Expanders can use pressure reduction to drive rotating machines. Information on how to evaluate the potential benefits of installing an extender can be found here.
Typically in the chemical process industry (CPI), “a large amount of energy is wasted in pressure control valves where high pressure fluids must be depressurized” [1]. Depending on various technical and economic factors, it may be desirable to convert this energy into rotating mechanical energy, which can be used to drive generators or other rotating machines. For incompressible fluids (liquids), this is achieved using a hydraulic energy recovery turbine (HPRT; see reference 1). For compressible liquids (gases), an expander is a suitable machine.
Expanders are a mature technology with many successful applications such as fluid catalytic cracking (FCC), refrigeration, natural gas city valves, air separation or exhaust emissions. In principle, any gas stream with reduced pressure can be used to drive an expander, but “the energy output is directly proportional to the pressure ratio, temperature and flow rate of the gas stream” [2], as well as technical and economic feasibility. Expander Implementation: The process depends on these and other factors, such as local energy prices and the manufacturer’s availability of suitable equipment.
Although the turboexpander (functioning similarly to a turbine) is the most well-known type of expander (Figure 1), there are other types suitable for different process conditions. This article introduces the main types of expanders and their components and summarizes how operations managers, consultants or energy auditors in various CPI divisions can evaluate the potential economic and environmental benefits of installing an expander.
There are many different types of resistance bands that vary greatly in geometry and function. The main types are shown in Figure 2, and each type is briefly described below. For more information, as well as graphs comparing the operating status of each type based on specific diameters and specific speeds, see Help. 3.
Piston turboexpander. Piston and rotary piston turboexpanders operate like a reverse-rotating internal combustion engine, absorbing high-pressure gas and converting its stored energy into rotational energy through the crankshaft.
Drag the turbo expander. The brake turbine expander consists of a concentric flow chamber with bucket fins attached to the periphery of the rotating element. They are designed in the same way as water wheels, but the cross-section of the concentric chambers increases from inlet to outlet, allowing the gas to expand.
Radial turboexpander. Radial flow turboexpanders have an axial inlet and a radial outlet, allowing the gas to expand radially through the turbine impeller. Similarly, axial flow turbines expand gas through the turbine wheel, but the direction of flow remains parallel to the axis of rotation.
This article focuses on radial and axial turboexpanders, discussing their various subtypes, components, and economics.
A turboexpander extracts energy from a high-pressure gas stream and converts it into a drive load. Typically the load is a compressor or generator connected to a shaft. A turboexpander with a compressor compresses fluid in other parts of the process stream that require compressed fluid, thereby increasing the overall efficiency of the plant by using energy that is otherwise wasted. A turboexpander with a generator load converts the energy into electricity, which can be used in other plant processes or returned to the local grid for sale.
Turboexpander generators can be equipped with either a direct drive shaft from the turbine wheel to the generator, or through a gearbox that effectively reduces the input speed from the turbine wheel to the generator through a gear ratio. Direct drive turboexpanders offer advantages in efficiency, footprint and maintenance costs. Gearbox turboexpanders are heavier and require a larger footprint, lubrication auxiliary equipment, and regular maintenance.
Flow-through turboexpanders can be made in the form of radial or axial turbines. Radial flow expanders contain an axial inlet and a radial outlet such that the gas flow exits the turbine radially from the axis of rotation. Axial turbines allow gas to flow axially along the axis of rotation. Axial flow turbines extract energy from the gas flow through inlet guide vanes to the expander wheel, with the cross-sectional area of the expansion chamber gradually increasing to maintain a constant speed.
A turboexpander generator consists of three main components: a turbine wheel, special bearings and a generator.
Turbine wheel. Turbine wheels are often designed specifically to optimize aerodynamic efficiency. Application variables that affect turbine wheel design include inlet/outlet pressure, inlet/outlet temperature, volume flow, and fluid properties. When the compression ratio is too high to be reduced in one stage, a turboexpander with multiple turbine wheels is required. Both radial and axial turbine wheels can be designed as multi-stage ones, but axial turbine wheels have a much shorter axial length and are therefore more compact. Multistage radial flow turbines require gas to flow from axial to radial and back to axial, creating higher friction losses than axial flow turbines.
bearings. Bearing design is critical to the efficient operation of a turboexpander. Bearing types related to turboexpander designs vary widely and can include oil bearings, liquid film bearings, traditional ball bearings, and magnetic bearings. Each method has its own advantages and disadvantages, as shown in Table 1.
