Engine Design

[Jim Hathaway]

I was wondering what benefit the mixed reciprocating engine/turbine installation gave to be used in the Olympic Class-
I am speaking versus a straight reciprocating or straight turbine installation.
I was under the impression that the installation in Laurentic was partly experimental to gain experience with turbines, it suprises me that White Star would specify it for what would become thier flagships unless there was a concrete benefit.
Was it simply conservative engineering?
Jim Hathaway

 

[Michael H. Standart]

More like economical engineering. White Star wasn't trying to compete with the Cunard speed queens, so there was no benefit to an all turbine plant. The combination reciprocating/turbine plant was just more economical.

 

[Jim Hathaway]

Thanks, Michael, it makes sense, but still a strange installation for a production ship-
Have a great Christmas-
Jim

 

[Michael H. Standart]

Strange by our reckoning to be sure, but it made sense for White Star when what they were looking at was the bottom line. The reciprocating engines did the lions share of the muscle work with the exhaust steam being used once more to run the turbine. The result was more efficient use of the steam and a more economical ship to operate.

 

[Jim Hathaway]

Hi Mike-
They were probable economical, there was also a weight saving, I would bet-
Thanks!
Jim

 

[Jeremy Lee]

The Cunard engines of the Mauretania and Lusitania were much heavier and were not as economical I guess?

Titanic's engines were much less powerful, especially in terms of horsepower.

 

[Michael H. Standart]

I don't know that the turbine plant on the Mauritania and Lusitania was heavier, but it certainly had a lot more power. The *catch* is all that extra power to give you all that speed extracts a price in the fuel you guzzle down. In short, speed costs. White Star was not really interested in record breakers. What they wanted were large comfortable ships that were fast enough to carry the mails and maintain a service with weekly sailings from both sides of the Atlantic. For this purpose, 21 knot ships were plenty fast enough and were not as expensive to operate.

 

[Jim Hathaway]

I was just able to answer my question with a paper available on-line.
The Rede Lecture by Sir Charles Parsons, in it's 3rd section has a few paragraphs about mixed powerplants using a low pressure turbine such as Titanic and Laurentic had.
In the paper he states "a slow speed turbine expands low pressure steam much furthur and more economically than any reciprocating engine. Under this system, the turbine generally is made to develop about 1/3 of the whole power.
He mentions the first combination vessel, the Otaki with twin screws driven by triple expansion reciprocating engines exhausting into a center low pressure turbine driving a screw.
Parson's paper says Otaki consumed 12% less coal than her sister driven by four reciprocating engines.
He goes of to say that Laurentic, in service realized a 14% savings of coal consumed over the same speed as her sister Megantic with a pure reciprocating engine plant.
Mauretania's turbines were of high pressure design, Titanic's was of a low pressure design operating on 9 psi exhaust steam.
The link to this paper is below-
http://www.history.rochester.edu/steam/parsons/index.html
Jim Hathaway

 

[Allan T Condie]

The main reason for using reciprocating engines on the Olympic Class was for economy. Reduction Geared Turbines were not an issue in 1911 and all Turbine installations were direct drive with high propellor speeds and they were heavy on steam therefore heavy on fuel compared with later designs. The disadvantage of turbines is you need a separate set to go astern. Titanic's main engines could,as with any recip. set be reversed easily and when maneovring the turbine did nothing - two large gate valves diverted steam from the recip. engines direct to the condenser.

 

[Jim Hathaway]

Thanks, Allan,
The paper mentions reduction geared turbines, but I got the impression that it was still experimental when the paper was delivered (1911)
I printed it out, it will provide interesting reading today-
Jim Hathaway

 

[Jim Hathaway]

Because the turbine engine was using exhaust steam from the boilers, it would be proportional to the power being produced by the reciprocating engines.
I am curious if the turbine had any provision to be throttled independently, or whether the turbine was either on line or off line.

 

[Allan T Condie]

It would appear that the turbine would only be operational once the two reciprocating engines had been started and exhaust steam was available. In order to take the Turbine off line two very large valves were fitted to divert exhaust steam from the LP cylinders of the reciprocating engines either a) to the condensers, or B) to the turbine. What fine control these valves allowed I have yet to research. Bedtime reading for Christmas Night!

 

[Jeremy Lee]

BTW, if I remember correctly, Titanic and Olympic's engines could produce 45,000hp while Mauretania and Lusitania's could do up to 70,000hp.

A great difference!

 

[Samuel Halpern]

The turbine was engaged when they ran the reciprocating engines at 50 rpm and greater. This corresponded to half ahead. There appears to be no throttling provision between reciprocating engine exhaust and turbine input. The piston changeover valves were either set to direct reciprocating engine exhaust to the turbine or directly to the main condensers. The reciprocating engines exhausted steam at a pressure of 9 psia. Main condensers were kept at a pressure of 1 psia with the vacuum pumps. You would not be able to keep a differential of 8 psi between turbine input and turbine output unless the changeover valves completely blocked steam from going to the main condensers when the turbine was put in operation.

 

[Scott R. Andrews]

Sam,

You are correct that there was no throttle control per se for the turbine; basically, the turbine simply responded proportionally to the amount of exhaust being received from the reciprocating engines. The changeover valves did, however, provide some limited speed control as there was an automatic override of the starting platform's turbine controls whereby the turbine's Proell centrifugal governor actuated the Brown's engine which operated the two piston valves. If the speed of the rotor reached 10% above the maximum number of revolutions dialed in by the engineerings, the piston valves redirected steam directly to the condensers until the speed dropped below the preset limit. That's about the extent of the engineer's ability to fine-tune the speed at which the turbine rotated.

Regards,
Scott Andrews