**Abstract:** This paper presents an analysis of the leakage issues encountered in the outlet check valve of a high-pressure liquid ammonia pump (P101A/B) used in a urea plant. It discusses the root causes of the failure and outlines the repair procedures implemented to restore the valve’s functionality and ensure safe operation.
**Keywords:** Liquid ammonia check valve, leakage, maintenance, repair, high-pressure pump
**I. Overview**
The high-pressure liquid ammonia pump P101A/B is a critical component in the urea production process. Manufactured by the American SUNDSTRAND company in the 1980s, it is a HMP-5112 high-speed centrifugal pump designed for continuous operation. The pump operates at a discharge pressure of 27 MPa with a shaft speed of 20,445 RPM. Its outlet check valve is a single-disc swing-type, self-sealing high-pressure valve that prevents backflow of the medium when the pump stops, thereby protecting the impeller from reverse rotation. After over a decade of service, two valves experienced significant leakage, which posed a serious risk to the safety and efficiency of the entire urea plant. Prompt disassembly and repair were necessary to avoid potential operational failures. This paper focuses on the repair of one such valve, detailing the challenges faced and the solutions applied.
**II. Maintenance Issues**
(1) Due to prolonged operation, the valve disc became worn, causing the valve stem to wear into an oval shape. This led to improper seating and frequent damage to the sealing surface, resulting in severe leakage, as illustrated in Figure 1.
(2) Additionally, manufacturing defects and corrosion from the liquid ammonia media caused cracks on the outer surface of the valve body, further contributing to leakage, as shown in Figure 2.
**III. Overhaul Plan**
Since the valve is a welded unit and cannot be removed for offline repairs, on-site restoration was required. Two main aspects were addressed: external leakage and internal leakage.
1. **External Leak Repair Procedure**
(1) The damaged areas on the valve body were manually ground using a small electric grinder, and visual inspection was conducted until all cracks and imperfections were removed.
(2) A402 electrodes (chromium-nickel stainless steel) were used for hot spot welding on the sealing surface.
(3) The weld area was then manually ground to match the original surface level.
(4) A custom grinding ring was fabricated, as shown in Figure 3.
(5) The inner diameter of the sealing surface was ground to 160.7 mm using the custom ring.
(6) A new sealing ring was manufactured based on the final dimensions, as displayed in Figure 4.
2. **Internal Leak Repair Procedure**
(1) The seat sealing surface was ground using a portable grinder to restore its flatness and ensure a circular shape with cemented carbide.
(2) The original valve core was reamed on a lathe, aligned through the center hole, as shown in Figure 5.
(3) Two rings made of 1Gr18Ni9Ti were installed and machined to fit precisely around the shaft, ensuring proper alignment and securing them with spot welding, as seen in Figure 6.
(4) The shaft was also machined to fit the new components.
(5) A tungsten carbide D547 molybdenum valve head was welded onto the valve, and the sealing surface was adjusted according to the repaired seat.
(6) A trial run was conducted, followed by a sealing test to confirm the effectiveness of the repair.
**IV. Conclusion**
After implementing the above overhaul procedures, the high-pressure liquid ammonia pump P101A/B's outlet check valve was successfully restored, effectively resolving the leakage issue. Following more than a year of continued operation, the valve has performed reliably, demonstrating the success of the repair approach. This case study provides valuable insights and practical solutions for maintaining similar high-pressure check valves in industrial settings, offering a reference for future maintenance and repair operations.
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