Turbines, steam seals

Figure 14-24. Steam sealing systems for condensing and noncondensing steam turbines. (Used by permission Bui. 8908-EMD. Dresser-Rand Company.)...

Figure 7.18. Heavy-duty centrifugal, axial, and reciprocating compressors, (a) Section of a three-stage compressor provided with steam-sealed packing boxes (DeLaval Steam Turbine Co.), (b) An axial compressor (Clark Brothers Co.), (c) Double-acting, two-stage reciprocating compressor with water-cooled jacket and intercooler (Ingersoll-Rand Co.).

Start-up and monitoring of turbine-generator auxiliaries such as lubrication oil, hydraulic, and steam seal systems... 

Space needs to be provided for the auxiliaries, including the lube oil and seal systems, lube oil cooler, intercoolers, and pulsation dampeners. A control panel or console is usually provided as part of the local console. This panel contains instmments that provide the necessary information for start-up and shutdown, and should also include warning and trouble lights. Access must be provided for motor repair and ultimate replacement needs to be considered. If a steam turbine is used, a surface condenser is probably required with a vacuum system to increase the efficiency. AH these additional systems need to be considered in the layout and spacing. In addition, room for pulsation dampeners required between stages has to be included. Aftercoolers may also be required with knockout dmms. Reference 8 describes the requirements of compressor layouts and provides many useful piping hints. 

The mechanical seal was developed ar rhe beginning of the last century. Its development coincided with the invention of the steam turbine, rhe dynamo and the electric motor. 

Steam turbine, 53, 146, 282-92, 179 back pressure, 282 blade deposits, 479 condensing, 282 efficiency, 288 extraction, 282 induction-type, 282 paitial admission, 288 rating, 290 reliability, 478 selecuon variable, 275, 285 speed, 278 stage losses, 286 steam temperatures, 284 steam velocity, 288 trip and throttle valve. 479 Step unloading system, 80 Stiffness coefficients, 385 Stodola slip, 153, 155 Stonewall, 186 Straight labyrinth. seal leakage, 532... 

If HjS is continuously present in the flare gas or if the flare seal drum also functions as a sour water disengaging drum, then the effluent seal water must be routed to a sour water stripper, desalter, or other safe means of disposal. Withdrawal from the drum is by pump in place of the normal loop seal arrangement. Two pumps are provided one motor driven for normal use, and the other having a steam turbine drive with low pressure cut-in. The seal drum level is controlled by LIC with high and low alarm lights plus an independent high level alarm. 

Axial or propeller blade fatis are eithet Irelt or gear driven. Some drivers are variable speed tnoiors. and some fatis have variable pitch blades. Iti s iecial circumstances, steam turbine, gas or gasolitie etigine drivers are used. Geais should be carefully specilied to avoid overload and should be special ) sealed to prevent moisture entering the case. 

It is important to seal the glands on the turbine shaft, and a typical arrangement is illustrated in Figure 14-24. Thrust bearing failures can be serious problems for steam turbines and other mechanical drives, as well as the bearings used on the driven equipment. 

The flange leak was taped over, and the exhaust-steam pressure dropped back to 100 mm Hg. The steam required to drive the turbine fell by 18 percent. This incident is technically quite similar to losing the downcomer seal on a distillation tower tray. Again, it illustrates the sort of field observations one needs to combine with basic technical calculations. This is the optimum way to attack, and solve, process problems. 

For one thing, steam produced from hot-lime-softened water will have some amount of silicates. These silicates tend to deposit on the rotor blades of turbines, which use the motive steam as a source of energy. The silicate fouling of the turbine blades reduces the turbine s efficiency. But, more importantly, from an operator s point of view, the silicate deposits eventually break off of the blades. This unbalances the rotor. An unbalanced rotor is the fundamental cause of vibration, Vibrations lead to damage of the shaft bearings and seals. Eventually, vibrations will destroy the turbine s internal components. 

Having replaced the loop seal piping, (some units use a steam trap instead of this loop seal), I started steam flow to the turbine. But the vacuum in the surface condenser, which had started out at an excellent 27 in Hg, slipped down to 14 in Hg. This loss in vacuum increased the backpressure in the turbine case. The higher pressure in the turbine case reduced the velocity of the steam striking the buckets on the turbine wheel, which reduced the amount of work that could be extracted from each pound of steam. 

The gas that accumulates inside the surface condenser is called the noncondensable load to the steam jets. Some of the noncondensable load consists of C02 accidentally produced when the boiler feedwater is vaporized into steam. Air leaks through piping flanges and valves are other sources of noncondensable vapors. But the largest source of noncondensable vapors is often air drawn into the turbine case, through the shaft s mechanical seals. To minimize this source of leaks, 2 or 3 psig of steam pressure is ordinarily maintained around the seals. However, as the turbine s shaft seals deteriorate, air in-leakage problems can overwhelm the jet capacity. This will cause a loss of vacuum in the surface condenser. 

Condensate, however, may not always be usable as boiler feedwater, due to the presence of certain process contaminants (see Table 1). When steam is used for heating, to power equipment, and for varied process applications, condensate comes into contact with oil and other impurities. Contamination from fuel-oil heaters or leaking seals in steam-driven equipment, such as pumps, turbines or compressors, is common. Also, the raw water used as plant makeup, may contain significant levels of solvents, uel oils, gasolines and greases. In addition to these general contamination sources, each industry has its unique one. (Table II). If any of these contaminants become entrained in the returned condensate stream, numerous and sometimes unpredictable problems in the boiler and related systems cs.n occur. 

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