Extent of use of ambient air flammable gas monitors ?

[mrtangent]

Dear All,

We are considering install flammable gas monitors for one of our production units which may automatically turn on deluge/air extraction units.

I am curious as to how many other sites use these to activate air extraction / ventilation units (disperse flammable vapours ...). Our system flash points are moderate 40dec C+.
Are these common on oil sites/ refineries/ or rarely used? only for inside buildings at the moment we are only considering an inside unit.

Typically are they maintenance hastle free or does it depend on the chemical in the atm ?
Our chemicals combust to form residues , has anyone had these problems ...

 

[JimCasey]

Flammable gas monitors frequently rely on a platinum or Pt/Ir catalytic bead as a sensor. When exposed to flammable vapors, the catalytic bead gets hotter and an imbedded RTD experiences an increase in resistance.

What can screw it up?
Water splash is high on the "things-to-avoid" list. Most Flammable gas monitor companies supply a rain shield device to help. MOST combustible vapors are heavier than air so the monitors should be in sumps, trenches, or at baseboard level. Low places are subject to flooding.

Indifference of the painting crew: If the paint guys don't have the FGMs deenergized before painting with a solvent based paint, the solvent fumes can overtemp and destroy the catalytic bead. OR if water/based(latex) paint, they can obstruct the flame arrestor around the bead with a paint film and it will remain functional but isolated from the atmosphere it is intended to monitor.

Silicones poison the catalytic bead basically by decomposing into a layer of glass on the catalyst. Other chemicals are also poisonous to catalysts; hence the removal of tetrethyllead from automotive gasoline.

If you are considering the initiation of automatic shutdown on alarm, investigate the voting modules that are offered by the manufacturers of the Flammable gas monitors. A voting module waits until there are enough alarms to verify a problem, so your plant is not just tripping off due to random noise or somebody passing gas near a sensor.

 

[unclesyd]

I don't have the specifics but here are the generalities.

We use outdoor monitors in an operating area where we have two trains air oxidizing cyclohexane, (700,000 gals in each system). As I understand it if one monitor detects flammable vapors only an alarm in the control room is sounded. If two monitors detect flammable vapors several things take place. The feed to the system is shut off, isolation valves are closed, air is turned off, an area fire alarm is sounded throughout the plant. At this point the operator has makes a decision whether to trip the deluge system (dry). If a third monitor is activated the deluge system is tripped automatically. There are some other interconnected detectors and sensors that add redundancy to this system.

Anecdotal: The system works. During a January shutdown with the temperature in the mid twenties the fire protection people reset the system to check something. Being a shut down there was enough vapors to trip several detectors. Along with about many others I had to swim out of the building. Needless to say we were none to happy beside being cold and wet as we watch the fire chief and his merry man ride off in the warmth of their little red truck.

In another area where we have vaporized heating media in an enclosed space we have vapor detection systems that monitor a very congested equipment area. The use of these monitors has changed in scope from the initial installation for just detecting vapors and sounding an alarm in two control rooms to one that controls the amount of ventilation air in the area. There is a normal air flow in the area that is used to prevent any concentration of vapors and lower the ambient temperature. These detectors when in an alarm condition now dramatically increase the air flow through the area. I don't think vapor detection system can trip the deluge system.

 

[unclesyd]

I must have been asleep and working in the dark. I called to check on the vapor detection system for the enclosed space and was reminded of the following.

We also have flammable vapor detectors around two butane storage sphere and the truck and railroad unloading station. The activate the plant alarm system.

We have detectors around the butane vaporizing system. these trigger both an alarm and deluge system.

Where the butane is used in an air oxidation process the area is cover with flammable vapor detectors. These trigger both an alarm and a in certain case trip the deluge system.

They are also installed on our wharf where we unload cyane, ammonia, and now I think butane. These initially only trigger an alarm, usually about twice a week.

We have a vapor detection system around our cryogenic NH3 storage tanks and vaporizing area.

We have several systems around our rail loading and unloading facility.

In my discussions this morning a point was make that I was not aware of is that over 95% of our fire alarms are triggered by these system.

 

[epoisses]

95%... so they're either very useful or very useless, depending on whether there's a real fire whenever the alarm goes..?

Re maintenance, I would guess these systems need regular (monthly?) calibration.

 

[jsummerfield]

Offshore platforms include extensive use of combustible gas, toxic gas (H2S for sour gas platforms) and fire detection. Voting is common with one detector to alarm and two to trip equipment, etc. This can vary - perhaps one toxic gas detector initiates deluge. Monthly calibration is the norm. The control system requirements are another controversy - UL approval, integration with process shutdown system, etc. Check API RP 14G.

John

 

[kenvlach]

First, rent a portable unit or two – environmental sampling firms & labs have them. There are several operating principles, some better for trace levels & others for flammable/explosive limits.

Then measure your existing background levels. Also, check valves, etc. for local higher concentrations -- you may detect leaks. This will help determine where to install permanent units; there are also OSHA & Fire Codes on quantity and location.

unclesyd, what kind of building did you have to swim out of?

 

[unclesyd]

kevlach,

The swimming aspect was somewhat a figure of speech as I tripped over a curbing used to contain any normal wash down and found myself trying to get up in a swiftly moving stream of cold water. You can say my breathing was labored. The buildings have a centerline fire wall with individual fire wall bays for each reactor. All the water runs to the side of the wall I was on. I can't recall the capacity of the deluge system, but it is huge. The sprinkler heads are fed from both ends of the building by 2 twelve headers on end. For this type trip all the water resources on site are diverted to this building.

This is same process that was at the root of the Flixborough Disaster.

Click the bottom left picture and you can see the configuration.

http://www.acusafe.com/Incidents/Flixborough1974/incident-flixborough1974.html

http://www.answers.com/topic/flixborough-disaster