Steam Turbines

One of the first questions the designer must answer concerns which type steam turbine should be used. The back pressure turbine is selected when process steam demands are greater than the steam required for process drivers such as large compressors. This type turbine is also selected when various steam levels are required by the process.

The condensing turbine is selected when steam demand for process drivers is greater than the low-pressure process steam requirements. It is also selected when no high pressure steam is available.

The induction-type turbine is selected when excess steam is available at an intermediate pressure.

The extraction turbine is selected when there is a demand for intermediate-pressure steam and, in particular, when there is a variation in the amount of steam required. The extraction turbine generally falls into two classes:

1. The controlled type, which is selected when there are large intermediate pressure process requirements with fluctuations in demand.

2. The uncontrolled type, in which there are small intermediate pressure process requirements with little change in demand.

The guidelines are quite general, but will at least act as introduction to the types of turbines available to the process designer. Sometimes the decision on which type turbine to select is not obvious. The back pressure turbine is most frequently selected. It has lower capital cost, simple construction, is the most suitable turbine for high speeds, and is generally more reliable.

The condensing turbine has several advantages and disadvantages over the back pressure turbine. The advantages are that it requires less change in the live steam for various turbine loads and is therefore easier to control. it also requires less steam because the enthalpy drop is larger. Finally, only one steam level is affected for a change in power requirements.

The disadvantages are that the condensing turbine has a high capital cost because it is larger than a back-pressure type. It develops high specif ic volumes of steam in the exhaust end, and there is the additional cost for a condenser and other auxiliary equipment. The condensing turbine has a lower overall reliability because the condenser, ejectors, extraction pumps, and other auxiliary equipment add to the complexity of the operation. Because high specific volumes are developed in the condensing turbine, longer exhaust blades must be used. In comparison, the back pressure turbine uses shorter blades with decreased risk of blade excitation.

The condensing steam turbine has a relatively low thermal efficiency because about two-thirds of the steam enthalpy is lost to cooling water in the condenser. Expensive boiler feedwater treatment is required to remove chlorides, salts, and silicates, which can be deposited on the blades causing premature failure. The blades are already under erosion conditions because of water drops present in the condensing steam. Even with these disadvantages, the condensing turbine is still selected, especially in a process that requires very large compressor drivers and relatively low amounts of process steam.

A rule of thumb: Select an extraction steam turbine when 15 to 20% of the driver power requirements can be supplied by the extracted steam.

Both extraction and induction turbines provide some significant advantages to process plant designers. Some of these are as follows:

1 Process steam can be supplied at two or more pressures without having to purchase boilers operating at different pressures or having to throttle steam, which is a waste of useful energy.

2. Process steam requirements can be controlled at a suitable pressure and volume required by the process and maintained at these conditions by extraction or induction turbines.

3. It is easier to make a plant steam balance using extraction or induction turbines.

4. Process steam requirements and driver steam requirements can be optimized.

5. Process plants usually operate under fluctuating flow and pressure conditions. Extraction and induction turbines provide the flexibility for these fluctuating operating conditions.

Extraction or induction turbines are not without their problems. Some of these are as follows:

I. Turbine blades can be excited by steam flowing through the intermediate nozzles, which can cause premature blade failure.

2. Excess steam extraction can cause starving in parts of the turbine, which will result in overheating.

3. To control the intermediate pressures, extra valves are required.

4. Extra nozzles require a longer turbine shaft, which increases the span between bearings which can produce rotor dynamics problems.

5. Extraction and induction turbines may be up to 5% less efficient than back-pressure turbines.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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