referred to as a FID). I have no practical experience with this type of sensor, so I can't help there.

IR sensors are available that will measure the hydrocarbon content in the area (either open path or point type). They will measure the gas even if there is no O2 in the area. This is a product of the IR energy absorbtion that the sensor looks for. It is typically designed to look for a relatively narrow IR band where hydrocarbons absorb the most energy. Advantages: these sensors cannot be poisioned; calibrations can be spread out over a longer period of time. Disadvantage: LEL reading even when there is no O2 in the area. If this in in an open area this may be a moot point, if its is an enclosed area that may not be opened very much it may be an important point. If considering this type of sensor, an O2 sensor (typically an electrochemical type) may be justified to add to the scheme and some type of logic applied w/in the controller to cross check.

I agree with Bill in that the catalytic type of LEL sensor is a good general LEL sensor. It requires O2 and the fuel gas to produce a signal, so the reading is a representation of the flammability of the surrounding environment. It does have a few drawbacks, it can be poisioned (desensitized) by compounds such as silicones, high sulfur containing compounds, and lead (eg. leaded gasolines). It also requires periodic calibrations to ensure accuracy. If this is to be used as a safety monitoring device, the calibrations can be used as a means to verify operation (this can also be used to justify the labor expense to do the calibrations).

In any case, whatever type of technology chosen and whomever you get it from, you want to keep these items in mind:

What are the gasses I want to monitor for (this includes the constitutients of the flammable gas)?

What is the recommended cal interval?

What are the known poisions to this sensor?

What is the recommended calibration gas (the type may vary depending upon the makeup of the monitored gas)?

Area classification where the sensor will be placed and area classification of the sensor (CL I Div 1 Gr B,C, & D etc... sometimes this is not readily apparent from the manufacturer's literature). Also what type of approvals (CSA, UL, CENELEC) does the equipment carry (get copies of the certifications and not rely upon the spoken word or manufacturer's lit).

Type of area where the sensor will be placed (dirty, dusty, very humid/wet etc...).

These are a few items I can think of right now... If anything else pops into mind I'll add to the post.

Bob Kuzmich, Test Engineer, Industrial Scientific Corporation, 7848 Steubenville Pike, Oakdale PA 15071, 1-800-338-3287
 
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