Class 1 Division 2 Ventilation Sequence of Operation?

[John_187]

There is a building that is defined as class 1 division 2 rated. It is a building with gas compressors. Does the class 1 div 2 ventilation (6 air changes per hour) only apply to when the gas compressors are on? Or does it mean the ventilation fan that does 6 ACH should be on continuously 24/7, even when the the gas compressors (which are the source of the class 1 div 2) are off? Thanks for the help

 

[chicopee]

So, what type of gas is being compressed?

 

[John_187]

The gas being compressed is natural gas, thanks

 

[GT-EGR]

Is the source of combustion (the natural gas) still present in the room in a large enough quantity to cause an explosion if accidentally released even when the compressors are off? If so, it is always division 2 and should always be treated as such.

In my experience, you always run the 6 air changes, and then additionally you monitor the gas concentrations in the locations they are likely to collect - and then you run a secondary purge air change rate (20 air changes but this is subjective) anytime your gas concentrations begin approaching unsafe concentrations.

 

[John_187]

Thanks for the reply. I don't know how I would determine that, I just thought it might be a black and white answer for class 1 division 2 scenarios. That it had to be ventilated at least 6 air changes per hour continuously 24/7, 365. Let me know, thanks

 

[GT-EGR]

The main point of the 6 air changes (or higher) is to continuously dilute the air, in case there is a small source of leakage. If you simulate a pinhole leak in a pipe, at the pressure you operate, that is a possible operating scenario that could happen since a tiny leak may not be noticeable to your system, but will slowly build up in the room if un-ventilated. In the past I think the program I used was Pipe-Flo (although it's been a while and I don't have that now).
Once you determine a possible gas leakage rate, you make sure you have a high enough fresh airflow rate to keep that gas from building up in your room to a concentration above the explosive limits.

The higher air change/purge is for the less likely events where the gas piping is actually damaged, and you have a big release. It's likely that air change number doesn't dilute it while the release is happening, but it will slow it down, and also once it's fixed, it will clear the room out faster.

Also the 6 air changes are likely cooling your space as well, so make sure that airflow is keeping your room temperature acceptable if you have a limit.