Hazardous Classification for Enginee Room in Tanker

Please refer to the standards of the classification society under which the vessel will be registered (ABS, DNV, Lloyd's, Davy Jones's Locker, etc.)

<...never see enginee room in Hull which classified area....>

Diesel does not generally precipitate the need for hazardous area electrical equipment - unless there is something about this particular installation that has not been shared with the forum.

If in doubt, consult a qualified Chemical Engineer locally.

As PWSlack has noted, diesel is not normally considered a high hazard, although if atomized (pressurized fuel line leak) it can be so.

Your fresh air make-up system cannot be considered in determining the hazardous classification of the area. The worst case must be considered, ie...fan failure.

I can see a couple of hazardous potentials that would need to be covered. (And I'm way out of my depth here.)

The area would probably be considered a "confined space" depending on the number of entry points and general clearances.

It could also be hazardous for conatinment of Oxygen free/displaced situations where release fo fire suppressant materials, or accumulation of heavier gasses (with fan failure) might cause issues.

What about accumulation of CO, CO2, Sulphates and Nitrates from exhaust fumes?

What about thermal issues (Radiated heat contained) in the space?

You also mention dilution below LFL, but what about human exposure constraints for volatile elements like Tolluene and such often found in fuels?

As previously said, contact those that are underwriting/insuring/licencing the final unit for their endorsement and interpretations.

Update:

There is a concept of explosion protection called "purge-pressurise", which involves taking fresh air from the safe area and passing it through an enclosure in the hazardous area with a view to diluting and excluding flammable materials from that space. When certain criteria are fulfilled, such as an overpressure above a setpoint for a set period of time, non-hazardous-area electrical equipment within the purge-pressurised enclosure can then be safely energised; should the purge fail of the overpressure is lost beyond a certain length of time, then the non-hazardous-area equipment becomes de-energised until the purge has been re-established for a certain length of time. The concept can be applied to equipment enclosures all the way up to whole buildings located inside the hazardous area. In that way, non-hazardous-area equipment can be used inside the enclosure in safety. Without seeing this particular installation, it sounds as though this concept could be in use.

It is not known here whether this method of protection is applicable to the diesel areas on-board a vessel in general or to this particular installation, however, if it is, there will be design guidance documents elsewhere for similar installations that may be followed with a certain level of knowledge and caution, and applied to this particular installation. Consult these documents for futher guidance.

If in doubt, consult a qualified hazardous area electrical equipment designer.

In any case, do not design, select, install, test, commission, alter or dismantle hazardous area electrical equipment without having previously passed through suitable training on this topic.