General Engineering Knowledge

H D McGeorge

• Publisher: CRC Press
• Year: 2012
• Print ISBN: 9780367148607
• eBook ISBN: 9781136077173
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This book covers the general engineering knowledge required by candidates for the Department of Transport's Certificates of Competency in Marine Engineering, Class One and Class Two. The text is updated throughout in this third edition, and new chapters have been added on production of fresh water and on noise and vibration. Reference is also provided to up-to-date papers and official publications on specialized topics. These updates ensure that this little volume will continue to be a useful pre-examination and revision text. - Marine Engineers Review, January 1992

Chapter 1

Centrifugal Pumps and Priming—Coolers and Cooling Systems—Pipelines and Corrosion

Centrifugal Pumps

The simple centrifugal pump is used for sea water circulation and other duties where self priming is not a requirement. When installed for bilge pumping or ballast duty, these pumps require a primer i.e. some means of removing air from the suction pipe so that the liquid to be pumped is caused to flow into the pipe and so to the eye of the impeller.

Single Stage Centrifugal Pump

For general duties the impeller is of aluminium bronze keyed and secured to a stainless steel shaft. The impeller shown (Fig. 1) is fully shrouded and of the single entry type. The renewable wear rings are of aluminium bronze and the casing is normally of bronze or cast iron. The cover has a hub containing the shaft bearing at the bottom and, above, either a packed gland or a mechanical seal. The shaft bearing is of phenolic resin asbestos, lubricated by the liquid being pumped except for pumps operating on high static lift. These have grease lubricated bronze bearings to ensure adequate lubrication during the priming period.

Fig. 1 Single stage centrifugal pump (Hamworthy Engineering Ltd)

A spigotted coupling spacer connects the motor half coupling to the pump shaft. When this is removed, the pump cover, together with the impeller and shaft assembly can be lifted out of the pump casing for inspection or maintenance.

Impellers

The fully shrouded, single entry impeller in the pump shown (Fig. 1) is the type most widely used. It consists of a number of vanes curving backwards from the direction of rotation. The vanes are supported on one side by shrouding connected to the hub. The shrouding supporting the vanes on the other side, has an entry at the centre. When the pump is operating, liquid in the casing is swirled by the rotating impeller. The swirling action causes the liquid to move towards the outside and away from the centre (in the same way that stirred coffee moves to the side of the cup, tending to spill over the rim and leaves a dip at the centre). The backward curving vanes and the rotation give the liquid a combined radial and circular motion.

Casing

The section of the volute casing shown in the sketch (Fig. 2) increases, thus allowing unrestricted flow from the impeller. The volute also acts as a diffuser, converting kinetic head into pressure head. Some pumps have a double volute casing which gives radial balance and reduced wear on the bearings.

Fig. 2 Section through impeller and casing

Pumps designed to produce high pressure, have a diffuser ring so that a greater quantity of kinetic energy in the liquid can be converted to pressure.

Suction

When a centrifugal pump is operating, the liquid leaving the impeller produces a drop in pressure at the entry or eye of the impeller. This causes liquid from the suction pipe to flow into the pump. In turn, there is a movement of the liquid to be pumped. The latter is normally subject to atmospheric pressure. A centrifugal pump will maintain a suction lift of four metres or more once it has been primed, because of the water passing through.

The water in a pump acts like a piston for water in the suction pipe and an empty pump will not operate.

A pump which is required to initiate suction from a liquid level below itself, must be fitted with an air pump.

Air Pump Arrangement

The diagram (Fig. 3) shows a primer coupled to the top of an electric motor and centrifugal pump set. A pipe from the pump outlet, provides cooling water for the primer. This returns through another pipe to the pump suction.

Fig. 3 Centrifugal pump with primer

The main pump suction pipe has a float chamber fitted. The float operates a valve on the pipe leading from the float chamber to the air pump suction. With no liquid in the suction, the float drops, opening the valve and allowing the air pump to evacuate the air from the suction pipe. This partial vacuum causes the atmospheric pressure to force liquid into the suction pipe. The rising liquid will lift the float and close the valve on the air pump suction. Air pumped out, passes to atmosphere.

Air Pump Principle

The air pump or water ring primer, as the simple plan view shows (Fig. 4) consists of an elliptical casing which contains a vaned rotor and has a covering plate with ports cut in it. The casing is partly filled with water. The rotor is coupled to the electric motor so that when the pump is running the water spins with the rotor and being thrown outwards, takes up an elliptical shape. The tips of the vanes are sealed by the water and the volume between them varies during the rotation. Beneath the suction ports, the volume increases so that air is drawn from the float chamber. Under the discharge ports, the volume decreases, forcing air out.

Fig. 4 Water ring primer or air pump

Cooling water is necessary to prevent overheating of the sealing water from the action of the vanes in the liquid. Interruption of the coolant supply results in vapour from the sealing water destroying the vacuum effect, so that air is no longer pumped.

The internal passages of a typical air pump are shown in the sectional sketch (Fig. 5). The right side shows the operating passages and the path of the air being pumped. It is drawn from the suction float chamber of the main pump and through the pipe and passages to the suction ports of the primer. The discharge ports are not shown but the air from them is discharged into the top of the outer casing. From the outer casing the air is discharged to atmosphere.

Fig. 5 Air pump showing operating passages

The primer runs continuously but can be unloaded, when the pump has been primed, by the arrangement on the left. The shut-off handle rotates a ported chamber to the position shown, so that the partial vacuum is broken and the water is free to circulate.

