Actuators and Hydraulic Power Control Units are designed to provide the valve with the desired action. Two main type of actuator can be defined, according to the type of action they carry out:
- linear actuator, designed to provide a linear movements to valve disc;
- rotary actuator, designed to provide the valve disc with a rotating action.
The necessary power to perform the required stroke is given by a dedicated Hydraulic Power Unit, carefully designed for a safe and reliable service.
Power generated by the Hydraulic Power Control Unit is applied to the valve disc by a mechanical drive section device, which drives the valve disc, allowing the valve functionality. It holds both hydraulic and mechanical functionality and can be considered as a multi-type redundant device.
During normal operation, the mechanical drive section works in hydraulic mode. The hydraulic piston receives high pressure oil from the Hydraulic Power Control Unit and starts moving. This movement makes the disc move too and allows the valve operation.
Redundancy is available in mechanical mode; the actuator can be switched to manual operation and then operated by the hand-wheel.
The actuator can be driven in mechanical mode also by an air motor (optional), which is installed on the actuator main frame and allows a faster operation, keeping the same reliability of the whole mechanical drive section.
Hydraulic cylinder features
- Cylinder design: Heavy Duty - ISO 6020/1 with cushion in full open and full close position.
- Cylinder rod : overlaid by Nichrom
- Cylinder rod seals: Viton®
- Hydraulic cylinder is provided with hydraulic cushions in both opening and closing directions. Cushions can be adjusted according to the necessity, in order to have always an high performance brake effect.
The position transmitter is a redundant device to provide both the local panel and the DCS with a reliable information. The selected primary transmitter provides the position control signal to be used by the local control system in a position control loop (P-amplifier/controller). In case of failure of the primary transmitter, the local control system automatically switches to the secondary one, still keeping the system in operation. A switch to select either one of the two position transmitter is provided.
In Hydraulic mode the double effect cylinder receives high pressurized oil from a port (C1 or C2), while the other port allows the oil discharge from the other side of cylinder. The differential pressure, between the two hydraulic cylinder cameras, generates the thrust required to move a valve disc.
In order to work in Mechanical mode every hydraulic function needs to be disabled; it is also necessary to manually engage the mechanical mode in order to avoid any possibilities of hydraulic and mechanical contemporary use. The system engagement is made by rotating the engaging lever. Once rotated, two bronze split nuts clamp an ISO screwed bar and the clamp lock assembly starts moving according to the screw rotation.
Once engaged the mechanical drive, the field operator can manouver the actuator by a rotation of the actuator handwheel.
The actuator can be also driven by the Air Motor (optional) , which allows a faster operation.
The worm gear-motor run in lubricant medium and are sealed by Viton® type material seals.
The mechanical drive section case is designed to withstand two main actions:
- its own weight;
- the actuator thrust reaction.
These actions bend and compress the case, so that it is designed as a cantilever structure, verified also against the buckling. For an additional degree of safety, the case has been checked with a finite element analysis, performed with NE-Nastran solver, pre and post-processed by Femap.
International and local codes compliance available upon request.
