Turbo Expander

Turboexpander is the most effective and efficient way to reduce the pressure of a fluid. The use of this equipment has been started in the 60s in oil and gas plants. The Turboexpander takes energy from the fluid flow, produces power, reduces the pressure and lowers the temperature of the fluid. The obtained power can be used to rotate the compressor, pump or generator. The work resulting from the expansion process increases the thermal efficiency of the plant.

Turbo Expander

Turbo Expander is the most effective and efficient way to reduce the pressure of a fluid. The use of this equipment has been started in the 60s in oil and gas plants. The turbo expander takes energy from the fluid flow, produces power, reduces the pressure and lowers the temperature of the fluid. The obtained power can be used to rotate the compressor, pump or generator. The work resulting from the expansion process increases the thermal efficiency of the plant.

What is a Turboexpander?

Turboexpander is the most effective and efficient way to reduce the pressure of a fluid. The use of this equipment has been started in the 60s in oil and gas plants. The turbo expander takes energy from the fluid flow, produces power, reduces the pressure and lowers the temperature of the fluid. The obtained power can be used to rotate the compressor, pump or generator. The work resulting from the expansion process increases the thermal efficiency of the plant.

Using  Turboexpander in cryogenic applications

Application of compressor brake expanders in natural gas cryogenic plants began in the second half of the 20th century. The requirements to increasing the thermal efficiency of the plant, minimizing the feed consumption in liquefied natural gas (LNG) plants, and reducing CO2 pollution, led to the development of plant process designs by considering employing turbo expander. Often the economic benefit of the turbo expander in cryogenic applications is related to the reduction of fluid enthalpy and consequently the temperature reduction. The low temperature fluid leaving the turbo expander plays a key role in enhancement of the thermal efficiency of the plant. In addition, the expander’s shaft power which obtains from the fluid enthalpy reduction, can led to the reduction of plant’s input power. Each of these cases causes a reduction in the specific power consumption of the cooling cycle and leads to a reduction in costs which makes the project economically more feasible.

Using the Joule Thomson valve in parallel with the Turboexpander

In cryogenic expander systems, a Joule-Thomson (JT) valve is placed in parallel with the expander. This valve is set in such a way as to prevent a complete shutdown of the process if the expander fails.

 Using Turboexpander in hot gas applications

The idea of converting combusted gas sensible heat into power has been studied for about a century. Energy costs, environmental issues, pollution control and carbon flow management are the reasons why Turboexpanders are widely used in modern plants. There are fundamental obstacles for designing an expander with high reliability in hot gas applications. These challenges can be listed as follows: very high temperature, chemical corrosion and erosion caused by hot gas that all impose high tensions on the turbo expander system.

Corrosion is one of the most important phenomenon that can make destructive effects on turbo expanders. Corrosion occurs due to the high temperatures at which the expander is supposed to operate. The effect of creep and corrosion caused by hot gas is significant in these applications. If these factors are not considered, they will cause the disc or its blades to fail.

Figure 1: Main components of hot gas expander
Figure 1: Main components of hot gas expander

Velocity triangles in radial flow Turboexpanders

In radial flow turbo expanders, energy is transferred from the fluid to the turbo expander blades. Figure 2 shows a schematic of radial flow Turboexpanders.

Figure 2: Velocity triangle in radial flow
Figure 2: Velocity triangle in radial flow

The fluid leaving the Turboexpander, has high kinetic energy (high speed shown with C4 vector), so a diffuser is employed to recover its kinetic energy. Figure 2 shows the speed triangles in this case. As shown in this figure, W3, which is the relative velocity of the input, is radially inward and C4, which is the absolute velocity component at the exit of the rotor, is in the axial direction. This velocity triangle arrangement is used for many radial flow turbo expanders. In a radial flow turbo expander, there are three steps in which the energy conversion takes place:

  1. The potential energy of the fluid is converted into velocity in the inlet vanes. The efficiency of this energy conversion is estimated about 95 percent.
  2. The remaining potential energy is converted into mechanical power in the Turboexpander.
  3. The speed of output stream is relatively high and its value is regulated in the output diffuser.
Figure 3: Turboexpander in the testing phase
Figure 3: Turbo expander in the testing phase

Gas expanders thermodynamic

Figure 4 shows three reference expansion processes between higher pressure P0 and lower pressure P5. This figure shows that the lowest exit temperature occurs for an isentropic process where energy is extracted from the fluid. On the other hand, as illustrated in the figure, the outlet temperature for the constant enthalpy expansion process (for example, in the Joule-Thomson process) will be higher than the real and isentropic processes. The temperature difference between 5a and 5h actually represents the additional temperature drop that can result from employing turbo expander instead of a Joule-Thomson valve. Turboexpander efficiency can be defined as the ratio of static temperature to total temperature in the way where temperature reduction is the main purpose of using turbo expander: Ƞ exp = (T (0) – T (5a) )/ (T t (0) – T t(5s) ). The above definition of efficiency can be used to understand how effective it is to extract energy from the input fluid which results in temperature reduction of the outlet stream.

Figure 4: Isentropic expansion process (0-5s), real expansion process (0-5a) and constant enthalpy expansion process (0-5h)
Figure 4: Isentropic expansion process (0-5s), real expansion process (0-5a) and constant enthalpy expansion process (0-5h)

Classification of Turboexpanders

COMPRESSOR-LOADED: This type of Turboexpander is one of the most common ones. After the separation of heavy liquid hydrocarbon compounds in the separation tower, the produced gas is re-compressed in the centrifugal compressor.

This type of Turboexpander is often used in natural gas purification or in ethylene plants. The generator prevents the turbine from expanding. This device is connected to the grid and allows electricity to be generated. Various designs exist, including planetary gear or parallel shaft gear designs, with a choice of synchronous or asynchronous generators brake. This model is used when the cold production is relatively low (less than 100 kW) and when the gas does not need to be re-compressed.

Two types of bearings in Turboexpander

Oil bearing: In an oil bearing, a thin layer of oil is used between the bearing surfaces, typically around or under the rotating shaft to insulate. Oil bearings are relatively cheaper compared to similar bearings.

Active Magnetic bearing: This type of bearing carry the load using magnetic plates. These bearings support moving parts without physical contact, which causes very little friction and no mechanical wear. These types of bearings support the highest speed of all types of bearings and do not have a relative maximum speed.

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Frequently Asked Questions

Turbo Expander is the most effective and efficient way to reduce the pressure of a fluid. The use of this equipment has been started in the 60s in oil and gas plants. The turbo expander takes energy from the fluid flow, produces power, reduces the pressure and lowers the temperature of the fluid. The obtained power can be used to rotate the compressor, pump or generator. The work resulting from the expansion process increases the thermal efficiency of the plant.

The economic benefit of the turbo expander in cryogenic applications is related to the reduction of fluid enthalpy and consequently the temperature decrement. The cooled fluid leaving the turbo expander helps to increase the thermal efficiency of the plant. In addition, the shaft rotational energy obtained from the fluid enthalpy decrement can help the reduction of input power to the plant. Each of these cases causes a reduction in the specific power consumption for the cooling cycle and leads to a reduction in costs and, as a result, an increase in the economic feasibility of the project.

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