Steam Turbines

The basic principle of the turbine is as old as the wheel, linear energy is directed onto deflectors mounted on a central shaft and rotational energy is imposed on the shaft. It is the basic principle of the windmill and the waterwheel.

By the late 1800's the steam reciprocating engine had reached it's pinnacle, there seemed to be no further avenue to explore to improve efficiency, to get more power the engine had to be bigger, and the size of the moving componants was imparting vibration both to the engine, causing it to breakdown, and to the ship, which was particularly unwelcome in lavishly decorated ocean liners.

Turbines of one description or another had been experimented with, Watt himself had considered it and discounted it. The problem was one of basic engineering: the reciprocating engine utilises the pressure of steam, but the turbine principle uses the speed of steam, and that is fast, 2,000 mph is fairly typical of a moderate power boiler. In order to utilise that energy the turbine blades have to rotate at least have the speed of the steam jet. Even by the 1880's it was just not possible to construct a device that could rotate at those speeds without melting or flying apart, probably both.

The turbine transforms linear energy, such as wind, into rotary energy.

Charles Parsons patented the first workable turbine in 1884, and his genius looks simple in retrospect, like all great ideas. He decided since he could not create a device that could rotate at the speed of steam, he would slow the steam down.

The speed of the steam jet is dependant on the rate of expansion, it had long been known and built into reciprocating engines, that a vacuum, or partial vacuum, at the exhaust end of the engine would create a higher pressure ratio between input and output and so impart more energy to the pistons, this vacuum is the main function of the condenser in which cooling steam is used to create a drop in pressure below that of atmosphere.

Parsons idea was to reverse that function and create an exhaust that was pressurised, but still below that of the feed. This effectively resulted in a smaller pressure drop between feed and exhaust and a slower steam jet. By repeating this process a number of times most of the energy from the steam jet can be extracted without the turbine having to destroy itself.

A simple turbine schematic of the Parsons type, rotating and fixed stators alternate and steam pressure drops by a fraction of the total across each pair, the stators grow larger as pressure drops.

The method he used still forms the basis of turbines today, including the gas turbines used in jets and modern warships. Parsons took a tube and down it’s inner length he set rings of angled blades. He then set a cylinder in the tube which also had rings of angled blades. Steam is fed into the tube at one end, passes through the first set of fixed blades and hits the first set of blades of the cylinder at an angle. Rotational velocity is imparted on the cylinder and it begins to spin. The steam passing through the rotating blades hit the next ring of fixed blades and it is this effect that impedes the steam, causing a pressure to build. By careful design of the interacting blades the ideal pressure differential is created to spin the cylinder. The steam passing on down the tube encounter the next set of blades attached to the rotating cylinder and the process repeats, in the diagram the blade sets get bigger each time, the principle is the same as in the triple expansion engine, as the available steam pressure drops larger areas are needed to extract the energy at best efficiency.

There is a very basic flaw in the turbine that may be obvious at this stage: since the interaction of the fixed and rotating blades is critical to the efficient operation of the turbine it is not possible to create a device with variable speeds. Later developments will include a degree of variable pitch on the blades, but at this stage just getting the blades to hang on is a challenge enough! Another problem is the turbine will only operate efficiently at high speed, in the order of thousands of revs, in order to fully utilise his turbine Parsons also had to invent a better method of gearing down high speed shafts to a more useful speed.

Ten years after filing his patent and with turbines making their mark on land Parsons built the Turbinia, a 44 ton yatch a 100 feet long, 9 ft beam and 3 ft draught and demonstrated it at the 1897 Diamond Jubilee of Queen Victoria where it tore up and down the ranks of warships at 34 knots when the fastest ships of the day were limited to 27 knots.

Turbinia, preserved at the Discovery Museum in Newcastle, to overcome cavitation she had nine propellers on three shafts.
The admiralty refused to invest in the project even so but allowed Parsons to equip two new destroyers with turbine engines at his own expense, they were HMS Viper and HMS Cobra which stunned a sceptical audience by achieving 37 knots over a speed trial, Parsons had only promised 30 knots.

Both Viper and Cobra were lost in accidents at sea, confirming the doubters suspicions, but Parsons again invested his own money and built another destroyer, HMS Velox, and finally the admiralty agreed to equip a cruiser in 1902, HMS Amethyst. Amethyst was one of four sister cruisers being built, the other three received standard reciprocating engines and the performance comparison deeply shocked even the conservative admiralty, a committee was formed (of course) to look into the matter and in 1905 recommended that all future warships be equipped with Turbines. Jackie Fisher had become First Sea Lord the year before and heartily agreed, rushing through his concept of an all big gun battleship and marrying it with turbines to create HMS Dreadnought in 1906.