Many turboexpander manufacturers select magnetic bearings as their “bearing of choice” due to their unique advantages. Magnetic bearings ensure friction-free operation of the turboexpander’s dynamic components, significantly reducing operating and maintenance costs over the life of the machine. They are also designed to withstand a wide range of axial and radial loads and overstress conditions. Their higher initial costs are offset by much lower life cycle costs.
dynamo. The generator takes the rotational energy of the turbine and converts it into useful electrical energy using an electromagnetic generator (which can be an induction generator or a permanent magnet generator). Induction generators have a lower rated speed, so high speed turbine applications require a gearbox, but can be designed to match the grid frequency, eliminating the need for a variable frequency drive (VFD) to supply the generated electricity. Permanent magnet generators, on the other hand, can be directly shaft coupled to the turbine and transmit power to the grid through a variable frequency drive. The generator is designed to deliver maximum power based on the shaft power available in the system.
Seals. The seal is also a critical component when designing a turboexpander system. To maintain high efficiency and meet environmental standards, systems must be sealed to prevent potential process gas leaks. Turboexpanders can be equipped with dynamic or static seals. Dynamic seals, such as labyrinth seals and dry gas seals, provide a seal around a rotating shaft, typically between the turbine wheel, bearings and the rest of the machine where the generator is located. Dynamic seals wear out over time and require regular maintenance and inspection to ensure they are functioning properly. When all turboexpander components are contained in a single housing, static seals can be used to protect any leads exiting the housing, including to the generator, magnetic bearing drives, or sensors. These airtight seals provide permanent protection against gas leakage and require no maintenance or repair.
From a process standpoint, the primary requirement for installing an expander is to supply high-pressure compressible (non-condensable) gas to a low-pressure system with sufficient flow, pressure drop and utilization to maintain normal operation of the equipment. Operating parameters are maintained at a safe and efficient level.
In terms of pressure reducing function, the expander can be used to replace the Joule-Thomson (JT) valve, also known as the throttle valve. Since the JT valve moves along an isentropic path and the expander moves along a nearly isentropic path, the latter reduces the enthalpy of the gas and converts the enthalpy difference into shaft power, thereby producing a lower outlet temperature than the JT valve. This is useful in cryogenic processes where the goal is to reduce the temperature of the gas.
If there is a lower limit on the outlet gas temperature (for example, in a decompression station where the gas temperature must be maintained above freezing, hydration, or minimum material design temperature), at least one heater must be added. control the gas temperature. When the preheater is located upstream of the expander, some of the energy from the feed gas is also recovered in the expander, thereby increasing its power output. In some configurations where outlet temperature control is required, a second reheater can be installed after the expander to provide faster control.
In Fig. Figure 3 shows a simplified diagram of the general flow diagram of an expander generator with preheater used to replace a JT valve.
In other process configurations, the energy recovered in the expander can be transferred directly to the compressor. These machines, sometimes called “commanders”, usually have expansion and compression stages connected by one or more shafts, which may also include a gearbox to regulate the speed difference between the two stages. It can also include an additional motor to provide more power to the compression stage.
Below are some of the most important components that ensure proper operation and stability of the system.
Bypass valve or pressure reducing valve. The bypass valve allows operation to continue when the turboexpander is not operating (for example, for maintenance or an emergency), while the pressure reducing valve is used for continuous operation to supply excess gas when the total flow exceeds the expander’s design capacity.
Emergency shutdown valve (ESD). ESD valves are used to block the flow of gas into the expander in an emergency to avoid mechanical damage.
Instruments and controls. Important variables to monitor include inlet and outlet pressure, flow rate, rotation speed, and power output.
Driving at excessive speed. The device cuts off flow to the turbine, causing the turbine rotor to slow down, thereby protecting the equipment from excessive speeds due to unexpected process conditions that could damage the equipment.
Pressure Safety Valve (PSV). PSVs are often installed after a turboexpander to protect pipelines and low pressure equipment. The PSV must be designed to withstand the most severe contingencies, which typically include failure of the bypass valve to open. If an expander is added to an existing pressure reduction station, the process design team must determine whether the existing PSV provides adequate protection.
Heater. Heaters compensate for the temperature drop caused by the gas passing through the turbine, so the gas must be preheated. Its main function is to increase the temperature of the rising gas flow to maintain the temperature of the gas leaving the expander above a minimum value. Another benefit of raising the temperature is to increase power output as well as prevent corrosion, condensation, or hydrates that could adversely affect equipment nozzles. In systems containing heat exchangers (as shown in Figure 3), the gas temperature is usually controlled by regulating the flow of heated liquid into the preheater. In some designs, a flame heater or electric heater can be used instead of a heat exchanger. Heaters may already exist in an existing JT valve station, and adding an expander may not require installing additional heaters, but rather increasing the flow of heated fluid.
