Wushihua II fertilizer plant is a large-scale ammonia production facility that uses natural gas as its primary raw material. The steam system of the plant is well-balanced, and superheated steam generated by the first-stage reformer (pressure 12.5 MPa, temperature 515°C) is used to drive a steam turbine. During normal operation, excess steam is directed to other medium-pressure systems, requiring temperature reduction and pressure drop. The steam is cooled down to 355°C and reduced 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 insufficient or during operation. Any leakage would directly impact the efficiency of the steam-driven turbine.
The original valves were designed and manufactured by ABB in Italy, starting from April 7, 1997. However, numerous issues arose with the initial design, making the original temperature and pressure relief valve (one) non-reusable. By the end of 1998, the Swedish company BTG was chosen to supply the temperature and pressure regulator. After an overhaul in September 1999, the new valves were installed online and have been operating smoothly for over 400 days without any accidents, leaks, or energy loss. This has significantly contributed 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, pressure drop of 8.7 MPa; inlet temperature of 515°C, outlet temperature of 350°C, and saturation temperatures of 328.21°C and 248.83°C respectively. The inlet size is DN 150mm, and the outlet is DN 100mm. These valves feature a linear characteristic, with a spray water inlet at 290°C and 15.1 MPa, and a diameter of DN 50mm. They are single-seat, balanced sleeve valves with a spray system.
The single-seat sleeve valve relies on a flexible graphite guide seal ring. Internal leakage typically occurs 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 problem reoccurred within a month. Another replacement was made during a shutdown, but again, internal leakage returned within a short period. In early 1998, packing leakage occurred again, requiring pressure and plugging measures. Due to long-term gelling in the stuffing box, the valve stem failed, leading to complete loss of control.
During the 1998 overhaul, a double-cylinder support and instrumentation interlock control system were introduced, and the valve was put back into service in September 1999. To ensure smooth operation of the two chemical processes, the control valve was replaced with one from BTG, effectively solving the previous issues.
The causes of internal leakage included: â‘ poor material selection andä¸åˆç† matching of the valve, leading to corrosion and wear. â‘¡ the stem design was too thin, resulting in low bending strength and frequent jamming. â‘¢ the stuffing box design was inadequate, especially under high-temperature and high-pressure conditions. ABB’s stuffing box was only 50–80 mm deep, compared to the standard 100 mm, which could not withstand extreme conditions. â‘£ the actuator design had multiple flaws, including excessive height (up to 2.2 m), leading to instability and vibration. ⑤ the joint between the valve stem and packing was weak, with insufficient coupling strength and poor heat treatment, causing slippage and misoperation.
BTG’s temperature and pressure regulator model VST-SE offers high reliability, wide load adjustment range (from 2% to 100%), precise temperature and pressure control, long service life, and minimal maintenance. Its design features a forged steel body, smooth internal and external curves to reduce thermal stress, and a pressurized self-sealing bonnet for easy removal. Multi-step decompression via cage-style spools and multi-layer overflow vessels ensures controlled pressure reduction, minimizing vibration and noise. The stem is installed with an interference fit, preventing loosening or falling off. The flexible seat ensures long-term leak-free operation, and patented steam atomization technology enhances performance.
BTG valves also feature a long valve seat with reliable guidance, high concentricity, and an elastic valve seat that compensates for rapid temperature changes. The valve core is made of stainless steel with nitriding treatment, and the stem and packing are made of expanded graphite for reliability and ease of disassembly. The hollow steel structure of the connecting bracket helps reduce vibration and heat, improving stability and reducing stem bending and packing damage.
Compared to conventional designs, BTG valves offer superior concentricity, stability, and sealing performance, significantly extending their service life. According to usage at Daqing Fertilizer Plant, BTG valves can last 8–12 years before overhaul. To prevent bolt rust due to prolonged maintenance, bolts should be coated with molybdenum disulfide every 3–5 years for easier future maintenance.
Since installation in September 1999, HV-112 has operated smoothly for over 400 days without affecting normal production. It has reduced routine maintenance and ensured continuous, stable operation of the plant.
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