LEL determination in Class 1 Div 1 area

[Prchick]

Hello everyone. Looking for some guidance on LEL monitor design.

We currently have several LEL monitors located in our truck unloading area. These monitors are located in Class 1 Div 1 area since there may be some flammable material from disconnects (maybe a drop or two). We are trying to determine the alarm limits the monitors should be going off on for us to take serious action on. We will have a number of alarms go off and by the time someone goes out to investigate, there is no sign of a leak and the portable monitor reads 0. Is there any guidance on what these should be at? We have other monitors in Class 1, Div 2 areas at lower values since we shouldn't normally see gases in the area.

 

[Heaviside1925]

OP,
Just consider this post may be more appropriate in a chemE forum since this appears to be more of a PSM question. I understand that the Class/Div is referred to a lot in electrical design and is sometimes referred to as the electrical classification but how those boundaries are determined are PSM driven and in your case of truck unloading, I would expect API or NFPA referenced.  

Do you know why is this area is classified as Div 1 (API guidance, Hazop)? It sounds like, from your description, the LEL meters are being used for leak detection, which I would not consider the best method. Div 2, yes for LEL but for Div 1, consider that under normal operations vapors can be found in ignitable quantities in a Div 1, so 100% LEL would not be considered an abnormal condition.

If the monitoring of the Div 1 space is only for leak detection, consider a hydrocarbon (I am assuming this is a hydrocarbon) liquid detection monitor, I think there are some smart IR technologies out there too, but I am not familiar with them. The issue with atmospheric monitoring is its dependent on ambient conditions and the fluid properties, so on a hot day you may be setting an LEL meter all the time but on a cold day, you could have a steady leak and not even know it.

If the monitoring is industrial hygiene / exposure driven, then a continuous monitoring system should be used for that chemical.

 

[Prchick]

Thanks for the response. I wasn't sure where to put this since I was thinking it was more an instrumentation question.

This is in hydrocarbon service. They have it classified as Div 1 because there is always some material present when disconnecting the truck from the unloading arm as it's pressurized liquid. We usually see these alarms go off more during the winter when the material can condense.

Part of my struggle is figuring out why they would place them at these locations.

 

[Heaviside1925]

OP,
There should be an MOC you can track, otherwise they may have been put there by maintenance as a solution to some problem and only created another. If you can track down the MOC, I would talk to your PSM coordinator about this problem because it doesn't make much sense to have an LEL detector in a location that's already known to be at the LEL. That said, there may be a specific reason for the LEL detectors and in that case alternate instrumentation could be considered.

 

[Heaviside1925]

OP,
Just one more quick note.
They have it classified as Div 1 because there is always some material present when disconnecting the truck from the unloading arm as it's pressurized liquid.
Are you sure that's the reason or is that what you were told is the reason? If you look in API or NFPA guidance on hazardous locations, the area (I think is like a 3' sphere) around the loading/unloading nozzles on a tanker truck is recommend to be Class I Div 1. So, it may be code driven and not the result of anyone's opinion that it should be classified that way based on an activity. If that area is already classified in that way, then it should always be treated as such and not just whenever a detector goes off.