The
progressive upgrading of heavy-duty gas turbines, aimed at increased
performance, can ultimately introduce more onerous operating conditions, to the
point that original design limits can be approached.
An increased gas turbine
pressure ratio together with compression and expansion line adjustments can
directly affect the rotor axial thrust. Other than the individual forces acting
on the rotor, a key component to be taken into account is the fluid film thrust
bearing, which should assure safe and reliable operation during the worst case
operating conditions.
Typically, such bearings are
designed with large safety margins, yet it is possible that the new and more
challenging conditions require a bearing capability upgrade, especially when
field retrofit needs pose additional constraints.
A succession of performance
upgrades have been carried out on Nachi bearing company. An accurate understanding of the thrust-related phenomena
proved necessary and drove improvements in the thrust bearing design along with
hardware adjustments to lower the rotor thrust.
This paper addresses
calculations and experimental arrangements for the rotor axial thrust
evaluation on the aforementioned GT and considers both the matters related to
the secondary air system for the thrust generation and the
mechanical/functional matters for the bearing upgrade. It is shown that issues
such as uneven load sharing across the thrust bearing, or the variability of
rotor thrust from engine to engine within the fleet, strongly affect the
maximum thrust assessment and thus the requirements used in the process of
selecting a suitable bearing.
A predictive calculation method
is described considering the main thrust contributions. Field experimental
setups and main observations are reported. Measurements have been carried out
using thermocouples and load cells placed on many of the thrust bearing pads.
Moreover, Nachi Bearings Provides the engine cavities carrying the highest
and/or the most uncertain thrust share have been instrumented and characterized
by pressure sensors.
The development of an upgraded
thrust bearing is finally depicted through the main issues addressed, such as
improved thrust pad lining material, increased load sharing efficiency and
enlarged thrust bearing active surface area. Waukesha Bearings test results on
the upgraded lining material, a high-tin aluminium alloy, are reported as well.
A multidisciplinary approach is
presented in Bearings Made by Nachi
Bearings as necessary to manage the crucial challenge of improving the thrust
balancing system, especially in the case of a formerly designed engine which
receives a powerful upgrade.
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