Britannic turbine rotor stats: expansion stages, blade lengths, and other dimensions


A. Gabriel

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Jun 13, 2018
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As there are presently no known plans of the Parsons turbine on Olympic or Titanic, I have decided to write up an exhaustive description of Britannic's turbine rotor, as Britannic's machinery is the only set for which detailed schematics are known to exist thanks to the February 1914 issue of Engineering. The hope is that by comparison with known specifications for Olympic's and Titanic's respective turbines, it may be possible to reverse-engineer the detailed specs and dimensions of the various parts by scaling down those of Britannic wherever it may be deemed appropriate or possible.

Unless otherwise noted, all numeric values in this post are taken from these schematics, and all quotations are taken from the Engineering article on Britannic.

The Parsons turbine of the Britannic, designed to develop 18,000 horsepower at 170 rpm, takes steam at 10 psi absolute from the reciprocating engines and expands it through six expansion stages, "exhausting [it] into two condensers at about 28 in. to 28-1/2 in. vacuum, with a 30-in. barometer". I think this implies that the steam that exits the turbine for the condenser comes out at a pressure of 1.5 to 2 inHg, which works out to around 0.74 to 0.98 psi absolute. Compare this with the specs for Olympic's (and therefore presumably Titanic's) turbine, designed to develop 16,000 horsepower at 165 rpm, which takes steam at 9 psi absolute from the reciprocating engines and expands it down to 1 psi absolute, with "the condensing plant having been designed to attain a vacuum of 28-1/2 in. (with the barometer at 30 in.)" --Engineering, 21 October 1910.

The combined length of all six expansion stages, from the forward end of the rotor drum to the aft end of the final expansion stage (which I think might be the aft edge of the last row of moving blades?), is 14 feet 11-11/16 inches*. However, the aft end of the rotor drum protrudes beyond this into the steam exhaust chamber by 2-13/16 inches, so the total length of the rotor drum from end to end (excluding the conical portions of the end wheels) is 15 feet 2-1/2 inches. From the forward end of the rotor drum to the forward edge of the first row of fixed casing blades is a space of 1-5/8 inches, and from the first fixed row to the forward edge of the first row of moving blades is a distance of 1-1/2 inches, so that the distance from the forward end of the rotor drum to the forward edge of the first row of moving blades is 3-1/8 inches. The rotor length measured between the extreme edges of the first and last row of blades would therefore be 14 feet 8-9/16 inches. Compare this with Olympic, where "the length of the rotor between the extreme edges of the first and last ring of blades is 13 ft. 8 in." (Engineering, 21 October 1910)

*For some reason the text of the Engineering article states the length of the rotor drum to be 14 ft. 11-1/4 in., despite the actual plans provided in Plate XXXIII clearly marking out the length over all six expansion stages as 14' 11-11/16", excluding the aforementioned protrusion into the exhaust chamber.

The diameter of the rotor drum for Britannic's turbine is 12 feet 6 inches, while for Olympic it is 12 feet precisely. The blade lengths for Britannic's turbine rotor vary from 16 inches to 26-1/2 inches, which is verified by the values given in the plans for diameters from shaft center to inner face of casing: 15 ft 2 in. for the first expansion stage, 15 ft 11 in. for the second expansion stage, and 16 ft 11 in. for the third through sixth expansion stages. Converting all diameters to radii and subtracting the radius of the rotor drum from each one gives blade lengths of 16 inches, 20-1/2 inches*, and 26-1/2 inches. Blade lengths for Olympic for comparison vary from 18 inches to 25-1/2 inches as per the October 1910 issue of Engineering, with the intermediate length remaining a mystery for lack of mention.

*The RMS Titanic Manual by Haynes Publishing claims 21-1/2 inches as the blade length for Britannic's second expansion stage, contrary to the length calculated from the Plate XXXIII dimensions.

The method of converting diameters to radii and subtracting can also be used to work out the depths of the blade roots in each stage, because Plate XXXIII has dimensions given for the diameters at the base of the rotor blades (12 ft 4-1/4 in. at 1st stage, 12 ft 4 in. at 2nd, 12 ft 3-3/4 in. at 3rd-6th) as well as the diameters at the base of the casing blades (15 ft 3-1/2 in. at 1st, 16 ft 0-1/2 in. at 2nd, 17 ft 0-1/4 in. at 3rd, 17 ft 0-3/4 in. at 4th and 5th, 17 ft 1 in. at 6th).

From hereon out, all measurements between rows of blades are given to or from the forward edges of the rows involved unless otherwise noted.

