LEL 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.

 

[EmmanuelTop]

The only way to get the exact (or as close to exact as possible) values of alarm set points, is to perform quantitative risk assessment - by analyzing possible/potential scenarios. This means: location(s) of the release, pressure, hole size, ambient conditions, site geometry, etc. This will tell you how the loss of containment occurs and what concentration profiles you can expect. There are various software that can do this, and there are specialized companies that can do this. See which option is more effective in your case.

An example of FERA (Fire & Explosion Risk Assessment) Guideline is available on the link:

https://psmegypt.com/wp-content/upl...-Explosion-Risk-Assessment-FREA-Guideline.pdf

 

[1503-44]

In my opinion that's going about things kind of backwards.
Between a monitor and a drop of gasoline you will always find a LEL threshold. If your monitor is within that threshold, it triggers. But if 1 drop of gasoline is not a serious event, its a false alarm. Proximity detection is not what you want to know.

Why not do as Emmanuel suggests, first determine what events constitute the dangerous conditions that you want the monitors to sense? Then you can simulate the resulting LEL envelop, locate your monitors in appropriate places and set them to detect a given concentration, such as 50% LEL persisting for 5 minutes, or some similar criteria.

You should be able to easily make a basic cloud dispersion analysis, or you might even try a physical simulation by setting out a few open top barrels of gasoline in a simulated loading area and testing your monitor responses to various distances and settings over time under no, or light wind conditions.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[georgeverghese]

This could only occur where you've got top loading gantries for volatile liquids, I suspect. Speak to loading arm suppliers if there is some other type of top loading arm which compensates for the downward movement of the tank ( and maintains a tighter fit to the tank top fill nozzle) as it fills up.

 

[Heaviside1925]

OP,
I would assume the driver for the Div I rating is from an API recommended practice or NFPA 497. For top loading of a tanker truck (I think, I don't have it in front of me) it's a 5 meter sphere around the loading hatch. These recommended geometries are used because the plant doesn't need to perform dispersion modeling around every possible leak point in the facility and it aids in simplifying design for electrical classification, consideration of ignition sources as well as EHS driven exposure guidelines. It's this last reason is what I suspect is why there are LEL meters in a Div 1 area.

Before going into this last reason see the following from API 505 - Annex G: If an area contains equipment that may release flammable gases or vapors within the area during normal operations, gas detectors are not a feasible alternative unless some degree of ventilation is provided, since frequent alarms or equipment shutdowns, or both, are likely to occur.

My suspicions about why there are LEL monitors in a Div 1 area are due to a project ~10 years ago involving both truck and rail loading and what was a simple instrument installation project that turned into a full-blown rewrite of companywide SOPs. If you review operator SOPs I would guess that EHS requires operators to either not be in Div 1 areas or if they are, a high level of PPE is required. So how is an operator supposed to visually gauge the tank filling from outside of this Div 1 area? Operations gets maintenance to install some electrically rated LEL meters in the Div 1 location, so they are able to justify working in a Div 1 area and still "meet" EHS SOPs. From the outside, this seems to be a logical way of keeping all the stakeholders happy but defeats the whole purpose of EHS using the area hazard rating to limit exposure. It can also justify unsafe work practices. It was discovered in the project I previously referenced that the operators had ways of gauging that did not involve them being exposed to the hazard area, but they didn't want to continuously gauge, so they would instead squeeze between the guardrails and move the fill lid over a bit so they could see how close they were getting prior to them starting to gauge. Bear in mind exposure limits do not necessarily correspond to LEL. As other posters mentioned, a few drops likely do not constitute an unsafe condition or are even considered outside of normal operations. Recommended practice are very helpful from a design, operations and EHS perspective but like many on size fits all practices there are always exceptions to the rule. I think you may have some detective work ahead of you but as a general rule, I would strongly recommend against LEL detectors in an already classified Div 1 location.

 

[Prchick]

Thanks for the responses. I've done so modeling for different butane / propane releases and the ones I'm concerned about are those where the truck driver leaves the vent valve open and the LEL cloud is quite large. The instances where there is a drop or two has a circle, but not large enough I believe to go near any points of ignition. We've also seen that the drips only trip our lowest limit and this is what drives the operators crazy.