Pressure drops Turbine exhaust

Catalyst contamination from sources such as turbine lubricant and boiler feed water additives is usuaUy much more severe than deactivation by sulfur compounds in the turbine exhaust. Catalyst formulation can be adjusted to improve poison tolerance, but no catalyst is immune to a contaminant that coats its surface and prevents access of CO to the active sites. Between 1986 and 1990 over 25 commercial CO oxidation catalyst systems operated on gas turbine cogeneration systems, meeting both CO conversion (40 to 90%) and pressure drop requirements. 

The reaction turbine, shown schematically in Figure 2-2, is generally more efficient. In its primary (stationary) nozzles only half the pressure energy of the gas stream is converted to velocity. The rotor with a blade speed matching the full-jetted stream velocity receives this jetted gas stream. In the rotor blades the other half of the pressure energy is used to jet the gas backward out of the rotor and, hence, to exhaust. Because half the pressure drop is taken across the rotor, a seat must be created around the periphery of the rotor to contain this pressure. Also, the pressure difference across the rotor acts on the full rotor area and creates a large thrust load on the shaft. 

It is also important to note that the lowest design pressure of any section of the casing must be specified to be no lower than the pressure which it may be subjected to under the PR valve relieving conditions. This is necessary to recognize pressure drop within the casing. The PR valve should be sized to pass the normal steam flow to the turbine, but credit may be taken for steam flow which is withdrawn from an intermediate turbine stage if it would not be blocked by the same contingency as closure of the exhaust. 

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. 

Other inefficiencies are due to pressure drops of 2-5%, loss of 1-3% of the enthalpy in the expander, and 1% or so loss of the air for cooling the turbine blades. The greatest loss of energy is due to the necessarily high temperature of the exhaust gas from the turbine, so that the overall efficiency becomes of the order of 20% or so. Some improvements are effected with air preheating as on Figure 4.2(b) and with waste heat steam generators as in Example 4.2. In many instances, however, boilers on 1000°F waste gas are... 

JO A portable power-supply system consists of a 30-liter bottle of compressed nitrogen, connected to a small adiabatic turbine. The bottle is initially charged to 13,800 kPa at 27°C and in operation drives the turbine continuously until the pressure drops to 700 kPa. The turbine exhausts at 101.33 kPa. Neglecting all heat transfer to the gas, calculate the maximum possible work that can be obtained during the process. Assume nitrogen an ideal gas for which CP = (7/2)R. 

Rate of loss of availability. In the scheme of Fig. 13.4 for reboUing a tower with low-pressure exhaust steam from a turbine, factors that reduce the power output of the turbine are (l)the temperature difference across the reboiler, which causes the turbine exhaust pressure to be higher than the tower bottom pressure, and (2) the steam pressure drop through the tower, which causes the tower bottom pressure to be higher than the tower top. We shall focus attention on the second of these inefficiencies and shall derive an expression for the reduction in turbine power caused by steam pressure drop through the tower. 

For the turbine there are three pressure drops to consider. One for the compressor discharge AP2, one for the practical throttling effect in the combustion chamber AP23 and one for the turbine exhaust pressure due to ducting AP4. The two pressure drops at the inlet to the turbine can be combined as,... 

The amount of energy that the steam turbine extracts from the steam depends on the enthalpy drop across the machine. The enthalpy of the steam is a function of its temperature and pressure. One can use a Mollier diagram as a graphic tool to determine the amount of energy available under a particular set of conditions. If in Figure 2.131 the inlet conditions correspond to point and the outlet conditions to point P2, a line drawn between these two points is called the "expansion line" and represents the operation of the turbine as it is extracting energy from the steam. In an ideal turbine, the steam would expand at a constant entropy (isentropically) and the condition of the exhaust steam, from an ideal machine (which has no losses), would correspond to point 3. 

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