Document 44DqvJezMp4J97dm68D77p30j

steam path may be washed with satu rated steam without dismantling the turbine. For deposits consisting mostly f silica scale or other insolubles, me chanical cleaning is the most satisfac tory; this requires dismantling the unit. During the war some shipyards and chemical-cleaning concerns on the West Coast removed turbine rotors and cleaned them in vats of cleaning sol vents and weak acid solutions. Excel lent results were reported. This method shonld be satisfactory if proper neutral ising solutions are used after the clean ing process. Air blast entraining flyosh and other abrasives bsve also been used. IUBRICATINO SYSTEM Oil temperatures leaving the bearings should be recorded on the log sheet. This gives o running check on the lubricatlng-system operation. A gradual increase in this temperature may be caused by (1) rising cooling-water temperature or (2) oil-cooler fouling. An Increase in the oil temperature rise through the bearing (temperature dif ference of bearing and oil-cooler out lets) may ba caused by (1) increased bearing loading from a change in align ment (2) increaaed bearing friction from oil deterioration (3) decreased oil-flow rate (4) partial wiping by dirt entrained in the oil. These events call for prompt action. Look for steam blowing along the shaft from pocking boxes. This steam finds its way into bearing housings and the condensate will deteriorate the oil. The lubricating system must be kept as free from moisture as possible. TURBINE VIBRATION An operator accustomed to the nat ural hum of a turbine can usually detect increased vibration by the change in sound, but he should be equipped with Instruments to measure vibration am plitude. Readings should be taken at one point daily, say on the No. 1 bear ing, and recorded. When vibration in creases occur, lake additional readings and prompt steps to locate the cause. The manufacturer's represcntslive should be called in to investigate when vibrations approach a detrimental mag nitude. At least six types can occur: 1. Vibration at running-speed fre quency, cause: rub or unbalance. 2. Whipping, a vibration of criticalspeed frequency, happens when the unit operates at a speed olher'lhan the criti cal. This form of vibrolion, primarily caused by oil action in tho bearings, can be determined by Frehm indicators. 3. Half-speed frequency vibrations GENERATOR AND EXCITER require as careful followup during operation os 0* turbine even though mechanical construction Is simpler and temperatures town take place when the critical speed Is other than about half running speed. A packing rub generally causes It 4. Double-frequency vibrations on the rotor are of such small amplitude that they are hard to detect. These ere caused by difference in flexibility of a 2-pole generator field in a plane through the pole-piece centers and a plane at right angles to it. In 2-pole machines the double-frequency vibra tion appears in the stator frame when the field is applied. The poles, 160 deg apart, exert o magnetic force on the stator Irsme tending to distort it into an ellipse. The distortion rotates with the field and the two high and two low points on the ellipse's circumference cause the vibration. 5. Vibrations of very low frequency result from restricted bearing-oil flow. 6. The pump-frequency vibrations *ri caused by incorrectly cut pump gesn! or by the gears rubbing on their caifof Unbalance causes most common hrations, but balancing does not eliid;. note the other types of vibrations, t GENERATOR OPERATION Ceneretor temperatures should- regularly read and recorded. .In *. closed ventilating system, rising to*) peratures with a constant electrical lo*l moy be caused by (1) reduced eooliel; water flow (2) increased cooling-wet0 temperature (3) fouled air-cool0, tubes. Steps should be taken to cot', reel rising temperatures before lM reach tbe maximum value allowed bf the manufacturer. Most ac generate0 are designed to operate at the follow*# temperature rises: - j 40 (28) POWER pbni*rv I** FREQUENCY OF INSPECTION varies from hourly, for euch unit la dismantled for Internal examination. Hare are readings as oil temperatures, to annually or longer when some vital points to wateh, Insuring safety and economy Armtture, 60 C measured by Im bedded detectors. Field, 85 C by resistance. Temperature rises are based oo an ambient temperature not exceeding 40 C In open-ventilation systems,, atmos pheric air enters the generator through a screen or filter. Here a gradual rise In generator temperature for tbe same load and inlet-air temperature indicates that dirt accumulates in eir passages. . Keep collector rings clean, and main tain brush pressures within limits speci fied In manufacturer's instruction books. Rxdtcra. Most modem exciters are totally enclosed and pipe-ventilated from the main generator, see Illustra tion. The air filter, mounted over the r outlet at the exciter-commutator end, should be replaced when the accumu lated dust doubles the pressure drop of the air flowing through it Any accumulation of dirt or metal dust inside the exciter housing should be prevented. When fitting new brushes or cleaning the commutator, take spe cial precautions to remove all traces of carbon or copper dust from all parts of the. exciter. Compressed air cleans most effectively, but be sure no water U entrained. Should dirt and dust be lodged with oil, dean it out with carbon tetra chloride and carefully dry the surfaces. Carbon tetrachloride vapor has detri mental effect on commutation, so all traces must be removed before starting the machine. Wipe studs and lands and clean the brush holder* to remove all traces of metal dust. See that brushes move freely In the holder*. Brush pressure should be maintained within manufacturer's limits, about 3 lo 3Vi pai of contact surface. New brushes should be fitted care fully on the commutator. Keep tbe commutator dean and maintain its pol ished surface; wipe it occasionally with' a piece of bard canvas. ACCESSORY DEVICES Failure of safety devices to function causes practically ell steam-turbine no cidents. It is important that these.de* vicea be checked periodically to Insure their being in working order. The emergency overtpeed governor must be examined and tripped at regu lar intervals. Whenever' a machine la shut down this should be done by the hand trip rather than by manual clos- (Continued on page 150) POWER Jtnu,(y I,,, |N| l