Three screw pumps are com- monly used as lube oil pumps for compressors, gearboxes,
turbines and other rotating equipment.
Their popularity stems from the efficient, reliable and long-lasting service
While most lube oil pumps are only
exposed to ambient pressure at the
pump inlet port, there are some applications where the pump sees high inlet
pressures. This poses a huge challenge
for the pump seal solution and pump
construction. In many pump designs it
can also lead to high bearing and seal
loads that negatively affect the pump
and seal life.
High inlet pressure applications
for lube oil pumps can be found in
flooded screw compressors and tur-
bo expanders. The pressure of the
lube oil entering the pump is at the
same level as the compressed gas,
which can reach 1450 psi (100 bar)
Such high inlet pressures can create high axial forces on the main
pump drive screw (power rotor) that
lead to high bearing loads.
This needs to be considered in the
pump design. Reinforced bearings,
liquid channels for hydraulic balance,
mechanical bracing and other measures tend to significantly increase
the pump cost.
Since the lube oil pump is used to
boost the system pressure of the lube
oil by a few bar (around 72. 5 to 217.5
psi [ 5 to 15 bar]) the outlet pressure is
not significantly higher.
Three screw pumps are often driven
by an electric motor, which is con-
nected to the shaft of the main screw.
The shaft exits through the pump cas-
ing. Therefore, the main screw must be
sealed to prevent leakage. The most
common sealing solution is a mechani-
cal seal. Due to the high inlet pressure,
only special sealing arrangements, of-
ten with balanced mechanical seals,
can achieve this task. The pump cost
continues to soar.
There is, however, a new approach
to high inlet pressure applications that
is able to meet the technical challenges
and even provides lower cost solutions
— magnetically coupled lube oil pumps.
Compared to a mechanically sealed
pump, a magnetically coupled pump
has no rotating seal. The pump is
hermetically sealed and leak-free. No
seal maintenance or seal change is
required, leading to a much more reliable and durable pump. The magnetic
coupling does not wear and therefore
requires no maintenance under normal
circumstances. This creates a more reliable pump solution at a lower cost.
How does a magnetic
The outer rotor — a magnetic assembly containing permanent magnets — is attached to the motor shaft.
The pump housing and drive screw
are hermetically sealed by a containment can (canister). The containment
can is made from highest quality materials (hastelloy, titanium and others) to
minimize eddy current losses, thereby
assuring high efficiency. It is also able
to withstand high pressure. The inner
rotor is attached to the pump shaft
and contains strong and durable earth
magnets. The motor torque is transferred through the containment can
Pulling For A New
Approach > Magnetic coupling meets challenging high inlet pressure lube oil pump applications
BY THOMAS FLAUGER
Thomas Flauger is marketing director at
KRAL AG in Lustenau, Austria. Contact
him at: firstname.lastname@example.org.
n The magnetic coupling replaces
the mechanical seal.
continued on page 48