Count On Steam Turbines

These long-lived low-maintenance drivers suit many applications

By Amin Almasi, rotating equipment consultant

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Steam turbines extract thermal energy from pressurized steam and deliver mechanical work by rotating output shafts. These turbo-machines often are good choices to drive compressors, pumps, electrical generators and other equipment.

Steam turbines come in a wide array of designs and complexities to match the desired application and performance specifications. For industrial services, units may have a single casing or multiple casings (e.g., three — low, medium and high pressure).

Steam turbines are expensive to make, requiring precision manufacture and special quality materials, but generally provide long service lives, often exceeding 40 years. They are inherently rugged, reliable and low-maintenance drivers. Moreover, the turbines are easy to control and offer enclosed non-sparking operation suitable for some explosive atmospheres or corrosive environments. The units provide fast, reliable starting capability and are particularly adaptable for direct connection to driven equipment (such as compressors, pumps, etc.) that rotate at high speeds. Steam turbine drives can handle continuous duty under severe operating conditions. Their reliability and availability records usually are excellent; many steam turbines experience only one shutdown every 3–6 years.

The Basics

A steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency from the use of multiple stages in the expansion of steam, which results in a closer approach to the ideal reversible expansion process. However, no steam turbine affords a truly isentropic or constant entropy process; typical isentropic efficiencies range from 70–91% based on the particular application.

The steam turbine operates on basic principles of thermodynamics using a portion of the Rankine cycle. Steam, which could be a superheated vapor or dry saturated vapor (depending upon application), enters the turbine at high temperature and high pressure. It encounters several sets of blades, or buckets as they are called in the steam turbine industry, which convert the energy of the steam into kinetic energy. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage. The fluid exits the turbine as a low-energy steam (which could be nearly saturated vapor or a liquid/vapor mix depending upon application) at a lower temperature and pressure than it entered; the discharged fluid often goes to a condenser for cooling.

Except for low-power applications, turbine blades are arranged in multiple axial stages (in series), a technique called compounding, which greatly improves efficiency at part-load operation (low speeds). A significant advantage of steam turbines is their variable-speed and variable-load capabilities; such flexibility is extremely important in many applications, particularly variable-speed mechanical drive services such as compressors or pumps.

An impulse steam turbine has fixed nozzles that orient the steam flow into high-speed jets. These jets contain significant kinetic energy, which is converted into shaft rotation by bucket-like shaped rotor blades, as the steam jet changes direction. A pressure drop only occurs across the stationary blades (nozzles), with a net increase in steam velocity across the stage. As the steam flows through the nozzle, its pressure falls. Due to the relatively high ratio of expansion of steam, the steam leaves the nozzle with a relatively high velocity. Impulse steam turbines are also known as constant-pressure turbines.

In a reaction steam turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of a stator; it leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to that of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor.

The rotor blades of the reaction turbine are shaped more like aerofoils, arranged such that the cross-section of the chambers formed between the fixed blades diminishes from the inlet side toward the exhaust side of the blades. The chambers between the rotor blades essentially form nozzles so that as steam progresses through the chambers its velocity increases while at the same time its pressure decreases, just as in the nozzles formed by the fixed blades. Thus, the pressure falls in both the fixed and moving blades. As the steam emerges in a jet from between the rotor blades, it creates a reactive force on the blades that, in turn, causes the turning moment on the turbine rotor.

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