MARINE AUXILIARIES

Topic 1 : Pumps Malaysian Institute of Marine Engineering Technology (MIMET) Marine Auxiliaries / WAS / Jan 2020



Description of Pumps.  A pump is a machine used to raise liquids from a low point to a high point.  The pump provides the energy to develop the head and overcome any losses in the system to enable to flow or build up the pressure. Malaysian Institute of Marine Engineering Technology (MIMET) Marine Auxiliaries / WAS / Jan 2020

Why pumps is important onboard ship? • A ship consists of various types of fluids moving inside different machinery and systems for the purpose of cooling, heating, lubrication, and as fuels. • These liquids are circulated by different types of pumps, which can be independently driven by ship power supply or attached to the machinery itself. • All the systems on board ship require proper operational and compatible pump and pumping system so that ship can run on its voyage smoothly.



• The selection of a type of pump for a system depends on the characteristics of the fluid to be pumped or circulated. • Characteristics such as viscosity, density, surface tension and compressibility, along with characteristics of the system such as require rate of fluid, head to which the fluid is to be pumped, temperature encountered in the system, and pressure tackled by the fluid in the system, are taken into account.

The pumping action can be achieved in various ways such as; 1. according to the type of pump employed. 2. The arrangement of pipework. 3. The liquid to be pumped and its purpose will result in certain system requirements. 4. Head-Flow’s characteristics that must be met by the pump.

A pumping system on the ship is consists of ; 1. Suction piping, 2. A Pump 3. Discharge piping (shown in Figure 6.1). The system is arranged to provide a positive pressure or head at some point and discharge the liquid. The pump provides the energy to develop the head and overcome any losses in the system.



Why there is a losses in the pump system? Losses (H) are mainly due to friction within the pipes and the difference between the initial and final liquid levels. The Total System Losses ( HTOTAL) are found as follows: HTOTAL = HFRSUCT + HFRDIS + HDISTANK + HSUCTTANK where : HFRSUCT = friction head loss in suction piping (Losses) HFRDIS = friction head loss in discharge piping HDISTANK = height of discharge tank level above pump. HSUCTANK = height of suction tank level above pump (negative when tank level is below pump suction) All values are in meters of liquid. H= System Loss

Types Of Pumps Onboard Ship

PUMP CLASSIFICATION (CATEGORY) Kinetic energy: Displacement: Energy associated with motion moving of something from its place or position Rotor: Rotating component of a (ROTOR DYNAMIC) machine Dynamic: A process characterized with constant change DME - UniKL MIMET 12

Positive Displacement & Kinetic (Rotor Dynamic) Positive displacement Means :- A specific amount of fluid passes through the pump for each rotation. Pump Type : Gear Pump. DME - UniKL MIMET 13

Positive Displacement & Kinetic (Rotor Dynamic) Kinetic (Rotor dynamic) Means :- No specific amount of fluid flow per rotation; Flow depends on speed of rotation driver (Mechanical Driver) Pump Type : Centrifugal Pump DME - UniKL MIMET 14

There are 2 main classes of pump in marine use: 1. Positive Displacement Pump Positive displacement pumps are self priming pumps and are normally used as priming devices. They consist of one or more chamber, depending upon the construction, and the chambers are alternatively filled and emptied. The positive displacement pumps are normally used where the discharge rate is small to medium. They are popularly used where the viscosity of the fluid is high. They are generally used to produce high pressure in the pumping system. 2. Rotor Dynamic In dynamic pressure pump, during pumping action, tangential force is imparted which accelerates the fluid normally by rotation of impeller. Some systems which contain dynamic pump may require positive displacement pump for priming. They are normally used for moderate to high discharge rate. The pressure differential range for this type of pumps is in a range of low to moderate. They are popularly used in a system where low viscosity fluids are used.

