Hydrocarbon Processing - October 2021 - GP-30

TURBOMACHINERY
wall " (see FIG. 3 and FIG. 8). The heat barrier wall is manufactured
from a glass epoxy laminate that provides mechanical
strength, as well as insulation of the bearing cavity from the
expander section at very low temperatures.
On the compressor side of the machine, the stationary seal
element (called a compressor seal, see FIG. 3) can be made of
different materials, depending on the application, but an aluminum
alloy is common. From a repair perspective, a typical
finding is that the seal areas-both on the heat barrier wall
and the compressor seal-have experienced some degree of
wear due to rubs. Rubs are not uncommon and are related to
transient excursions of rotor lateral vibrations, usually during
startup. They can be repaired by boring out the damaged
section(s), installing a blank insert of a matching material and
re-machining the seal features to specification.
Inlet guide vanes (IGVs). The inlet guide vanes are the
variable stators of the expander stage. The airfoil-shaped mechanical
elements provide an efficient aerodynamic flow path
for the expander gas to enter the rotating wheel. They also
function like a valve, by closing to pinch the flowrate. OEMs
differ widely in their chosen designs for IGVs. Some choose
to have four or five vanes with simple adjustment mechanisms.
Others opt for a larger number of vanes with complex
linkage mechanisms.
Parts inside the IGV are exposed to two wear mechanisms:
namely, friction and erosion. The friction stems from repeated
sliding motion against contacting parts that are pressurized
against each other. Erosion wear is fairly common in the surfaces
of the nozzle segments in direct contact with the highspeed
part-liquid, part-gas flow; therefore, these parts are
often designed with hard coatings, such as HVOF tungsten
carbide, to provide a smooth, solid surface for friction protection
and a hard surface to slow down erosion. Under normal
wear conditions, these components can be repaired by removing
and reapplying the coating.
How should turboexpanders be monitored for health?
A common question among users is how turboexpanders
should be monitored for health. Frequent monitoring of the
machine after commissioning and during the first year of operation
is critical. Operators should keep a log of important data:
* Process temperatures and pressures in the expander
and compressor (if available) streams
* Bearing support system-i.e., lube oil system
(if it is an oil-bearing machine)
* Rotor vibrations
* Seal gas filters
* Valves.
Such information is instrumental in providing visibility to
mechanical problems or performance deficiencies. Data can
be retrieved from the human machine interface (HMI) and/or
the distributed control system (DCS), or even written down
and kept for future reference. While control systems provide
warnings and alarms when the expander operates outside of
its design constraints, historical data logs may highlight operational
changes not easily observed in the HMI. Operations
and maintenance personnel should gather a baseline of how
the unit performs to more easily identify unfavorable trends
over time, and gauge performance after plant shutdowns or
upsets. It is important to remember that process drifts and
performance deterioration can occur slowly over a period of
years and may go unnoticed as the changes turn into norms.
Another common question is whether turboexpanders
have an hourly runtime or a periodic overhaul cycle. The answer
is neither, as these machines are designed to run practically
indefinitely. A turboexpander can be expected to perform
as designed for years without issue as long as some operational
conditions are met:
* The process remains largely stable and without upsets
* The seal gas and seal gas filters are kept clean
* The lube oil is regularly tested for
contaminants and viscosity
* The lube oil filters are kept in good condition
or replaced in a timely fashion
FIG. 8. Variable inlet guide vane assembly. Image courtesy of
L.A. Turbine.
* Contamination in the lube oil system or
the process is kept to a minimum.
However, if a condition creates the need to increase the level
FIG. 9. Inlet guide vane example. Image courtesy of L.A. Turbine.
30 SEPTEMBER/OCTOBER 2021 | GasProcessingNews.com
and frequency of preventative maintenance actions, operators
should not hesitate to do so. For example, if a plant's process
has known contaminates that pass to the expander and cause
erosion of the wheel blades, then adding inspections, cleaning
and possibly performing changeouts of the expander inlet,
along with removing and replacing an MCS at regular intervals,
may be necessary. Some issues to consider include frequency
of system interruptions, a change in the source for process or
seal gas, age of equipment, duration of operation since last inspection
and particular application (e.g., natural gas processing,
air separation, geothermal power generation).
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Hydrocarbon Processing - October 2021

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