Classified area determination

[LPL]

An unclassified area has a gas concentration < 0.25 x LFL. LFL is defined as concentration of a combustible gas in air.

Assume a process containing a certain combustible gas concentration and no oxygen. When considering gas leakage to atmosphere, is it permitted to compare the gas's diluted concentration in air up to the required minimum oxygen concentration required for combustion vs 0.25 x LFL? Or must one compare the gas concentration within the process (even though no oxygen is present) vs 0.25 x LFL.

Thanks in advance for your input.

 

[georgeverghese]

The subsection "Limiting Oxidant Concentration" on page 26-54 of Perry Chem Engg Handbook 7th edn may be of interest.
When gas leaks to the open atmosphere, gas leakage may be low flow low velocity Gaussian dispersion or it could range up to high flow high velocity jet dispersion.

 

[shvet]

What is the design code? EI, API, IEC?

 

[pierreick]

Hi,
You may find pointers in the link attached:

https://www.hse.gov.uk/comah/sragtech/techmeasareaclas.htm

Pierre

 

[LPL]

The code is NEC 500.

 

[shvet]

An unclassified area has a gas concentration < 0.25 x LFL. LFL is defined as concentration of a combustible gas in air.

The topic related to unclasssified distances was taken from a company's internal practice

The extent of the zone depends on the estimated or calculated distance over which an explosive atmosphere exists before it disperses to a concentration in air below its lower explosion limit. A safety factor shall be included depending on the uncertainly of the assessment.
Annex D.3 of IEC 60079-10-1 provides a chart for estimating the hazardous distance [m] as a function of the release characteristic, for:
a) An unimpeded jet release with high velocity;
b) A diffusive jet release with low velocity or a jet that loses its momentum due to the geometry of the release or impingement on nearby surfaces;
c) Heavy gases or vapours that spread along horizontal surfaces (e.g. the ground).

Notes to chart:
- Not recommended for zone of Negligible Extent
- Curves based on zero background concentration and do not apply to indoor low dilution situations.
- Not intended to identify zones.
As indicated the chart is an estimation only and can be used for evaluation purposes of zoning by third parties. It is recommended to seek Expert Advice in the accurate determination of the extent of the zone.
If the source of release is located outside an area or adjacent to, the penetration of a considerable amount of flammable gas or vapour can be prevented by measures, such as:
a) physical barriers (wall or other obstruction);
b) maintaining a sufficient overpressure
c) purging the area with sufficient flow of fresh air

Note that dedicated software exist for determining ex-zones dimensions, e.g.

https://bit.ly/4dLI1Yh