COMPARISON DME - UniKL MIMET 16

The main difference between kinetic and positive displacement pumps ? The main difference between kinetic and positive displacement pumps lies in the method of fluid transfer. A kinetic pump imparts velocity energy to the fluid, which is converted to pressure energy upon exiting the pump casing A positive displacement pump moves a fixed volume of fluid within the pump casing by applying a force to moveable boundaries containing the fluid volume. DME - UniKL MIMET 17

DME - UniKL MIMET 18

1. Displacement pump The displacement pumping action is achieved by the reduction or increase in volume of a space causing the liquid or liquid (or gas) to be physically moved. The method employed is either by using :- Piston in a cylinder using a reciprocating motion or a rotating unit using vanes, lobe, gears and screws. A reciprocating displacement pump is shown diagrammatically in next slide

Example of reciprocating displacement pump

 It achieved by the reduction or increase in volume of a space causing the liquid to be physically moved.  The method employed is either a piston in a cylinder using a reciprocating motion.  Increasing volume space during suction stroke would cause liquid draw in via suction non return valve.  Reducing volume space during discharge stroke would force the liquid out via the discharge non return valve and close suction valve.  When starting the pump the suction and discharge valves must be opened. It is important that no valves in the discharge line are closed.

Fig 6.3 The operating principle of Reciprocating Displacement Pump- (double acting pump use piston).

The operating principle of displacement pump fig 6.3 The pump is double-acting, that is liquid is admitted to either side of the piston where it is alternately drawn in and discharged. As the piston moves upwards, suction takes place below the piston and liquid is drawn in, the valve arrangement ensuring that the discharge valve cannot open on the suction stroke. Above the piston, liquid is discharged and the suction valve remains closed. As the piston travels down, the operations of suction and discharge occur now on opposite sides.

An air vessel is usually fitted in the discharge pipework to dampen out the pressure variations during discharge. As the discharge pressure rises the air is compressed in the vessel, and as the pressure falls the air expands. The peak pressure energy is thus 'stored' in the air and returned to the system when the pressure falls. Air vessels are not fitted on reciprocating boiler feed pumps since they may introduce air into the de-aerated feedwater.

Reciprocating displacement pumps; Advantage 1. self priming, 2. will accept high suction lifts, 3. produce the discharge pressure required by the system 4. can handle large amounts of vapor or entrained gases. Disadvantages 1. Complicated in construction. 2. A lot of number of moving parts. 3. Requiring high attention and maintenance.

A relief valve 1. Always fitted between the pump suction and discharge chambers to protect the pump being overpressure due to operating with a valve closed in the discharge line. (refer to picture below). 2. In addition to relieving excess / dangerous pressure from closed-top vessels or piping systems. 3. These normally-closed valves provide system control benefits as shown. Depending upon the function performed, they are given different names. 4. \"Pressure Relief Valve\"- to protect a system, such as pump, pipe segment or tank) from excessive pressure (in excess of the set point).





Displacement Pump 1a. The Rotary Vane Type The pumping action in each case results in the trapping of a quantity of liquid (or air) in a volume or space which becomes smaller at the discharge or outlet side. It should be noted that the liquid does not pass between the screw or gear teeth as they mesh but travels between the casing and the teeth.

rotary vane type cont'  The starting procedure is similar to that for the reciprocating displacement pump.  A relief valve will be fitted between suction and discharge chambers.  The particular maintenance problem with this type of pump is the shaft sealing where the gland and packing arrangement must be appropriate for the material pumped.  The rotating vane type will suffer wear at a rate depending upon the liquid pumped and its freedom from abrasive or corrosive substances.  Usually found onboard on fuel oil system due to it can maintain pressure regardless of fluctuations in flow rate, also they can self-prime from dry, have excellent priming capability and easy to maintain, they are low vibration and extremely quiet when operating compared to other pump types. They can also run dry for small periods of time and handle small amounts of vapour.

Example of rotary vane pump

displacement pump cont' 1b.The screw pump:  A screw pump is a type of rotary pump which is equipped with screws that mesh together and rotate within a cylindrical cavity or liner.  The fluid enters from the suction side of the pump and moves linearly along with these intermeshing screws to the discharge side of the pump.  The clearances between the screws and the liner are very small hence the fluid gains pressure while moving through the pump.

