LEL Detection vs Area Classification

[controlnovice]

Currently, we have area classification extending 10 ft away from three open top vessels (Class I, Div II), with a 3ft bubble around the openings and drains (Class I, Div I). These are interior tanks with point ventilation around the top (for adding dry material), and the building has excellent floor ventilation as well.

The plant wants to decrease the Class I, Div II area to 5 ft around the vessel (instead of 10ft). They measured all around the vessel, and even inside, at different times in the process with an LEL detector, and it never increased above 3%.

How is it explained that it is the possibility that hazardous vapors may be present so the LEL detector is not a sufficient enough test? Or is the LEL test sufficient to reduce the area classification boundary?

 

[TTheHammer]

You still need to go back to the NEC article 500 for the definitions of Class I, Div 1 and 2, to make sure if your conditions of flammable mixture presence is still the same i.e still occur under upnormal conditions.

To the best of my knowledge 10 ft and 5 ft and others are recommendation depending on factors: severity, hazardous material (Gas, benzene, propane, etc..), density, wind effect, and location

you can check this for some IEC classification examples and recommendations.

 

[controlnovice]

I guess my question is: Can a one time LEL test be enough to satisfy the declassification of an area? These tanks always have an open top.

 

[jsummerfield]

Use NFPA 497 and API RP 500 for guidance.

What installation differences are considered for the area between the five and 10-foot radius? Typically, most electrical installations are the same. Differences may include braided flex conduit instead of sealtight.

A sniffer is not a substitute for a continueous area montor to detect hydrocarbon.

At the end of the day, if an explosion occurs, OSHA will inspect the installation.

John

 

[weh3]

The classification of an area depends merely on the level of expectation that hazardous vapors will exist. In a Division 2 area, by definition, you will find no hazardous levels under normal operation, so the measured level is immaterial.

The code sets minimum boundaries; sometimes the engineer or a company standard increases them for convenience of design or for safety. For example, the military requires that the interior of pumphouses handling aviation turbine fuels be classified as Class I, Div. 1, even though the NEC is less stringent, and even though the exact same fuel does not require the same strictness if it is destined for use in a diesel engine.

Why does the company want to declassify?

Cost? The incremental cost of a 5-ft. radius of NEMA 7/9 stuff is negligible.

Perhaps they want to put some heat source in there and can't find any other real estate? It sounds as though this risks becoming a political issue.

Use your judgment, and if you are stamping the design, then you had better stick to the code as a minimum.

Wm

 

[jsummerfield]

An unclassified utility area added new equipment. The specifications required Division 2 equipment. The reasons stated are that nearly everything else in the plant is rated Division 2; and perhaps something hazardous might be added adjacent in the future.

John

 

[DRWeig]

One more opinion: If you classify an area in a less stringent manner than recommended in the industry standards and an explosion occurs, you have no defense. Even if you study the area for extended periods of time with a monitoring device and have lots of logic to support your decision, the standards will be presented as minimums in court.

ISA has a good set of recommended practices. I would go by what they (or IEC) recommend.

IMHO that is.

Let us know what you decide!

Old Dave

 

[DRWeig]

Oops, I left out NFPA 497... Thanks jsummerfield!