Wushihua II fertilizer plant is a large-scale ammonia production facility that uses natural gas as its primary raw material. The plant's steam system is well-balanced, and the superheated steam generated by the first-stage reformer (12.5 MPa, 515°C) is utilized to drive a steam turbine. Under normal operating conditions, excess steam is redirected for use in downstream sections and other medium-pressure equipment, requiring temperature reduction and pressure drop. This process reduces the steam temperature to 355°C and the pressure to 3.8 MPa. The valves HV-112 and PV-172A (identical in type and parameters) must be fully closed and leak-free when steam is not available or during operation. Any leakage could directly impact the efficiency of the steam-driven turbine.
The original design of these valves was manufactured by ABB from Italy, starting from April 7, 1997. However, the initial design had multiple issues, making the original temperature and pressure relief valve (one) non-reusable. By the end of 1998, the plant opted for a temperature and pressure regulator produced by Swedish company BTG. It was installed online during a major overhaul in September 1999 and has operated smoothly for over 400 days since then. The system has achieved zero accidents, no leaks, and energy savings, significantly contributing to the safe and stable operation of the fertilizer plant.
PV-172A and HV-112 are temperature and pressure control valves with the following parameters: normal flow rate of 100,000 kg/h, inlet pressure of 12.5 MPa, outlet pressure of 3.9 MPa, a pressure drop of 8.7 MPa; inlet temperature of 515°C, outlet temperature of 350°C, with saturation temperatures at 328.21°C and 248.83°C respectively. Inlet size is DN 150mm, outlet is DN 100mm. The valves feature linear characteristics, with a spray water inlet at 290°C and 15.1 MPa, with a diameter of DN 50mm. Both valves are single-seat, balanced sleeve valves with a spray system.
Single-seat sleeve valves require flexible graphite guide seals. Internal leakage can occur due to two main reasons: damage to the sealing surface between the seat and spool, or damage to the guide ring on the spool. In August 1997, PV-172A experienced severe internal leakage. The Italian representative replaced the sealing ring for free, but the leakage returned within a month. After another replacement during a shutdown, the same issue occurred again within a short time. In early 1998, packing leakage occurred again, leading to pressure-sealing measures. Due to prolonged exposure to high temperatures, the packing became gelled and hardened, causing stem failure and loss of control over the valve.
During the 1998 overhaul, the plant used a double-cylinder support and instrumentation interlock control system, which was implemented in September 1999. To ensure smooth operation of the two chemical fertilizer units, the control valves were replaced with those from BTG, effectively solving the long-standing issues.
Causes of internal leakage included:
1. Poor material selection and mismatched valve design, leading to corrosion and flushing.
2. A thin stem design with low bending strength, causing jamming and bending.
3. An unreasonable stuffing box design, especially for high-temperature and high-pressure valves. ABB’s stuffing box was only 50–80mm deep, insufficient for the harsh environment.
4. Issues with the actuator design, including excessive height (2.2m), leading to vibration and potential damage to the stem and packing.
5. Weak joint between the valve stem and the coupling, with inadequate heat treatment and frequent slippage, resulting in valve failure.
BTG’s VST-SE temperature and pressure regulator offers significant advantages. It features high reliability, a wide load adjustment range (2% to 100%), precise temperature and pressure control, long service life, minimal maintenance, and low noise and vibration. The valve body is forged steel, with smooth curves to reduce thermal stress. The bonnet uses pressurized self-sealing for easy removal and leak-proof performance. Multi-step decompression via cage-style spools and multi-layer overflow vessels ensures controlled pressure reduction, minimizing vibration and noise. The stem and valve stem are installed with interference fit, preventing loosening. Flexible seats ensure long-term leakage-free operation, while patented steam atomization technology enhances performance.
BTG’s valves also have a long valve seat with reliable guidance, high concentricity, and an elastic seat design that compensates for rapid temperature changes. The valve core is made of stainless steel with nitriding treatment, and the stem is made of expanded graphite for durability and ease of disassembly. The hollow steel structure of the bracket helps reduce vibration and heat, improving stability and reducing stem bending. The valve cover design enhances coaxiality and stability, resulting in better performance compared to standard valves.
Since being installed in September 1999, the HV-112 valve has operated without affecting normal production for over 400 days, reducing maintenance needs and ensuring smooth operation. According to usage at Daqing Fertilizer Plant, BTG valves can last 8–12 years before overhaul. To prevent bolt rust, bolts should be coated with molybdenum disulfide every 3–5 years to facilitate future maintenance.
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