Maquinas Motrices Para Unidades Mecanicas

SECTION 15

Prime Movers For Mechanical Drives
"Prime movers for mechanical drives" are used as a common name for machines made for transferring mechanical energy to pumps and compressors, including: • Steam turbines • Gas turbines • Electrical motors • Internal combustion engines Special considerations for the use of prime movers as drives for generators are not included in this chapter.Mechanical Drive Steam Turbines
TURBINE TYPES
Mechanical drive steam turbines are major prime movers for compressor, blower, and pump applications. Steam turbines are available for a wide range of steam conditions, horsepower, and speeds. Typical ranges for each design parameter are: Inlet Pressure, psig 30 – 2000 Inlet Temperature, °F saturated – 1000 Exhaust Pressure, psig saturated – 700Horsepower 5 – 100,000 Speed, rpm 1800 – 14,000 Steam turbines used as process drivers are usually required to operate over a range of speeds in contrast to a turbine used to drive an electric generator which runs at nearly constant speed. Significant hardware differences exist between these two applications. Only variable speed process drivers will be covered here. Mechanical drive steam turbines arecategorized as: • Single-stage or multi-stage • Condensing or non-condensing exhausts • Extraction or admission • Impulse or reaction a set of turning vanes between the first and second row of buckets to redirect the steam flow (Fig. 15-3). A multi-stage turbine utilizes either a Curtis or Rateau first stage followed by one or more Rateau stages. Single-stage turbines are usually limited to about2500 horsepower although special designs are available for larger units. Below 2500 horsepower the choice between a single and a multi-stage turbine is usually an economic one. For a given shaft horsepower, a single-stage turbine will have a lower capital cost but will require more steam than a multi-stage turbine because of the lower efficiency of the single-stage turbine.Condensing/Non-Condensing
The energy available in each pound of steam which flows through the turbine is a function of the overall turbine pressure ratio (inlet pressure/exhaust pressure) and inlet temperature. Condensing turbines are those whose exhaust pressure is below atmospheric. They offer the highest overall turbine pressure ratio for a given set of inlet conditions and therefore require the lowest steam flowto produce a given horsepower. A cooling medium is required to totally condense the steam. Non-condensing or back-pressure turbines exhaust steam at pressures above atmospheric and are usually applied when the exhaust steam can be utilized elsewhere.

Single Stage/Multi-Stage
In a single-stage turbine, steam is accelerated through one cascade of stationary nozzles and guided into the rotatingblades or buckets on the turbine wheel to produce power. A Rateau design has one row of buckets per stage (Fig. 15-2). A Curtis design has two rows of buckets per stage and requires

Extraction/Admission
Some mechanical drive steam turbines are either extraction or admission machines. Steam is extracted from, or admitted to, the turbine at some point between the inlet and exhaust

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15-1

FIG. 15-2 Rateau Design

FIG. 15-3 Curtis Design

FIG. 15-4 Extraction / Admission Flow Turbines

15-2

(Fig. 15-4). Admission or extraction...