Area oxygen meter location 1

[schnipp]

During a recent review of some PSM areas in our plant, a question came up about location of oxygen sensors for area atmospheric monitoring.

We have a nitrogen blanket inside two shrouded pieces of equipment (that normally run under vacuum). But since they are in an enclosed room, it was decided long ago to install area oxygen sensors to make sure the nitrogen doesn't displace oxygen in the building. They O2 sensors were installed about 6 feet off of the floor (normal breathing location). Our plant PSM coordinator thinks that nitrogen is heavier than "normal" air, and that the sensors should be located 2 to 3 feet from the ground.

We all think he is off his rocker on this one since normal air is 78% nitrogen anyways.

What is your opinion?

 

[JLSeagull]

I'm with you schnipp. The gasses would more likely mix than separate. This is a partial pressures thing that can be explained by the chemical types better than an electrical type. A good example is hydrogen sulfide. H2S monitoring is common in facilities handling sour gas. However, a release of sour gas would not instantly separate the heavier H2S from the lighter methane. Most would place the H2S detectors about 0.5 meters (18-20 inches) above the grade elevation because H2S is heaver. This is not necessarily the right place but it is the practice of most safety people associated with the fire and gas detection business.

After you fail to persuade the individual that the normal breating elevation is rational, apply multiple detectors at the low elevation knowing that dispertion likely mixes the gasses.

 

[dcasto]

nitrogen is lighter than air. At small leak rates the the N2 will disperse very quickly and dilute the the air to a low O2. At a huge release rate, the N2 will force the air (O2) out and again leave a low O2 atmoshere. The N2 will not really sink like the H2S does, if it did, we'd never be able to cross the Mojave desert.

Having the sensor at 6 foot is fine, having it close to the potebtial leak source will give a quicker response time.