Page 90 - Improving Machinery Reliability

62 Irnpioving Machinery Reliability

adjustment and speed variation of the order of 10%-15% not only improves the part-
load efficiency of the compressor but increases the stable operating range.9 Appre-
ciable differences in energy consumption may result and savings of hundreds of
horsepower realized if the optimum control mode is selected for axial equipment in
the higher horsepower categories.
On-stream torque-measuring devices will also prove effective in determining
whether increased energy input to the driver is caused by performance decay of the
driver or the driven equipment. How the issue can be resolved by torque meters is
best illustrated by the simple case of a gas turbine driving a centrifugal compressor.
If high driver fuel consumption and high coupling horsepower are noted, the driven
machine could be more highly loaded, mechanically deficient, or internally fouled.
Diagnostic instrumentation or analytical procedures are available to determine which
of these three possible causes is most probable. If high driver fuel consumption and
normal coupling horsepower are noted, the most probable cause of the efficiency
decay would be turbine fouling.

Hot-Alignment Monitoring Systems. Excessive misalignment between driver and
driven machine can result in excessive vibration, high bearing loading, shaft break-
age, and coupling failure. Proper alignment of turbomachinery requires knowledge
of thermal growth in the radial direction of both machine casings. If diaphragm cou-
plings are involved, changes in the axial position of the two machine shafts must be
known as well.
One of the latest and potentially most advantageous methods of on-stream moni-
toring involves laser optics. The system shown in Figure 2-6 consists of a semicon-
ductor laser (transducer) emitting a beam in the infrared range (wave-length 820
mm), a reflecting prism system, and a display system.'O The laser beam is refracted
through a prism and is caught by a receiveddetector. It is an intrinsically safe long-
term monitoring system for continuous observation of alignment.
The system allows monitoring of critical machinery during operation so that
changes such as foundation movements may be detected and dealt with in time.
It also facilitates measurement of changes in alignment due to thermal growth.
This information may then be used to align the machines in the cold state with exact-
ly the correct amount of compensating misalignment so that perfect alignment is
reached when the machine warms up during operation. These measured values also
allow users to compare and correct calculated growth figures for their machinery.
Each system permits two distinct alignment parameters to be observed at once.
This means that two systems are mounted at 90" to one another to form a complete
monitoring facility.
The system indicates information via a single display unit. Two distinct alignment
parameters or other information, such as temperature, may be obtained from the dis-
play, whose display mode is switched by simply interrupting the laser beam (e.g.,
using one's hand). This display allows use of the monitoring system for general mea-
surement tasks where minute displacements must be measured (e.g., civil engineer-
ing projects). (For more alignment information, see pages 463-477.)