Page 7 - Industrial Technology EXTRA 19th July 2021

put on the rotor and its bearings. Rotating at speeds expanders, and as such, similar technologies can be
up to 11’000 rpm, the rotor needs to be supported applied to both. Developments in blade coatings
by bearings that can withstand the operational can reduce fouling and oxidation in both the rotating
stresses and provide long-term reliability. A retrofit and stationary sections.
project can upgrade the cylindrical sleeve bearings
to tilt pad components. Spherical seated bearings Gas turbines
can be replaced with a design using cylindrical Important pieces of equipment, such as gas
rollers, offering better long-term performance. turbines can provide both primary and back-up
The rotor itself has a multi-stage design where a power supplies, so, their reliability is paramount.
hollow shaft with internal cooling passages holds Over time, design improvements have been
the turbine disks and blades in position with a developed both by the OEMs and independent
single thru-bolt. This bolt is subjected to maintenance providers and these can be applied to
considerable thermal stress and is often machined the installed equipment through retrofit projects.
from stainless steel in the original build For example, in the Siemens V-Series gas
specification. turbines, the flame tube F-ring has a tendency to
The part can be upgraded to 450 stainless steel oxidize and produce scale that can be ingested by
alloy that allows increased bolt stretch and greater the turbine during normal operating conditions. To
bundle crush on the rotor assembly. The dimensions improve this situation, the component can be
are machined to exactly match the original using machined to expose fresh material and allow an
the 3D CAD drawings and can be installed as part Inconel overlay to be welded to the new surface
of a rotor refurbishment. before being machined back to the required
diameter. Inconel 82 is a nickel chromium alloy
Improved blade performance with high strength and an outstanding corrosion
Hot gas expanders can also benefit from a reduction resistance that will improve the durability of the
in blade tip leakage, which accounts for substantial component.
losses in efficiency in legacy equipment. Installing Similarly, the combustion section of the gas
an abradable coating on the stator inner surface turbine can be protected with a thermal barrier
provides improved clearance control for the rotor coating (TBC) applied to all of the flame tubes and
blades. Clearly this sort of upgrade requires the other hot gas path components. A ceramic TBC
expander to be under maintenance for an extended
period, so is best suited to a planned shut-down
period.
Further improvements can be achieved by
upgrading the materials used to manufacture the
blades themselves. In applications requiring high
strength and good corrosion resistance up to
1,300°F (700°C) the use of a super alloy, such as
A286, in the blades can be beneficial.
Of course, with turbine blades operating in high
temperature environments, gas turbines have
several characteristics in common with gas

July 2021 • INDUSTRIAL TECHNOLOGY EXTRA • p7