The value of the turbine cannot be measured in speed alone, power for power the turbine was lighter and more compact than the reciprocating engine and with less vibration made for a stabler gun platform which effectively increased the accuracy and range of the guns. At top speed the reciprocating engine was at the limit of it's capability and suseptable to mechanical failure, but the turbine reached it's peak effieciency at top speed.

HMS Amethyst, the first cruiser to be fitted with turbines.

HMS Dreadnought, the ship that re-wrote the concept of a Battleship

This activity was not lost on the great merchant entrepreneurs of the time. In 1901 the King Edward became the first Turbine Passenger vessel, operating on the Clyde, several smaller ships followed but it was in the big liners that the Turbine proved it’s full worth. These ships operated at full power for days on end, their coal consumption per ship was equivalent to that of the entire Royal Navy Home Fleet! The Virginian and Victorian were the first turbine equipped liners, each of 13,000 tons, they were followed by the 30,000 ton Cunard Carmania, she had a sister ship the Caronia and the comparison in speed, fuel consumption and engine space occupied spelled the death of the reciprocating engine in high performance ships. Lusitania and Mauretania of 38,000 tons were to follow with Turbine engines giving 70,000 SHP.

Although the turbine was not efficient in slower cargo ships and never fully replaced the reciprocating engine there it did augment many. Parsons invented a turbine which utilised the waste low pressure steam of these engines, geared down to existing shafts the turbine improved fuel economy on long journeys in the order of hundreds of tons of coal per voyage. This "Parasitic" Turbine was also sometimes used to drive a seperate shaft, as on the Titanic whose central shaft was turbine driven and the outer two reciprocating engines.

Crankshafts for the Titanic's reciprocating engines, the sheer weight of the moving parts in the high power marine reciprocating engines caused problems with vibration, wear and reliability, comparable turbines were a fraction of the size.

A significant problem with the turbine as a maritime engine was discovered quickly by Parsons when his Turbinia failed to achieve the expected speeds in her first trials. Convinced his engine was right he turned to the propellers and constructed a glass tank and a strobe light to study the effects of the propeller at high speed. He soon discovered what is now known as Cavitation, at the 2,000 RPM output by Turbinia the outer tips of the propellors were turning so fast they were unable to form a grip on the water and were generating pockets of vacuum instead, in extreme circumstances the whole propeller would simply spin around and produce nothing but bubbles, no movement at all.

Modifying the propeller design helped, as did using smaller propellers, in the end Parsons fitted nine propellers on three shafts in order to goose the Turbinia along at 34 knots. But in practical use the only option was to run the turbine at less efficient slower speeds and use small propellers. In warship design this was acceptable as the advantages were great still, not least the ability to fully mount the engines below the waterline, getting weight down low in the ship and providing better protection. But as an example a coal burning triple expansion engine would use 1.54 tons of coal per hour for every SHP produced, at low speed a coal fired turbine was up to 2.4 tons as opposed to 1.2 tons at high speed.

The turbine needed to be geared down, but gear wheel manufacture at the time was not up to the job and Parsons himself had to invent a new method of manufacturing gear wheels before they were of sufficient precision to handle the speeds involved.

Part gearing (no, I don't know what that means either!) was fitted to the Acheron Class Destroyers HMS Badger and HMS Beaver in 1911 and then full single reduction gearing to the Laforey Class Destroyers HMS Leonidas and HMS Lucifer in 1913, when employed in the latest Dreadnoughts speed was increased to such an extent, despite heavier armour and guns, that a new class came into being, the Super Dreadnought.

Typical arrangement of a High Pressure and Low Pressure Turbine through a double reduction gearbox. A seperate turbine was needed for reversing the ship, this was almost always on the LP turbine where fitted.

The combination of gearbox and turbine made the engine expensive, and they were inefficient at low speeds, though the invention of the cruise turbine which was optimised to run at lower steam pressures helped conserve fuel when ships were running at low speeds helped. But in the merchant service only the big liners with their long distant sprints across the oceans really utilised the turbine. The development of the oil fired furnace was given a great boost during WW1 and between the wars the oil furnace - reciprocating engine pairing challanged the turbine, particularly in the area of fuel consumption.

