AN INTRICATE JOB

Dominion, Volume 29, Issue 221, 15 June 1936, Page 10

AN INTRICATE JOB

Renewing Superheater

Tubes

RANGATIRA’S BOILERS

Long and Tedious Task

One of the most interesting and intricate engineering jobs ever undertaken in Wellington is that of the renewal of tlie superheater tubes in the boilers of the inter-island steamer Rangatira. The job was started while

the ship was on the floating dock undergoing repairs to her hull, but the work is slow and tedious, and is being completed at the Clyde Quay Wharf.

The Rangatira has six large watertube boilers which work at a pressure of 4201 b. bo the square inch, the steam being superheated to a temperature of 700 degrees before it leaves the boilers to do its work in the turbines. The boilers are oil-fired, the combustion taking place in large bricklined chambers giving a very large heating surface to the series of watertubes whicli pass through them to the massive forged steel drums in which the steam is collected at a temperature of 445 degrees. The steam then passes through the superheaters in which it is raised to the high temperature of 700 degrees, finally leaving the boilers as an intensely hot dry gas.

Highly Efficient Turbines. The propelling machinery of the Rangatira consists of two turbo-alter-nators which at 30S0 revolutions per minute develop 5120 kilowatts at 3150 volts, three phase. These generators supply current to two large motors, each about 15 feet in diameter and direct-coupled to the propeller-shafts. Each motor develops 6500 shaft horsepower at the full speed of 220 revolutions per minute. Thus the reduction between the speed of the generators and that of the motors is in the ratio of 14 to 1. The turbines driving the alternators are remarkably small, considering their high output of 6500 horse-power, each measuring six feet in length and slightly less in width at the greatest diameter. They are actually smaller in area than their exhausts into the condensers Upon which they are seated. The efficiency of these high-speed turbines is indicated by the fact that the steam which enters their nozzles at a velocity of 1500 feet per second and a pressure of 4001 b. (25 atmospheres) passes into the exhausts at a pressure of only ,51b., while its ternttiHiiiiiiitiimiiitttiiiiiiiitiiiiiiiiiiiiiiiiiiiiiiiitiitiittiiiimimtt

perature falls from 700 degrees to S2 degrees. The three tiny turbines driving the direct-current generators for lighting purposes, run at SOOO revolutions per minute, mechanical gearing reducing the speed of the generator to 1000 revolutions per minute. Superheater Described. Briefly described, tlie superheater consists of a large steel drum 17 feA long (the length of the boiler) with an internal diameter of about two feet. Fitted into the drum are hundreds of steel tubes which extend diagonally Auto the combustion chamber of the boiler. There are eight "stages” of superheater tubes, through which the steam is directed by means of "baffies” fitted inside the drum.

The tubes of the superheaters, which are "solid-drawn” from special steel, are shaped like a huge hairpin about six feet in length. They have an internal diameter of seven-eighths of an inch, the metal itself being one-eighth of an inch in thickness. Each superheater carries 308 of these tubes, so that there are altogether 1848 in the six boilers. An Intricate Job. The work of renewing the tubes is an exceedingly intricate and tedious job, as, owing to the very confined space, only two men can work inside the suiterheater drum —and they have to lie full length and head to headwhile two others work in the combustion chamber of the boiler fitting the tubes into place and holding them up while the men inside the drum secure them.

First of all. however, the old tubes have to be burned out of their sockets by oxy-acetylene torches, an operation that has to be repeated 616 times in each boiler, there being two ends to each of the 30S tubes. The ends of the boilers have to be removed to give access to the tubes—another awkward job owing to the very limited space in each boiler-room, which also coitiplicates the task of getting rid of the old tubes and the bringing in of toe new ones. The fitting of the new tubes is an even more tedious job. as, when they are fitted, the two ends of each of the 308 tubes have to be secured by the men working inside the narrow drum by means of special expanding tools which give them a bell-shaped "mouth” and so prevent their drawing () ut of their respective holes or sockets Finally, when the 616 ends of the lubes are secured, ami the baffle plates in position, the drum is closed and the "job” is tested by an hydraulic pressure of 6501 b. to the square inch. It speaks well for the exactness of the tubes and their fitting into the drum that not a “bead” of water shows through, even under this great pressure.

The final supply of tubes arrived by the Kuahine last week, and at the rate of fitting, some 30 a day. it is expected that the work will be completed during the next fortnight. Practically the whole of the repairs to the Raugatira arising out of her mishap are now completed. A good deal of survey work and refitting is also being carried out, and the ship should be ready for recommissioning about the end of this month.

(Letters to the Editor on Page 2.)