Ch 5- Inert Gas Systems (c) Flashcards
What are the two inert gases?
- Nitrogen
- Flue gas
Which inert gas is readily available and cheap?
Flue gas
Which inert gas is expensive?
Nitrogen
Which inert gas is more suitable for high-quality cargo or as an emergency supply?
Nitrogen
Where is flue gas sourced from?
Exhaust from a boiler
What is an inert gas generator?
A device in which fuel is burnt to create exhaust gases which contain less than 5% oxygen
What is the purpose of a scrubbing tower?
- Cools the flue gas
- Cleans the flue gas
How many blowers must you have in an IG system?
Two (or one plus spare parts)
What is the minimum capacity of blowers?
125% of MAXIMUM cargo discharge rate
What does a blower do?
Directs IG to tanks
What is a deck seal?
It prevents backflow of vapour from cargo tanks to the gas free area
What is the most common type of deck seal?
Wet
What is the more reliable type of deck seal?
Non-mechanical
What does a liquid-filled P/V breaker do?
- Prevents over or under pressurisation of cargo tanks
- Actuates at pressures in excess of PV valve settings but less than tank test pressures
What is a liquid filled P/V breaker filled with?
Fresh water or anti-freeze
Describe what happens in a liquid filled P/V breaker when there is positive pressure?
Pressure acts on the liquid within the P/V breaker
This forces liquid up the central column
What is a mast riser used for?
Venting excess pressure
Keeps vented gas clear of deck
What is at the top of a mast riser?
Flame screen
Describe using IG in a voyage cycle
- Primary inerting (8% O2 and positive pressure)
- Loading (vent off excess pressure)
- Top up on passage if required
- Discharge (system fully running)
- Tank washing
- Purging for gas freeing
This then repeats.
What are the steps of action in event of IG failure with crude oil?
- STOP all operations
- Close IG deck isolation valve
- Open vent between isolation valve & gas regulating valve to relieve pressure
- Fix the problem or obtain an alternate supply
What are the steps of action in the event of IG failure with products?
- STOP all operations
- Decide if repair is practicable & availability of alternative supply of IG
- If repair not practicable operations can resume but:
- Check flame arrestors on vents are in good condition
- Open vent risers
- Do not allow free fall of water or slops
- Take anti-static precautions when ullaging, dipping or sampling
- Additional precautions if tank cleaning
Describe the order of an IG system (12)
- Exhaust gases source
- Inert gas isolating valve
- Scrubbing tower
- Demister
- Gas blower
- I.G pressure regulating valve
- Deck seal
- Mechanical non-return valve
- Deck isolating valve
- PV breaker
- Cargo tank isolating valves
- Mast riser
