Turboexpander Basics | Design | Working Principle | Operation
The turboexpander for this plant is a compound machine supported by accessory systems. It consists of a centrifugal compressor and a radial inflow expander mounted at opposite ends of a common shaft. The machine is called a turboexpander or compander.
The expander operates at cryogenic temperatures and is mounted inside the cold box. The expander consists of an impeller wheel, a passive zero clearance nozzle assembly, an abradable shroud, and a shaft seal, all mounted inside the expander case. The nozzle assembly has adjustable vanes that control inlet flow and, thereby, affect the expander horsepower. The nozzles are controlled by a electric actuator that moves a lever which indicates relative nozzle position on a protractor scale. The electric actuator is controlled from the plant control system.
The bearing case, which contains the bearings and the common shaft, and the compressor operate warm and are not insulated. The compressor consists of a casing, an impeller wheel, an abradable shroud, and a shaft seal. The energy required by the compressor is provided by the gas expanding through the expander. As the expanding gas does work driving the compressor, its temperature drops, providing refrigeration.
1-2. EXPANDER NOZZLE ASSEMBLY
The nozzle assembly serves as the expander flow control. Flow is controlled by opening and closing the expander nozzles. The nozzles are teardrop-shaped vanes that pivot with motion of the nozzle assembly mechanism. A series of these vanes are mounted around the circumference of the expander wheel and are swept in the direction of expander wheel rotation. Closing the nozzles causes the trailing edges of the adjacent vanes to move closer together creating a smaller orifice (throat) and allowing less gas to pass through. Opening the nozzles has the opposite effect: increasing the throat and allowing more gas to pass through.
Nozzle adjustment is accomplished by an actuator. In cases where the expander inlet nozzles are used as a process variable control, it is important that nozzle adjustments are kept to a minimum. Dead bands around the control setpoint will reduce the nozzle movement. Maximizing the dead band ranges to achieve fewer nozzle adjustments and larger percentage changes is preferred. Manufacturer recommendation is for no more than two nozzle adjustments per hour and no less than a one percent change in position.
The actuator mounts to the outside of the plug-in cartridge bearing case and receives a signal from the plant control system. Linear motion of the actuator is converted to angular motion of the internal nozzle assembly mechanism through the actuator shaft and linkage.
NOTE
Constant hunting or frequent nozzle adjustments will reduce the service life of the nozzle mechanism and actuator.
CAUTION
If moisture or oil passes through the expander and contaminates the nozzle assembly, the nozzles may become sluggish or even freeze. Where moisture is involved, defrosting the expander will free the nozzles and they may remain free. In the case of oil or grease, as soon as the process gets cold, the assembly will lock up again. Therefore, if freezing persists, the only way to ensure that the nozzle assembly remains free is to fully disassemble and clean all moving nozzle components.
1-3. TURBOEXPANDER SEAL
FIGURE 1-2. TURBOEXPANDER SEAL GAS FLOW AND CONTROLS SCHEMATIC
The turboexpander seals are a close clearance labyrinth design. The seals on the expander and compressor ends of the turboexpander keep bearing oil from getting into the process piping, minimize process gas loss, maintain acceptable bearing temperatures, and also regulate rotor thrust.
Figure 1-2 shows how seal gas flows through the machine. During normal operation, seal gas is taken from the compressor suction to the expander seals, from the compressor discharge to the compressor balance piston, and from within the expander. The expander cannot be sealed using only its own process gas because the cold process gas would freeze the expander bearing. Therefore, seal gas for the expander is obtained from the compressor inlet and piped to the expander via external piping and the seal gas crossover valve. This warm gas enters behind the expander wheel, combines with the cold expander process gas, and is bled back through the labyrinths with the majority of the gas being vented to the seal gas return line. The remaining gas flows to the lube oil reservoir and is vented through the oil mist eliminator. The crossover valve should be adjusted so that the temperature of the gas venting matches the setpoint as read on the crossover temperature indicator listed in the Product Definition Instrument Summary. This ensures that no warm seal gas is bled into the expander process, which would degrade performance.
Seal gas for the compressor is obtained from the compressor discharge line. The seal gas enters behind the compressor wheel, where it acts against a collar behind the compressor wheel to control rotor thrust. Some of the seal gas bleeds through into the compressor process stream, and the rest bleeds back towards the bearings. The major portion of the seal gas bleeding toward the bearings is vented to the seal gas return line. The remaining gas flows to the lube oil reservoir and is vented through the oil mist eliminator.
