4

Figure 11-5g Straight Flow Back-Pressure Turbine.

Figure 11-5g Straight Flow Back-Pressure Turbine.

11-5g) operate with an exhaust pressure equal to or in excess of atmospheric pressure. Exhaust steam is used for heating, process hot water, or other purposes. Back-pressure turbines may be of single- or multi-stage design. Because all of the unused steam in the power generation process is passed on to process application and, therefore, not wasted, mechanical efficiency is not a major concern. In some cases, where low-pressure steam applications are limited, multi-stage turbines are used to maximize power output.

An extraction turbine (Figures 11-5c, d and h) is a multi-stage machine with the added design feature of having one or more outlets to allow intermediate pressure steam (intermediate between inlet pressure and exhaust pressure) to be withdrawn. Automatic-extraction turbines are generally designed in three configurations: single, double, and triple, depending on the number of pressures at which steam is extracted. Adding automatic stages adds significantly to cost and complexity. As a result, the single-automatic extraction design is the most common.

Extraction turbines are generally designed for applications in which there is a need for discharged steam at different pressures or when there are varying low-pressure steam process requirements. While back-pressure turbines are somewhat inflexible, since shaft power and process steam requirements must be closely matched, extraction turbines can cope with changes in these variables and satisfy requirements over a broad range. If the mass flow requirement of process steam is reduced, an extraction turbine can pass additional steam through the low-pressure stage to maintain shaft power output.

Figure ll-5h Non-Automatic Extraction Back-Pressure Turbine.

Figure 11-5i Controlled Extraction/Admission Back-Pressure Turbine.

Admission (or induction) turbines (Figure 11-5e) include steam input at two or more pressures at two or more locations. This configuration can allow byproducts from in-house processes to be used to increase turbine power output. Extraction and admission features can also be combined into a single unit knows as an extraction/induction turbine (Figures 11-5f and i).

Extraction and admission turbines may be either controlled (Figures 11-5d, e, f, and i) or uncontrolled (Figures 11-5c and h). A non-controlled extraction or admission steam turbine has one or more openings in the turbine casing for extraction or admission of steam, but does not have means for controlling the pressure of the induced or extracted steam.

The pressure at the bleed opening of a non-automatic extraction (or bleeder) turbine (Figure 11-5c) is proportional to the reduction in flow through the stages following the bleed. Under low-load conditions, the pressure in each stage will decrease. Therefore, the bleed opening must be located in a stage where the pressure at minimum turbine load will be adequate for the desired extraction load. At full power, the pressure in the bleed stage may exceed the desired pressure and a pressure-regulating valve must be used. In order to prevent steam from entering the turbine through the bleed opening, a non-return valve is also required.

An automatic-extraction turbine (Figure 11-5d) is designed to hold pressure in the extraction line constant, regardless of load, by regulating the flow of the steam to the turbine stages following the extraction or admission open-ing(s). Automatic extraction turbines do not experience the power loss associated with non-automatic units that require a pressure-reducing valve. Automatic extraction turbines can be designed to accommodate extraction flows from zero to about 9% of inlet flow. A small portion must always be maintained to flow through the low-pressure stages of the turbine to carry away the heat generated by windage. Generally, automatic units, which are far more costly than non-automatic units, are used if the extraction or admission steam flow is greater than 15 or 20% of the total steam flow.

Often, two or more types of turbines are applied to a specific application. For example, condensing and backpressure turbines can be used in combination, rather than depending on a single extraction turbine. Extraction-admission turbines can also be used to balance out variable low-pressure steam conditions through the ability to either admit or extract steam, depending on operating conditions.

Industrial steam turbines ordinarily include a single casing with all rotating blades attached to one shaft and

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