CENTRIFUGAL PUMPS
WHAT IS THE PRINCIPLE OF WORKING OF A CENTRIFUGAL PUMP?
Ans.
The principle used for centrifugal pump is the centrifugal force in the form of dynamic pressure which is generated by rotary motion of one or more rotating wheels called the impellers.
SKETCH LAYOUT OF A CENTRIFUGAL PUMP INSTALLATION AND LABEL IT.
STATE THE APPLICATION / FEATURES OF A CENTRIFUGAL PUMP
· On most tankers, the main cargo pumps are centrifugal pumps, located at the bottom of a pump room at the after end of the cargo tanks.
· These may have rated capacities of 1000m3/hour on medium size tankers.
· These are powered by a drive shaft from a turbine or electric motor located in the engine room.
· Some smaller tankers and particularly chemical or product carriers are fitted with deep well centrifugal pumps in some or all cargo tanks.
· While both of these pump types operate on centrifugal force principles, their operation is quite different
STATE THE PRECAUTIONS TO BE OBSERVED WHILE USING A CENTRIFUGAL PUMP
· The pumps must be carefully checked before starting to ensure that their chambers are full of cargo.
· Keep discharge valve fully shut before starting the pump.
· Increase the rpm gradually and open the discharge valve.
· Confirm no unusual rise in the temperature of pump or its casing. Also Confirm no vibrations.
· As discharge proceeds, it is important to maintain a good trim by the stern so that the pump inlet remains below the cargo suction inlet.
· Cargo officer must know the height of the ship's bottom framing and slow down the pump while there is still a meter of cargo above that level.
· The pump speed control and the pump discharge valve can then be manipulated to maintain pump suction and cargo flow until the tank is nearly empty.
· Main cargo pumps should not be used to drain the tank. The stripping pumps are provided for that purpose.
· Vapour/air extraction device, if fitted, should be operational when the cargo level is low and continued till Cargo pump is finally stopped or changed over to next tank.
DEEPWELL PUMPS
· Deep well pumps are centrifugal pumps designed to be mounted in the cargo tank.
· They have a special application in product/chemical carriers, where each tank is fitted with a dedicated pump. This improves the flexibility of the chemical/product carrier and minimizes the possibility of contamination.
· Because the number of pumps is larger, the individual pumps themselves are smaller and individually require less power.
· With a deep well pump installation, the need for a pump room is eliminated, along with the hazards and complications of that space.
· The more common method of evacuating the deep well is with an air or nitrogen purge system. The gas is piped through a small-bore line at the base of the pump and pushes the contents up the main discharge riser.
· When a cargo tank is being stripped (or drained) with a deep well pump, the pump cycles as its self-priming function is activated, dropping speed and pitch as it fills with liquid and then increasing in speed and pitch as is loses suction and drops the contents of its discharge column back into the well to re-prime itself.
· Pump speed should be reduced when stripping (if speed control is available). If the pump is powered hydraulically, it can be regulated to any speed desired to permit maximum cargo recovery.
RECIPROCATING PUMP
DEFINITION :
· Reciprocating pumps are those which cause the fluid to move using oscillating pistons, plungers or diaphragms.
· These pumps are also called positive displacement pumps because they are fitted with a system of suction and discharge valves to ensure that the fluid is displaced in a positive direction.
APPLICATIONS :
· These are small capacity (say 100 to 250 m3/hour capacity) pumps which are used for stripping of lines, pumps and tanks.
· Though rated capacity is less, they can handle very high pressures
WORKING PRINCIPLE :
The working principle can be explained by considering below reciprocating pump :
· Assume that the intake side of the pump is connected to a supply of liquid.
· When we move the piston to the right, lower pressure is created in the chamber formed by the piston. Higher pressure on the fluid outside the chamber forces fluid in through the inlet port and fills the chamber.
· Moving the handle forward in the opposite direction forces the fluid out. A check valve at the inlet port prevents flow there and, since the fluid must find an outlet somewhere, it is forced out through the discharge port.
· The check valve at the discharge port prevents the entrance of fluid into the pump on the subsequent suction stroke.
· The back-and-forth movement of the piston in the pump is referred to as reciprocating motion and this type of pump is generally known as a reciprocating-type piston pump
PRECAUTIONS WHILE USING A RECIPROCATING PUMP
• The pump should be started against partially open delivery valve.
• The pump should never be started or operated against closed delivery valve.
· Close monitoring of strokes must be done to prevent damage to the pump.
· Pump must not be operated above its rated discharge pressure
· Pump shafts must be regularly lubricated if auto lubrication system is not functional.
· OOW must be standby to stop the pump when stripping operation is nearing completion.
COMPARISION BETWEEN THE CENTRIFUGAL AND RECIPROCATING PUMP.
Ans.
Centrifugal Pump Reciprocating Pump
1. Flow is smooth 1. Flow is intermittent
2. Compact and 2. Comparatively needs
Need less space more space
3. Initial cost is less 3. Initialcost is 4 to 5
times more.
4. Easy Installation 4. Instal lation is
Difficult.
5. Low maintenance 5. High Maintenance
6. Requires Priming 6. Does not require
Priming
CARGO EDUCTOR
WORKING PRINCIPLE
· The Eductor works on Bernoulli’s principle.
OPERATION/FEATURE OF EDUCTOR
· Eductor is operated by the drive fluid entering through the pressure nozzle, producing a high velocity jet. Drive pressure is normally between 8 to 10 kg/cm2.
· This jet action creates a vacuum in the suction line, which causes liquid to flow up the body of the Eductor where it gets mixed with the drive fluid.
USAGE ON BOARD TANKERS
· Eductor are used to empty tanks, pumps and pipelines.
· Medium size tanker are fitted with educator of rated capacity of 100 to 300m3/hour.
PRECAUTIONS WHILE USAGE
· When shutting down an Eductor, the suction valve should remain open as this prevents the Eductor creating a vacuum on the suction line.
· If the Eductor drive pressure falls below the designed operating pressure, the Eductor suction valve should be closed to prevent any backflow of the driving liquid into the tank.
ADVANTAGES
· Eductors have a low initial cost
· Eductors are self-priming
· Eductors have no moving parts
· Eductors are easy to install
· Eductors require little or no maintenance
SCREW PUMP
WORKING PRINCIPLE
· A screw pump is a positive displacement pump that use one or several screws to move fluids or solids along the screw(s) axis.
· Screw rotates in a cylindrical cavity, thereby moving the material along the screw's spindle.
· There may be single, double or triple screw type pumps but each ‘screw pump’ operates on the same basic principal of a screw turning to isolate a volume of fluid and convey it.
DESIGN
· The individual turns of the screw seal a set volume of fluid by direct contact, with the stator.
· The stator has a shape that corresponds to the outside surface of the screw.
· The interference fit between the screw or pumping element and the flexible stator create a unique pump design that can handle a variety of fluid characteristics, including high levels of solids and variable viscosities.
· However, the interference fit creates a need for lubrication and cooling of the stator material.
COMPARISION BETWEEN SCREW PUMP & CENTRIFUGAL PUMP
· Compared to centrifugal pumps, positive displacements (PD) pumps have several advantages.
· The pumped fluid is moving axially without turbulence which eliminates foaming that would otherwise occur in viscous fluids.
· They are also able to pump fluids of higher viscosity without losing flow rate.
· Also, changes in the pressure difference have little impact on PD pumps compared to centrifugal pumps.