Philosophy of shutdown key logc

[Xianglu]

We have a few buildings each has the following shutdown key logics on the ventilation louvers:

To open the louvers: H2S Hi, and LEL Hi
To close the louvers: H2S Hi Hi, LEL Hi Hi and fire detection eye

Can anyone help on what are the reasons for such logic; is this because of the code requirement, company standard or industrial practice?

Thanks,
Henry

 

[zdas04]

At H2S Hi, you want to ventilate the building with the expectation that at the Hi limit the atmosphere is not immediately hazardous to life and health (and the dilution effects outside will not be a risk to people outside the louvers). At H2S Hi Hi, the atmosphere inside the building is dangerous and the potential for the concentrations immediately outside the louvers to be hazardous is too high, so the problem should be contained in the (evacuated) building. You turn off the fire eye to keep it from inadvertently misidentifying H2S as "smoke" and calling the fire department into a hazardous environment.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[Xianglu]

Thank you David,
Is this practice or code requirement, if code which code?

thanks,
Henry

 

[zdas04]

Couldn't tell you if it is in a code or not. It is simply an application of Engineering judgement. A vanishing concept.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[SNORGY]

Did I misread the OP, or is the intent to close the louvers on fire detection, i.e., the fire detection eye activates to close the louvers?

If so, this may be configured that way in an attempt to limit the amount of fresh air available to the fire, should it exist. The bigger the building, the less effective this will be, in my view.

I can agree with zdas04 and the rationale to close the louvers on H2S Hi-Hi. It does depend a bit on the H2S ppm at the Hi-Hi setting; in upstream O&G I typically have seen Hi at 10 ppm and Hi-Hi at 20 ppm. I am not sure I'd be too concerned at 20 ppm, but if the Hi-Hi setting is, say, 100 ppm or more then zdas04's assessment makes perfect sense to me. Otherwise, at a Hi-Hi of, say, 20 ppm I'd be trying to get MORE air into the building, not less - but then what happens above the 20 ppm (a "Hi-Hi-Hi" ?)?

The handling of the LEL Hi (typically 20% x LEL) and LEL Hi-Hi (typically 40% LEL) is less easy for me to accept. At 40% LEL (Hi-Hi) I have seen this coincident with an ESD setting, but to me, the effect of closing the louvers will be to accelerate the rise to 100% LEL. It's worth knowing if the combustible gas is less dense or more dense than air, relative to the louver position (high or low) in the building. When I specify buildings where both H2S and combustible gas may be present, I have two sets, a high louver and a low louver. I also have exhaust air fans opposite the louvers to try to effect a sweep through the building, but the OP makes no reference to these.

My thought is that many standards and design guidelines specify a shutdown of the ventilation system at Hi-Hi settings for gas. The settings alluded to by the OP might be that way simply to be consistent with that mindset.

 

[zdas04]

Hi Hi H2S forcing the louvers shut and the fans off is a response to not knowing. If the concentration has gone from 10 ppm to 20 ppm you just don't know if it has reached a new plateau or if it is on its way to 20% (and you don't have any more levers to pull after the Hi Hi decision). You just know that the actions at Hi did not reverse the increasing trend so you lock it down.

LEL is a touchier subject. Personally I agree with shutting louvers and ignition sources at Hi Hi. My thinking is that the explosive range for natural gas is so narrow (5-15% by volume) that you could easily get above the UEL with a leak into a sealed building (lowering your fire risk), running the fans and opening the building could just get you to the middle of the explosive range with sparking motors running. It is a judgement call, but that is the same one I'd make. Hi Hi LEL or H2S are generally associated with an ESD.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[SNORGY]

David, your post in initial response to this caused me to go to bed an hour late thinking about it. Knowing the source, I knew I had better think it through carefully. It is obvious that you did, it just took me longer to get there. The controls and safeguards, as configured, appear to be behaving in the best way, not knowing what some of the other factors are.