3.5 Mechanically-operated trips
Included under this heading are any trips which may be implemented by means of a direct mechanical or hydraulic action onto the latching trip lever.
The low lubricating-oil pressure trip is sometimes implemented in this manner (Fig 2.45). Lubricating oil under pressure is supplied to a single-acting spring-loaded trip cylinder. The oil is supplied via a deadweight accumulator common to front and rear channels. This smooths out fluctuations in oil pressure and, in the event of a sudden loss of oil pressure, will maintain the pressure to the trip cylinder for several seconds, thus allowing possible recovery by starting of the emergency pumps.
Similarly, a direct high exhaust-pressure trip is often implemented using the same spring-loaded trip cylinder as shown in the upper part of Fig 2.45. For the front tripping system, there are two sets of detectors labelled 1 and 2. Lubricating oil is supplied to the detectors via an orifice plate. The downstream pressure is controlled by a leak-off valve, actuated by a bellows unit. The top half of the bellows is evacuated and sealed at a reference pressure close to zero mbar absolute. The lower half of the bellows is connected to the condenser. The leak-off valve is normally fully seated, since the force supplied by spring A exceeds that exerted by the condenser pressure. As the condenser pressure rises, a point is reached where the spring force is counter-balanced and a further pressure increase causes the leak-off valve to unseat. As lubricating oil pressure is lost within the vacuum trip unit, plunger В is moved downwards by spring force and the pressure in the short length of pipe С is lost. If detector 2 also trips, lubricating oil supplying the front trip cylinder is connected to drain, thus releasing the trip latch. A similar pair of exhaust pressure detectors operate into the trip cylinder of the rear system. On-load testing is enabled by a mechanical interlock on the trip test lever, so that when the front protection system is isolated, its two associated exhaust pressure detectors can be tested in turn by slowly raising the pressure by a needle valve admitting atmospheric air. This permits resetting of the trip point if desired.