CMC Hot Work Procedures 3 — Questions and Answers
Question 1: Under NFPA 306, what classification is assigned to a space that is safe for workers but NOT safe for hot work?
- Class I
- Class II
- Safe for Entry (Correct answer)
- Restricted Entry
Correct answer: Safe for Entry
'Safe for Entry' means atmospheric conditions allow worker entry but hot work is NOT authorized; a separate 'Safe for Hot Work' classification is required for welding or cutting.
Question 2: A welder reports that the flame on the torch repeatedly goes out during work inside a tank. What does this most likely indicate?
- Fuel gas pressure is too low
- Oxygen concentration in the tank is dropping, possibly indicating displacement by inert gas (Correct answer)
- The torch tip is clogged with slag
- The tank temperature is too high for the fuel mixture
Correct answer: Oxygen concentration in the tank is dropping, possibly indicating displacement by inert gas
A repeatedly extinguishing flame can indicate falling oxygen levels caused by inert gas ingress, which is a serious life safety hazard requiring immediate evacuation.
Question 3: Which type of hot work presents the GREATEST risk of igniting vapors in adjacent spaces on a vessel?
- Electric arc welding on an exterior deck
- Cutting through a bulkhead that separates two spaces (Correct answer)
- Grinding of a handrail on an open deck
- Plasma cutting of removed deck plate on a barge
Correct answer: Cutting through a bulkhead that separates two spaces
Cutting through a bulkhead creates a direct opening and introduces heat and sparks into the adjacent space, which may contain flammable vapors.
Question 4: Prior to issuing a hot work certificate, a Marine Chemist detects hydrogen sulfide at 5 ppm in a cargo tank. What is the appropriate action?
- Issue the certificate since H₂S at 5 ppm is below the IDLH of 50 ppm
- Deny hot work certification and require additional ventilation until H₂S is reduced (Correct answer)
- Issue a conditional certificate requiring SCBAs for all workers
- Allow hot work since H₂S is non-flammable at that concentration
Correct answer: Deny hot work certification and require additional ventilation until H₂S is reduced
H₂S presence indicates incomplete gas-freeing and potential for higher concentrations; hot work should not be authorized until the space is fully gas-free and H₂S is at safe levels.
Question 5: A Marine Chemist is called to re-certify a tank after hot work was suspended for lunch. What is required before re-issuing the certificate?
- Only a visual inspection of the space is needed
- A full atmospheric re-test of the work space and all adjacent spaces (Correct answer)
- Re-test only if the ventilation was turned off during the break
- Chief officer approval is sufficient to resume without re-testing
Correct answer: A full atmospheric re-test of the work space and all adjacent spaces
After any work suspension, a full atmospheric re-test is required before hot work can resume, regardless of whether ventilation remained on.
Question 6: What is the significance of the 'Lower Explosive Limit' (LEL) in the context of marine hot work?
- It is the concentration at which a gas mixture is too lean to ignite
- It represents the minimum vapor concentration that can sustain an explosion if ignited (Correct answer)
- It indicates the oxygen level required for combustion to occur
- It defines the maximum allowable vapor level for confined space entry
Correct answer: It represents the minimum vapor concentration that can sustain an explosion if ignited
The LEL is the minimum flammable vapor concentration (vol%) that can propagate a flame if an ignition source is present; hot work is prohibited above 10% LEL.
Question 7: When a Marine Chemist certificate specifies 'continuous mechanical ventilation required,' what is the consequence if the blower fails mid-operation?
- Workers may continue for up to 15 minutes while the blower is repaired
- Hot work must cease immediately and the space must be evacuated (Correct answer)
- Work can continue if the hatch remains open for natural ventilation
- The fire watch assumes responsibility and work may continue at reduced pace
Correct answer: Hot work must cease immediately and the space must be evacuated
Ventilation is a condition of the certificate; failure of the required ventilation immediately invalidates the hot work authorization and requires evacuation.
CMC Hot Work Procedures 3
Under NFPA 306, what classification is assigned to a space that is safe for workers but NOT safe for hot work?