Friday, July 6, 2012

Pyrophoric Ignition Hazards in Typical Refinery Operations

A pyrophoric material is a liquid or solid that, even in small quantities and without an external ignition source, can ignite within 5 minutes after coming in contact with air.

In oil and petrochemical industry, this only partially defines the concern. We also need to be concerned with the fact the pyrophoric material can create heat which can ignite residual hydrocarbons associated with the equipment containing the pyrophoric material.
  • Example pyrophoric materials include alkali metals and many organometallic compounds such as alkylmagnesiums, alkylzincs, and of course pyrophoric iron sulfide.  Nickle carbonyl in some catalysts.
          Pyrophoric compound + oxygen (typically air)  --->  Oxide of the compound + heat
  • Sometimes with several intermediate reaction steps
  • Can be very reactive or very slow to react
  • Can vary with conditions, humidity, temperature, particle size, degree of disbursement in air, etc.

Conditions required to form pyrophoric iron sulfide
  • H2S concentration > 1% (can form at lower concentrations but typically not in concentrations that are a concern)
  • Iron scale or rust (FeS)
  • Less than a 1:1 ratio of oxygen to H2S (some oxygen is required to form the rust but if insufficient oxygen is present the reaction with H2S cannot go to completion)

Pyrophorics have been known to form in refinery equipment such as :
  • Crude oil tanks
  • Asphalt tanks
  • Sour water tanks
  • Vessels in sour service such as coke drums, distillation columns, inlet separators, pig  receiver / launchers
  • Reactors
  • API Separators
  • Marine tankers and barges
  • Portable tanks and tote bins
Mitigation methods :
  1. Most effective method is chemical neutralization before opening the equipment; potassium permanganate solution (typically around a 1% solution, circulate and check for color)
  2. Keeping the deposits and scale wet until it can be safely removed to a remote area and allowed to dry
  3. Maintain a constant air ventilation to ensure there is plenty of oxygen to allow the reaction to go to completion, preventing the formation of the pyrophoric intermediates
  4. Replace components that contain sulfur compounds
  5. Use nitrogen or other inert gases to keep oxygen out (obviously difficult and adds hazards of its own)
  6. Quickly move scale and potential pyrophoric deposits to a remote
  7. area and monitor in case ignition does occur
Source :
Doug Jeffries
Chief Fire Protection Engineer , Chevron