This is really a site operations and permit policy decision, but as a start point, the safe area / hazardous area calculations undertaken by the electrical codes are probably the best way to start. However these only cover normal operations and for tanks there is no specific distance but depends on the contents, venting arrangements, location of flanges, valves etc

In my experience, anything inside a plant which constitutes hot work still needs a hot work permit, but when issuing and approving, the location will clearly govern whether it is permissible or the amount of precautions required. Normally a gas detector is required on permanent duty to warn of a gas cloud arriving as a result of some abnormal operation or occurrence (leak, rupture, spillage etc) occurring elsewhere, when hot work needs to be immediately extinguished together with cooling or extinguishing equipment.

If you are commissioning a tank then during that operation and afterwards, it then makes that part of the plant "live" and may change the permit requirements. It is often difficult on a new plant to get everyone to recognise that things change once you introduce hazardous substances when they have become used to not thinking about those things, but this is the responsibility of operations / construction management IMO to implement new rules as things change.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

Thanks LittleInch. I figured it would probably have something to do with the electrical zone ratings, was hoping for a "20m from pumps" kind of an answer :) They didn't do zone classification for this plant - working in Africa is veeeery different....

Ummm, it should still be designed by someone otherwise what sort of electrical equipment do they have and working in a plant without a work permit system when it's live is a bit beyond the pale...

As it depends on the substance, pressure, type and size of vent / flange it all needs to be calculated, but normally 10 to 15m max from any potential leak source (flange, relief valve vent, pump etc should be a good start point. For tanks it depends on the vent arrangement and whether the vapour is lighter than air or heavier....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

It's gasoil & mogas...

I am not sure if it was designed properly, though I know everything is EX except motors are IP56. Thanks for the help though!

Will definitely have to see if they are even using work permits o.O Hopefully they are. But I know it's so remote that they don't have water to hydrotest, and generally when it's remote, safety standards are not as they should be.

You seem to keep mixing "hot work" - which in construction and repair is "work that creates an explosive/flamable reaction" such as welding, grinding, arc cutting, plasma work, and gas-fired heating (postweld heat treatment with thermal blankets (although "hot") is not considered flame-producing usually) - with electric motor design and operation.

Unless the motor is sparking, construction (commissioning of the tank, other fabrication around the tank) doe not usually consider electric motor operation hazardous. Oil and refinery operations of course, need special electric motor rules and reg's to avoid just that sort of ignition of the vapors around the refinery.

Every OSHA construction site is required to monitor its own hot work rules and processes. Once your tank is built, installed to the pipe network, and the red tags are released to turn the tank back over to operations, the plant needs to re-set its hot work permit process around the "new tank" as required.