There are many types of pumps namely Centrifugal pumps, Axial flow pumps, plunger pumps, mixed pumps that are used on ships for different applications.
Marine pumps can be used for cooling, Fuel Oil transfer, Lube Oil transfer, F.O., and F.S. cargo pumping, cargo stripping, Hydrophore tank unit, freshwater generator, oil-water separator, incinerator, sewage treatment unit, ballast, circulating, general service(G/S) Fire fighting, boiler feed water, condensate water, fresh(drinking) water, sanitary water, Sludge and bilge, and so on.
Pumps in itself are very vast territory, in this post I’ve covered all the pumps that are in general pump classification
The pump is a machine which converts mechanical energy into potential energy, kinetic energy, thermal energy, and pressure energy of the fluid, and is used to raise liquid from low level to high level. The pump serves as a means of transporting fluid.
Use of Pumps On Ship
Pumps are widely used on-board ships for the following purpose:
1) Domestic freshwater supply to the bathroom, washbasin, galley, etc.
2)Fire main/Sanitary supply of seawater.
3)Machinery cooling by seawater/freshwater.
4) Pumping out the bilge and flooded compartments.
5) Supply of fluid for operation of steering gear, stabilizer, and hydraulic machinery.
6) Circulation of brine in air conditioning plants.
Pumping action can be achieved in various ways, depending upon the type of pump employed.
Different Types of Pumps
Pumps are broadly classified into two main types namely; Positive displacement pump and Dynamic pressure pump (Roto-dynamic pump).
1. Positive Displacement Pumps
Positive Displacement pump is defined as the pump, which creates suction by increasing the volume by mechanical variation at the suction side and achieve discharge by mechanically reducing the volume of the chamber in which fluid is trapped.
Positive displacement means that once the fluid is trapped in the chamber by suction has no way to go back to suction or churn, it has to be discharged, which means it is absolutely necessary to operate a Positive displacement pump with its discharge valve open.
If Positive displacement pump works against the closed discharge valve, the pressure will increase tremendously until there’s a burst in pump piping or severely damaged to the pump itself -or both.
To prevent such a mishap, relief, or safety valve has to be provided at the discharge line, they may be internal or external. An internal safety valve is provided as the precaution whereas the external relief valve feedback to the suction line.
1.1 Reciprocating pumps :
Reciprocating pump is the term that is used for the pumps that perform to and fro motion to displace fluid from suction line to discharge line. Reciprocating pumps move water or other liquids by a plunger or piston that travels back and forth inside a cylinder.
Under Reciprocating pumps comes. (a)Piston Pumps (b)Plunger Pump (c)Bucket pumps.
Piston pump: This pump consists of a piston, that moves to-and-fro to pump fluid from low level to high level, this may further be divided into two types, single acting, and double-acting.
This type of pump is in common use as a bilge pump or tank stripping pump. The design is simple, robust, and reliable. Materials are very much dependent on the usage but bronze is common for larger parts and stainless steel for piston rods
Plunger pump: This pump consists of a plunger (Cylinder), that moves to-and-fro to pump fluid. this pump is similar to the piston pump, the only difference is that a piston is replaced with the plunger. A smooth cylinder slide and High-pressure seal prevent any leakage from the pump.
Bucket Pump: A simple bucket pump is similar to the lift-Disc pump without the use of a pipe to draw the liquid. Conventional bucket pumps are used in a remote village where there is no electricity and low water content in the ground.
Historically bucket pumps were used in old turbine ships and now used as a high-pressure complex bucket pump for lubrication. The modern bucket pump is used on a ship to insert or disperse a predefined volume of Greece or lube oil while the pump’s plunger is operated.
Construction and working
The major visible parts of a bucket pump are:- pump handle, lever arm, and the piston rod. All the pumping mechanics is connected to this piston rod a long with the piston inside. If you disassemble this pump system; you can clearly see the piston connected into the piston rod assembly with a plastic disc having holes in the middle of it.
When the handle arrangement is up & submerged in the fluid, this section get filled up with the fluid. When the handle is pressed, low pressure is created below the piston which further sucks in the liquid and then traps into the piston cylinder. When the handle is moved upward.
So after consecutive attempts of pump stroked using pump handle. The fluid pressure generated on the top side of the piston lifts the metal flap on the top against the spring force. The water or fluid thus flows from the top section of the piston to the nozzle and finally ejected out.
1.2 Semi-Rotary Pump:
As the name suggests ‘Semi-Rotary’, which means the pump only moves in an arc and not fully rotates, these pumps are used on the small off-shore vessel to pump a small quantity of fluid.
- Used for bilge pumping
- Topping up of diesel oil, lube oil, crankcase of a diesel engine with lube oil
- To pump gasoline, bio-diesel, ethanol blends.
- You can use this pump for water, lube oil, diesel oil. the only precaution you’re going to take before using it for different fluid is just to flush the pump thoroughly so that there’s no contamination.
- These pumps are not designed for extremely thick fluid like grease.
1.3 Rotary Pumps
Rotary Pumps are those pump which pumps fluid by rotary motion, don’t get it confused with centrifugal pumps, because in this pump the fluid is trapped in a cavity as the pump rotate and the only flow of fluid is forward and not backward that is toward the suction neither it can churn.
Rotary Pumps are further divided into (a) Constant Delivery Pump (b) Variable Delivery Pump
Constant Delivery Pump
A Constant Delivery Pump is a pump where the discharge rate is constant throughout the working of the pump.
( i ) Gear Pump: Gear Pump works on the principle of meshing in and out of gears, the fluid is trapped in the cavity that is formed between the gear and the casing of the pump.
