Marine Auxiliary Machinery.pdf

1. Introduction Auxiliary Machinery covers everything onboard except the Main Engine and Boiler and includes almost all the pipes and fittings as well as main items of equipment providing the following functions: i. ii. iii. iv. v. vi. vii. viii.

Supply the requirements of the main engine, circulation, force lubrication, feed heating, coolers, condensers, air compressors, oil reception, transfer and treatment. Keep the ship dry and trimmed, bilge and ballast system. Supply the domestic requirements, fresh, salt, sanitary and sewage system, and HVAC. Supply the main power for propulsion and maneuvering, shifting, propel steering gear and stabilizers. Supply the ship with electric power and lighting, steam and diesel generator engine. Moor the ship and handle cargo windlass, capstan, winches and liquid-cargo oil pumps. Provides for safety, fire detection and fighting, life boat engines and launching gears, water-tight doors. Provides for data logging, remote control and automatic action, pneumatic, electropneumatic, and electric equipment, self-regulating apparatus, etc.

2. Pumps A pump is a machine for raising liquids from a low level to a high one, also for increasing the energy in liquids to allow it to flow or (i.e. increasing the pressure). A pumping system on a ship will consist of a suction piping, the pump, and a delivery (discharge) piping as shown in below:

The system is arranged to provide positive pressure or head. The pump provides the needed energy to overcome any losses in the system.

Where,  

: Total losses in the system, : Frictional head loss in the suction piping,



: Frictional head loss in the discharge piping,



: Height of the suction tank above the pump (negative if below),



: Height of the discharge Tank above the pump.

All values are in meters of liquid. The system (head loss-flow) characteristic can be drawn as shown below.

The system flow rate or capacity will be known and the pump manufacturer will provide a (head—flow) characteristic for his equipment which must be matched to the system curve.

To obtain the best operating conditions for the pump it should operate over its range of maximum efficiency. The determination of Net Positive Suction Head (NPSH) is undertaken for both the system and the pump. Net Positive Suction Head is the difference between the absolute pump inlet pressure and the vapor pressure of the liquid, and is expressed in meters of liquid. Vapor pressure is temperature dependent and therefore NPSH should be given for the operating temperature of the liquid. The NPSH available in the system is found as follows:

Absolute Pump Inlet Pressure All values are in meters of liquid. Notes:   



The pump manufacturer provides NPSH required characteristics for the pump (provided in mwater head). The pump in the system must be matched in the terms of NPSH such that the following relation is achieved (NPSHrequired < NPSHavailable). An insufficient value of NPSHrequired will result in cavitation (i.e. forming of collapsing bubbles in the liquid which will affect the pumping operation and may damage the pump. 1 atm = 1.13 bar = 760 mmHG = 10.3 mmWater.

2.1.

Pump Types

Reciprocating

Positive Displacement

Piston Gear

Rotary

Pumps

Screw Axial Flow Kinetic

Centrifugal Radial Flow

2.1.1. Positive Displacement Pumps They are most suitable and efficient when dealing with small volumes. They can pump flows of high viscosities. They can develop high pressure differential. The displacement pumping action is achieved by the reduction or increase in volume of a space causing the liquid (or gas) to be physically moved. The method employed is either a piston in a cylinder using a reciprocating motion, or a rotating unit using vanes, gears or screws. 2.1.1.1.

Reciprocating (Piston)

It may be single-acting or double-acting. The one shown below 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.

2.1.1.2.

Rotary (Gears)

It’s usually a motor driven through a chain or wheel drive. There are no suction or discharge valves. The two toothed gears shown below mesh together and fit in the casing. The fluid is carried round between the tooth and the casing. Such pumps are fairly efficient, runsmoothly and are best suited for pumping oil, particularly for boiler oil pressure feed.

2.1.1.3.

Rotary (Screws)

Considering the figure shown below, it’s seen that the flow enters the outer suction manifold and passes between the casing of the pump and the screws, which are motordriven, up to the central discharge manifold of the pump.

Timing gears are fitted to some screw pumps to ensure that correct clearance is maintained at all timings between the screws, preventing overheating and possible damage.

2.1.2. Kinetic Pumps 2.1.2.1.

Centrifugal (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 below. It’s efficient, simple in design and reversible. 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. A water cooled thrust bearing is then required.

2.1.2.2.

Centrifugal (Radial Flow)

In a centrifugal (radial flow) pump liquid enters the center or eye of the impeller and flows radially out between the vanes, its velocity being increased by the impeller rotation. A diffuser or volute is then used to convert most of the kinetic energy in the liquid into pressure. The arrangement is shown below. The impeller volute casing design will depend on the required duty (e.g. head to lift, pressure, quantity, etc.).

For the velocity to be constant, the volute is made so that the cross-sectional pipe area increases uniformally from cut water to throat.