A COP or cargo oil pump is an integral part of a complex cargo discharge operation. Located in the pump room, the system comprises of a number of pumps including; eductors, steam reciprocating pumps, screw pumps and centrifugal pumps working as a cargo oil pump. But the most common arrangement is the one with centrifugal pump; driven by a steam turbine.
Due to their wide use and design factors; a cargo pump is also referred as cargo oil pump turbine assembly. The type of pump used for the cargo operation depends largely upon the required pumping rate and pressure. The amount of cargo to be discharged also plays a role while deciding the pump used; for example eductor pumps are used only for stripping purposes.
When the large amount of oil is pumped out; the low oil content in tank leads to loss of suction. This is when the pumps such as eductors are used to suck in the left overs and flush the hose with air blow. In most ship designs a pump room consist of a number of centrifugal pumps as the main cargo pump; with an eductor as the secondary cargo pump.
The whole assembly is connected such that; it can take suction from the slope tank and the bottom of the tank. This comes handy when transferring cargo from one tank to another; while discharging altogether at ports. In cargo pumps where the centrifugal pump is driven by steam turbine; the pump lies in the pump room with the turbine part resides in the engine room.
Role of Cargo Oil Pump In Cargo Operation
The complete cargo operation specially that for discharging consists of three main points; cargo pump operation, inert gas system and pre planning ( planning in advance ). Various key features such as cargo sequence, temperature, characteristics, quantity, pressure, rate of pumping; tanks to be discharged, manifold connections to be used, modes of communication and emergency procedures are all pre planned and agreed upon for a specific cargo operation.
The chief officer monitor and control the cargo operation from a dedicated place on automated ship’s called cargo control room. Once all the lines are lined up and Inert gas system on stand by; the chief officer start the cargo pump at a slow speed. This is done to ensure there is no leaks or fault at any junction or deck in cargo or I.G system.
Once ground crew confirms the normal operation confirming safety checks; the officer start other pumps in parallel and increase the pump speed and pressure. The cargo pump should not be started till the inert gas system is operational with; all cargo and slope tanks in common ( in parallel ). Furthermore for cargo discharge; all the vent valves should be closed with deck isolation valve open.
Once it is confirmed that the shore facility is receiving cargo at their end; slowly increase the speed of the pump to its nominal full rpm. In event of more than one cargo pump running in parallel; it is required to have same running speed and pressure to maintain the cargo flow. Any variation will results in pump’s overheating with no flow output or loss of pumping.
Types Of Cargo Pumps And Their Working Principle
1 ) Centrifugal Pump As Cargo Oil Pump Turbine ( C.O.P.T )
A centrifugal pump creates suction by the virtue of pressure difference created by; the impeller action on the fluid medium in a volute casing. The fluid is sucked in; rotated at an high speed and then discharged through the output nozzle following a volute casing structure. A centrifugal pump consists of parts such as; shaft, impeller, a casing, bearings and sealing structure.
The complete pump structure can be divided into two key parts; rotary and stationary parts. The impeller and the rotating shaft make up the rotary structure; while volute casing accounts for stationary part. A centrifugal pump is best suited for handling low and medium viscous fluids. They are best known for their high capacity output, dynamic head, ease of use and low cost operation.
Furthermore, being a type of dynamic pressure pump; it allows to control the output without changing its operational speed or dangerously pressurizing the system. For centrifugal pumps used as cargo oil pump they are fitted with additional non return valve at output to avoid backflow.
I have already written in details about the construction, working and maintenance of centrifugal pumps in details in one of my old post. You can click and read that post from link over here.
A centrifugal pump turbine or C.O.P.T in short when used as main cargo oil pump have two main parts; a turbine and a centrifugal pump. The steam turbine consists of parts such as boiler, feed pump, condenser and turbine working together in a closed loop system. The high pressure steam inters the turbine casing through a series of inlet nozzle.
These nozzles convert the high pressure steam into high velocity jet directing them on turbine blades; to rotate along with the shaft to generate rotational power. The generated power is then transmitted to the centrifugal pump through a connecting shaft; while exhaust steam goes back to the boiler via condenser and feed pump.
On another hand a centrifugal pump works on the simple law of centrifugal forces. The pump impeller throws the fluid outwards away from the central axis. The process transfers part of the kinetic energy of the impeller to the fluid; which then pass through the volute casing to leave under pressure.
The pressure difference created due to sudden change in fluid volume creates a partial vacuum behind the impeller eye; this creates the much needed suction pressure to continue the process.
