Early Steam Turbines

1886. See text relating to C J. Hanssen's letter

At the turn of the 19th/20th centuries, the steam turbine quickly came to displace the reciprocating steam engine for large power generation and high speed marine propulsion applications. Credit for this revolution belongs to Charles Algernon Parsons.

The potential for high velocity steam jets to produce rotary motion had been known for centuries. Hero (or Heron) of Alexandria recorded a form of reaction turbine in the second century BC.

In 1629 An Italian architect named Branca described a form of impulse turbine in which a steam jet impinged upon vanes on a wheel.

A variety of other forms of steam turbine were proposed by inventors including Kircher, Wolfgang de Kempelen, James Watt, James Sadler, Richard Trevithick, John Ericsson, James Pilbrow, Von Rathen, William Avery, Ruthven, William Gorman, Robert Wilson, Tournaire, John S. Raworth, Alexander Morton.

A number of manufacturers produced small, simple, inefficient steam turbines commercially in the 19thC. They were used for driving fans, circular saws, etc. By 1860 the North Moor Foundry Co were producing steam turbine driven fans. In 1884 the Butterley Co installed merchant saws driven by steam turbines, which were presumably of substantial construction.

In 1886 C. J. Hanssen wrote to Engineering, pointing out that he had made an experimental multi-stage turbine in 1870:
'Sir,— Under press of business I only of late noticed in Engineering of May 1, 1885, the drawing and description of Mr. Parsons’ high-speed motor. I take much interest in this invention, and congratulate Mr. Parsons on his success. However, I beg to say the idea is not quite new; already in 1870 I schemed an engine on exactly the same principle, and had it made and tried at the engine works of Messrs. Burmeister and Wain, in Copenhagen. I beg to hand you inclosed a drawing of my experimental engine, which, as shown, had four wheels in succession, each wheel 2 ft. 6 in. in diameter. Unfortunately I have mislaid my notes on steam pressure used and number of revolutions and brake power obtained ; the result did not come up to my expectations, and other engagements forced me to discontinue my experiments.
I have, however, always been of opinion that the problem could be solved, and shall be glad to see more of the results obtained with Mr. Parsons’ engines.
Yours faithfully.
C. J. Hanssen, C.E.
Kolding, Denmark, March 18, 1886.'

In 1889 Gustaf de Laval patented and produced a sophisticated type of geared impulse steam turbine which gained wide acceptance. The steam underwent a single stage of expansion, leaving the nozzle with a very high steam velocity. This required a correspondingly high peripheral speed of the turbine blades in order to obtain resonable efficiency. The high speed limited the diameter of the turbine disc, thereby limiting the power output obtainable. The turbine speed was too high for most applications, but de Laval incorporated an efficient double helical reduction gearbox which reduced the speed considerably.

Earlier, Charles Algernon Parsons had appreciated that to gain wide application as a prime mover, a steam turbine would need to be run at a moderate speed, and that this could be achieved by dividing the overall heat drop into a number of stages of small fractional expansions, i.e. by compounding.

The first Parsons turbine ran in 1884, driving a dynamo, also of Parsons' design, to develop 10 HP at 18,000 rpm. Much larger turbines were soon developed for land and marine application.

Parsons' turbines were of the reaction type (actually impulse-reaction). Nurmerous stages were needed, each with numerous blades. For efficiency, radial and axial clearances had to be very small to minimise leakage.

A number of other inventors developed the de Laval turbine in compound form. Far fewer stages were needed than with the Parsons turbine. All the pressure drop took place in the fixed blading, and none in the moving blades. Consequently there was no significant axial (thrust) loading on the rotor. The pressure drop across the fixed blading was relatively high, and in order to minimise interstage leakage, the rotor diameter was minimised between the moving blade rows, and the fixed blades were carried in 'diaphragms'. Thus the compounded impulse turbine was characterised by 'disc and diaphragm' construction. Interstage leakage was minimised by having labyrinth glands in the diaphragms. The earliest prominent exponents were Curtis, Rateau and Zoelly.

It was soon realised that it could be advantageous to have one or several impulse stages at the inlet end, followed by reaction stages. This is because blades at the inlet end of a normal Parsons turbine were very small, and consequently the stage efficiency was low, due to the end effects (unfavourable flow regime at tip and root) and the relatively large leakage area compared with the blade height. By using a larger diameter wheel with several impulse stages at the inletin place of a series of reaction stages, a large pressure drop could be obtained before the steam was admitted to the reaction stages. A further advantage, with superheated steam, is that a large temperature drop occurs in the impulse stage, so a smaller portion of the rotor and casing is subjected to high temperatures. This arrangement also reduced the length of the turbine.

Over time, the distinction between 'impulse' and 'reaction' turbines became less clear, all rows of blades having some degree of both impulse and reaction.

Marine Turbines

Charles Parsons foresaw the advantages, and difficulties, of applying the steam turbine to high speed marine propulsion. The turbine's high speed, so advantageous for electric generators, posed considerable difficulties for propulsion. After a great detail of experimentation and development, Parsons provided a stunning demonstration of the marine turbine's capabilities with the Turbinia.

Very much larger marine turbines soon followed, propelling warships and passenger ships. Merchant vessels followed, having reduction gearing to obtain a suitable propeller speed.

Impulse-type steam turbines soon found application for marine propulsion, whether as pressure- or velocity-compounded turbines in their own right, or for the inlet end stages of Parsons-type turbines.

Both impulse and reaction types had their own advantages and disadvantages in marine propulsion. The Parsons type was bulkier and more susceptible to blading damage in the event of debris ingress or rubbing contact between rotor and stator (small clearances, large number of blade rows), but it was regarded as more efficient, and was probably less susceptible to blade fatigue failure.

Impulse-type turbines had fewer blade rows, and blade/stator clearances could be much larger. However, fewer blades meant higher steam bending loads per blade. This was more significant on marine applications than for power generation turbines. In general terms, this is because marine turbine blades were subject to high steam bending forces at a variety of speeds, whereas turbines driving alternators only carried high loads at the normal synchronous speed. The danger comes when high steam bending stresses are applied when a row of blades is being excited to vibrate at one of its many resonant frequencies. The situation is more complex on 'disc and diaphragm' turbines than on those with Parsons-type drum rotors, because the discs themselves introduce 'wheel-type' vibration modes.

The problems was exacerbated on Curtis- and Rateau-type turbines because they tended to have partial arc admission at the inlet end. This was advantageous for efficiency, compared with throttle control, but it did mean that blades passed jets of high velocity steam followed by quiescent areas, subjecting them to cyclic 'once per rev' excitation. However, the effects of partial arc admission only persist in the first few stages, where blades are relatively short.

Geared Marine Turbines

Parsons recognised the need to address the conflicting requirements for a high turbine speed and low propeller speed. One approach was to interpose double helical gearing. It was first applied in a small boat in 1897. Subsequently, considerable development work was needed before gearing for high speeds and high powers was produced which offered tolerable noise and vibration levels. The Parsons Marine Turbine Co purchased the merchant ship Vespasian in 1909 and installed geared turbine propulsion machinery. Trials carried out before and after conversion showed steam consumption with the turbine to be up to 19% lower (depending on the propeller speed).

See Also

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