TURBOEXPANDER MONITORING, INSPECTION, AND MAINTENANCE
1. GENERAL
Operating experience is the basis for determining if machine overhaul is needed. Wheel and nozzle erosion and other wear can be caused by “carry-in” of solids from the process, seal gas, and oil streams. Parts erosion or wear is indicated by performance falloff, increased vibration, or increased seal gas consumption or vent flow.
Scheduled maintenance of the turboexpander and accessory systems is not required. However, as described below, regular inspection of various components on the lube oil and seal gas systems is recommended.
CAUTION
If any replacement hardware is used (for example, nuts, bolts, etc.), make certain that it is of equal or better quality than the item being replaced.
2. TURBOEXPANDER
2-1. MONITORING.
A trend of process data is required to help identify an efficiency falloff of the turboexpander. The key information required is turboexpander flow, turboexpander inlet and outlet pressures and temperatures, turboexpander speed, nozzle vane position, oil supply pressure and temperature, and oil return temperature.
Collection, review, and comparison of all turboexpander trends immediately following a shutdown and following restart is crucial in troubleshooting any problems experienced with the turboexpander(s). This is of particular value following an uncontrolled shutdown of the turboexpander(s).
2-2. INSPECTION.
In general, if the turboexpander is not exhibiting any problems, routine inspection of the turboexpander internals is NOT required or recommended. However, if an inspection is warranted, rotor axial and radial float can be inspected by removing the plug-in cartridge. Inspection of the expander wheel, expander shroud, nozzle vanes, and nozzle mechanism requires the plug-in cartridge to be pulled and partially disassembled. Inspection of the expander seal can be done by measuring the seal gas consumption or a visual inspection of the seal. To be able to visually inspect the expander seal or the expander bearing, the plug-in cartridge must be removed and considerable disassembly must be performed. It is recommended that only a qualified CryoMachinery representative perform inspections requiring considerable disassembly of the turboexpander.
An inspection of the turboexpander external components and its surrounding area is recommended. This includes a visual inspection of the turboexpander to ensure that no ice is forming, which might inhibit the motion of the nozzle actuator. Also ensure that frosting or icing is not occurring on any of the seal gas, lube oil, or instrument lines. If the turboexpander is located in an area that has little noise, the sound coming from the turboexpander may help in identifying the early stages of a problem with the turboexpander.
2-3. MAINTENANCE.
Routine maintenance of the turboexpander is not required. If the turboexpander needs servicing, work should be performed in strict accordance with the Turboexpander Technical Manual, Turbo Assembly Drawing, and Outline Drawings by a qualified CryoMachinery representative.
3. INLET SLAM VALVE
3-1. MONITORING.
Normally, the inlet slam valve is either fully open or fully closed and may go for long periods of time without being stroked. Because it is crucial that this valve function flawlessly on a turboexpander shutdown, while the turboexpander is in operation, a visual inspection is recommended to ensure that no ice has formed on the valve or valve stem and that all other mechanical components are free and clear to move. Also the solenoid vents and actuator quick relief vent valves should be inspected to ensure that the vents are clear of debris and are not collecting water.
3-2. INSPECTION.
Immediately following a shutdown, the inlet slam valve should be looked at to ensure that the slam valve has closed properly and the turboexpander speed has gone to zero. A visual inspection of the inlet slam valve is also recommended to ensure that no ice has formed on the valve or valve stem and that all other mechanical components are free and clear to move. Also the solenoid vents and actuator quick relief vent valves should be inspected to ensure that the vents are clear of debris and are not collecting water.
Just prior to start-up with the manual valve upstream of the inlet slam valve closed, the inlet slam valve should be cycled open and closed. The valve should go from fully open to fully closed in less than two seconds.
WARNING
The system including the section of pipe between the inlet slam valve and the upstream manual valve should be depressurized when doing this. Also the lube oil and seal gas should be established in case the turboexpander spins.
3-3. MAINTENANCE.
Maintenance of the valve and related equipment should be performed in strict accordance with the manufacturers’ recommendations.
4. LUBE OIL SYSTEM
4-1. MONITORING.
The control system monitors supply oil temperature and supply oil pressure.
WARNING
Operators should watch for sudden changes in oil level. This may indicate excess oil carryover or process contamination and must be addressed immediately.
4-2. INSPECTION.
Check the sight glass in the rundown tank return line to verify that lube oil is flowing to the rundown tank. The rundown tank is called upon only on a shutdown where the lube oil pump trips. Prior to every startup, check the rundown tank to ensure that it is functioning properly as follows.
- With the seal gas in service, the expander inlet slam valve closed, and the lube oil pump stopped, record the oil supply pressure reading (system settle-out pressure).
- Start the lube oil pump and wait until oil is detected by the sight glass in the rundown tank return line.
- Stop the lube oil pump and monitor the oil supply pressure decay. The pressure should NOT go from the operating pressure to the system settle-out pressure immediately. Instead there should be an initial rapid decay and then the pressure should slowly move toward settle-out pressure over a minimum of 10 seconds.
A general visual inspection of the accessory system should be made routinely to ensure that the system has not been compromised in any way. Check the following.
- that oil levels are within operating range
- that all fill and inspection ports are properly sealed
- that pump and motor group are free of objects.
- Fix oil leaks and address oil spills.
4-3. MAINTENANCE.
Routine maintenance of the lube oil system is not required. Any maintenance performed on the individual components should be in strict accordance with the original equipment manufacturers’ recommendations. Perform an annual oil sample analysis for breakdown or contamination (that is, water; particle count; improper rust, foam, or oxidation inhibitor levels; change in viscosity, etc.). Change the filter elements if the differential pressure exceeds the recommended value.
5. SEAL GAS SYSTEM
5-1. MONITORING.
The plant control system monitors seal gas supply pressure. No additional monitoring of the seal gas system is required.
5-2. INSPECTION.
The seal gas system should be inspected to ensure that the seal gas supply valves are in the proper position and that the seal gas supply lines have not been blocked out. This includes the start-up and operating seal gas supply lines.
WARNING
The seal gas vent should normally be venting instrument air. If the vent line or any other seal gas line is frosted, that line may be flowing process gas. If these lines are frosted, investigate immediately and take the necessary precautions to correct the problem. If the problem cannot be corrected, shut down the expander.
5-3. MAINTENANCE.
If the inspections reveal that the seal gas pressure has dropped, the filter should be inspected and cleaned. Routine maintenance of the seal gas system is not required. Any maintenance performed on the individual components should be in strict accordance with the original equipment manufacturers’ recommendations.
6. SHUTDOWNS
6-1. MONITORING. Shutdowns are monitored to confirm the trips immediately following a shutdown. The items recommended to be confirmed on a regular basis are
- that the turboexpander inlet slam valve actually closes when called for
- that the lube oil pump actually shuts off when the seal gas pressure is insufficiently low.
6-2. INSPECTION. Shutdown inspection and testing should be done in accordance with the plant’s general inspection and testing guidelines.
6-3. MAINTENANCE. Routine maintenance on the shutdown circuits and their components is not required. Any maintenance performed on the individual components should be in strict accordance with the original equipment manufacturers’ recommendations.