2 ) Reciprocating Cargo Pumps
A reciprocating cargo oil pump is a steam or motor driven positive displacement pump; with a plunger moving within the pump cylinder. It is well known for its constant volume pulsating output at higher pressure. The pump takes in the fluid medium via a suction valve and discharge at a higher pressure through the discharge valve.
The plunger or piston within the pump cylinder is in close contact with the liner with the help of the piston ring. The most common design is with a double acting piston driven by a reciprocating steam pump. This is done by connecting the piston rod of the reciprocating steam piston; to that of the pump via a linkage or beam.
The modern reciprocating cargo pumps come with an additional dampener at its output to reduce the pulsating effect. Furthermore being a positive displacement pump; it is required to have a relief valve in place to avoid damage due to excessive pressure. Based on its design it can be a simplex, duplex, single acting or a double acting pump.
A positive displacement pump works on the simple principle of pressure that accounts for force per unit volume. This is achieved by the up and down motion of the plunger or piston. A reciprocating pump works on the principle of positive displacement pump; and thus produce pressure exerting force on liquid in confined area.
Thus for a reciprocating cargo pump the output pressure depends on the piston area and its speed within the pump cylinder. As the piston retracts backwards; the cylinder suction valve is opened against the spring tension under the effect of negative pressure inside. Now as the piston starts to contracts moving upwards; the suction valve close and pressure starts to build under impact of driving force.
When the pressure reached to a desired limit; the discharge valve operates and fluid is discharged. While the driving force can be given; in the form of steam pressure or from a prime mover. The return of the piston is ensured by the pair of steam valve and its gear; as with direct acting steam pump or through crankshaft and connecting rod in case of prime mover.
3 ) Centrifugal Pump As Framo Pump
A Framo pump is nothing but a centrifugal pump put upside down in a cargo tank. It is located such that the impeller lies on the bottom most part of the cargo tank. They are used largely in chemical tankers where dedicated pumps are required for each tanks. They are mounted such that; only a minimum clearance left between the floor and the pump impeller suction.
This results in advantage in form of shorter period of stripping. And relief from unnecessary suction problems; due to faulty or chocked suction pipes.
The pump consists of parts such as hydraulic motor, impeller, volute casing, wear rings, sleeves, mechanical and cargo seal along with the bearings. All the main components of the pump is secured well within the deck trunk welded to the ships deck.
The pump is driven by the hydraulic motor; under the influence of high pressure hydraulic oil from power packs. A power pack is an independent system consisting of; a hydraulic oil tank and high pressure pump along with an electric motor to run.
The complete system is based on the balance between the two fluid medium; cargo and the hydraulic oil. If the cargo gets in to the hydraulic system it damage the pump; while if hydraulic oil leaks it contaminates the cargo. Thus a coffer dam is provided to avoid the contamination of cargo with hydraulic oil in the pump system.
The hydraulic system control panel sends signal to the hydraulic power unit to start the discharge operation. The hydraulic power unit consists of a hydraulic oil tank and a high pressure feed pump.
The pump starts to take in suction from the hydraulic oil tank and discharge at very high pressure to the main hydraulic line. It is driven by an electrical motor; controlled by the hydraulic system control panel.
A low pressure return line is connected; to the hydraulic oil tank to maintain the same oil level at all times.
Upon each operation; the coffer dam lines for each Framo pump is to be purged out. This is done to remove any cargo or hydraulic oil leaked; from cargo and hydraulic oil seal to the coffer dam. Then the remote assembly is signaled by the main controller; to open valves to the hydraulic motor.
This create the necessary torque needed to drive the shaft of the centrifugal pump. The pump then takes suction from the suction well and discharge to the shore facility via a dedicated cargo pipeline.
4 ) Screw Pump
A screw pump is a type of positive displacement pump. Much like gear pump with a major difference that; it pumps fluid axially along its spring axis.
The pump consists of two main parts; the prime mover and the screw pump assembly. Two or more screws are coupled to the shaft with a fine clearance; which when combined with shaft motion creates suction pressure.
A screw pump can be divided into eight major parts; the driver screw, driven screw, timing gear, relief valves, bearing, driving shaft, rotary and stationary seals with suction and discharge ports.
Based on the number of screw; the pump can be called a single screw, twin screw or multi screw pumps. With better suction capability and lower maintenance cost; the pump is a good fit for those who look for a cargo pump which can handle all kinds of cargo. But due to its high installation cost it is often restricted to high pressure high viscosity fluid application.
I have already written a post in past explaining in details about screw pump; its constructional parts with their specific function, use and maintenance procedure. You can click and read that post from link Over Here.
