I am working on a document about pyrophorics and how to extinguish fires involving them.

 

I have read that:  for pyrophoric solids (and for active metal fires involving water – which “behave” like pyrophorics) one can use a Type D extinguisher.  This makes sense to me since the Type D material will largely exclude air and probably cool the solid (a bit).  For liquids, a Type ABC extinguisher is recommended, and this makes sense to me since the ABC agent will cover the liquid (excluding air) and reduce vaporization.

 

For pyrophoric gases, the situation is less clear.  The 8^th edition of the GHS Purple Book does not have pyrophoric gases as a hazard category.  (There are pyrophoric solids and pyrophoric liquids/solutions.)   The hazard statements for a “pyrophoric gas” will include H220 (extremely flammable gas) and H250 (catches fire spontaneously if exposed to air).

 

I have poked around a bit using phosphine as the test case.

 

The Linde SDS for phosphine (phosphine-ph3-safety-data-sheet-sds-p4643.pdf (lindeus.com)) revised in 8/31/18 says:  “Suitable extinguishing media : Carbon dioxide, Dry chemical, Water spray or fog”.  This is shocking to me since none of these materials would adequately eliminate a leg of the fire triangle (or tetrahedron).  On a lucky day, I can imagine that CO2 could momentarily displace enough air to extinguish a small volume of the burning gas but assuming that the gas is leaking from some system, it would reappear and catch fire. The SDS continues, (colored fonts added by me):

 

DANGER! Toxic, flammable, corrosive, liquid and gas under pressure.   If venting or leaking gas catches fire, do not extinguish flames. Flammable vapors may spread from leak, creating an explosive reignition hazard. Vapors can be ignited by pilot lights, other flames, smoking, sparks, heaters, electrical equipment, static discharge, or other ignition sources at locations distant from product handling point. Explosive atmospheres may linger. Before entering an area, especially a confined area, check the atmosphere with an appropriate device.

 

The phrase in red seems appropriate to me.  Important, too, since the natural intuition for chemists and emergency responders is to put out the fire. 

 

The section in green seems entirely inappropriate since no “ignition source” is needed for a pyrophoric substance.

 

As a former firefighter the phrase in blue is not helpful since only 0% oxygen would be safe, and this is not realistic at any fire scene. 

 

FYI, the 2021 DOT Emergency Response Guide lists phosphine with a Guide #119.  It says:

• DO NOT EXTINGUISH A LEAKING GAS FIRE UNLESS LEAK CAN BE STOPPED.

Small Fire

• Dry chemical, CO2, water spray or alcohol-resistant foam.

Large Fire

• Water spray, fog or alcohol-resistant foam.

• FOR CHLOROSILANES, DO NOT USE WATER; use AFFF alcohol-resistant medium-expansion foam.

• If it can be done safely, move undamaged containers away from the area around the fire.

• Damaged cylinders should be handled only by specialists

 

Hmm? 

 

I found two useful documents from the University of Nebraska-Lincoln:

Safe Operating Procedure (unl.edu)

For (pyrophoric) gases, close the valve at the cylinder if it is safe to do so. If not, evacuate the area. Most burning pyrophoric gases are difficult to extinguish. Silane is reported to be impossible to extinguish.

 

UNIVERSITY OF NEBRASKA LINCOLN (UNL)

DO NOT use carbon dioxide or water fire extinguishers as these types of extinguishers can actually enhance the combustion of some pyrophoric compounds.

(Phosphine might not be applicable for this statement.)

 

It seems that the best remedy for a burning gas (presumably arising from a leak in a system) is to isolate the leak or close the valve – if these steps are even possible.

 

Perhaps in some situations there is no way to stop the fire except to let it burn itself when the pyrophoric gas concentration is exhausted.  Best to be working in a hood, I imagine.  (Preferably an uncluttered hood not storing bottles of flammable liquids!)

 

Additional thoughts are welcome.

 

Dave

 

 

David C. Finster
Professor Emeritus, Department of Chemistry
Wittenberg University