Operation

Centrifugal pumps for bilge, ballast and general service are usually fitted with primers. Before starting such pumps, the primers must be checked, to ensure that the sealing water is at the correct level. Fresh water is used for topping up. Suction valves between the liquid and the pump are opened and a check is made that other valves on the suction side of the system are closed. The delivery valve is kept shut and the pump is started. The centrifugal pump can be started with the discharge valve closed, but it is an exception. (A propeller pump should not be started with the discharge shut or overload results. This is shown by comparison of characteristic curves. Obviously positive displacement pumps would not be started with closed discharges.) If priming takes a long time, the primer will become hot unless cooling water is passed through it. The sea water suction can be opened to allow cooling.

Exhauster for Central Priming System

Several centrifugal pumps can be primed from a central vacuum tank as an alternative to being fitted with individual water ring primers. The pumps are connected to the vacuum tank through the same sort of float chamber arrangement as is used with the individual water ring primer. There is also a shut off cock for isolating the pump and a non-return valve.

The primer has two electrically driven air pumps (Fig. 6) which evacuate the tank. Starting is by means of pressure switches through suitable starters, and air pumps are automatically stopped by the switches when the required vacuum is reached. The pumps run intermittently as demand makes necessary and not continuously. Thus, on a vessel where priming is needed for a number of pumps, the use of a central primer would reduce the number of air pumps and the running time for them.

Fig. 6 Exhauster for central priming system

The primer shown, consists of a gunmetal casing of oval shape with a rotor and ported plate, as described for the individual water ring primer. Water in the casing forms a seal and, because it takes up the elliptical shape of the casing when the rotor turns, produces a pumping action. The two suction ports are connected by passages in the cover to the suction pipe from the vacuum tank. The two discharge ports are connected via an aperture to the sealing water tank. Air from the vacuum tank, together with make-up water, is drawn into the suction and discharged to the sealing water tank. The water remains in the tank, while the air passes to atmosphere through the outlet/overflow pipe.

The sealing water reservoir also keeps the air pump cool and is cooled in turn, by a sea water flow through the cooling coils shown. If the water ring temperature rises, then the function of the primer will be destroyed by the presence of water vapour.

Tube Coolers

Tube coolers for engine jacket water and lubricating oil cooling are normally circulated with sea water. The sea water is in contact with the inside of the tubes and the water boxes at the cooler ends. Two pass flow is shown in the diagram (Fig. 7) but straight flow is common in small coolers.

Fig. 7 Tube cooler with two pass flow (Serck)

The oil or water being cooled is in contact with the outside of the tubes and the shell of the cooler. Baffles direct the liquid across the tubes as it flows through the cooler. The baffles also support the tubes.

Tubes of aluminium brass (76 per cent copper; 22 per cent zinc; 2 per cent aluminium) are commonly used. Ordinary brasses and other cheap materials have been used with unsatisfactory results. The successful use of aluminium brass has apparently depended on the presence of a protective film formed along the tube length by corrosion of iron in the system. Thus unprotected iron in water boxes and other parts, while itself corroding, has prolonged tube life. This was made apparent when steel was replaced by other corrosion resistant materials or protected more completely. The remedy in these systems has been to fit sacrificial soft iron or mild steel anodes in water boxes or to introduce iron in the form of ferrous sulphate fed into the sea water. The latter treatment consists of dosing the sea water to a strength of 1 ppm for an hour per day over a few weeks and subsequently to dose before entering and after leaving port for a short period.

Early tube failures may be due to pollution in coastal waters or to turbulence in some cases.

Many coolers are fitted with tubes of 70/30 cupro-nickel. More expensive materials are available. Tubes are expanded into tube plates and may be further bonded by soldering

Tube Plates

Naval brass tubeplates are used with aluminium brass tubes. Tube stacks are made up to have a fixed tube plate at one end and a tube plate at the other end which is free to move with the expansion of the tubes (Fig. 8). Other materials found in service are gunmetal, aluminium bronze and sometimes special alloys.

Fig. 8 Detail of cooler expansion arrangement

Tube Stack

The tube stack shown is fitted with alternate disc and ring baffles. The fixed end tube plate is sandwiched between the casing and the water box. If the joints leak at the other end the special ‘tell tale’ ring will allow the liquids to escape without mixing. The joint rings are of synthetic rubber.

Water Boxes and Covers

Easily removable covers on water boxes permit repairs and cleaning to be carried out. The covers and water boxes are commonly of cast iron or fabricated from mild steel. Where they have been coated with rubber or a bitumastic type coating, the iron or steel has been protected but has provided no protection for the tubes and tubeplate. Uncoated ferrous (iron) materials in water boxes provide a protective film on the tubes as the unprotected iron itself corrodes, the products of corrosion coating the tubes. The iron also gives some measure of cathodic protection.

Water boxes of gunmetal and other materials are used but these, like the coated ferrous metals, give no protection. Soft iron or mild steel anodes can be fitted in the water boxes and provided they cause no turbulence, will help to give cathodic protection and a protective film.

Shell

The shell or cylin...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright
• Contents
• Preface
• 1. Centrifugal Pumps and Priming—Coolers and Cooling Systems—Pipelines and Corrosion
• 2. Hazards in Enclosed Spaces—Tankers—Cargo Pumping
• 3. Fire Protection
• 4. Fuel—Handling and Treatment—Self Cleaning Purifier—Automatic Combustion System for Auxiliary Boiler
• 5. Refrigeration—Air Conditioning—Heating
• 6. Metallurgical Tests
• 7. Stern Tubes, Seals and Shafting Systems
• 8. Steering Gear
• 9. Pollution Prevention—Monitoring—Oily Water Separator—Sewage Treatment
• 10. Production of Water
• 11. Noise and Vibration
• Index