A screw pump add axial movement to the fluid locking it into its fine clearance area. The continues motion of the pump; trapping and pushing fluid axially forward build up the output pressure. The driving and driven screws of the pump works in phase with the timing gear. It not only keeps both screws at pace but also avoid any contact in between. Any such contact can lead to wear and tear of the screw edges; negatively affecting the pump performance.
Both driving and driven screws in the pump are of opposite types i.e one is left hand type while other right hand type. This ensures to trap air or fluid within fine clearance; building suction pressure.
A screw pump at start traps the air in between the fine clearance of its drive and driven screw. Pushing the air axially towards the output; it create differential pressure which helps suck in the fluid medium into pump medium. As more and more fluid is trapped and then pushed towards the output; pressure increase along the pump discharge.
5 ) Cargo Eductors
An eductor is a simple structure with no moving parts. It’s basically consists of a nozzle, throat and a diffuser. An eductor can be placed in the main cargo pipeline or with a separate piping arrangement. Basically it is a less efficient low cost alternative for other stripping pumps.
An eductor use the under pressure fluid ( liquid and gas ) to generate pumping action. This is done by the flow of the fluid through a nozzle; followed by a diffuser or a diverging output. This can be used with almost all kinds of pumps with different fluid medium. An eductor can be used to handle any cargo even explosive medium; as long as it is compatible with the flowing medium.
In cargo eductors it is a common practice to use the same cargo as the flowing medium to generate pumping action. Being a simple design it is reliable; maintenance free and is self primed. In most case it is operated by the flow of cargo by the C.O.P.T.
An eductor works on the simple principle of “Bernoulli’s principle” or venturi effect in specific. The Bernoulli’s principle states that; the velocity or flow rate of a fluid is inversely proportional to its pressure. this can be represented by the following equation:
For a fluid flowing in a tube at constant speed; it must maintain its flow rate to keep a constant volumetric discharge. But when a sudden narrow path restricts its flow; reducing its velocity. By the law of conservation of energy we knew the energy lost in form of reduced velocity has to convert into another form; which in this case in pressure energy.
This process creates a differential pressure between the inlet and outlet of the conversing cross sectional area. The venturi effect combined with the principle of mass continuity; generates suction pressure at its suction inlet. In most scenario the driving fluid or medium in the case is the cargo from the next tank pumped through a C.O.P.T.
Alarms And Trips Associated With Cargo Oil Pump
A cargo oil pump can be damaged in a number of events such as; overheating, no oil in the tank, closed suction, chocked suction, excessive back pressure etc. Thus various alarms and trips are fitted to safeguard pump operation. While different cargo oil pump types have different set of alarms and trips; but they also have many which are common to all.
- High Wear Rate
- High Casing temperature
- Low lube oil Pressure
- High Motor Current
- Low lube oil Level
- High bearing Temperature
- High Lube oil Temperature
- Low Hydraulic Oil Level ( In Framo Pumps )
- High Hydraulic Oil Level ( In Framo Pumps )
- H.P.U Oil Leakage ( In Framo Pumps )
- Cooling System Failure
- Power failure
- Low Low Lube Oil Level
- Local Hand Trip
- Remote Trip
- Automatic Emergency Trip
- Casing Over Heating
- Priming Pump failure
- High hydrocarbon Content
- Low Inert Gas pressure
- Over speed
- Emergency Stop Centrifugal Pump
- Emergency Stop For Hydraulic Oil Pump ( In Framo Pumps )
- Low Low Hydraulic Oil Level ( In Framo Pumps )
- Very High Back Pressure
How To Start A Cargo Oil Pump ( C.O.P.T ) In 14 Simple Steps
- Check and drain water from the lube oil sumps for the cargo oil pump turbine.
- Check and Open Sea water overboard Valve
- Open Vent valve for the Condenser.
- Open The discharge and Suction Valves for the Sea water pump for vacuum cooling evaporator.
- The Fresh water inlet and outlet valves are to be opened for the air ejector.
- Start and warm up the air ejector system.
- Close the vent valve and monitor steam pressure.
- Now using the Turning bar; rotate the turbine spindle to check free movement.
- Crack open the turbine warming line for some time.
- Now slowly open the main steam valve to the turbine assembly; try keep this process as slow as possible.
- Observe the slow increase in turbine R.P.m till the governor take control.
- Once reached to its operational R.P.M; inform the cargo control room and the pump is ready to be used.
- Check Whether the I.G system are operational.
- Now start the Cargo oil Pump To start the discharge Process.
Note: This article is produced on request from “Dodla Sivarama Prasad“.
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