1st expansion stage: eight rows each of fixed and rotating blades
  • 1-5/8 inches -- space from forward end of rotor drum to first row of casing blades (forward edge)
  • 1-1/2 inches -- from first row of casing blades to first row of rotor blades
  • 1 foot 9 inches -- from first row of casing blades to last row of casing blades (divide by 7 to find space between successive rows of casing blades)
  • 1 foot 9 inches -- from first row of rotor blades to last row of rotor blades (divide by 7 to find space between successive rows of rotor blades)
  • 3 inches -- from forward edge of last row of casing blades to end of stage, where a sudden discontinuity in the inner face of the turbine casing is used to accommodate the increased blade length of the 2nd stage
  • 2 feet 1-5/8 inches -- overall length of stage

2nd expansion stage: eight rows each of fixed and rotating blades
  • 1-3/4 inches -- space from end of previous stage to first row of casing blades (forward edge)
  • 5-1/32 inches -- from last row of rotor blades from previous stage to first row of rotor blades in this stage
  • 2 feet 0-15/16 inches -- from first row of casing blades to last row of casing blades (divide by 7 to find space between successive rows of casing blades)
  • 2 feet 0-15/16 inches -- from first row of rotor blades to last row of rotor blades (divide by 7 to find space between successive rows of rotor blades)
  • 3-9/16 inches -- from forward edge of last row of casing blades to end of stage, where a sudden discontinuity in the inner face of the turbine casing is used to accommodate the increased blade length of the 3nd stage
  • 2 feet 6-1/4 inches -- overall length of stage

3rd expansion stage: eight rows each of fixed and rotating blades
  • 1-3/4 inches -- space from end of previous stage to first row of casing blades (forward edge)
  • 5-5/8 inches -- from last row of rotor blades from previous stage to first row of rotor blades in this stage
  • 2 feet 5-5/16 inches -- from first row of casing blades to last row of casing blades (divide by 7 to find space between successive rows of casing blades)
  • 2 feet 5-5/16 inches -- from first row of rotor blades to last row of rotor blades (divide by 7 to find space between successive rows of rotor blades)
  • 4-3/16 inches -- from forward edge of last row of casing blades to end of stage (first row of casing blades in the next stage)
  • 2 feet 11-1/4 inches -- overall length of stage

4th expansion stage: seven rows each of fixed and rotating blades
  • 4-11/32 inches -- from last row of rotor blades from previous stage to first row of rotor blades in this stage
  • 2 feet 3 inches -- from first row of casing blades to last row of casing blades (divide by 6 to find space between successive rows of casing blades)
  • 2 feet 3 inches -- from first row of rotor blades to last row of rotor blades (divide by 6 to find space between successive rows of rotor blades)
  • 5-9/16 inches -- from forward edge of last row of casing blades to end of stage (first row of casing blades in the next stage)
  • 2 feet 8-9/16 inches -- overall length of stage

*Plate XXXIII includes a seemingly out-of-place dimension that I make note of here -- between the aft edge of the last row of 4th-stage rotor blades and the forward edge of the first row of 5th-stage casing blades is a space of 1-1/8 inches.

5th expansion stage: six rows each of fixed and rotating blades
  • 5-9/16 inches -- from last row of rotor blades from previous stage to first row of rotor blades in this stage
  • 1 foot 10-1/2 inches -- from first row of casing blades to last row of casing blades (divide by 5 to find space between successive rows of casing blades)
  • 1 foot 10-1/2 inches -- from first row of rotor blades to last row of rotor blades (divide by 5 to find space between successive rows of rotor blades)
  • 4-1/2 inches -- from forward edge of last row of casing blades to end of stage (first row of casing blades in the next stage)
  • 2 feet 3 inches -- overall length of stage

6th expansion stage: five rows each of fixed and rotating blades
  • 5-3/16 inches -- from last row of rotor blades from previous stage to first row of rotor blades in this stage
  • 1 foot 11-1/2 inches -- from first row of casing blades to last row of casing blades (divide by 4 to find space between successive rows of casing blades)
  • 1 foot 11-1/2 inches -- from first row of rotor blades to last row of rotor blades (divide by 4 to find space between successive rows of rotor blades)
  • 5-1/2 inches -- from forward edge of last row of casing blades to end of stage (where the section that holds the fixed blades gives way to the exhaust chamber)
  • 2 feet 5 inches -- overall length of stage
*Plate XXXIII gives a few more seemingly enigmatic dimensions at this end of the turbine rotor:
  • 3-3/4 inches from the forward edge of the last row of rotor blades to a line somewhere on the rotor drum's 'protrusion' into the exhaust chamber
  • 1-3/16 inches from the end of the last stage to the same line
  • 1-5/8 inches from this line to the very aft end of the rotor drum (again, as mentioned earlier, excluding the conical portion of the end wheel)
 
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