Cargo Handling Systems Flashcards
What are the advantages of double hulled tankers?
offer a degree of protection against collision and stranding damage by protectively locating the cargo tanks
what degree of longitudinal sub-division of cargo tanks is in the earlier double hull tankers?
none
what degree of longitudinal sub-division of cargo tanks is in the newer and larger double hull tankers?
either 1 or 2 sub-divisions
where are most of the longitudinal strength members located?
in the segregated ballast tanks
why are most of the longitudinal strength members located in the segregated ballast tanks?
gives flush cargo tanks - easier to drain + clean
how do you segregate different grades or types of chemical in cargo tankers?
- smaller tanks
- more tanks
- more complex pipeline system
how are tanks separated?
- some by cofferdams
- more sophisticated ships each cargo tank will be served by its own pump + pipeline
what are the three basic pipeline systems used on tankers?
- direct line
- ring main
- free flow
what is the function of the pipeline system?
to direct the oil to and from the terminal and within the cargo tanks
what controls the flow of oil?
valves located within the system
what is the Manifold?
- the interface between the ship and shore pipelines
- runs athwartships, usually amidships
what are ‘Chiksans’?
- the actual connection of the Manifold
- usually made of flexible rubber hoses or metal arms
why do modern systems use hydraulic clamps as a connection to the ships Manifold?
allows fast decoupling in the event of an emergency
what part of the Manifold allows for the vertical movement of the ship whilst loading at a terminal?
the Chiksans
how is seawater ballast drawn to the pumps through sea?
suction valves located in the pumproom
why is a double valve segregation used in drawing seawater ballast to the pumps from the sea?
prevent leakage to the sea - especially important if the ballast system is connected to the cargo system
what determines the bottom line layout in a ship?
- which trade the ship is to be operated in
- ship size
- economic factors
what is a direct line layout of pipes in a tanker?
- crude tankers
- typically three pipe lines
- each serve its own group of cargo tanks
- allows three grades of oil to be carried with total segregation
- if only carrying one grade, you can open the crossover valves between the lines
what is a ring main line layout of pipes in a tanker?
- not a common system used now!!
- product tankers where a high degree of segregation is required
- system is more complex, hence more expensive
- great flexibility is achieved
- usually any of the ships cargo pumps can serve any cargo tank
- crossover lines interconnecting each side of the ‘loop’ due to segregation valves on branch lines
what is a free flow system line layout of pipes in a tanker?
- simple system
- some crude oil tankers
- doesn’t rely on pipelines to direct oil to the tanks
- uses a series of sluice valves to adjoin tank bulkheads are fitted
- reduced costs
- cargo is loaded through a simple ring main line
- sluice valves located at adjoining cargo tank bulkheads are opened to discharge cargo
- oil flows from forward to aft and the cargo pump suctions, in the after most centre cargo tank
- discharge the cargo via the deck lines
- as the cargo flows from one tank to the next - mixing occurs
- high discharge rates are achievable + very little residues are left
- this reduces the stripping time at the end of discharge
what does the term stripping time mean?
the removal of the small quantities of oil remaining after the main cargo pumps have discharged as much of the cargo as possible
If stripping lines aren’t fitted, what else can be used and why?
- main lines
- due to the advent of self-priming units fitted to main cargo pumps
how do centrifugal pumps work?
- imparting energy to the liquid in the form of increased velocity
- centrifugal force is imparted to the liquid by rotation of a single, or possibly double, impeller rotating at about 1200 rpm
- the velocity produced is converted to a discharge pressure by slowing it down in a suitably shaped casing known as a volute
- the impeller blades are curved and operate to fine tolerances within the pump casing
- the drive mechanism for the pump is via a drive shaft from the engine room into the pump room
- pumps are usually driven from the ships boilers
what are the advantages of using a centrifugal pump system?
- capable of pumping large bulk quantities > 5,000 m (cubed)
- relatively compact size - allows for small pump rooms therefore larger cargo carrying capacity
what are the disadvantages of using a centrifugal pump system?
- not good at handling gas/air/liquid mixtures (can be overcome by fitting a self-priming device
- not good at creating pressures below atmospheric
what are positive displacement pumps?
- known as a reciprocating pump where the standard main cargo pump for many years until the increase in ship size rendered them redundant for this purpose
- still used on oil tankers as stripping pumps
- steam driven
- consists of two pistons in separate chambers
- steam forces the steam piston up
- the linkage connecting the two pistons forces the oil chamber piston to the left displacing the oil in the chamber out to the discharge pipe
what are the advantages of the positive displacement pumps system?
- simple construction, therefore easy maintenance
- very good suction characteristics
what are the disadvantages of the positive displacement pump system?
- not suitable for high flow rates
- pumping capacity is directly related to the size of the cylinders
- not good for pumping against high back pressures
- may result in having to strip internally to the slop tanks + using a centrifugal pump to discharge ashore from there
what are submerged pumps?
- essentially these are centrifugal pumps
- usually hydraulically driven (have their impeller located in the bottom of the tank)
- typically found on chemical tankers where total segregation is required
- each tank has its own pump, load/discharge line + manifold connection
- as pumps are submerged in the cargo, good stripping performance is achieved
- air or nitrogen is blown into the top of the stack + cargo is force out through the small-bore line forward of teh (closed) discharge valve
what are the advantages of the submerged pump line system?
- eliminates the need for a pump room, so increasing space available for cargo
- if individual pumps and lines total segregation is achieved
- reduced piping in cargo tanks
- good stripping performance
what are the disadvantages of the submerged pump line system?
- initial high installation costs
- bulk discharge rate may not be as high as with conventional centrifugal pumps
- how to deal with pump failure while cargo is still in the tank (can supply the ship with a portable pump that can be lowered into the tank)
what is the Eductors pump system?
- used for stripping tanks during discharge + tank cleaning
- pump with no moving parts
- relies on Bernoulli’s principle to function
- a suction is created by a constriction in part of the eductor
- this increases flow velocity + creates a low pressure (suction)
what are the advantages of the Eductor pump system?
- no moving parts, hence reliable
- simple construction
- self-priming
what are the disadvantages of the Eductor pump system?
- requires a motive liquid to ‘drive’ it
- during discharge, the two liquids combine and so must be of the same grade
- efficiency depends on driving pressure and low back pressure
- usually used for internal stripping + not directly ashore
