Pyrophoric Ignition Hazards in Typical Refinery Operations

Transcription

1 Pyrophoric Ignition Hazards in Typical Refinery Operations CAER Safety Summit Meeting December Doug Jeffries Chief Fire Protection Engineer

2 Agenda. Definitions and chemistry of pyrophorics Conditions required to form pyrophoric iron sulfide Where pyrophorics have been known to form in refinery equipment Possible methods to mitigate and prevent pyrophoric related incidents A couple example pyrophoric related incidents 2

3 3

4 Definitions 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 1 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 4

5 Chemistry of Pyrophorics 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. Bottom line: pyrophorics can be a very elusive and tricky animal to recognize and capture until it bites you 5

6 6

7 Conditions required to form pyrophoric iron sulfide H 2 S 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 H 2 S (some oxygen is required to form the rust but if insufficient oxygen is present the reaction with H 2 S cannot go to completion) Fe 2 O 3 + 3H 2 S = 2FeS + 3H 2 O + S 4FeS + 3O 2 = 2Fe 2 O 3 + 4S + heat 4FeS + 7O 2 = 2Fe 2 O 3 + 4SO 2 + heat 7

8 Where pyrophorics have been known to form in refinery equipment 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 8

9 Pyrophorics and the Fire Triangle FUEL OXYGEN IGNITION 9

10 Example 1 Marine tanker cargo hold. Cargo was light crude with low, sufficient vapors to reduce oxygen levels to near zero High humidity accelerated rust and formation of pyrophoric iron sulfide As crude cargo was off loaded, the normal inerting with engine exhaust was halted Air was allowed to enter causing the pyrophoric iron sulfide to react, heat up, and ignite Fortunately, the vapor space was relatively small so the damages were slight and there were no injuries 10

11 Example 2 Filter cake deposits ignite. Powdered filter cake is added to a mix tank to assist filtration after mixing Powder accumulates in the vapor space of the tank, particularly near the baffles Reaction begins to occur but is disrupted when product and more filter cake dust covers the deposits Vibration from turning on the tank mixers or disturbance while cleaning the tank exposes the unreacted layers in the deposits The reaction generates heat igniting vapors in the tank The tank is severely damaged but thankfully there were no injuries 11

12 Example 3 Reactor catalyst ignites in drums. Reactor is normally flooded with water prior to dumping catalyst In a trial to recover more precious metal and reuse the catalyst, it is decided to dry dump Process stream contains hydrogen sulfide and nickle carbonyl and possibly other pyrophoric compounds are formed in the catalyst Reactor is purged with nitrogen while dumping catalyst and removing internals Upon exposure to air catalyst heats up, igniting residual hydrocarbon in the catalyst Fortunately, drums are moved away from the reactor so no damage and no injuries 12

13 Example 4 Sour water tank roof blown off. Sour water is high in H 2 S and is a wet environment Odor complaints cause operators to alter the tank vent and pressure / vacuum valve Normal ventilation of the tank vapor space is changed resulting in formation of pyrophoric iron sulfide When the tank vent and P/V valve are returned to normal service, oxygen is allowed to enter Pyrophoric iron sulfide reaction occurs, sufficient heat is generated to ignite the vapor space blowing the roof off the tank A similar incident occurred when the tank was opened for cleaning and inspection 13

14 Example 5 API Separator explosion. An API Separator in an upstream producing facility is in operation in a sour producing area The separator is gas blanketed to prevent oxygen from getting inside Pyrophoric iron sulfide forms inside the separator The separator is taken out of service for maintenance No washing or chemical neutralization is performed prior to opening the separator Air enters, a pyrophoric reaction takes place, igniting residual hydrocarbon inside the separator 14

15 Example 6 Hot oil tank fire. A refinery has a hot oil tank and hot oil system for heating process streams and storage tanks The synthetic hot oil compound is replaced with medium cycle oil from the catalytic cracker (which happens to contain H 2 S and a high level of particulates) The particulates accumulate in the horizontal hot oil tank, mandating a clean out. The oil is drained and the decision is made to put a man inside to soften the deposits with diesel The manway and vent are opened, the man enters Air sweeping across the top of the tank produces a pyrophoric reaction which heats and ignites hydrocarbon The man inside the tank receive fatal burns 15

16 Mitigation methods Most effective method is chemical neutralization before opening the equipment; potassium permanganate solution (typically around a 1% solution, circulate and check for color) Keeping the deposits and scale wet until it can be safely removed to a remote area and allowed to dry 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 Replace components that contain sulfur compounds Use nitrogen or other inert gases to keep oxygen out (obviously difficult and adds hazards of its own) Quickly move scale and potential pyrophoric deposits to a remote area and monitor in case ignition does occur 16