ORNL-4191 - the Molten Salt Energy Technologies Web Site

head and flow characteristics of the impeller-
diffuser design, (2) to measure radial hydraulic
forces acting on the impeller (needed for designing
the molten-salt bearing), (3) to measure and re-
duce to an acceptable level the axial forces acting
on the impeller, and to determine the relationship
between the axial clearance at the bottom end of
the impeller and the axial force, (4) to observe
the fluid behavior in the pump tank, and to make
the necessary changes to reduce gas entrainment
to an acceptable level, (5) to assure that the
mocked-up molten-sal; bearing will run submerged
under all operating conditions, and (6) to check
the point of cavitation inception and the required
net positive suction head of the impeller.
Molten-Salt Bearing Tests
The present layouts of the MSBE and MSRR
salt pumps require a molten-salt journal bearing
near the impeller. A molten-salt bearing presents
three important considerations: (1) the hydrodynamic
design of the bearing to provide the
requisite lubricating film, (2) the selection of the
kind and form of the bearing materials, and (3)
the design of a hearing mounting arrangement
which will preserve the lubricating film despite
thermal distortions between pump shaft and
casings.
We are studying the use of hard, wear-resistant
coatings on the journal and bearing surfaces.
Such coatings present advantages over the
sintered, solid-body journal and bearing inserts
most often used in high-temperature process fluid
lubrication. The hard coatings are convenient to
apply and hopefully eliminate the differential
thermal expansion problems. Mechanical Technology,
Inc., of Latham, New York, has been
engaged to produce Hastelloy N specimens with
each of four different hard coatings: (1) cobalt
(6 to 8%) bonded tungsten carbide, (2) nickel
(mj bonded tungsten carbide and mixed tungstenchromium
carbides, (3) nickel.-chromium (15%)
bonded chromium carbide, and (4) molybdenum
(7%) bonded tungsten carbide.
‘These coatings will be subjected to corrosion
and thermal cycling tests in molten salt at ORNL.
A test in molten salt will be made with a 3 x 3 in.
bearing using one of these coatings, if one should
prove satisfactory.
A layout is being made of a tester to accommodate
a full-scale molten--salt bearing for the
100
MSBE fuel salt pump. The tester will be capable
of subjecting the bearing and its mounting arrange-
ment to start-stop wear tests and thermal cycling
and endurance tests in molten salt.
Rotor- Dynami cs Feosi bi I ity investigation
Mechanical Technology, Inc., is performing an
analysis (Reactor Division subcontract No. 2942)
of the rotor dynamics of the preliminary layout of
the MSBR fuel salt pump to determine its flexural.
and torsional critical speeds and flexural response
to a dynamic unbalance. Interim results’ of the
analysis show that the pump will operate between
the fourth and fifth flexural critical speeds of the
pump system, which includes the pump shaft, inner
and outer pump casings, and the drive motor.
The third and fifth system criticals are essentially
the first and second simply supported beam criticals
of the shaft. The critical-speed results also
show that the pump-system criticals are relatively
independent of the hearing stiffness over a range
representative of practical bearing designs. The
stiffness characteristics of the drive motor
coupling also have little effect on system ciiticals.
The synchronous response amplitudes resultiiig
from a “bowed-shaft” unbalance condition have
been calculated over the complete range of pump
speeds, The response results show only one system
critical to be significant from a bearing load
standpoint ..- namely, the “first shaft critical”
which occurs at ahoiit 700 rpm.
In addition to passing through one shaft critical
speed, three additional system criticals must be
traversed as the pump accelerates to design speed.
These three criticals are basically cantilever
resonances of the outer casing. The first two
cantilever inodes occur at quite low speeds and
hence should not be a problem from a steady-state
standpoint. However, if the pump system should
be transiently excited during normal operation,
these cantilever beam modes would be the primary
contributors to the resulting transient vibration
response of the pump system.
The third cantilever beam mode of the outer
casing also excites a simply supported resonance
of the inner casing. This mod:: occurs between
‘P. W. Curwen, Rotor-Dynamic Feasibility Study of
Molten Salt Pumps for MSBR Power Plants, MTI-67TR48,
Mechanical Technology, Inc., August 6, 1067.