Auto Blow Downs on Gas Detection

[maddocks]

Is anyone aware of a code and/or published guideline that requires an auto-blowdown of a natural gas facility when a gas detection event is triggered? We often do this at 20 ppmv or 40% LEL but is this a code requirement or just common practice. The plant is 24 hour attended and the owner is asking us to remove the auto-blowdown feature and let the board operator make the decision. Operating pressures range from atmospheric right up to 1400 Psig with sour gas. Any thoughts?

 

[EmmanuelTop]

In majority of plants where I used to work, this function does exists but its automatic triggering has been disabled. Handling sour gas is something special, and definitely one of the reasons why you need to raise the Owner's attention with regards to possible consequences. You may wish to consider performing a dedicated QRA for cases when prolonged release(s) occur, and quantify the associated risks - maybe they are acceptable, maybe they are not.

One possible way to reconfigure the system is to allow the operators for a certain amount of buffer time, upon initiation of an ESD. One of the plants where I worked (but handling gas with no H2S) has had an ESD system where automatic blowdown was delayed for 90 seconds - a relatively sufficient time for operators to investigate if gas/fire detection was false, or not. This was happening if only one of the detectors has got activated. If two, or more detectors, have got activated simultaneously, blowdown was instantaneous.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[dcasto]

I would investigate programming a system where it takes multiple detection devices adjacent to each other before taking any shutdown and eventual blowdown. I've done this with acid gas compression where there is no methane just H2S and CO2.

 

[LittleInch]

I'm not aware of any specific code requirement, but you could find it in a company recommendation. Last few years I've seen more and more actions like this dependant on "confirmed" fire or gas where either two detectors or a manual plus a detector have been activated or the controller has a confirmed button to activate based on a radio call from an operator or visual from a camera. Before that there is an alarm and sometimes a delay to allow the alarm to be confirmed or cancelled.

So like the other posters, I would be somewhere in between. A single detector no-auto blowdown, two or more - automatic as if it's gone horribly wrong you want to take the action out of the hands of the overworked controller....

My motto: Learn something new every day

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