A turbine is a machine that develops rotary power by directing a high-velocity fluid against blades mounted around the rim of a wheel.
Turbines are classified by the type of fluid that turns them. Turbines may be driven by water, steam, gas, or wind. Since most turbines in process plants are driven by either steam or gas, we will describe only these classification.
As a high-velocity fluid passes through a turbine, it pushes against the blades and causes the wheel to turn. The rotation of the wheel turns the axle, which drives the connected machinery.
Turbines use two principles to convert the energy in the motive fluid into rotary power:
1. Power is developed by impulse action, which is the striking of the blades by the motive fluid.
2. Power is also recovered by the reaction, where the high-speed fluid escaping from the blades propels the wheel.
Steam turbines
Steam is the most commonly used driver fluid for turbines used onboard ships. The expansive force of steam is the greatest of any of the common gases used for power. Steam turbines are operating with an inlet pressure as high as 3500 psi, and/or with a temperature as high as 1200°F (649°C). The heat energy contained in steam at this pressure and temperature is extremely high.
How a Steam Turbine Works
Steam is admitted to the turbine wheel through a streamlined nozzle that efficiently expands the steam to very high velocity. The blades of the turbine wheel are curved in such a manner to receive the impact of the steam on the nozzle side, and to form a backwards jet on the side of the wheel opposite the nozzle. In this manner, both impulse and reaction forces are put to use.
Turbines may be single stage, with one wheel, or they may be multi-stage, with many wheels mounted on the same shaft. In multi-stage turbines, the steam enters at one end and passes from wheel to wheel in series until it exhausts from the last stage. At each stage, the steam partially drops in pressure, so that the pressure drop of the steam is divided among the stages.
Exhaust Configurations
Steam turbines may be either condensing, noncondensing or extracting, depending on how the steam leaving the turbine is used.
Condensing Turbines
A condensing turbine exhausts into a water-cooled condenser that turns all of the exhaust steam into water. The condensing action recovers water for re-use in the boilers, and it creates a vacuum to achieve the lowest possible exhaust pressure for the turbine. This helps force steam through the turbine and reduces the amount of steam required to operate the turbine.
Noncondensing (Topping) Turbines
The exhaust steam from noncondensing turbines is exhausted into a lower-pressure system. If the turbine exhaust is used for process steam requirements, the turbine is classified as a “back pressure” turbine.
Extracting Turbines
Steam may be extracted from a mid-stage in a multi-stage turbine to drive an intermediate-pressure steam source. Extracting turbines may be either condensing or topping. For example, a multi-stage turbine operating with 600-psig steam pressure and exhausting to a condenser may have a mid-stage connection to draw steam at 100 psig. This steam may be furnished to a process for heating purposes.
Multi-stage Steam Turbine
As steam passes through a multistage turbine, it expands to as much as 1000 times its original volume. Each successive stage of the turbine is, therefore, larger than the previous one in order to make efficient use of the expanding steam. This arrangement of larger and larger stages gives steam turbines their characteristic conical shape.
Critical Speed
In large multi-stage turbines there is a speed, referred to as the critical speed, that is in tune with the natural vibrating frequency of the turbine shaft. If the critical speed is below the operating speed, the turbine must not run within the critical speed range. Severe vibration and damage to the turbine will result.
Speed Control - Governor Systems
Governor systems are speed-sensitive control systems that are integral with the steam turbine. They control normal operation by varying the amount of steam to the turbine. They commonly consist of spring-opposed rotating weights, a steam valve, and an interconnecting linkage or servo motor system.
The turbine speed is controlled by varying the steam flow through the turbine by positioning the governor valve. Variation in the power required by the load and changes in the steam inlet or exhaust conditions alter the speed of the turbine, causing the governor system to respond to correct the operating speed.
Control Systems
Control systems respond to pressure changes in the process system and then reposition the turbine governor valve to maintain the preset pressures.
Overspeed Trip System
The overspeed system usually consists of a spring-loaded pin or weight mounted in the turbine shaft or on a collar, a quick-closing valve that is separate from the governor valve, and interconnecting linkage. The centrifugal force created by rotation of the pin in the turbine shaft exceeds the spring loading at a preset speed. The resultant movement of the trip pin causes knife-edges in the linkage to separate and permit the spring-loaded trip valve to close. This mechanism stops the turbine when the speed has exceeded the control range of the operating governor.
Lubrication System
Oil Ring Lubrication System
The oil ring lubrication system employs an oil ring that rotates on the shaft with the lower portion submerged in the oil contained in the bearing case. The rotating ring transfers oil from the oil reservoir to the bearings. The oil in the bearing-case reservoirs is cooled by water flowing in cooling water chambers or tubular heat exchangers.
Pressure Lubrication System
The pressure lubrication system consists of an oil pump driven by the turbine shaft, an oil reservoir, a tubular oil cooler, an oil filter, and the interconnecting piping. Oil is supplied to the bearing cases under pressure. The oil rings may be retained in this system to provide oil to the bearings during start-up and shutdown, when the operating speed and bearing design permit.
Gas turbines
Gas turbines use combustion gases to drive them, instead of steam. The turbine, governor, and lubrication system for a gas turbine are similar to these items in a steam turbine. The extra equipment needed are the items that provide the combustion gases.
Besides the turbine, gas turbines require the following for generating the combustion gases:
• An axial flow air compressor for combustion air
• A combustion air preheater, or regenerator
• A combustion chamber where the fuel is fired, including the ignition system
How a Gas Turbine Works
The air compressor and turbine are at the opposite ends of a common shaft. There is space between the two devices that houses multiple combustion chambers, arranged in a circle around the shaft. The inlets to the combustion chambers receive the preheated compressed air from the compressor and regenerator. The outlets of the combustion chambers are directed at the turbine blades through special nozzles.
Compression of the Combustion Air
The compressor draws in fresh air and compresses it to a pressure of 50 to 75 psig. The air is drawn by the compressor through a heat exchanger (the regenerator) where it is preheated before entering the compressor.
Combustion of the Fuel
In the combustion chamber, the compressed air combines with fuel and the resulting mixture is burned. An electric igniter starts the ignition.
Rotation
The burning gases expand rapidly and rush into the turbine, where they cause the turbine wheels to rotate. Hot gases move through a multistage gas turbine in much the same way that steam moves through a steam turbine.
Regeneration
The hot gases leaving the turbine may be circulated to a regenerator. There, the gases warm the incoming air before it enters the compression chamber. Preheating the air reduces the amount of fuel needed for combustion.
Operating Conditions
Gas turbines run at even hotter temperatures than steam turbines. The hotter a gas turbine runs, the more efficiently it operates. The temperature in many gas turbines is 1600°F or higher. The turbine cannot transmit its entire power output to the load, since a substantial portion is needed to drive the compressor. The turbine is started with the aid of an electric motor, which first has to set the compressor in motion. When a supply of compressed air is available to the combustion chamber, the turbine can start running.
