Can some one tell me about this? A pressure switch is installed on a fuel transfer pipe line. (diesel, benzene, naphtha, kerosene, etc). The switch has spring-loaded plunger against the fluid pressure operates to switch a sealed micro-switch. There is a neoprene rubber or vtron sealing to prevent the liquid reaching the switch compartment.

<b>moderator's note: I think he means viton seal</b>

Now my question is : how to you classify this pressure switch to be in zone 1 or zone 0? As a complete apparatus, the pressure switch is in continuous contact with the fluid. So is it Zone 0?

kamath

 

Hi Kamath,

The right question to ask is - "Is the switch a possible source of release?" When classifying an area, the possible release sources have to be identified, and then the associated zone can be assigned based on factors like the expected release rate. The zone definition considers whether concentration in the flammable range is likely, not just if a release can occur.

A switch that is designed and selected to be compatible with the expected maximum pressure and temperature of the fluid, and is not susceptible to corrosion or other effects of the fluid, process conditions (eg vibration or pulsation) or the surroundings is not expected to provide a source of release under normal conditions. At worst it would be a secondary grade source of release "which is unlikely to occur in normal operation and, if it does, the release will be infrequent and only for short periods". The flanges or other fittings on the pipeline, and the impulse piping and isolating valves between the line and the switch, are also likely to provide the same grade of release.

Generally, a secondary grade source of release will give rise to a Zone 2 area unless it is in a confined space when a Zone 1 may result. The zone extents will generally be based on failure of the impulse piping - so the switch does not have to be treated separately when looking at the classification.

What does have to be considered when using a switch of the types you have described - with a single seal between the process fluid and the electrical compartment - is that failure of the seal can cause the pressure in the electrical compartment to build up. This can force fluid along the cable and into the (usually non-classified) space at the other end of the cable, and a barrier gland or other sort of seal is needed at the switch end of the cable.