1-4. TURBOEXPANDER ROTOR ASSEMBLY
An expander wheel, a shaft, and a compressor wheel comprise the rotor assembly. The expander and compressor wheels are typically made of aluminum, and the shaft is machined from an alloy steel bar.
Wheel designs are selected to meet process, efficiency, and any other design requirements of a particular project. Under normal operating conditions, the wheels are subject to thermal and centrifugal stresses. The centrifugal stress varies with the square of the turboexpander speed. This is why overspeed protection of the turboexpander is so critical. (Ref: A 3.5″ tip diameter wheel rotating at 55,000 rpm has a tip speed of 256 m/s (840 ft/s). 256 m/s = 922 km/hr (840 ft/s = 573 mi/hr).
In addition to avoiding overspeed, there are “no run zones” that should be avoided. These “no run zones” are specific to each wheel design.
CAUTION
If the rotor spins in a no run zone, a natural frequency of the wheel/rotor will be excited and damage will result. Because of this, when a machine is being brought up to speed, it should be brought quickly through the no run zone speeds.
These no run zones, as well as the No Run Zone Logic, are listed in the CryoMachinery Product Definition Instrument Summary.
From a rotor dynamics perspective, the shaft is a stiff shaft design, meaning that it operates below the first bending critical. Functionally the shaft is designed to transmit energy (torque) from the expander through the pin drive and into the compressor. The shaft must also transmit the radial and thrust load of the rotor to the bearings. Special features of the shaft include the machining of turboexpander seal teeth, and instrument pick-up locations for speed, axial position, and vibration.
NOTE
The shaft has six speed pick-up holes in the shaft. Turboexpander speed to be determined as follows:
Actual Shaft Speed (rpm) = 60 * Speed Probe Signal (Hz) / 6.0 (that is, 60,000 rpm = 6000 Hz)
The speed pick-up supplied with the turboexpander is coupled with a tachometer. The tachometer is tied into the plant control system to monitor speed and provide overspeed shutdown protection. The tachometer relay shall be hardwired directly to the expander inlet trip valve to ensure the fastest possible response time.
1-5. TURBOEXPANDER BEARINGS
The bearings are oil-lubricated journal and thrust. Oil is supplied to the expander and compressor bearing by the lube oil system and through a common plug-in cartridge bearing case connection. Adequate lubrication of the bearings is critical during all stages of operation and most critical during transient upsets, shutdowns, defrosts, and start-ups. The rundown tank is sized to provide a minimum of 10 seconds of run-down oil for the bearings. This system is not designed to support continued operation of the turboexpander if a lube oil pump failure, trip, or loss of power occurs.
CAUTION
Failure to establish seal gas and then oil prior to pressurizing and/or defrosting the turboexpander may result in damage to the unit. ALWAYS confirm that seal gas and lube oil have been established when opening and closing valves in the turboexpander process circuit or passing any gas through the turboexpander.
Because of the cryogenic temperatures of the process, the expander bearings are likely to cool during a shutdown. If the expander bearing temperature drops below the permissive setpoint with oil and seal gas operating, an expander defrost is required. A proper expander defrost will ensure that oil is flowing to the bearings.
CAUTION
Failure to defrost the turboexpander prior to start-up will result in extensive damage to the unit if the expander bearing is frozen. Maintaining seal gas pressure and oil flow during a shutdown helps to prevent the bearing from freezing.
The following guidelines may be helpful during a turboexpander shutdown:
- On a shutdown where the oil and/or seal gas are interrupted, reestablish the seal gas and then oil as soon as possible.
- If the turboexpander is going to be down for any length of time, maintain seal gas and lube oil if possible, block in the expander, and depressurize that system. Defrost as necessary to keep the expander bearing warm.
NOTE
Each system design has slightly different heat transfer characteristics. Plant operators should familiarize themselves with the shutdown cooling characteristics of the expander and act accordingly using the above guidelines.
1-6. TURBOEXPANDER LUBE OIL SYSTEM
The major system components include a lube oil reservoir, an immersion heater, lube oil pump(s), pump suction strainer(s), pump safety relief device(s), a shell-and-tube oil cooler, a temperature control valve, lube oil filter(s), an oil rundown tank, and lube oil instrumentation.
The lube oil reservoir is a fabricated stainless steel tank with fill and drain connections, a reservoir temperature indicator, a level indicator, and an internal baffle. A safety relief valve protects the lube oil reservoir from overpressure.
