Ch 4- Venting arrangements (c) Flashcards
What happens when liquid enters a tank?
The vapour in the space is compressed
What happens in a tank if there isn’t a venting system?
The pressure will rise
Who dictates the maximum and minimum allowable tank pressures?
Class society rules
What does SOLAS require cargo tank venting to be able to deal with? (3)
- Day-to-day variations in pressure caused by localised heating and cooling
- Full flow venting conditions during loading or discharge of cargo
- Allow the tanks to be purged and/or gas-freed
What are PV valves designed to do?
Deal with daily variations in pressure caused by local heating and cooling
At what height must PV valves be?
2m above deck level
What are PV valves not designed to cope with?
Full flow venting during loading
What are the venting arrangements for loading (5)?
- Can be separate for each tank
- Can be combined with other tanks
- Can be incorporated into the IG piping
- Capacity=125% of max loading rate
- isolation valves,(if fitted), locked open or closed
What height is required of a mast riser?
6m
What does HVVV stand for?
High Velocity Vent Valve
What is a HVVV capable of dealing with?
Full flow and daily variations
What is guaranteed with a HVVV?
Vapour exit velocity
What is the minimum speed for a HVVV?
30 m/s
What is exit velocity of an HVVV controlled by?
Variable diameter outlet
What is an HVVV combined with?
Vacuum relief valve
What is the minimum height of a HVVV above deck?
2m
What does PV stand for?
Pressure-Vacuum
SOLAS requires an approved venting system for loading, plus either:
- Secondary means of venting
or - Pressure gauges with alarms for each cargo tank
Name four ways flammable vapours are controlled
- All tank outlets/vents are minimum 10m from any air intake
- High outlet velocities
- Height of outlet above deck
- Wind speed across deck
What is a mast riser?
A vertical pipe fitted to the common venting pipelines of all the cargo tanks
What two valves does a PV valve consist of and what do they do?
- Pressure valve that lifts under a set positive pressure
- Vacuum valve that lifts under a set vacuum pressure
What issue do PV valves solve?
Preventing the mixing of different grades of oil’s vapours
What is the purpose of a PV valve?
to protect cargo tanks from excessive over/under pressure
What is created in a cargo tank during discharge?
A vacuum
What happens within the PV valve during discharge?
As the vacuum reaches the set negative pressure of PV valve, the vacuum side disk of the PV valve will lift and allow the air to come inside the cargo tank to reduce the vacuum
What percentage of oxygen needs to be maintained in tanks of flammable cargo?
8%
When is the passage of air inside tanks only allowed?
If the tank is not in inert condition
How are vacuums resolved in tanks that contain flammable cargo?
By continuously introducing inert gas in the cargo tank during discharging
What does SOLAS Chapter II, Reg 11.6.3.2 state, in regards to secondary means of venting?
A secondary means of allowing full flow relief of vapours, air or inert gas mixtures shall be provided to prevent over pressure or under pressure in the event of failure of primary means of venting
What is the purpose of a pressure sensor?
To alert the operator by an alarm if the primary method of venting fails
Are pressure sensors permitted to be considered as an alternative to the secondary means of venting (according to SOLAS)?
Yes
How is the oxygen levels of tanks maintained?
Inerting the tanks
What are the alarm settings for PV valves for inerted tanks?
- The positive pressure is more than the PV valve lifting pressure while loading the cargo. So the setting for positive side will be 10% more than the PV valve lifting pressure.
- The tank is under vacuum when discharging the cargo. So the setting for vacuum side will be any positivevalue close to zero. Usually 100 mmWG is set in this case.
When are vapour return lines used?
When the vapours of the cargo are considered to be toxic
(Sometimes it is used for non-toxic cargoes too because of terminal requirement)
What do vapour lines do?
Allows the connection between ship tank’s vapour space and shore tank’s vapour space
What is a PV breaker?
Another mechanism that acts as a secondary means of venting on crude oil tankers
Where are PV breakers located and connected?
To the common IG line of the ship
On what principle does a PV breaker work?
On the principle of a set water column filled in it.
It allows the pressure to release from common IG line by emptying out the filled water in the PV breaker
It also allows to break the vacuum by allowing the air inside the tanks through common IG line
VOC Emission Control During Loading (4)
- Cargo tank vapours sent ashore during loading
- The vapours will be either burnt or condensed back into liquid
- Burning gives no commercial benefit
- Vapour recovery has some financial payback
What is MARPOL Annex VI?
Air pollution from ships
What are five operational hazards with vapour lines?
- Misconnection of vapour recovery lines
- Over and under pressurisation
- Cargo tank overfill
- Static electricity
- Ignition of vapours connected to ships tanks
What does NMVOC stand for?
Non-Methane Volatile Organic Compounds
What does VOC stand for?
Volatile Organic Compounds
What four requirements shall be on a VOC Management Plan?
- Provide written procedures for minimixing VOC emissions during the loading, sea passage and discharge of cargo
- Give consideration to the additional VOC generated by crude oil washing
- Identify a person responsible for implementing the plan
- For ships on international voyages, be written in the working language of the master and officers (and translated into English, French or Spanish if this is not one of the spoken languages)
