ARAPUNI SECRET

Otago Daily Times, Issue 21975, 9 June 1933, Page 6

ARAPUNI SECRET

VIBRATION OF UNIT BAFFLING PROBLEM SOLVED,

The most stubborn of Arapuni’s secrets —the vibration in No. 1 generator—has been satisfactorily solved by the discovery of a defect in the runner, or spinning part of the turbine. The error has been remedied and the unit is now operating with its maximum of efficiency, thus solving a problem which has puzzled engineering experts for three years.

It is now possible (says the New Zea- - land Herald) to disclose the exact s nature of the defect. In the manufact ture of the runner, which is the huge ~ steel water-wheel whose rotation is set . up by the water entering the power ) station from the penstocks, one side of the bottom ring was accidentally made 1 heavier than the other. The raanu- , facturers, ipgtead of restoring the 3 balance by adding a corresponding weight to the lighter end of the wheel, 2 added v the weight to a smaller but £ parallel steel ring attached higher up T the shaft. The result was that while the bottom ring was perfectly balanced when stationary, in motion several disturbing forces were set up that accounted ! for the vibration of the unit. 3 WEIGHT ADDED IN WRONG PLACE. A top may be balanced statically if an added weight near the bottom is set off by a weight added to the upper part of the top on the opposite side, but as soon as the top is spun the weights, being in different planes, will cause the top to tremble as it spins. The same thing applied to the runner. As an engineer would say, it was balanced statically but not dynamically. Had the counter-balanced weight .been added to the lower ring, both weights, . namely, that added accidentally and J . that deliberately, would have revolved . at equal speeds in the same plane. But . because one weight was added to the s upper and smaller ring, the two weights > travelled at different speeds. As a matter of fact, taking into consideration j i the different circumferences of the two i wheels, the heavy part of the lower ’ i wheel was travelling at the rate of j 100 feet a second as against the 50 feet ' j a second of the weight added to the ’ I upper wheel. Thus the two weights ' I were repeatedly pulling against each ’ j other, while at the same time, because ; j they were both situated in different ■ i planes, a bending tendency was exerted ' | on the shaft when spinning was at high ; i speeds. ( It was in April, 1930, that the vibra- ! i tion was first noticed. - It may seem ; 1 strange that the cause, not finally deter- | mined until three years later, should have i proved so elusive, but many things happened to put the investigators off the scent. Two months after the vibration was noticed leakage occurred in the headrace. RUNNING TESTS IMPOSSIBLE. On June 11, 1930, the works closed down and the machinery was not in motion again until April 10, 1932, after the Hornell repairs had been carried out. During all this time running tests were impossible. Indeed, the first obvious avenue of investigation lay in the direction of discovering whether the vibration was connected in any way with the movement of the country produced by the leakage. This was finally disproved by the observations of extremely sensitive levels. It must be borne in mind that the discrepancies in the weights of the runner rings were not visible to the eye. It was impossible, for instance, on inspection, to see the extra heavinessin the steel of the lower ring, while the piece added to the upper ring was covered and out of sight. '* Unevenness in the weight of the runner was one of the original suspected causes, but when stationary it balanced perfectly, and it was impossible to test it again in motion for nearly two years owing to the closing down of the works. Moreover, other theories had to be examined. There was, for instance, the very widespread credence given to the rumour that the foundations of the power-house were unstable, that underground streams were in the neighbourhood and that the station was actually “ floating ” on water. Other theories that had to be examined related to possible vibration in the penstock and the condition of the draught tube of the turbine. VIBRATION ELIMINATED. Over a year ago it was announced that the cause of the trouble was probably a flaw in the manufacture of the runner. Since then elaborate tests have enabled the Public Works Department’s engineers to state precisely to what extent the runner was dynamically out of balance and to remedy the fault. The principal tests that led to the confirmation of the flaw theory involved the removal of the suspected runner and its replacement by the runner from No. 3 unit. No. 1 unit thereupon worked perfectly with no sign of vibration. Two tests of this nature were carried out, and in the end the engineers had the problem finally, solved. ; The defective runner from No. 1 unit has been adjusted by taking away the weights added by the manufacturers to 1 the upper ring and balancing the lower 1 ring properly by adding corresponding weights in The same plane. It is now 1 operating in No. 3 unit and is working perfectly. The responsibility of correcting the ‘ defective unit would normally have ■ rested with the original contractors, 1 Messrs Armstrong, Whitworth and Company, who, in the contract with the New ( Zealand Government, undertook to give ' a performance guarantee for all machinery installed. The runner in question, however, was installed after 1 the contract was cancelled. ,