WOODWARD 8918-118

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The WOODWARD 8918-118 is not a standalone actuator. Instead, it is a specialized and complete Dual Redundant Skid for Current-to-Pressure Conversion, specifically for 10 Bar systems in Zone 2 hazardous locations.

WOODWARD 8918-118: CPC-DX Dual Redundant Skid (10 bar, Zone 2)

The WOODWARD 8918-118 is a pre-assembled, highly reliable system designed for critical industrial applications, particularly in steam turbine control. Its primary function is to provide a fault-tolerant interface that accurately converts electrical control signals into precise hydraulic pressure to operate turbine valves.

Product Overview

This part number signifies a packaged solution that integrates two Woodward CPC-II Current-to-Pressure Converter units onto a single manifold, configured for continuous, bumpless operation even in the event of a single component failure.

• Product Type: Dual Redundant Skid System for Current-to-Pressure Conversion
• Manufacturer: Woodward
• Part Number: 8918-118
• Model Designation: CPC-DX (Dual Transfer Skid)
• Components: Typically includes two CPC-II Current-to-Pressure Converters mounted on a common manifold that incorporates an automatic transfer valve.
• Function:
  • Accepts a 4–20 mA pressure demand signal (and typically redundant input signals) from an electronic control system (e.g., a turbine controller or Distributed Control System – DCS).
  • Precisely regulates hydraulic oil pressure to control single-acting steam turbine governor valves.
  • Ensures redundant operation: if one CPC-II unit experiences a fault, the other unit automatically and seamlessly takes over control, maintaining continuous and stable turbine operation.
• Output Pressure Range: 0 to 10 bar (0 to 145 psi). This is a key distinguishing feature of this specific model.
• Hazardous Area Rating: Certified for Zone 2 hazardous locations, meaning it meets safety standards for environments where flammable gases, vapors, or mists may occasionally be present.

Detailed Product Description

The Woodward 8918-118 CPC-DX Dual Redundant Skid is engineered for maximum reliability and uptime in critical power generation and industrial processes. Its key features and benefits include:

• Dual Redundancy (Master/Slave Configuration): The skid houses two CPC-II units. One unit typically operates as the “master,” actively controlling the output pressure, while the second unit acts as a “hot-standby slave.” The slave continuously monitors and tracks the master’s operation. Should the master unit fail, the transfer valve automatically and “bumplessly” (without interruption) switches control to the slave unit, ensuring uninterrupted operation of the turbine.
• Precision Pressure Control: Each CPC-II unit within the skid features a highly accurate internal pressure sensor and a sophisticated PID (Proportional-Integral-Derivative) controller. This ensures precise and stable control over the hydraulic output pressure, which directly translates to accurate positioning of the turbine valve.
• “Dirt Buster” Technology: The integrated CPC-II units incorporate Woodward’s patented “Dirt Buster” or “Silt Buster” technology. This robust design, featuring a strong actuator, corrosion-resistant materials, a unique single-moving rotary valve, and a self-cleaning port design, makes the unit highly resistant to contaminated oil (capable of handling up to ISO20/16 cleanliness). This is crucial for long-term reliability in industrial hydraulic systems.
• Enhanced Dynamics: The CPC-II units are designed for rapid and stable response to changes in control signals, which is vital for maintaining effective turbine speed and load control, especially during load changes or grid disturbances.
• Fail-Safe Mechanism: Each CPC-II unit has an internal valve-return-spring that forces it to a failsafe position (typically valve closure, or minimum output pressure) in the event of an internal unit failure, loss of electrical input power, pressure sensor failure, or processor failure. The dual redundant setup further enhances this safety aspect by having a backup unit immediately available.
• Comprehensive Diagnostics: The system provides extensive diagnostics, including status indications for pressure sensor health, input demand signal health, internal unit health, and input power quality. This allows operators to monitor the system’s operational status effectively and proactively identify potential issues.
• Simplified Installation and Maintenance: As a pre-engineered and factory-tested skid, it simplifies installation compared to integrating individual components. Woodward offers specific repair kits for CPC replacement within the skid (e.g., Kit – CPC Replacement, 10 Bar, Zone 2, for CPC-DX Skid: 8923-1996).

Technical Specifications (for the 8918-118 Skid)

Typical Applications

The WOODWARD 8918-118 CPC-DX Dual Redundant Skid is primarily utilized in critical heavy industrial applications where continuous and highly reliable operation of steam turbines is paramount:

• Steam Turbine Valve Control: Its core application involves accurately and redundantly positioning the control valves of single-acting steam turbines, which directly regulates steam flow and, consequently, impacts turbine speed and power generation.
• Power Generation Plants: Integrated into the most critical control loops of large steam power plants, often in situations demanding the highest level of reliability and fault tolerance to prevent costly shutdowns.
• Combined Heat and Power (CHP) Plants: Where continuous steam and power output are essential for process integrity and economic operation.
• Petroleum and Chemical Industries: Utilized in processes that rely on steam turbines for power or compression, where unexpected shutdowns can lead to significant production losses and safety concerns.