AN INTRODUCTION TO MARINE GAS TURBINE
The aim is to be able to identify and explain the function of gas turbine major engine component and discuss the mode of operation of simple and complex cycle gas turbine engines compared with diesel and steam plant.
Definition of a Gas Turine
A continuous cycle self contained heat engine using a gas as the working fluid.
Breakdown of definition
1. Energy output. After providing the power to drive the compressor(self contained) the energy output may be in the form of :
· Jet thrust
· Shaft power
· Compressed air from compressor
· Heat
2. Self contained. Once running it is not dependent on any other machine.
3. Gas may be.
· Air – Open cycle
· Hydrogen, helium etc closed cycle . In this case heat is added by means of a heat exchanger.
4. Energy source. This may be provided by
· Oil fuel
· Natural gas
· Sewage gas
· Waste gas
· Waste heat (eg blast furnance)etc .
5. Compressor. This is usually either axial or centrifugal, but free piston and reciprocating compressor have been tried, although the process then is no longer continuous.
6. Continuous. Unlike a diesel which operates with gas in a similar way the GT uses a constant pressure process which is continuous.
Comparison of cycles
In the services we are concerned with three main forms of providing power (see below). These all have particular advantages and disadvantages, some of which can be readily appreciated by a quick comparision of the ideal cycle on a T-S chart.
The following factors show themselves:
1) Steam Plant
- Operates across saturation envelope.
- Closed cycle – recovery process needed to condense and re-use steam, hence much additional machinery.
- Small work to compress fluid. Good work from expanding vapour- large exess.
2) Diesel
· Fairly good available work output after compressing gas.
· Open cycle.
· Non-continuous cycle means valves and moving parts. Stress problems.
3) Gas turbines
- Very poor specific work output available after compressing gas. High powers only available if:
- Component efficiencies high
- High mass flow
- High mass flow possible because process is continuous.
Conclusions
From the above comparisons the factors which stand out about gas turbines are:
- Unlike The others, an overall power output is possible purely because the constant pressure lines diverge with increase in entropy.
- Its specific output is very small and dependent upon the efficient operation of the various components. Mass flows are high hence small increases in component efficiencies will give relatively large increases in output.
Marine gas turbine (GT)
An aircraft can use the energy in the gas stream directly to provide the propulsive power eg in a jet. In the marine environment this is not feasible because of the very nature of the surroundings , the noise, the requirement to reverse and limitations imposed by the construction of the ship. The GT must be used to drive a propeller/water jet/generator. Because of this requirement we may consider the marine GT to be split into two main parts.
· The gas generator- that providing the high energy gas stream.
· The power producer- that converting the energy in the stream into a useful form of power eg shaft power(power turbine)
Hence it is possible to adapt an aircraft engine for marine use by the addition of a power turbine eg Olympus SMIA.
In addition marine GTs are not nearly so restricted for space as the aero versions and therefore it is theoretically possible to attempt to improve the performance by using one or more of the following:
- Intercooling
- Reheat
- Heat exchanger
- Water injection
- Waste heat recovery
REASONS FOR THE ADOPTION OF GAS TURBINE
- The aim to be able to explain the reasons why the royal navy and other navy is adopted gas turbines as main propulsion units in major surface warships, the typical problems associated with running them at sea and their solutions.
- The resons for using gas turbine in warships:
· High power/weight ratio.
· Quick startup capability
· Comparatively low development cost.(benefits from aero engine development)
· Low onboard maintenance requirement.
· Ease of upkeep by exchange of critical parts.
· Reduced watchkeeping manpower.
· Good SFC at high power.
· Availability
· Reduced underwater noise(fewe hull openings)
- Typical problems and solutions
· Distortion of combustion chambers- ongoing design effort, regular inspection.
· Combustion of naval fuels-redesign of combustion system.
· Compressor fouling-air filtration, regular washing.
· Surge and rotating stall- air bleeds, variable geometry blades.
· Turbine Disc failures – Design effort, defined service life.
· Bearing failures- Uprated bearings, earlier detection of possible failure.
· Fuel consumption at part load- multispool variable geometry engines, higher operating temperature.
· Practical problem of a complex cycle- no solutions at present.
· Poor life of aero types –rapid engine change capability-comprehensive repair/rebuild facilities, life continually being upgraded.
List the various factors which must be present for a scavenge fire to start?
For any fire to begin there must be present a combustible material, oxygen or air to support combustion and a source of heat of a temperature high enough to start combustion. In case of scavenge fires the combustible material is oil. The oil is usually cylinder oil which has drained down from the cylinder material is oil. The oil is usually cylinder oil which has drained down from the cylinder spaces, in some cases the cylinder oil residue may also contain fuel oil. The fuel may come from defective injectors, injectors with incorrect pressure setting, fuel particles striking the cylinder, and other similar causes. The oxygen necessary for combustion comes from the scavenge air which is plentiful supply for the operation of the engines. The heat in the scavenge space, around the cylinder, brings the oil to a condition where it is easily ignited. The high temperature required to start combustion may arise from piston-ring blow past.
How would you become aware of a scavenge fire? How would you deal with a scavenge fire?
The first indication of a scavenge fire may be a slight reduction in the engine speed due to the reduction in power which comes about when a fire starts. Other indications are a higher exhaust temperature at the cylinders where the scavenge fire has started and irregular speed of turbo-blowers. External indications will be given by a smoky exhaust and the discharge of sooty smuts or carbon particles. If the scavenge trunk is oily the fire may spread back from the space around or adjacent to the cylinders where fire started and will show itself as very hot spots on areas of the scavenge trunk surfaces. In ships where the engine room is periodically unmanned, temperature sensors are fitted at critical points within the scavenge spaces. On uniflow-scavenged engines the sensors are fitted round the cylinder liner just above the scavenge ports. A temperature higher than reference or normal then activates the alarm system.
If a scavenge fire starts, two immediate objectives arise; they are to contain the fire within the scavenge space of the engine and to prevent or minimize damage to the engine. The engine must be put to dead slow ahead and the fuel must be taken off the cylinders affected by fire (see note). The lubrication to these cylinders must be increased to prevent seizure and all scavenge drains must be shut to prevent the discharge of sparks and burning oil from the drains into engine rooms. In allows the fire to burn itself out without damage. Once the fire is out and navigational circumstances allow it, the engine should be stopped and the whole of the scavenging port examined and any oil residues found round other cylinders removed. The actual cause of initiation of the fire should be investigated. If the scavenging fire is more major nature. It sometimes become necessary to stop the engine and use the steam or extinguishing arrangement fitted to the scavenging trunk. The fire is then extinguished before it can be spread to surfaces of the scavenging trunk. Where it may cause the paint to start burning if special non-flammable paint has not been used.
How can the incidence of scavenge fires be prevented or reduced?
One of the first things that must receive attention is maintaining the scavenge space in as clean a condition as possible. This can be done by keeping scavenge drain pipes clear and using them regularly to drain off any oil which comes down into scavenge space drain pockets. The scavenge space and drain pockets should also be cleaned regularly to remove the thicker carbonized oil sludge which don’t drain down so easily and which are a common cause of choke drain pipes. The piston rings must be properly maintained and lubricated adequately so that ring blow-by (blow-past) is prevented. At the same time one must guard against exceed cylinder oil usage. With timed cylinder oil injection the timing should be periodically checked. Scavenge ports must be kept clear.
The piston-rod packing ring and scraper rings should be regularly adjusted so that oil is prevented from entering
