Page 82 - Improving Machinery Reliability

54 hiproving Machinery Reliability

Extraction control-single and double extraction
Impellers-structural design and performance
Material selection
Nozzle design
Number of stages and staging arrangement
Power-transmission components (design and arrangement)
Rotor dynamics
Sealing system

In summary, major machinery vendors should excel through past satisfactory
experience in design, execution, and after-sale service of their product. To stand the
most critical examination, a vendor should show cooperation with the purchaser’s
review engineer. The joint efforts of both parties are needed to demonstrate and veri-
fy the vendor’s competence and to lead to reliable equipment selection.

Applying and Reviewing Machinery Reliability
Improvements Derived from Modern Electronics

Although it is beyond the scope of this text to deal extensively with electronic
auxiliaries, the topic of machinery reliability improvements cannot be adequately
covered unless the importance of modern electronics is highlighted and review tech-
niques are mentioned.

Compressor Surge Protection and Control

Improved efficiency and reliability of dynamic compression equipment will result
if a soundly engineered electronic surge-control system is implemented. The benefits
resulting from modern electronic-hardware-based surge-control systems are summa-
rized in two well-written technical papers and listed below: ’,*

Less damage to clearances, seals, bearings, etc., which will help maintain the origi-
nal design efficiency of the machine.
Reduced recirculation or blow-off when compressors operate under partial loads.
More stable pressure or flow control at partial loads.
A more efficient machine design becomes possible, Le., the machine design may
be chosen for maximum energy efficiency rather than for surge safety, as long as
effective surge control is assured.

The strategy and instrumentation for surge protection have seen very significant
improvements since the late 1970s. Realization that surge constitutes very high
speed flow oscillations or even flow reversals makes it imperative to discontinue the
use of slow-speed transmitters and recorders. Modern plants should, therefore, uti-
lize high-speed transmitters and antisurge controllers. Fast-action antisurge valves
must be chosen to provide full-stroke opening in 1.5 seconds or less. These valves
should be sized for full design flow and consideration be given to using two or more