Overspeed

The trip valve is provided as a second line of defense in case of overspeed The trip valve is frequently equipped with a trip-actuating solenoid which can be operated by push button, by low od pressure, or by some other process upset. When the speed control functions as described above, the trip will not be actuated by load dump. 

Safety features such as overspeed trip, low-od trip, remote-solenoid trip, vibration monitor, or other special monitoring of temperature, temperature changes, and casing and rotor expansion... 

Sticking of valve stems is common if solids are present in the steam. The steam mast be without sohds. (Note comments later under loss of power.) It is important that units operating on a steady load for long periods be checked for sticking stems at regular intervals. The records show that in several cases deposits have caused the stem of both the governor valve and the trip valve to stick when there was a loss of load. The effect of the loss of load was destructive overspeed. 

Various protective instruments are used to provide a shutdown signal (to a fast-acting trip valve at the expander inlet) that senses various things, such as overspeed, lubricant pressure, bearing temperature, lubricant temperature, shaft runout, icing, lubricant level, thrustbearing load, and process variables such as sensitive temperatures, levels, pressures, etc. However, too many safety shutdown devices may lead to excessive nuisance shutdowns. 

In contrast to steam turbines, in which runaway overspeediug is always a problem, pump-turbines operating at design head go to zero torque at about 130 to 140 percent of design speed. Thus, overspeed protection may not be necessary if the pump-turbine can withstand 140 to 150 percent of design speed and it is the sole driver. When a steam-turbine helper is used, it should be provided with the usual overspeed trip-out mechanism. 

Potential problems associated with centrifuges include mechanical friction from bearings vibration leaking seals static electricity and overspeed. 

In downhitt conveyors, running mostly on stored potential energy by gravity, the motor may overspeed beyond e.xcessive limits unless prevented by a brake or a lachogenerator relay. 

Cut-oul wind speed is the maximum w ind speed beyond which the prime mover may overspeed above its permissible limits. As the structure and the blades are designed for a particular maximum speed, a wind speed higher than this may exceed their mechanical endurance and become unsafe. At this speed the brakes apply and the machine is disconnected from the grid. The cut-in and cut-out speeds define the wind speed limits within which the turbine will work safely through the generator. Rated wind speed is the speed at which the prime mover rotates at the rated negative slip and generates the rated power. 

Momentary overvoltages due to a sudden load rejection, which may overspeed the generator and develop higher voltages or... 

Descending loads may overspeed the motor and iwerexcite the capacitor when connected across the motor due to motor generator action above the synchronous speed (Section 6.21). Such a situation may damage the motor as w ell as the capacitor and ntust be avoided. 

A TPG block diagram is shown in Figure 4-86. It is similar to the FCC diagram except a second inlet valve is added to assure trip action and a bypass valve is added to reduce overspeed and aid in startup. The only rotating elements are the expander and generator and, possibly, gear (Figure 4-87). 

A generator load drop of 100% is shown in Figure 4-89. For a valve elosure time of 1 see, a 5% overspeed is predieted. Note the eompari-son to the power reeovery string in Figure 4-83, where a 10% overspeed was predieted with a 10 see valve elosure. The mueh larger rotor inertia of the full power reeovery string allows the use of a slower and less expensive valve. 

Figure 4-90. Predicted overspeed versus valve closure time.

Steam turbine considerations. The speed at whieh the turbine must operate and the steam eonditions available influenee the operating effieieney of the unit. Double-ended steam turbines require speeial eonsideration of thrust bearing sizing, overspeed proteetion, and governing systems. 

Figure 4-143 represents a computer-generated plot that demonstrates the effectiveness of eddy current brakes in preventing overspeed of the string. The lower curve assumes the butterfly valve characteristic is linear from 60° open to the closed position. The rate of closure is 3.25 sec. (Butterfly valves are normally used to throttle the expander inlet gas.)... 

The top curve of overspeed versus time demonstrates that the string will accelerate to 22% overspeed due to expansion of the gas trapped between the valve and expander nozzle ring. However, if an eddy current brake were part of the string, the acceleration of the string would be reduced to 11% overspeed. This would provide an important margin of safety. 

The rotor speed must then be inereased to trip speed to eheek the overspeed trip setting, then redueed to maximum eontinuous speed and held for a minimum of 1 hr. Extended testing time should be alloeated if neeessary to reaeh satisfaetory equilibrium eonditions. Tlie lube oil must be held near design inlet temperamre during tlie meehanieal test. Data logging should be done in 15 min intervals for all measuring points. 

Overspeed is the speed at which the expander rotor is tested to prove its design capability for this service. Overspeed is 115% of maximum continuous speed. 

The expander shall be equipped with an eleetronie overspeed trip. The piekups shall sense the rotation of a 30-tooth wheel and be eonneeted to a 2-out-of-3 voting system. 

A typieal TPG valve arrangement is the five valve system (Figure 6-41). The two expander inlet valves provide regenerator pressure eontrol, overspeed proteetion, and flue gas shut-off to the expander. The expander exhaust valve enables expander isolation. The full and partial expander bypass valves permit aeeurate eontrol of the FCC proeess when the expander is not in operation. 

In the event that eertain faults oeeur in the eleetrieal equipment of the generator, the load eireuit breaker must be opened immediately. The result is that the maehine train is aeeelerated with the full power of the expander. Only if the inlet valves are elosed within 0.6 see ean exeessive overspeed be avoided. Eor this reason, both inlet valves must be able to elose within this time window in the event of an emergeney trip. 

As previously mentioned in Chapter 5, one of the most severe disturbances for the power recovery train (PRT) is a generator breaker opening. This event often causes the PRT to trip on overspeed or other process or machine conditions. A control solution has been developed to keep the PRT and the process under control during breaker opening. 

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