To overcome this Parsons demonstrated the advantage of using high pressure boiles, typical boilers of the time were at 200-275 psi, by doubling the pressure Parsons showed that turbine efficiency was hugely improved. The RN developed the three drum 500 psi boiler which became virtualy the standard fit for all warships up to and including WWII, two such boilers married to two sets of Parsons turbines with a Low Pressure and High Pressure turbine in each set would typically generate 40,000 SHP and push an unladen light Destroyer of the era along at 37 knots with dual reduction gearing. By comparison HMS Hood had 24 boilers and four turbines on four shafts generating just over 150,000 SHP which in 1920 shoved her 45,000 tons along at 31 knots.

An alternative to mechanical gearing was Electric, essentially the turbine was coupled to a generator which in turn powered an electric motor, giving much better control, but at a cost in efficiency due to the inherent losses in both generator and motor. But the system had it's uses and the Buckley Class Destroyers of the USN used a similar system in WWII, some of which were lend leased to Britain as Captain Class Frigates.

The actual shape and layout of a turbine varies a great deal, but this glimpse under the hood of a a high pressure turbine gives a rare peek.

Before I move on I need to cover something of the different types of turbine.

Impulse or De Laval

In this type of turbine the operation is rather more like that of waterwheel. In the waterwheel water is directed into buckets which fill, impart rotational velocity by gravity and then empty to rise and collect more. The impulse turbine has ring of fixed nozzles that blast onto bucket type vanes of a rotating wheel. The pressure drop is achieved in the nozzle which is flared and not by the rotating stator. the de Laval spings at typically 30,000 rpm and is ineffiecient at using the steam energy.

Reaction Turbine

The interaction of alternate fixed and rotating stator wheels forms nozzles as the blades align with each other, the moving stator is primarilly moved by the steam expanding into the virtual nozzle and then being forced by the shape of the blade to change direction, hence "reaction."

Impulse-Reaction or Compound or Parsons

In this type of turbine the blades are shaped to form a cross section inducive to impulse drive at the base and reaction at the tip. The Parson's turbine has a distinctive shape as the high pressure stators are smaller than the low pressure stators resulting in fan effect, in larger turbines the stators are mirrored and the steam fed to the centre to split either side and so reduce stress on the system.

Velocity Compound or Curtis

Curtis combined the de Laval and Parsons turbine by using a set of fixed nozzles on the first stage or stator and then a rank of fixed and rotating stators as in the Parsons, typically up to ten compared with up to a hundred stages, however the efficiency of the turbine was less than that of the Parsons but it operated at much lower speeds and at lower pressures which made it ideal for ships, the Curtis turbine was manufactured under license in Britain at John Brown's shipyard on Clydebank, hence the Brown-Curtis Turbine. Note that the use of a small section of a Curtis, typicaly one nozzle section and two rotors is termed a "Curtis Wheel"

Pressure Compund Multistage Impulse or Rateau

The Rateau employs simple Impulse rotors seperated by a nozzle diaphragm. The diaphragm is essentialy a partition wall in the turbine with a series of tunnels cut into it, funnel shaped with the broad end facing the previous stage and the narrow the next they are also angled to direct the steam jets onto the impulse rotor.

Now just to complicate things a typical maritime turbine set would be any mix and union of the above types. A marine turbine often traded off maximum efficiency to conserve space and weight, in particular the large low pressure turbine wheels of the Parsons type could be dispensed with for only a few percent loss of power and a gain of considerable reduction in weight and size of the turbine.

Principle of the basic Impulse Turbine
Curtis Wheel, typical arrangement is two rotating stators with one or more sets of blades, a stationary stator and a bank of fixed steam nozzles, used to extract power from the initial stage in a high pressure engine.

Rateau stage, each stage has a set of nozzles and an Impulse Wheel

A typical compact geared marine turbine. The gearbox is a signficant proportion of the engine and in some cases the largest single unit. The condenser is cooled with sea water in a process that is the reverse of what happens in a boiler. Sea Water is highly corrosive and "Condenseritus" is the old demon of steam ships where sea water gets into the feed water chain via corroded pipes and contaminates the engine and boiler.

Dual Axial Low Pressure Turbine with Reversing Turbines. Steam is fed to the centre of the Parsons Compound turbine and released through pairs of static and rotating blade wheels before venting to a condenser. To go in reverse the steam is diverted to a pair of reversing turbines, often a Curtis Wheel as it provides the most power for a compact unit.