Marine pumps can be used for for cooling, Fuel Oil transfer, Lube Oil transfer, F.O. and F.S. cargo pumping, cargo stripping, Hydrophore tank unit, fresh water 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 is very vast territory, in this post I’ve covered all the pumps that is in general pump classification
Pump is a machine which convert 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. Pump serves as a mean of transporting fluid.
Use of Pump
Pumps are widely used on-board ships for the following purpose:
1) Domestic fresh water supply to bathroom, washbasin, galley etc.
2)Fire main/Sanitary supply of sea water.
3)Machinery cooling by sea water / fresh water.
4) Pumping out 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 by various way, depending upon the type of pump employed.
Pumps are broadly classified into
1) Positive displacement pump
2) Dynamic pressure pump (Roto-dynamic pump)
Positive Displacement Pump
Positive Displacement pump is defined as the pump, which create 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 absolute necessary to operate Positive displacement pump with it 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 severe 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 feed back to suction line.
Dynamic Pressure Pump (Roto-Dynamic Pump) are pumps in which energy is continuously added to the fluid within the pump. This result in increased fluid velocity , at later stage this velocity may be reduced to increased the pressure. These pump may required priming and for this purpose reciprocating pump maybe employed.
First we’ll discuss about the Pumps that comes under positive displacement pump.
1) Reciprocating pump :
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.
(a)Piston pump : This pump consist of a piston, that moves to-and-fro to pump fluid from low level to high level, this may further divided into two type, 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
(b)Plunger pump : This pump consist of a plunger (Cylinder), that moves to-and-fro to pump fluid. this pump is similar to piston pump, only difference is that, piston is replaced with the plunger. A smooth cylinder slide and High pressure seal prevent any leakage from the pump.
(c) 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 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. Modern bucket pump is used on 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, a low pressure is created below the piston which further suck in the liquid and then trap 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 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.
2) Semi-Rotary Pump:
As the name suggest ‘Semi-Rotary’, which means the pump only move in an arc and not fully rotate , these pump are used on small off-shore vessel to pump small quantity of fluid.
- Used for bilge pumping
- Topping up of diesel oil, lube oil, crankcase of 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 pump are not design for extremely thick fluid like grease.
3) Rotary Pumps
Rotary Pumps are those pump which pump 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
a) Constant Delivery Pump
A Constant Delivery Pump are those pump hows discharge rate is constant through out the working of the pump.
- Gear Pump works on the principal 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 backwards. One gear is driving gear and another is Driven gear or idealer. They are of two type, external and internal gear pump
- Screw Pump uses one or more screw to move fluid from suction to discharge, screw rotate on its axis inside of cylinder, fluid is trapped in the cavity and is pushed forward. Application : Fuel oil transfer pump.
- Vane Pump are pumps whose vane are flexible and can extend and retracted when rotating on the shaft within a large circular cavity, the shaft is off-center from the pump casing causing eccentricity. On inlet side the vane extend and therefore increasing the area to allow for suction and on discharge side the vane retract causing discharge.
- Lobe Pump are used on-board ship where there is the chance of solid contamination, as these 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 is located in the gearbox.
(b) Variable Delivery Pump
A Variable Delivery Pump are those pumps whose discharge varies according to the need. It is used where continuous change in fluid flow is required. The principle behind 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 variable displacement pump. One thing to keep in mind is that, these pumps always has odd number of pumps. Application : Fin Stabilizer, Steering Gear.
All variable displacement pumps are positive displacement pumps.
Heleshaw pump comes under variable displacement pump
Although in many books and article the working of Hele shaw pump is described in Complex manner, but reality is, it working is simple.This pump is connected to an motor for rotation of the shaft at constant speed to which Cylinders is 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 rotate in eccentric state.
The eccentricity of floating ring determine the stroke of piston, floating ring accommodate slippers. As long as concentric state is maintained no suction or discharge occur, as eccentricity occur the suction and discharge starts. Direction of flow depends on the location of the floating ring.
2. VSG Pump
VSG pump are the advance 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, Drive shaft, Swash plate, 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 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 in between the swash plate surface and the piston shoe for the synchronous movement of the axial piston. During the one half of the rotation, piston moves out leading to increase in the volume inside the cylinder; while in the other half leading to decrease in the volume. This continues 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.
- It’s 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 then conventional hele-shaw pumps.
- It can be easily operated using Constant speed motor’s.
- It is very suitable for 4-Ram arrangements.
Dynamic Pressure Pump
Dynamic Pressure Pump (Roto-dynamic Pump) are pumps in which energy is continuously added to the fluid within the pump. This result in increased fluid velocity , at later stage this velocity may be reduced to increased the pressure.
These pump may required priming and for this purpose reciprocating pump maybe employed.
- Centrifugal Pump is a machine that uses rotation to impart velocity to a liquid, it then converts that velocity into flow. This definition tell us two thing about centrifugal pumps (1) Energy is imparted to liquid in form of velocity by the impeller. (2) Energy is getting converted into flow by the casing of centrifugal pump. This pump is an impeller and a casing working in union to create a steady pressured flow. Application : General service fire fighting main pump, Nozzle cleaning, Hydrophore supply…
- Volute casing Centrifugal Pump is the casing of centrifugal pump in which the cross-section of the casing increases gradually as fluid move toward discharge. This increase in cross-section result in decrease of fluid velocity, that is decrease in kinetic energy and convert it into pressure energy. The shaft is eccentric in volute centrifugal pump.
- Vortex casing Centrifugal Pump, in this pump there is circular chamber between 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.
- Diffuser casing Centrifugal Pump
2. Axial Pump are those pump in which generally a propeller is used to create an increases 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 pump provide very little resistance to flow when idling. Application : Fin Stabilizer, Bow thruster.