he high-pressure main steam valve is manually operated via a handwheel. Five high-pressure regulating valves and X extraction steam regulating valves are each driven by a hydraulic actuator through a lever mechanism.

A certain company in Chongqing utilizes a CB6MW extraction back-pressure steam turbine unit manufactured by Qingdao Steam Turbine Co., Ltd. Its configuration includes: one high-pressure main steam valve, multiple high-pressure regulating valves, and extraction regulating valves. The high-pressure main steam valve is manually operated via a handwheel. Five high-pressure regulating valves and X extraction steam regulating valves are each driven by a hydraulic actuator through a lever mechanism. The turbine's regulating system employs a pure hydraulic control system (comprising pressure transducers, differential oil valves, and feedback sleeves) based on the flow balancing principle. Since commissioning, the equipment has presented significant control challenges: high operational difficulty, substantial speed fluctuations (±15 rpm), difficulties in grid synchronization, and poor responsiveness to changing conditions (load response cycles of approximately 30–60 seconds). These issues are highly incompatible with production process requirements, causing considerable frustration for the operating personnel.

I. Analysis of System Speed Control Configuration Reveals the Following Shortcomings in the Original System:

● The rotational speed measurement component is a pulse pump, with the rotational speed measurement signal being pulse oil pressure. Pulse oil pressure is proportional to the square of rotational speed. At low rotational speeds, pulse oil pressure is negligible, making closed-loop rotational speed control technically challenging.

● In flow-balancing systems, a decrease in pulsed oil pressure Px triggers valve opening, while an increase causes valve closing. This means that if a rupture in the oil line or a gasket leak causes Px to drop, the valve moves toward greater opening—contrary to safety design principles.

● In crude oil engines, the low stiffness of mechanical springs results in significant valve response lag, failing to meet the requirements for rapid regulation:

First: The forces acting on the hydraulic motor's spool valve consist of spring force at the upper end and pulsating hydraulic pressure at the lower end. That is, one end experiences a constant force while the other end experiences a variable force. Consequently, when the pressure oil flow fluctuates (during rapid movement of the hydraulic motor), a parasitic feedback effect occurs. This parasitic feedback generates both positive and negative feedback effects, varying depending on whether the opening direction of the hydraulic motor aligns with that of the control valve. In essence, it is a factor contributing to hydraulic motor instability.

Second: Constrained by spatial dimensions and positioning, the spring stiffness cannot be made excessively high. Consequently, its capacity to overcome slide valve friction is limited, resulting in relatively significant friction-induced insensitivity.

Third: To minimize the impact of parasitic feedback caused by hydraulic pressure fluctuations, the secondary pulse pressure is set relatively low, resulting in reduced control sensitivity.

● The system exhibits load slippage in a certain load range, indicating poor output stiffness of the hydraulic motor.

● Extraction steam and power (backpressure) cannot achieve self-regulating control.

II. Technical Upgrade and Renovation Plan

Technical upgrades and modifications were implemented on the iterative product developed to address deficiencies in the original system and enhance new product R&D. This included adding a DEH electronic control system and and hydraulic speed control system upgrades. The original system's pressure transducers, throttle valves, feedback spool valves, and piston-type hydraulic motors—all transmission amplification mechanisms—were eliminated. The adoption of electro-hydraulic servo motors powered by an independent high-pressure oil source minimized the impact of transmission amplification coordination. An independent EH-dedicated oil station was added to reduce the original system's oil contamination effects:

(1) DEH Electronic Control System: Incorporates a dedicated DEH electronic control system.

(2) DEH Hydraulic Control System: Upgrades the high-pressure regulating valve motor and extraction motor to an electro-hydraulic servo motor system with an independent high-pressure oil source.

(1) DEH Electronic Control System

● Fully digital ESC design with built-in servo amplifier functionality, capable of directly driving common electro-hydraulic converters and integrating with various hydraulic equipment from original equipment manufacturers;

● Suitable for diverse power generation units including waste heat, waste-to-energy, biomass, and solar power plants, providing specialized solutions for isolated grid operation, thermal-electric interconnection regulation, and slip-pressure operation;

● Supports multiple communication protocols including MODBUS, DP, and Industrial Ethernet;

● Standard configuration includes a user-friendly human-machine interface and operator station.

(2) Electro-hydraulic servo hydraulic motor system;

● Highly adjustable performance enables comprehensive modification of the original unit's regulation characteristics, resolving numerous issues inherent in hydraulic systems such as load slippage and sticking. This ensures rapid responsiveness and precise control quality in regulation.

● Simple retrofit installation requires only removing the original hydraulic motor and regulation system components during on-site modification. The self-contained hydraulic motor is installed in the original motor's position and connected to the independent oil supply line, minimizing on-site work. High control precision: Utilizing a complete self-contained hydraulic motor system enhances output stiffness, ensuring stable operation of the DEH control system.

● Exceptional output stiffness delivers superior control accuracy and quality:

● Two servo hydraulic motors replace the original speed control and extraction hydraulic motors. The hydraulic motor system operates at pressures up to 14 MPa, completely eliminating constraints from the original unit's hydraulic system. Directly connecting valves via the steam distribution mechanism achieves exceptional control precision and stability. Full closed-loop positioning control delivers positioning accuracy of 0.01 mm, with dynamic response and closing speeds reaching 0.2 seconds—fully matching the control performance of high-pressure fire-resistant oil systems.

● The regulating system's oil supply is separate from the original turbine's oil supply system.

● The regulating system requires an electro-hydraulic servo system with high control precision, demanding extremely high oil cleanliness. An independent oil supply system, combined with multiple precision filters, fully ensures the required filtration accuracy.

(3) EH Dedicated Fueling Station

The oil supply system is configured as a modular unit. The primary advantage of this hydraulic motor system lies in its high working oil pressure of 14 MPa, which allows for a smaller servo hydraulic motor size while maintaining equivalent valve lifting force.

● Redundant Oil Supply Configuration

● The oil supply pumps are redundantly configured, with two sets serving as mutual backups to ensure reliable oil delivery. The two oil pumps operate in a one-active-one-standby configuration with online switching capability, featuring outputs such as low-pressure interlock and high-pressure alarm.

● Simple On-site Commissioning, Easy Fault Diagnosis and Resolution

● All hydraulic equipment undergoes factory testing and commissioning, ensuring that after on-site installation, only basic static adjustments are required before startup readiness. Multiple monitoring points are added to the hydraulic motors, enabling real-time local data monitoring for each unit and significantly simplifying fault troubleshooting.

● Reduced maintenance requirements

● Compared to low-pressure turbine oil systems, annual minor maintenance requires no disassembly of hydraulic motors. Only seals for hydraulic components in the pump station and hydraulic motors need replacement.

III. Operational Performance Following System Technology Upgrades and Modifications

Item	Value	Item	Value
Speed control range	Adjustable from 200 to 3600 r/min	Speed control accuracy	≤+1 r/min
Speed variation rate	Online adjustable within 3% to 6% range	Load control range	0 to 120%
Load control accuracy	≤+0.2% of rated value	Main steam pressure control accuracy	+0.1 MPa
Speed-up rate control accuracy	+0.1%	Control system insensitivity	<0.06%
Speed overshoot during load shedding	<7%, maintaining 3000 r/min	System availability	≥99.9%
Maximum climb rotation speed at full load	<8%h	Speed control cycle	<50ms

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