The clearance between the top-land and casing is in thous (1/1000 inch), this tight clearance with the high speed of rotation, effectively prevent the fluid from leaking backward. One gear is driving gear and another is Driven gear or idealer. They are of two types, external and internal gear pumps.
( ii ) Screw Pump: Screw Pump uses one or more screws to move fluid from suction to discharge, screw rotate on its axis inside of the cylinder, fluid is trapped in the cavity, and is pushed forward. Application: Fuel oil transfer pump.
( iii ) Vane Pump: Vane Pump is pumped whose vane is flexible and can extend and retract when rotating on the shaft within a large circular cavity, the shaft is off-center from the pump casing causing eccentricity. On the inlet side, the vane extends and therefore increasing the area to allow for suction and on the discharge side, the vane retracts causing the discharge.
( iv ) Lobe Pump: Lobe Pump is used on-board ship where there is the chance of solid contamination, as this pump has large pumping cavities. The lobe inside the pump does not get in contact. Application : Lube oil scavenging pump and Lube oil Circulating Pump
Lobe pumps are very similar to external gear pumps in working. Lobe pump unlike gear pump does not make contact. This contact is prevented due to timing gears which are located in the gearbox.
Variable Delivery Pump
A Variable Delivery Pump are those pumps whose discharge varies according to the need. It is used where a continuous change in fluid flow is required. The principle behind the variable displacement pump is to change the pump stroke in order to vary the amount of fluid displaced according to the requirement.
Floating ring, swash plate are two main component of the variable displacement pump. One thing to keep in mind is that, these pumps always has an odd number of pumps. Application: Fin Stabilizer, Steering Gear.
All variable displacement pumps are positive displacement pumps.
( i ) Heleshaw Pump :
Heleshaw pump comes under variable displacement pump. Although in many books and article the working of Hele shaw pump is described in Complex manner, the reality is, it works is simple.
This pump is connected to a motor for rotation of the shaft at a constant speed to which Cylinders are connected, the Floating ring is actuated/Moved by mechanical linkage (Such as Hunting gear) and this movement cause eccentricity of the concentric Floating ring and piston(Cylinders).
So, now the Suction and discharge occur as the pump rotating in the eccentric state.
The eccentricity of the floating ring determine the stroke of the piston, floating ring accommodates slippers. As long as concentric state is maintained no suction or discharge occurs, as eccentricity occur the suction and discharge starts. Direction of flow depends on the location of the floating ring.
( ii ) VSG Pump:
VSG pump is the advanced type of variable displacement pump with the axial piston arrangement which works on the principle that “A reciprocating piston draws in liquid during retraction and discharge while it extends”.
It consists of the following main parts Casing, Cylinder block, Piston, Driveshaft, Swashplate, Adjusting valve, and Inlet & Outlet ports.
Here the reciprocating motion of the axial piston is obtained by using a swash plate. A swash plate is a device used in variable displacement pumps to change the motion of rotating shaft to the reciprocating motion of the axial piston.
Due to its implications, VSG pumps are commonly known as the swash plate pump. These pumps are commonly used as a steering pump on large ships.
Due to swash plate arrangement in the VSG pumps, piston rotates around the shaft along with the piston barrel assembly. There is a contact between the swash plate surface and the piston shoe for the synchronous movement of the axial piston.
During the one half of the rotation, the piston moves out leading to an increase in the volume inside the cylinder; while in the other half leading to a decrease in the volume. This continuous phenomenon leads to the suction and discharge of the pump.
There are many advantages of using VSG pumps over the conventional hele-shaw pumps in the steering system.
- Its capacity can be easily controlled by changing the angle of swash plate. (Swash plate angle is proportional to the discharge).
- It is very economical and require less maintenance.
- It can run on more speed than conventional hele-shaw pumps.
- It can be easily operated using Constant speed motor’s.
- It is very suitable for 4-Ram arrangements.
2. Roto-Dynamic Pump
Dynamic Pressure Pump (Roto-Dynamic Pump) is a pump in which energy is continuously added to the fluid within the pump. This results in increased fluid velocity, at a later stage this velocity may be reduced to increased the pressure.
This pump may require priming and for this purpose reciprocating pump may be employed.
2.1 Centrifugal Pump
Centrifugal pump is a machine that uses rotation to impart velocity to a liquid, it then converts that velocity into the flow. This definition tells us two things about centrifugal pumps
(1) Energy is imparted to liquid in the form of velocity by the impeller.
(2) Energy is getting converted into flow by the casing of the centrifugal pump. This pump is an impeller and a casing working in unison to create a steady pressured flow. Application: General service fire fighting main pump, Nozzle cleaning, Hydrophore supply…
A. Volute Casing Centrifugal Pump
Volute Casing Centrifugal Pump is the casing of a centrifugal pump in which the cross-section of the casing increases gradually as a fluid moves toward discharge.
This increase in cross-section result in a decrease of fluid velocity, that is a decrease in kinetic energy and convert it into pressure energy. The shaft is eccentric in a volute centrifugal pump.
B. Vortex casing Centrifugal Pump
In this pump, there is a circular chamber between the impeller and the casing, the liquid first enter this circular chamber(Vortex) then volute casing, the eddy loss is considerably reduced by employing vortex casing and the efficiency of conversion from K.E. into pressure energy is increased as compared with volute casing.
2.2 Axial Pump
The axial pump is those pumps in which generally a propeller is used to create an increase in pressure by imparting axial acceleration to the fluid to be pumped. The flow rate of this pump is highest among all pumps. These axial flow pumps provide very little resistance to flow when idling. Application: Fin Stabilizer, Bow thruster.
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