If in building up speed a turbine starts to vibrate, slow down immediately. If the vibration ceases at a lower speed, run at that speed for several moments. Then again attempt to build up the speed.

Text 26. Vibration

If in building up speed a turbine starts to vibrate, slow down immediately. If the vibration ceases at a lower speed, run at that speed for several moments. Then again attempt to build up the speed. If no further vibration is noted, it may be assumed that the trouble was probably due to unequal heating of the rotor or to an accumulation of water in the turbine. If the vibration does not cease at a lower speed, the turbine should not be used except in an emergency, and steps should immediately be taken to locate and remedy the trouble. If, after running at reduced speed for some time, the vibration again manifests itself at the higher speed, a speed limit lower than that at which vibration is noted should be set. This speed should not be exceeded until the trouble has been remedied.

If, when the turbine begins to vibrate, a rumbling is heard coming from it, the trouble is undoubtedly due to water or other foreign matter in the turbine. If after slowing down or correcting faulty boiler operation, the trouble is not eliminated, the turbine must, except in case of emergency, be shut down. If a sharp metallic sound is heard after the rumbling noise, it may be assumed that, part of the blading has been damaged: the turbine must be shut down and not used

Mechanical power transmission means are often the cause of vibration. Gears which are inaccurately cut, worn or are not meshing properly will set up disturbing forces. A flat belt with a splice which is thicker than the belt or a chain not uniform in pitch will cause an impulse each time the non-uniform portion passes over the pulley or sprocket. If these disturbing forces fall in step with a resonant frequency of the machine, severe vibration will result.

Oil seals, baffles or guards rubbing on the rotating member will cause a chatter and this source of vibration can usually be detected by the local heating of the shaft. Oil seals must be set close to the shaft for the best results but clearances should be provided for the rise and lateral movement of the shaft due to the oil film. The inner edges of the seals should be thinned so that if there is contact with the shaft, the thin edge will break down to relieve the interference. Fiber oil seals should be given the same clearance as metal seals.

The origin of vibration may be at the bearings. Any condition that causes metallic contact between the shaft and the bearing will lead to vibration. The bearing may be misaligned or pinched in the housing or may be loose in the housing. There may be high spots on the journal or bearing bushing or the journal may be out of the round if the bearing does not have ample side clearance, enough oil may not get under the journal to maintain a complete oil film. Vibration arising from sources such as these will be accompanied by heating of the bearing.

When a machine equipped with antifriction bearings develops vibration, the cause may be worn or damaged bearing parts, and the balls or rollers of the bearings should be examined carefully for irregularities.

Bearing bushing – втулка подшипника

EXERCISES

I. Give synonyms to the following words:

to finish, sufficient, to provide, to flow, suitable, to obtain, to indicate, to allow, thoroughly, to lubricate, to drain, to occur, frequently, to revolve, to approach, to increase.

II. Give opposites to the following words: to disengage, bottom, low, slowly, impossible, to open, maximum, ahead, to warm up, roughly, to disconnect, complicated, workable, exhaust, wide, to stop, inner.

III. Translate the following sentences into Russian, paying attention to the function of the Infinitive:

1. Turbine blades and discs were water cooled to withstand the high gas temperature.

2. At that moment the gases are at a temperature which is much too high for the turbine blades to withstand.

3. The purpose of the diffuser is to convert the kinetic energy of the entering air into static pressure followed by a further static-pressure increase in the compressor.

4. Present practice is to supply this oil at 110 to 120°F and to allow 20 to 30°F rise of temperature in passing through the bearings.

5. Due consideration is given to the relative importance of plant performance, simplicity, weight, bulkiness and initial cost in determining which of the functions of reheat, intercooling and regeneration are to be incorporated in the design.

6. Metals having relatively high strength and withstanding high temperatures are used for turbine blades, but the best materials available limit the inlet temperature to about 1,100°F if the machine is to be reliable in service and show a satisfactory life.

III. Answer the following questions:

1. What question is of great importance with the constant pressure turbine?

2. What metals are used for the blades?

3. What is the task of the designers while designing gas turbines?

III. Translate the following sentences into English:

1. Повышение температуры газов на входе в турбину ограничивается напряжениями в лопатках.

2. Для турбинных лопаток применяются жаропрочные материалы.

3. Экономичность газотурбинной установки зависит от правильного выбора степени сжатия в цикле.

Дата добавления: 2015-11-04