Page 83 - Improving Machinery Reliability

Vendor Selection and Bid Conditioning 55

small valves in parallel where “false economy” would have perhaps dictated
installing one large valve alone.

Selection of Speed Governors and Controls
Mechanical drive turbines, Le., steam turbine drivers for large process plant com-
pressors, have traditionally employed hydraulic speed-governing devices. The inven-
tion of the transistor made it possible to design electronic speed-control devices even
more precise than the best hydraulic governors available. While this feature may be
highly attractive for parallel operation of synchronous generators, it is of no great
importance to most process plant compressors whose speed stability requirements
are far less stringent and have generally been met by conventional, high-precision
(NEMA-D) hydraulic governors.
The main reason for considering electronic governors seems to have been the
desire to get away from failure and downtime due to contamination of hydraulic
governor oil and/or governor drive failures. Hydraulic governors contain rotating
(mechanical) parts that are driven via worm-gear engagement by the turbine rotor.
These drives, which are commonly furnished by the turbine manufacturer, have not
always proven totally reliable. The question is now whether it is wiser to stay with a
perfectly satisfactory and reasonably simple NEMA-D hydraulic governor and to
focus attention on reliability/design improvements for the governor drive arrange-
ment, or get into the electronic governor business and hope for component reliability
to meet all your expectations.
Again, the decision should hinge on a full review of the vendor’s experience. Reli-
ability analyses could be performed by third parties with experience in the proba-
bilistic assessment of electronic component failures and redundancy design of elec-
tronic hardware. The failure or drifting of components could be detected by
electronic self-checking features and annunciated well before a machine shut-down
is imminent. Also, mechanical or hydromechanical redundancy features can be built
into a governing system.

Digital Control Systems Represent State-of-Art*

Control systems offered for steam turbines driving process compressors now pro-
vide many more functions beyond speed control. These systems can be custom pro-
grammed to provide whatever extent of control desired by the end user of the
machinery. Integrating the turbine and compressor control functions provides many
benefits to the end user, including a more responsive and capable control system,
increased reliability through redundancy, and a custom designed system capable of
stabilizing the process during upset conditions. In addition, an integrated system
reduces the number of components required for maintenance, training, digital control
systems’ comimunication, etc., resulting in economic savings. Among the functions
offered in an integrated turbine and compressor control system are the following:
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*Contributed by Gary Bostick, Woodward Govet nor Company, Dallas, Texas, sales office