Page 443 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Steam Turbines and Turboexpanders 421

Shutdown

The protective system for the expander includes alarm points for most
or all shutdown conditions. This is intended to provide time for the oper-
ator to correct a problem before it reaches emergency proportions and
the unit goes into shutdown condition. Alarm and shutdown signals are
normally provided for overspeed, high bearing temperature, vibration,
excessive bearing thrust load, low seal gas pressure, and low bearing oil
pressure. (The alarm on the latter also starts the auxiliary oil pump and
locks the circuitry so that restoration of oil pressure will not stop the aux-
iliary pump, which could result in cycling.)
If a condition reaches emergency proportions, the unit is shut down by
the automatic closing of the inlet trip valve, allowing the machine to
coast to a stop. Alarm and shutdown signals may also occur from other
sensors or plant protection circuits sensing low inlet gas temperatures,
high liquid level, low process flow, etc. If any one of these emergency
conditions is not critical then the shutdown circuitry may utilize a built-in
time-delay relay to prevent unnecessary shutdown.

Special Designs

In the past, uncontrolled problems often caused extreme wear and pre-
mature failure of mechanical parts in turboexpanders. With the feedback
from hundreds of units operating in the field for decades, continued re-
search and development has resulted in solutions to practically all operat-
ing problems. Improved, field-tested designs now offer increased relia-
bility and ease of maintenance to reduce costly downtime and product
loss.
One key problem area was overcome with the development of the dust-
free seal design (Figure 8-23). which has successfully eliminated erosion
of the seal behind the expander rotor. Previously, this was a frequent
problem during the startup period. Fine particles in the process stream
are now quickly removed from the seal and wheel labyrinth and directed
out through the balance holes in the rotor to the discharge gas stream.
The development of the automatic bearing thrust load control system
(Figure 8-24) solved another problem. Previously, machines were de-
signed for an equalized thrust load, but had no provision for thrust load
monitoring or load correction. A recent development in automatic thrust
load monitoring and control uses the thrust bearing oil film pressure to
indicate the actual thrust load on the meters, and to activate a control
piston which operates a control valve, maintaining a proper balance of
thrust forces. The control valve causes the gas pressure behind the com-
pressor impeller or a thrust balancing drum to increase or decrease as