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Evaluation and purchasing of turbo-compressors should involve an optimization process with respect to full array of compressor data. Otherwise, equipment may not be suitable over the full operating cases, in meeting reliability, maintainability, availability, commercial terms or power requirements for which the client expects.
2 COUPLING SELECTION
Usually the potential exists for torsional resonance and torsional fatigue failure. Coupling is the best available option for modification to tune the system. Coupling options are:
1- High-torsional-stiffness coupling (optimum if allowed by torsional analysis).
2- Direct forged-flanged rigid connection (no coupling), if allowed by alignment procedure.
3- Flexible coupling (more elasticity and damping and more maintenance).
Torsional vibration problems continue to occur in turbo-compressor trains. The main reasons are lack of comprehensive torsional vibration analysis, improper application (mainly flexible couplings), insufficient maintenance and lack of monitoring.
Some simple rules may be extracted and applied for turbo-machine design review. As a rule of thumb, for shafts coupled together, electric motor shaft diameter should be equal to or greater than the turbo-compressor (or gear unit) shaft diameter. The turbo-compressor shaft (or gear unit shaft) is usually fabricated from a higher material grade compared to an electric motor shaft.
3 CONDITION MONITORING
Condition monitoring should be particularly cost effective and include the necessary items to identify malfunctions at an early stage (lower maintenance costs and lower risk of accident).
Some practical notes on condition monitoring are noted here. Please do not forget to include the following:
A) Casing vibration: Velocity transducers may be preferred over accelerometers due to better signal to noise ratio. The optimum configuration is to use both velocity-transducers and accelerometers.
B) Rotor vibration monitoring.
C) Gas discharge temperature and pressure monitoring.
D) Temperature monitoring of critical components such as bearings.
E) Lube oil flow, pressure, temperature monitoring.
F) Cooling system monitoring.
4 DELIVERY AND BID EVALUATION
It is necessary to receive at least three proposals and have a minimum of two technically accepted machines. It is completely justified to extend the proposal dead time, clarification time, accept optimum configuration and associated deviations and attend extensive clarification meetings to have at least two clarified and technically accepted proposals.
Small and medium turbo-compressors should be delivered fully fabricated as one skid mounted package. For a large machine, it is better to deliver the machine prefabricated in several skid mounted packages. Each turbo-compressor casing (with gear unit, if any) would be mounted on a heavy-duty skid. The driver is usually delivered separately. It is recommended, as a first option, to ask the vendor to offer all site supervision work as a closed price.
Amin Almasi is senior rotating equipment consultant in Brisbane, Queensland, Australia.
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