The baffle and oil return diffuser are designed to allow the lube oil returning from the turboexpander sufficient time to degas. The reservoir vent is piped to a demister. The oil demister recovers most of the entrained oil from the seal gas that vents from the reservoir. The oil that accumulates at the bottom of the demister is returned to the reservoir.
The immersion heater maintains proper oil temperature in the reservoir when the turboexpander is not in operation and during cold weather. The heater is controlled by a thermostat. The thermostat setpoint is given in the CryoMachinery Product Definition Instrument Summary. Low lube oil level shuts down the lube oil heater.
Lube oil is supplied to the turboexpander at a fixed temperature through the use of motor driven oil pump(s), a shell-and-tube oil cooler, and a temperature control valve. Oil flows through the shell side or the oil cooler and water on the tube side. The temperature control valve mixes cool oil that has passed through the cooler with hot oil that bypasses the cooler to maintain the correct oil supply temperature to the compander.
CAUTION
If the temperature control valve heats up because of loss of cooling water to the oil cooler, the internal element may be damaged and the valve will no longer control at its set temperature. Replacement of the valve element is required.
The oil rundown tank provides a back-up oil supply to the turboexpander bearings. The system is designed to provide oil for approximately 10 seconds if the lube oil pump trips. This is sufficient time for the turboexpander to spin to a stop, provided that the inlet slam valve functions properly. See instructions for testing the rundown tank in Section 4 of this manual.
CAUTION
Loss of the lube oil pump and failure of the expander inlet slam valve to close will result in continued operation of the turboexpander without lubrication, and ultimately extensive damage to the internal components. The rundown tank is designed to provide only several seconds of lubrication to the compander for spindown. If the compander shutdown system does not function properly, damage to the compander is imminent.
The lube oil filter(s) installed in the turboexpander bearing oil supply protect the bearings from particle contamination that may be harmful to the rotor bearing assembly. A clean bearing oil supply for a high speed turboexpander is critical to ensure long life of the unit. Elements should be replaced when the pressure drop exceeds the value listed in the CryoMachinery Product Definition Equipment Summary.
Instrumentation associated with the turboexpander lube oil system protects the various components. Proper oil temperature and pressure are crucial. Oil that is too cool or too hot can damage equipment. The temperature of the oil in the reservoir controls the starting and stopping of the lube oil reservoir heater. Excessively hot oil could result in a bearing failure. Lube oil supply pressure instrumentation protects the turboexpander in case of a loss of oil.
1-7. TURBOEXPANDER SEAL GAS SYSTEM
(Refer to Figure 1-2 and the plant P&ID.)
The seal gas system consists of several key components including the seal gas filter, regulator, and the supporting instrumentation.
The seal gas filter ensures a clean supply of gas is supplied to the turboexpander. Ingestion of debris by the turboexpander may result in damage to the turboexpander seal and shaft. Therefore, a clean seal gas system is crucial.
Instrumentation to support the seal gas system includes a pressure regulator, a pressure transmitter that shuts down the lube oil pump and turboexpander if the pressure is insufficiently low, a seal gas return relief valve to help control thrust, and a back-pressure regulator designed to be closed during start-up and fully open during normal operation. Two seal gas pressure regulators are supplied if the process through the machine enters into an oxygen-enriched atmosphere (for example, the LP column). This is to provide extra protection to prevent the lubrication oil from entering into the process.
1-8. TURBOEXPANDER PROCESS SYSTEM
(Refer to the plant P&ID.)
The process incorporates a single turboexpander. The key turboexpander-related components of the process system are the automatic expander inlet trip valve, the expander nozzle actuator, and process instrumentation.
The expander inlet trip valve is probably the most critical piece of equipment for shutdown protection of the turboexpander. This valve is a high-speed valve that trips shut if a turboexpander shutdown occurs.
CAUTION
Failure of this valve to function properly will most likely result in extensive damage to the turboexpander. Regular inspection of this valve is highly recommended. During normal operation, the valve should be inspected to ensure that there is no ice build-up which might restrict the motion of the valve. Also inspect the solenoid vents to ensure that they are not obstructed. When the system is shut down, the trip valves should be cycled to ensure that the valves close from fully open to fully closed in 2 seconds or less.
Process piping free of contaminants is critical for the longevity of the turboexpander. The smallest of particles in sufficient quantity will erode the expander nozzles and wheel to the point where the expander efficiency can be affected and the wheel may fail.