Turbine case pressure, increase

The turbine case pressure was increased by raising the pressure in the air-cooled surface condenser. This was accomplished by shutting off several of the air fans, which, in turn, increased the condensing temperature of the exhaust steam. But why would raising the turbine case pressure drain the turbine, anyway After all, increasing the surface condenser pressure also increased the pressure in the drum that the turbine case drained to. 

The increase in unit size and complexity together with the higher turbine inlet temperature, and higher pressure ratio has lead to an increase in overall gas turbine efficiency. The increase in efficiency of 7-10% has in many cases lead to an availability decrease of the same amount or even more as seen in Figure 21-5. A 1% reduction in plant availability could cost 500,000/yr in income on a 100 MW plant, thus in many cases offsetting gains in efficiency. 

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. 

In the PCU during rapid transients the rate at which inventory must be changed according to the load schedule may not be physically achievable. The helium fill and bleed system has limited capacity. In this case turbine bypass control is used to more quickly vary the power output of the shaft. In this scheme the power output of the turbine is changed by bypassing high pressure compressor outlet coolant to the exit of the turbine. The pressure drop across the turbine is reduced so power is reduced while at the same time the frictional losses through the rest of the PCU circuit increase. The result is a rapid reduction in shaft power. However, PCU efficiency is reduced under turbine bypass control and so control is typically transitioned back to inventory control over time. 

The Brayton and Rankine power cycles, which are most commonly used in power generation, are identical except that the Rankine cycle employs a vapor with a phase change and the Brayton cycle operates on a single phase gas. In both cases, there is a pressure increase process (using a pump or compressor), a heat addition process, expansion in a turbine, and a heat removal process. Nuclear power generation currently uses the Rankine cycle almost... 

Another possibility is to open the speed or hand valves, as described in Chap. 24, "Steam Turbines." This will introduce more steam into the turbine case and slightly lower the pressure in the steam chest. Even though the governor-steam speed-control valve is wide open, the reduction in downstream pressure in the steam chest will increase the steam flow into the turbine. 

Dearation can be either vacuum or over pressure dearation. Most systems use vacuum dearation because all the feedwater heating can be done in the feedwater tank and there is no need for additional heat exchangers. The heating steam in the vacuum dearation process is a lower quality steam thus leaving the steam in the steam cycle for expansion work through the steam turbine. This increases the output of the steam turbine and therefore the efficiency of the combined cycle. In the case of the overpressure dearation, the gases can be exhausted directly to the atmosphere independently of the condenser evacuation system. 

Superheated steam may be needed where steam distribution pipework in a plant is over extended distances, resulting in a loss of heat and increase in wetness of the steam. Another case may be where a process requires a temperature above the working pressure of the plant. The third case is where steam is used for turbines. Here it improves the performance of the turbine, where for every 6°C increase in steam temperature it can produce a saving of about 1 per cent reduction in steam consumption. Superheaters may also be supplied as independently fired units. These may be used when either the amount of superheated steam required is much less than the boiler evaporation or is only needed on an intermittent basis. 

COMMENTS The effect of increasing the boiler pressure on the quality of the steam at the exit of the turbine can be seen by comparing the two cases. The higher the boiler pressure, the higher the moisture content (or the lower the quality) at the exit of the turbine. Steam with qualities less than 90% at the exit of the turbine, cannot be tolerated in the operation of actual Rankine steam power plants. To increase steam quality at the exit of the turbine, superheating and reheating are used. 

Smoke (carbon) formation, which apparently is due to incomplete combustion of portions of the fuel-air mixture (i.e., rich combustion), also can pose a serious public relations problem at civilian airports and, by radiant-heat transfer from incandescent carbon particles, can shorten the endurance life of combustion-chamber liners and adjacent parts (0). Smoke would also constitute a serious problem in the case of automotive gas turbines, because accumulation of carbon and other nonvolatile fuel components on the intricate passages of the heat exchanger could reduce turbine and heat-exchanger efficiency by reducing heat-transfer rate and increasing the pressure drop across the... 

First, it is instructive to examine the performance of a recuperated system that has only one compressor (i.e., remove the IC and C2 from Figure 8.2) and compare this to a simple cycle GT (i.e., also remove the recuperator from the diagram). Consider an isentropic compressor efficiency of 85%, isentropic turbine expander efficiency of 90%, recuperator effectiveness of 88% and no pressure losses. A fixed turbine inlet temperature of 1200 K will be assumed for various pressure ratios. This value is based on an assumed 1000 K SOFC inlet temperature, and a 200 K temperature rise from the SOFC inlet to the turbine inlet. The 200 K temperature increase from the cathode inlet to the turbine inlet is reasonable to assume given a cathode temperature difference across the cell of 150 K, and another 50 K temperature increase from anode exhaust combustion. Thus, 1200 K will be used as a base case for the turbine inlet temperature, and for sensitivity, values of 1100 and 1300 K will also be analyzed. 

Second, Table 8.1 shows that if a recuperator were used, heat exchange would only be possible up to about a PR of 12 1. Beyond this, the turbine exhaust gas temperature, EGT, is too close to the compressor exit temperature. The lower the pressure ratio, the greater the temperature difference and thus the more heat that can be recuperated (but requiring an increasing amount of heat exchange area as well). In this case, the pressure ratio would have to be quite low, between 1.5 and 2, in order for the EGT to be high enough to heat the compressor air. 

To understand the peak recuperated case efficiency, note that as the pressure ratio decreases, the turbine is taking less energy from the air, increasing the temperature going to the recuperator. Thus, ultimately less auxiliary fuel is needed at the postcombustor to keep the temperature at the cathode inlet fixed at 973 K. At a pressure ratio between 3 and 4, fuel to the auxiliary fuel is no longer necessary to preheat the air to the cathode inlet. This is the point of maximum efficiency for this system. The decrease in fuel cell efficiency with pressure is not enough to offset the improved recuperated GT efficiency, which is incidentally near its peak efficiency... 

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