STEEL TOWERS

Evening Post, Volume CVIII, Issue 45, 21 August 1924, Page 6

STEEL TOWERS

THE MANGAHAO LINE

AN EXPERT'S LECTURE

NITRATES FROM THE AIR.

The technological section of the Wellington Philosophical Society mot at the Dominion Museum last night, Mr. W. R. Holmes -presiding over a very large attendance. 'Pour short papers were :set down for reading, but it was found possible to deal with only t\vo, and the others, on the foundations of the Mangahao dam (Mr. F. W. Furkert) and on technical features of the telephone, system of Wellington (Mr. Smith), were postponed. *'"'", ■

Mr. A. Gavey gave a very interesting description of the steel towers used for the support o£ the Mangahao powertransmission . line in the KhandallahPaekakariki section, and, in particular, of the method of erecting them. . The steel towers, he said, were found preferable to wooden pole supports for this section, one important factor being the high • cost' of transporting the wooden ■ poles over the very difficult country iv this part of the line. His discussion of the relative costs of the two forms of .support, was. very., interesting. Steel towers,, he showed, cost, at five towers I ■Jar mile, approximately- £700 per mile of double line. Wooden poles, fourteen to the mile'of line, cost about £543 per mile, including in each case all charges j for transport and erection. But the cost of the line, using the wooden poles, was increased by the extra number of insulators required, so that the. wooden pole system \vould, in this section, have cost about £900 per mile. The cost of the wooden poles was greatly increased by the transport charges. . In ordinary country these averaged £1 per pole; but, whereas the Department's estimate for the hill section was £3, the lowest tender received was twice that amount. METHODS OF ERECTION. j The towers afe nominally 30, 50, and 70 feet high, and each carry two circuits, of three wires each. As the height is measured to the lowest of the three wires, nearly 30 feet of,, tower rises above the nominal.height. Two methods of erection were tried—that of building them on the ground and up-ending them proved unsatisfactory, chiefly owing to the ground being unsuitable" for the arrangement of the hauling gear, and most of the towers were built upright. The lecturer described and' showed illustrations o£ a very ingenious rising derrick, rigged inside the towers to lift, the members into position. Mr. Gavey also discussed the method of making foundations for the towers, and said that experience had shown that it was probably better to make considerable excavations on steep slopes, to get. a ) horizontal bench, than to construct extension legs for the tower. The aver' age time required for the erection of a tower was 150 man-hours, (making the cost, on a basis of'2s per hour, £15); the quickest erection was carried out in 30 man-hours. LIFE OF WOOD AND STEEL.

' An interesting discussion followed," in which the relative durability of steel and hardwood formed the chief topic. Several members spoke on this point, and cases were quoted of hardwood "structures fifty years o£ age or more, and. of galvanised iron or steel, which had lasted as long or longer. Mr. A. Orehistou referred to the rapid failure of poor qualities of iron, and to the curious, fact that iron of high electrical conductivity had a notably long life.in the open air, and remained sound when poorer material had utterly disappeared. It was clear on both sides of the argument that the life to be expected from either wood or steel depended upon the selection of the material. Mr. F. T. M. Kissell said that the question of durability was one of ' the most important unsettled questions in the technique of electrical transmission, arid there was need for more reliable ' information than was now available. He mentioned that whereas on the Lake Coleridge pole line the only replacements made had been those necessitated by the increase of load due to line changes, several steel . (ungalvanised) towers on the Hora Hora line had to be.renewed owing to "the failure of some of the,lighter members. Mr. Gavey said that probably 20 to ?5 years' service was to be expected from hardwood poles, and 40 to 50 years fiom steel towers.

The second paper was read by Mr. A. Orehiston, on the proposal to establish at. Milford Sound a -plant for the extraction of atmospheric nitrogen and its conversion into nitrogenous fertilisers -and-other-compounds. The nitrogen ex-•iraction—industry—in.-Nor.way... was the chief illustration used. Mr. Orehiston - briefly described the process, which conI sists in passing the air through an electric furnace in which the nitrogen is burnt into combination with the oxygen, to form a gas which' is readily converted into nitric acid. The : commercial sue- ! cess of the process .depends upon the availability of a . large' supply of very cheap power. Mr. Orehiston dwelt upon I the enormous amount of power which could readily bo obtained at. Milford Sound, beginning with.the harnessing of the Bowen Falls and subsequently utilising, many others. Tho only materials | which would have to be taken to the I .factory for the. carrying on of production and export of .the nitrates would be lubricant oil and hoop iron. He arj cued that" there was in New Zealand I and AustraJia^alone a market sufficient to absorb the output for -many years, i and that the industry would be worth in the neighbourhood of a million a year. Critics of the proposal had argued that the prosperity of the Norwegian industry was due to the demand for nitrates during .the;war; but the industry had, m fact, increased greatly^since the war, and the company was making very nigh profits. He contended that the concutions at Milford Sound were such that uitrates ■ could be produced 'at a price which would enable them to compete easily with those produced elsewhere,and that the natural nitrates of Chile, owing to the increased cost of production, would never recover their pre-war position in the fertiliser market. The paper was freely discussed from several points of view. One aspect raised was the difficulty which would be experienced in maintaining a staff in the , remote and in many ways'•inhospilablii ! surroundings of the Sounds, where scenery alone offered a poor inducement to continued residence. ' Mr. ' Orchislon countered the argument that (he works would destroy tho beauty of the sound by saying that they would cause very .little change, and that in any case the beauty of the Sounds was but a. small asset, compared with a million a year.