Example show a simple drawing of a screw- displacement pump

example of twin screw pump Application onboard ship :  Hydraulic and lubrication system : Many lubrication systems and hydraulic systems use a screw pump. These are usually small, triple screw pumps. Lubrication and hydraulic systems like this are used for supplying lube oil to large machinery, supplying hydraulic power to high- pressure hydraulic systems  Heavy Oil system – When pumping high viscosity crude oil, screw pumps can offer an ideal solution for moving it through pipelines. Able to pump higher viscosities than a centrifugal pump, and able to pump higher flows than a reciprocating pump, screw pumps are often used in these applications. In these scenarios, very large pumps are sometimes used to get to the flow rate required. These pumps are usually driven by large electric motors or diesel engines.

displacement pump cont'  A gear pump is a type of positive displacement (PD) pump. It moves a fluid by repeatedly enclosing a fixed volume using interlocking cogs or gears, transferring it mechanically using a cyclic pumping action. It delivers a smooth pulse-free flow proportional to the rotational speed of its gears.  How does a gear pump work? Gear pumps use the actions of rotating cogs or gears to transfer fluids. The rotating element develops a liquid seal with the pump casing and creates suction at the pump inlet. Fluid, drawn into the pump, is enclosed within the cavities of its rotating gears and transferred to the discharge. There are two basic designs of gear pump: external and internal. Refer figure 1



Positive Displacement Pump-Rotary Gear Pump In gear pump one gear is driven by a motor, whereas the other is an idler gear and rotates because of the meshing action between the two gears. The fluid is carried around the external teeth in both gears. Simple Gear Pump



1. AXIAL FLOW PUMP An axial-flow pump, or AFP, is a common type of pump that essentially consists of a propeller (an axial impeller) in a pipe. The propeller can be driven directly by a sealed motor in the pipe or by electric motor or petrol/diesel engines mounted to the pipe from the outside or by a right-angle drive shaft that pierces the pipe. Fluid particles, in course of their flow through the pump, do not change their radial locations since the change in radius at the entry (called 'suction') and the exit (called 'discharge') of the pump is very small. Hence the name \"axial\" pump.

axial flow pump cont' example of axial flow •An axial-flow pump uses a screw propeller to axially accelerate the liquid. •The outlet passages and guide vanes are arranged to convert the velocity increase of the liquid into a pressure. •A reversible axial flow pump is shown in Figure 6.5. The pump casing is split either horizontally or vertically to provide access to the propeller. •A mechanical seal prevents leakage where the shaft leaves the casing. •A thrust bearing of the tilting pad type is fitted on the drive shaft. The prime mover may be an electric motor or a steam turbine.



The efficiency is equivalent to a low lift centrifugal pump and the higher speeds possible enable a smaller driving motor to be used. The axial-flow pump is also suitable for supplementary use in a condenser scoop circulating system (refer figure 6.6)since the pump will offer little resistance to flow when idling. With scoop circulation the normal movement of the ship will draw in water; the pump would be in use only when the ship was moving slowly or stopped.

 The pump would be in use only when the ship is moving slowly or stopped figure 6.6

What is a centrifugal pump?  A centrifugal pump is a mechanical device designed to move a fluid by means of the transfer of rotational energy from one or more driven rotors, called impellers. How does a centrifugal pump work?  In a centrifugal pump liquid enters the centre or eye of the impeller and flows radially out between the vanes, its velocity being increased by the impeller rotation.  A diffuser or volute (the casing) is then used to convert most of the kinetic energy in the liquid into pressure. The impeller is the key component of a centrifugal pump. It consists of a series of curved vanes. These are normally sandwiched between two discs (an enclosed impeller). For fluids with entrained solids, an open or semi-open impeller (backed by a single disc) is preferred (Figure 6.9).





Design of the pump casing Two basic designs of pump casing: volute and diffuser. The purpose in both designs is to translate the fluid flow into a controlled discharge at pressure.  Diffuser designs. In this case, the fluid pressure increases as fluid is expelled between a set of stationary vanes surrounding the impeller (Figure 3).  Diffuser designs can be tailored for specific applications and can therefore be more efficient

 In a volute casing, the impeller is offset, effectively creating a curved funnel with an increasing cross-sectional area towards the pump outlet.  This design causes the fluid pressure to increase towards the outlet (Figure 2)

A vertical, single stage, single entry, centrifugal pump for general marine duties is shown in Figure 6.7 consist of; 1. The main frame and casing,together with a motor support bracket 2. House and the pumping element assembly. 3. A pump shaft 4. An impeller

Figure 6.7 Single-entry centrifugal pump