LEL and H2S settings for crude oil

[eeprom]

Hello.
I am trying to find documentation related to API 500 in order to determine the appropriate settings for H2S and LEL monitors for an enclosed building (with fans and vents). The building handles crude oil. The API standard says that an adequately ventilated building must be capable of preventing the accumulation of explosive vapors below 25% of their LEL. What I am looking for is what gasses are likely to be present so that I can figure out what 25% would be.

Thanks
EE

 

[BigInch]

4 to 6 changes of the enclosed volume of a building or room are usually adequate to keep explosive atmospheres from developing inside a natural gas transmission pipeline's compressor stations.

As far as what might be in the oil, it will usually be "natural gas" entrained in the crude which will bubble off. The problem is that natural gas can be composed of a number of gases of varying %Volumes. For the most part it will be methane, but other gases are possible. Percents by volume typically encountered are here,

I hate Windowz 8!!!!

 

[eeprom]

Thank you. I can find the LEL for CH4.

I read that 6 room changes per hour is required for "adequate" ventilation. I am confused by the requirement for ventilation. In API 500 6.3.2.1 It is stated that adequate ventilation is the ventilation required to keep explosive vapors (CH4) at levels below 25% of their LEL. But then in 6.3.4.2 it states that at least 6 room changes per hour is required to be "adequate."
I have LEL monitors in the room. Currently they are used to turn on/off fans, and for alarming.

If I take the first section of code, then I meet code because my fans are sufficient to keep the vapor levels below 25% of LEL. The LEL monitors turn the fans on at 5%; they shut off at 1/2%. But if I take then second section of code, this implies that I am only in compliance if the fans are running continuously.

Do you know, can I be in compliance having the automated exhaust system as described?

thanks,
EE

 

[Mechomatic]

So it sounds like the required ventilation is 6 or more room changes per hour, with higher ventialtion capacities required if 6 RC/h is insufficient to maintain vapor levels below 25% LEL. Kind of like a car warranty: 5yrs or 50k miles, whichever comes first. Only here it's "whichever is greater".

 

[BigInch]

I think the code doesn't specify how you accomplish the room air changes per hour (by natural ventilation or by forced air flow fans) but obviously YOU should. If you do with fans I'd make fans run continuously, or to start when a gas sensor picks up any small trace of gas which would be anything enough to tell you that you might some some possibility of a very tiny leak. I would not like to be anywhere close to an LEL before the fans started. I would not even like to think that I had a "leak", although I understand that sometimes gas does seem to be present without there being an actual leak, however that should be viewed as a technicality.

Speaking of fans starting and stopping, I would NOT like to turn the fans off if the gas level happens to drop (sufficiently) below the LEL. I'd be more concerned about first finding the leak that put you over the LEL.

I'd much rather have continuously running fans and a separate gas alarm, or gas alarm and shutdown.
My way of thinking says,

1) If you need fans, you admit explosion could become a problem. That's serious, so the design must always and continuously prevent that condition from being reached. Then if the condition was reached, you could presume that there was a safety system failure and the operation should shut down.
2) If you have a leak great enough, even though still well below LEL, to need a fan that is normally off to be switched on, that's a big problem, as you could easily presume that the LEL would be reached if the fans did not turn on.
3) If an accident did happen, you an be sure that the lawyers will ask you why you switched the fans off.
4) If you have a shutdown from a gas indicator, you should only restart with a manual switch.

Are the fan motors explosion proof?

I hate Windowz 8!!!!

 

[eeprom]

BigInch,
Thanks for your reply. Everything in the room is rated Class I Div 1. This includes the fans. And there is a gas monitor. I also prefer the idea of continuous fans. The reason I would argue against this is that it would be impossible to heat the building in cold weather. 6 room changes of air per hour would cause the heaters to run 24/7. The heaters would have to heat the room air from outside temperature to room temperature in 12 minutes.

According to the code: A location is classified as Class I Div 2 location if ...ignitable concentrations of gases or vapors normally are prevented by positive ventilation, and that might become ignitable through failure or abnormal operation of the ventilation system. Adequate ventilation is sufficient to prevent the accumulation of vapor - air/gas mixtures below 25% of the gas LEL of the gases present. The requirement of 6 air changes per hour is just one method of achieving the adequate ventilation.

It seems that having fans turn on and off based on gas levels (using set points below the 25% LEL) would maintain the classification.

EE

 

[BigInch]

No worries. Lots of process buildings aren't heated, nor do they generate enough heat to stay cozy. Just warm it up a little. Surely nobody works in that room continuously.

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