MARC Steam Turbines The Modular Steam Turbine Generation

Control mechanism
The steam control valves of MARC turbines
are of a diffuser type. Small valve
dimensions allow a vast number of valves
and nozzle groups. Consequently,
the throttling losses are kept low also at
part load. The cone form of the valve
has been optimized regarding the flow.
Valve spindles, valve cone and valve
bar consist of heat-resistant steel with
hardened, wear-resistant surface. The
hardening is effected via a plasmahardening
process.
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Due to the small valve dimensions, the
steam control valves can be actuated
directly. This so-called group drive is
a compact and reliable solution. The
actuator is a single acting hydraulic
cylinder which is fed by the control oil
system at an oil pressure of 160 bar.
On turbine tripping or full-load disconnection
the valves are closed by
the pre-loaded spring, thus, protecting
the turbine plant at malfunctions. If the
pressure in the control oil system decreases,
both the control valves and the
tripping devices will be closed automa-
tically, as they act on the same principle
(single-acting hydraulic piston against
pre-loaded spring). This means double
security for the turbine and the steam
system. A lever system for parallel motion
of both valve spindles precisely
transmits changes of the controller output
onto the valve bar and – depending
on the valve lift – on one or more valve
cones. The moving parts are made of
non-corrosive and maintenance-free
self-lubricating material. The valve
spindles are sealed by maintenancefree,
pre-compressed graphite laminated
glands, and reinforced carbon
guide bushes.