Page 84 - Improving Machinery Reliability

56 Improving Machinery Reliability

Speed control
Extraction steam pressure/flow control
Inlet/backpressure control
Electronic overspeed trip
Start-up sequencing (auto roll up)
Critical speed avoidance
Compressor anti-surge protection/control
Compressor load/capacity control
Compressor inlet temperature control (quench)
Drum level control
Lube oil monitoring
Compressor seal monitoring
Alarm and trip functions
Compressor valve sequencing
Emergency shutdown logic (ESD)
Digital technology also provides a more complete picture of the turbine and com-
pressor performance to the engineers and operators through the use of PC computers
and graphic display programs. Serial and Ethernet communications transmit and
receive data from external devices that can perform logging and trending of data,
event logging, sequence of event recording, and pictorial representations of the oper-
ating data specific to the turbinekompressor train and process. Even the compressor
performance map can be displayed to the operator, along with the on-line perfor-
mance data that pinpoint the operating point on the map. In this way, the operator lit-
erally has a picture of where the compressor is operating at all times.
Modern turbomachinery control systems integrate the control functions of the tur-
bine, the compressor, and the process in order to make the machinery perform in
such a way that it becomes an integral part of the overall process while at the same
time providing the necessary protection for the equipment. Many plants today are
required to operate from five to eight years between scheduled shutdowns, further
justifying the need for reliable controls that can best interface with plant operations.
Fault-tolerant, digital systems designed for high-speed control (Figure 2- 1) are the
logical choice for true state-of-the-art plants.

On-Stream Torque-Sensing and Hot-Alignment Monitoring Devices

Operating Principles. Most continuous torque-sensing devices for turbomachinery
are based on the torsional windup of a shaft operating under load. This torsional
windup results in angular deflection and strain, and both effects can be captured by
sensors that are customarily built into or around the coupling spacer tube.3
However, several competing approaches can be used to sense deflection and/or
spacer strain. One torque sensor consists of a four-arm strain gauge bridge laid to
detect torsional strain. As a twisting load is applied to the shaft, a signal is created by
the unbalancing of the strain gauge bridge. This signal is presented as voltage input
to a rotary module located in a rotating collar, and is used to modulate a constant-