Page 285 - Steam Turbines Design, Applications, and Rerating

264 Chapter Thirteen

the bearings, gears, and turbocoupling. Only minor dismantling and
reassembly is required for maintenance.

13.5.3 Working oil and lube oil circuits
In Fig. 13.19 the main oil pump (8) which is driven mechanically from
the gear input shaft draws the working fluid from the oil reservoir (1)
in the lower part of the housing and delivers it through the flow-control
valve (9) into the coupling’s working chamber (10). This chamber
formed by the primary and secondary wheels connects with the space
inside the coupling casing surrounding the secondary wheel. Because
of centrifugal force, the fluid inside the casing forms a ring (12) around
the periphery. The inside diameter of this ring is determined by the
radial position of the adjustable scoop tube (13).
The oil picked up by the scoop tube passes through a heat exchanger
(14) to the flow-control valve (9) and from there back to the working
chamber or the oil reservoir. The flow-control valve whose function
depends on the position of the scoop tube regulates the oil flow rate rel-
ative to the heat due to slip generated in the coupling. The excess oil
flows back to the oil reservoir. As a result the response time to a control
signal is very fast. On the signal increased speed the full flow of the oil
pump passes into the working chamber; on the signal reduce speed oil
flow to the working chamber is cut off.

13.5.4 Lubricating system
The lubricating system is separate from the working circuit although
the same oil is used for both. The main lubricating pump (3) is fitted
to the main working oil pump drive shaft. Lube oil is pumped from the
oil reservoir through an oil cooler (4) and a double filter (5) to the bear-
ings and the gears. A motor-driven auxiliary pump (2) is switched on
before start-up to ensure that all bearings are lubricated before the
turbocoupling runs. The auxiliary pump operates until the turbocou-
pling reaches its rated speed when the mechanically driven main lube
oil pump (3) takes over. The auxiliary pump is stopped automatically
on a signal from the instruments monitoring the lube oil circuit (6). If
the pressure in the lube oil circuit drops, e.g., if the drive unit is shut
down, the auxiliary pump is switched on again.
A double filter is fitted in the lubricating system to allow for contin-
uous operation. Only one of the two filters is in use at any time, and if
this becomes clogged a differential pressure gauge with alarm contacts
provides a visual or audible alarm to indicate the need to change fil-
ters. The dirty filter must then be shut down and cleaned.
The lubricating system of the geared variable-speed coupling can be
used if required for lubricating the driving and/or driven machine (7).