Lubricating oil and seal gas systems. As mentioned above, expanders can use different seal designs, which may require lubricants and sealing gases. Where applicable, the lubricating oil must maintain high quality and purity when in contact with process gases, and the oil viscosity level must remain within the required operating range of lubricated bearings. Sealed gas systems are usually equipped with an oil lubrication device to prevent oil from the bearing box from entering the expansion box. For special applications of companders used in the hydrocarbon industry, lube oil and seal gas systems are typically designed to API 617 [5] Part 4 specifications.
Variable frequency drive (VFD). When the generator is induction, a VFD is typically turned on to adjust the alternating current (AC) signal to match the utility frequency. Typically, designs based on variable frequency drives have higher overall efficiency than designs that use gearboxes or other mechanical components. VFD-based systems can also accommodate a wider range of process changes that can result in changes in expander shaft speed.
Transmission. Some expander designs use a gearbox to reduce the speed of the expander to the rated speed of the generator. The cost of using a gearbox is lower overall efficiency and therefore lower power output.
When preparing a request for quotation (RFQ) for an expander, the process engineer must first determine the operating conditions, including the following information:
Mechanical engineers often complete expander generator specifications and specifications using data from other engineering disciplines. These inputs may include the following:
The specifications must also include a list of documents and drawings provided by the manufacturer as part of the tender process and the scope of supply, as well as applicable test procedures as required by the project.
The technical information provided by the manufacturer as part of the tender process should generally include the following elements:
If any aspect of the proposal differs from the original specifications, the manufacturer must also provide a list of deviations and the reasons for the deviations.
Once a proposal is received, the project development team must review the request for compliance and determine whether variances are technically justified.
Other technical considerations to consider when evaluating proposals include:
Finally, an economic analysis needs to be carried out. Because different options may result in different initial costs, it is recommended that a cash flow or life cycle cost analysis be performed to compare the project’s long-term economics and return on investment. For example, a higher initial investment may be offset in the long term by increased productivity or reduced maintenance requirements. See “References” for instructions on this type of analysis. 4.
All turboexpander-generator applications require an initial total potential power calculation to determine the total amount of available energy that can be recovered in a particular application. For a turboexpander generator, the power potential is calculated as an isentropic (constant entropy) process. This is the ideal thermodynamic situation for considering a reversible adiabatic process without friction, but it is the correct process for estimating the actual energy potential.
Isentropic potential energy (IPP) is calculated by multiplying the specific enthalpy difference at the inlet and outlet of the turboexpander and multiplying the result by the mass flow rate. This potential energy will be expressed as an isentropic quantity (Equation (1)):
IPP = ( hinlet – h(i,e)) × ṁ x ŋ (1)
where h(i,e) is the specific enthalpy taking into account the isentropic outlet temperature and ṁ is the mass flow rate.
Although isentropic potential energy can be used to estimate potential energy, all real systems involve friction, heat, and other ancillary energy losses. Thus, when calculating the actual power potential, the following additional input data should be taken into account:
In most turboexpander applications, the temperature is limited to a minimum to prevent unwanted problems such as pipe freezing mentioned earlier. Where natural gas flows, hydrates are almost always present, meaning that the pipeline downstream of a turboexpander or throttle valve will freeze internally and externally if the outlet temperature drops below 0°C. Ice formation can result in flow restriction and ultimately shut down the system to defrost. Thus, the “desired” outlet temperature is used to calculate a more realistic potential power scenario. However, for gases such as hydrogen, the temperature limit is much lower because hydrogen does not change from gas to liquid until it reaches cryogenic temperature (-253°C). Use this desired outlet temperature to calculate the specific enthalpy.
The efficiency of the turboexpander system must also be considered. Depending on the technology used, system efficiency can vary significantly. For example, a turboexpander that uses a reduction gear to transfer rotational energy from the turbine to the generator will experience greater friction losses than a system that uses direct drive from the turbine to the generator. The overall efficiency of a turboexpander system is expressed as a percentage and is taken into account when assessing the actual power potential of the turboexpander. The actual power potential (PP) is calculated as follows:
PP = (hinlet – hexit) × ṁ x ṅ (2)
Let’s look at the application of natural gas pressure relief. ABC operates and maintains a pressure reduction station that transports natural gas from the main pipeline and distributes it to local municipalities. At this station, the gas inlet pressure is 40 bar and the outlet pressure is 8 bar. The preheated inlet gas temperature is 35°C, which preheats the gas to prevent pipeline freezing. Therefore, the outlet gas temperature must be controlled so that it does not fall below 0°C. In this example we will use 5°C as the minimum outlet temperature to increase the safety factor. The normalized volumetric gas flow rate is 50,000 Nm3/h. To calculate the power potential, we will assume that all gas flows through the turbo expander and calculate the maximum power output. Estimate the total power output potential using the following calculation: