In most natural gas applications, turboexpanders are operating close to or on the dew point line and a mixture entering the turboexpander impeller is a combination of liquid and gas phases of the natural gas. The presence of the liquid phase may induce rotordynamic instabilities during operation.
This article is a summary of the paper, “Subsynchronous vibrations on turboexpanders equipped with magnetic bearings assessment, understanding and solutions,” presented by Houman Shokraneh of L.A. Turbine, Laurent Richaume of TOTAL E&P Pau, France Bernard Quoix of TOTAL E&P Pau and France Matthieu Oliva of SKF Magnetic Mechatronics (S2M) France at the 2016 Turbomachinery Symposium.
In a liquid natural gas (LNG) rejection and gas injection on a Floating Production, Storage and Offloading (FPSO) barge at the N’Kossa field, the turboexpanders had accumulated more than 250 thousand hours in operation. An upgrade was proposed to adapt to new process conditions and to increase the power of the turboexpanders. After the re-wheeling of turboexpanders, numerous trips due to violent subsynchronous vibrations were experienced. The root-cause analysis revealed that the presence of liquids in the front wheel labyrinth seals caused fluid-induced non-synchronous vibrations (NSV).
In 2010, after almost 15 years of operation, additional gas volume to be processed by the expander-compressors required a complete re-staging of this equipment. Expander inlet pressure was reduced down to 79 bar Abs (original design was 82 bar Abs), with an outlet pressure of 26 bar Abs and the mass flow expanded was increased from 55 to 63 kg/sec. The volume flow to be processed through this expander-compressor was increased by almost 20%. This additional gas flow to be processed required a complete redesign of both the expander and compressor impellers.
Liquid presence is the first source for NSV and their consequences. Inlet separator upstream of the expander inlet shall therefore be carefully selected; especially the separation capacity shall not be affected when the expander is operated at partial load, or during startup phase with reduced flow. On the next step, expander impeller and the sealing design shall be carefully reviewed as well. Presence of liquid in the gas stream will impact expander behavior as liquid may accumulate between a rotating and a stationary part. An open impeller design seems to be a better choice for the expander which could be operated with a high amount of liquefied in the gas stream as it has one less sealing area in which liquid may be trapped.
If a shrouded expander impeller design is the only design option, then the sealing areas needs to be designed in a way to avoid liquid gathering and be released back to the stream. Impeller disk design, labyrinth seal position and gap between the impeller disk and the casing are some of the parameters which could play a significant role in the NSV phenomena. Nozzles are responsible for fifty percent of the process gas expansions. When a high amount of liquid is expected in the stream at the inlet of the expander case, the amount of liquid will increase significantly just after expansion at the nozzles and right at the OD of the wheel. This experiment proved that the amount of liquid at the OD of the wheel plays an even more important role than the amount of liquid at the inlet of the expander casing.
So when a high amount of liquid is expected at the inlet during any operating condition, the recommendation is to implement a controllable system, like the de-clamping system used in the FPSO, to be able to adjust the expansion across the nozzles and thus be able to control the liquid amount at the wheel OD. Finally, rotor dynamics with Active Magnetic Bearings (AMB) shall be carefully evaluated jointly with an AMB supplier, especially for operations at off-design conditions. Having the capacity to withstand subsynchronous excitation and maintain rotor radial vibration at an acceptable level will be beneficial to the expander operability. This, however, may depend on the nature and characteristics of the subsynchronous vibrations due to the external disturbances.
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