Trouble-free geared steam turbines for ships

In the previous article, the author discussed how the Parsons turbines were used in ships and highlighted how sea travel had changed dramatically. In this part, he talks about how Parsons used new machining tools and techniques to make more gears.

Parsons, in late 1894, became interested in driving cargo ships at about 10 knots with his steam turbines by using a speed reducing gear as the most economical way to proceed.

(The gearing in Vespasian cargo vessel)

The “Turbinia” episode of “Cavitation” then became known in 1895. Therefore, gearing looked more attractive to him to match the more optimum high RPM of the steam turbine to the needed low RPM of the props. There was a speed incompatibility problem. 

In 1896, he started his work with gearing and in 1897 used a 10-horse power geared steam turbine to drive a small 22-foot-long boat. The gear reduction was 14 to 1 with the prop running at 1400 RPM.  The speed obtained was 9 miles per hour and the performance was encouraging. He then, in 1898, powered an electric generator with a 2 to 1 reduction gear (9600 to 4800 RPM) with a 150 KW steam turbine with excellent results.

In 1909, 11 years later, Parsons Marine Steam Turbine Company purchased a 275-foot-long “Vespasian” cargo vessel equipped with a triple expansion condensing reciprocating steam engine. He overhauled the ship's engine and ran the ship to give base line data for comparison to use when he converted the ship to be driven by a geared steam turbine arrangement. He used a double helical gear with speed reduction ratio of 19.9 to 1 to drive a single prop with the HP turbine driving one side of the large central gear and the LP turbine driving the other side of this gear.

The water consumption (efficiency) was 15 to 20 percent less than that of the steam recip arrangement using the same boiler. By 1910, the ship had been run for 12,000 miles carrying cargo between the Tyne and Rotterdam. The results were outstanding and trouble free. There was no appreciative wear of the gear teeth which gave an efficiency of 981/2 %. The geared arrangement could then be used with confidence to drive electric generators, ships and other equipment whereby the speed of the steam turbines could be better matched with the speed of the driven equipment.

The gears had to be machined to a high precision and therefore Parsons had to develop new machining tools and techniques to make the gears. He developed a new process of rotating the gears while they were being machined and as a result could machine the gears with much greater precision.

Up to this time, through his reaction turbine design, Parsons was able to make larger and larger diameter turbines to reduce the RPM to dive larger and larger ships of all kinds to overcome the speed incompatibility problem. This worked out quite well, but was costlier and required many more blades and vanes for lager diameter rotating drums and discs. There was reluctance to driving Naval and merchant vessels using speed reduction gears.

Parsons realized that the steam turbines could be made much smaller and rotate at a much higher RPM if a speed reducing gear could be successfully built, applied and then accepted by the ship industry – Naval and commercial. Parsons was able to accomplish all three of these objectives to give the World the geared steam turbines for ships.

Steam turbines to drive ship props

It is interesting to note that the gas turbines used to drive fans with high bypass ratios of 12 require a speed reducing gear between the power turbine and the fan. Small gear driven fan engines have been used for a number of years for business planes made by Air Research and Lycoming, but to drive fans of 30,000 pounds of thrust and larger for airlines was slow in coming.

It took P & W working with its German partner some 10 years to come out with the new geared fan engine now being purchased in large numbers to drive commercial and commuter planes. The same incompatibility of RPM exists for fan engines as for Parsons steam turbines a hundred years ago to drive ships. At first R-R and GE both poo hood using a gear to drive the fan, but now R-R is reported to be following Pratt and is designed a geared fan engine. The same economic advantage of blade count and power turbine size is involved. It is not known what GE will do.

The commercial airlines have been reluctant to use geared fan engines in the past. However, helicopters and turboprop planes such as the C130 have used engines such as the Allison/R-R T-56 have used gears by necessity for years with great success. It looks like the commercial airlines are at last taking a close look at the new geared fan engine hitting the market and realize that there are economical advantages to be obtained. Money seems to have a way of talking.

Driving electric generators

Parsons first started making steam turbines to drive electric generators. His very first 6 KW tinker toy steam turbine built in 1884 drove an electric generator. He started out making steam turbines for power generation, but after the huge success of the “Turbinia” ship, he then started building many turbines for ship as well as for electric power. A parallel design and manufacturing path continued over the years. He used two separate manufacturing plants, one for power units and one for ships, and swapped designs back and forth. Parsons had to design and make the electric generators from scratch which was an accomplishment in its self that took a lot of ingenuity. 

Although the application of the steam turbine to power electric generators and the turbine to drive ship screws is similar, there is quite a difference. The ship props needed a slow RPM whereas the electric generators needed a much higher speed. Therefore, it was easier to apply steam turbines to drive electric generators than it was to drive ship props. There were several speed options for AC alternators that could be used, all of which were at a higher RPM than required for ships. There was not the incompatibility problem for electric power units to deal with.

Much could be written about the early development by Parsons in the design and manufacture of the electric generator, but this blog is about turbines and so I will skip over this fascinating evolution of electric generators. Those that are interested in this early effort and accomplishment can read all about it in the Parsons book.

In the next article, the author discusses about the first axial flow compressor built by Parsons.

Ivan G. Rice was past chairman of the South Texas Section of ASME (1974 - 75), past chairman of the ASME Gas Turbine Division (now IGTI) (1975 - 76). A Life Fellow Member of ASME and Life Member of NSPE/TSPE, he has authored many articles and ASME papers on gas turbines, inter-cooling, reheat, HRSGs, steam cooling and steam injection.

How modern day steam turbines came to be

Story of the first toy turbine

How Parsons marine steam turbines powered ships

Parsons turbine receives worldwide recognition

The 'Golden Age' of steam ships