1.5.3 Governor valve relay and control unit Type 3
Type 3 relays are similar to Type 2 but employ a different form of primary ram and servo-valve, known as a 'positioner'. The primary ram displacement is directly proportional to the input current to the servo-valve. The servo-valve employs a similar principle and configuration to that described in Fig 2.18, but feedback of ram position is introduced magnetically onto the force motor and subtracts from the armature deflection produced by the current coils.
The primary ram is used to drive the main ram via a feedback linkage and pilot valve as in the Type 2 relay, but there is also feedback of main ram position to the servo-valve amplifier by means of an LVDT. This ensures accurate fine control over the full range of valve openings.
The Type 3 relay is now superseding the Type 2 relay on new plant and some retrofit applications, with the advantage that it has reduced dependency on the LVDT signal, being able to operate without it. Additionally, the servo-valve has generally larger clearances than the Type 2 and is less susceptible to the effects of wear and contamination.
Types 1 and 2 relays have seen many thousands of hours service on large turbine-generators and have been developed to provide a high degree of reliability and freedom from problems. Nevertheless it is essential that all types of valve relay, and indeed all hydraulic components, are treated with care and that maintenance is carried out in a clean environment by experienced personnel. If plant is shut down or awaiting commissioning for more than three months, then fluid should be circulated round the system periodically to maintain a protective coating on components and prevent corrosion problems. Components kept in store must also be kept free from dirt and corrosion. The manufacturer will be able to provide further information to cover specific cases.