Valves governor

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. 

A governor valve controls the flow of steam to the turbine. This valve is actuated by the governor, which is operated by the speed of the machine. When the speed exceeds the set value of the governor, a trip-valve is actuated to completely shut off the steam supply. The trip valve may... 

If we close off one of the two horsepower valves, steam flow into the turbine will drop—initially by 20%. The turbine will slow. This will cause the governor valve to open. The pressure drop across the governor will decrease. The pressure in the steam chest will rise. The flow of steam through the 60 percent port nozzle and the remaining 20 percent port nozzle will increase. The velocity of the steam striking the buckets will also increase. The turbine wheel will now come back up to its set point. 

The net effect of this exercise will be to save not 20 percent of the motive steam, but 10 percent. The 20 percent reduction in nozzle area is partially offset by the opening of the governor valve. The inefficient, irreversible, isoenthalpic expansion and pressure drop across the governor speed control valve are reduced. The efficient, reversible, isoen-tropic expansion and pressure drop across the nozzles are increased. 

An ordinary American-type turbine is designed to run at 3600 rpm. Its overspeed trip will cut off the motive steam flow at about 3750 rpm. But the turbine can be run at any lower speed. There is usually a small knob on the left side of the governor-valve assembly, that is used to... 

Figure 17.4 shows a centrifugal pump, driven by a steam turbine. The correct operating speed for the pump and turbine is that speed that puts the process-control valve in a mostly open, but still controllable, position. As we slow the turbine to force open the process-control valve, the turbine s governor valve will close. Steam flow to the turbine will decline in accordance with fan laws ... 

Slowing a turbine closes the governor valve. This may now permit us to close an additional horsepower valve, without losing our flexibility to control the speed of the turbine. Closing that final horsepower valve will save us another 10 percent of steam. 

In extraction turbines, in addition to the governor valve, a second "valve" is required (Figure 2.134), which controls the steam flow rate that is extracted from the first stage of the turbine and is sent to the second stage. The extraction rate can be controlled either to keep the shaft speed or the pressure of the LP header constant, or a combination of the two. If the turbine incorporates the controls as a built-in feature, the turbine is referred to as an "automatic-extraction" type. Such turbines are generally designed to deliver 100% shaft power and to provide extraction steam only if the load requirements permit. This is the most common type of extraction machine. 

Figure 2.33 On-line coupled simultaneous DTA/DSC-EGD apparatus. 1, Gas cylinder 2, reducer valve 3, gas drying 4, governor valve 5, buffer 6, float meter 7, preheater 8, controlled temperature unit 9, bridge 10, fourway valve 11, gas conduit 12, crucible 13, furnace 14, infrared lamp 15, controlled temperature programmer 16, power compensation unit 17, DTA amplifier unit 18, three-way valve 19, soap film flow meter 20, rrxrorder for DTA/DSC 21, recorder for EGD 22, selective switch 23, tail gas...

Figure 2.37 Schematic diagram of the DTA-EGD-GC online coupled simultaneous apparatus (reproduced by permission of Tian Ping Instrumental Factory, China). 1, Gas cylinder for DTA system 2, carrier gas (or GC system 3, reducer valve 4, gas drying 5, governor valve 6, float meter 7, six-way valve 8, gasifier, 9, 5A molecular sieve 10, 401 organic support 11, soap film flow meter 12, sampling valve 13, infrared lamp 14, recorder for DTA/T 15, recorder for EGD-GC 16, tail gas...

Steam enters a turbine through the steam chest. The steam chest typically has a strainer on the inlet side to remove solids. Inside the steam chest is a device called the governor valve. The governor valve opens and closes to admit steam into the turbine. A governor system controls the position of the governor valve. An overspeed trip mechanism is attached to the rotor and will shut off the flow of steam into the turbine when it reaches 115% of its design limit. The shutoff valve is typically located in front of the governor valve. 

This technique was used daily on an alkylation unit that was refrigeration-compressor limited. During periods of cold weather, the compressor would change from horsepower-limited to speed-limited. Propane was then added to the refrigerant accumulator drum. The turbine governor control valve (i.e., the driver steam inlet valve) would open to maintain compressor speed. When the governor valve was fully open, the compressor was using maximum available horsepower. The optimum propane concentration had been reached. 

FIGURE 7.3 The Schema of a flare with scrubber dynamic 1 - scrubber dynamic 2 - governor valves 3 - the ignite burner, 4 - a pilot-light burner, 5 - a torch pipe / - waste gas // - fuel gas III - a condensate. 

They are so called because, with pressure held constant, a turbine governor valve movement necessarily causes a corresponding change in reactor power in the coupled system, but not in the decoupled system since the turbine valve is brought back to its original position by the pressure control. 

Because the SCWR has a once-through steam cycle, in which steam from the core outlet is directly supplied to the HP turbines, it has many similarities with BWRs. However, on a closer look there is a basic difference in the coolant flow path inside of the reactor that causes a difference of the steam cycle control. In a BWR the feed-water pump is controlling the liquid level in the reactor pressure vessel, the steam pressure is controlled by the turbine governor valve, and the core power is either controlled by the control rods or by the speed of the recirculation pumps. The SCWR concepts do not include any recirculation loop. The feed-water pump can control either the steam temperature at the core outlet, if the core power is controlled by the control rods, or it can control the core power if the steam outlet temperature is controlled by the control rods. Again, the steam pressure is controlled by the turbine governor valve in both cases. 

An example of control loops for operation in the load range is sketched in Fig. 8.16. Here the speed of the feed-water pump is controlled by the temperature of the superheated steam at turbine inlet, the mass flow of the HP steam extractions is controlled by the feed-water temperature, the reheat temperature is controlling the steam mass flow of the reheater, and the pressure at the reactor outlet is controlling the turbine governor valve. The thermal power of the reactor, and thus with some delay in the generator power, is controlled by the control rods of the reactor core. 

A supercritical fossil-fired power plant with a once-through steam cycle is usually operated with a sliding pressure the turbine governor valve is kept open in the upper load range and the boiler outlet temperature is kept constant such that the boiler outlet pressure increases proportionally with the steam mass flow and thus with load. Consequently, the boiler is operated at subcritical pressure below approximately 80—90% load. However, such control is not permitted for the SCWR because dryout... 

The trip throttle valve for No. 2 AFW pump was opened by the equipment operators at about 1 53 a.m. After the SFRCS was reset and tripped on low level by the shift supervisor, the AFWS aligned itself so that each AFW pump would feed only its associated steam generator, i.e., the No. 2 AFW pump would feed the No. 2 steam generator. Thus, the No. 2 AFW pump refilled the No. 2 steam generator and its pressure increased abruptly to the atmospheric vent valve relief set point. The turbine governor valve was fully open when the trip throttle valve was opened and the pump delivered full flow for about 30 seconds until the operator throttled the flow down. 

Governor valve lifting gear Steam temperature 600-825°F Nonsoap base lor2 265-340 500 Pass federal test method Standard No. 791 Method 5309.2... 

Deductions are made for presstme drop through the governor valve (12.5 Btu), loss due to supersaturation C (about 0.95), and 2 percent margin (0.98). The remaining enthalpy drop is called net available energy H . 

The available energy is 205 Btu subtracting a 12.5-Btu drop throu the governor valve leaves 192.5 net Btu, which corresponds to a theoretical steam velocity C = 223.8 x V192.5 = 3104 ft/s. 

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