A vessel has been defined under Zone 20/21. No Hazard outside Vessel.(not even Zone 22) Which implies Hazard could be present only in case of Spillage/Leakage. What we are planning to do is to use instrumentation certified for Zone 20/21 (or Intrinsically safe Instrumentation) for Instruments directly mounted on the these Vessel, but standard weatherproof instruments for instruments installed in the vicinity or other vessels which are not directly identified with Hazard zone. In other words what is the extent of Hazard zone . In case of Gases , it is rather simple to define but factors like cleanliness , housing keeping etc seem to reduce higher zones to No Hazard.

Are any literature or standards are available defining extent. Most of the literature defines Zones and seem to provide only very general guide lines.

Thank you for your assistance