Optimal Valve Selection for Efficient Urea Production

Introduction

Urea, with a nitrogen content greater than 46%, is the most efficient fertilizer in the nitrogen series. It is also an important chemical raw material used in manufacturing urea-formaldehyde resins, plastics, adhesives, coatings, as well as in the production of medicines and leather. Urea is synthesized from liquid ammonia and carbon dioxide under specific pressure and temperature conditions, with the overall chemical reaction expressed as:

2NH₃(liq) + CO₂(gas) → CO(NH₂)₂(liq) + H₂O(liq) + Q

Given that the reaction is reversible, the conversion of raw materials is never complete, leading to the development of various production processes focused on improving conversion rates and recycling. Among these, the most widely applied is the CO2 stripping process technology by Dutch company Stamicarbon. This article will introduce the valve selection for urea production plants.

1. Ammonia Preheater | 2. Ammonia Heater | 3. High-Pressure Ammonia Pump | 4. Air Blower | 5. Liquid Separator | 6. CO₂ Compressor | 7. CO₂ Stripping Tower | 8. High-Pressure Ammonium Carbamate Condenser | 9. Urea Synthesis Tower | 10. High-Pressure Scrubber | 11. High-Pressure Carbamate Pump | 12. Rectifying Tower | 13. Flash Tank | 14. Low-Pressure Scrubber | 15. Absorption Tower | 16. Desorption Tower | 17. First-Stage Evaporator | 18. Second-Stage Evaporator | 19. Low-Pressure Ammonium Carbamate Condenser | 20. Urea Solution Tank | 21. Urea Pump | 22. Granulation Tower | 23. Crusher | 24. Ammonia Water Tank

Overview of Valve Applications

For pipeline systems with moderate medium conditions, valve applications will not be further elaborated. Instead, the focus will be on specific valve applications in urea production plants.

Key Characteristics of Media Conditions

1. High Pressure:
   • According to the ASME B16.34 P-T table, valve pressure ratings will reach CLASS 900/CLASS 1500.
   • Low-pressure drop valves such as gate valves and ball valves are recommended. If globe valves are used, Y-pattern globe valves are preferred. Butterfly valves are not suitable for high-pressure environments.
   • Welding connections are preferred for valves, and gate valves, globe valves, and check valves of DN50 and above should use pressure-sealed bonnets.
   • Due to ammonia's toxicity and strong irritating odor, low-leakage structures and packing are necessary.
   
   
2. Presence of Liquid Ammonia:
   • Liquid ammonia can cause stress corrosion cracking, a severe form of metal failure. Therefore, it is crucial to control the hardness of pressure-bearing components and select appropriate valve internal materials.
3. Pressure Pulsation:
   • Liquid ammonia is pressurized using reciprocating pumps, which can cause mechanical vibration due to pressure pulsation. Valves with good flow characteristics, such as full bore ball valves, are recommended. Reducing the stress levels of valve pressure-bearing components is advisable to prevent material failure from low-cycle fatigue.
4. Condensation During CO2 Pressurization:
   • When the water content in the medium exceeds 5g/m³ (standard state), severe corrosion of carbon steel by CO2 occurs. Valves should be made of austenitic stainless steel in such cases.

Synthesis and Stripping Systems

Media Conditions

1. High Pressure:
   • The principles for valve applications under high-pressure conditions are the same as mentioned above.
2. Strong Corrosion:
   • In highly corrosive media conditions, selecting suitable corrosion-resistant metal materials is essential. For valve structures, avoiding crevice corrosion is crucial. The following table summarizes the commonly used materials for synthesis and stripping systems in actual engineering applications.

In media environments sensitive to crevice corrosion, avoid valve structures with crevices. For example, use butt-welded connections instead of socket-weld connections and lens-type flange sealing surfaces instead of ring-joint flange sealing surfaces. Additionally, valves at pressure reduction points should be angle valves.

Circulation Systems

Compared to synthesis and stripping systems, although the main process pipelines also contain the highly corrosive medium "NH₃ + CO₂ + Ammonium Carbamate + Urea + Water (with O₂)," the temperature is lower, and the corrosive agents cyanate and cyanamide are not present. Ammonium carbamate concentrations are also lower, resulting in a relatively milder corrosive environment. With lower system pressure, the risk of metal failure is also reduced. Valve selection should focus on appropriate pressure-bearing component materials and internal materials. For media containing ammonium carbamate, avoid structures sensitive to crevice corrosion. Due to the lower temperature, pipelines containing urea may experience urea crystallization. Valves without flow dead angles or even without valve cavities, such as plug valves, are recommended.

Urea Solution Evaporation and Granulation Systems

Although the main process pipelines contain the medium "Urea + Water," the temperature is relatively low (90-95℃), and the concentrations of ammonium carbamate and CO₂ are low, leading to milder corrosion. However, the system operates under vacuum, and molten urea is present, requiring valves with low flow resistance. Vacuum valves with a vacuum degree of 29kPa are recommended. To prevent urea crystallization from accumulating in flow dead angles or inside valve cavities, valves without flow dead angles or valve cavities, such as plug valves, are recommended.

Absorption and Desorption Systems

The main process pipelines contain urea, ammonium carbamate, ammonia, CO₂, and other media, but at lower concentrations. The temperature and pressure are also lower. Therefore, apart from selecting suitable corrosion-resistant metal materials, no other special requirements are necessary.

FAQs

1. Why are butterfly valves not recommended for high-pressure environments?
   • Butterfly valves typically have higher pressure drops and may not withstand high-pressure environments as effectively as gate or ball valves.
2. What materials are preferred for valves in the presence of liquid ammonia?
   • Austenitic stainless steel materials such as 316LMod, 310MoLN, and Ti are preferred to prevent stress corrosion cracking.
3. How can valve selection address the issue of pressure pulsation in urea plants?
   • Full bore ball valves with good flow characteristics are recommended to handle mechanical vibration caused by pressure pulsation.
4. What is the significance of using valves without flow dead angles in urea plants?
   • Valves without flow dead angles prevent the accumulation and crystallization of urea, which can block valves and disrupt operations.
5. Why is it crucial to consider crevice corrosion in valve selection?
   • Crevice corrosion can lead to severe damage in metal components. Avoiding structures with crevices helps in mitigating this risk.

Conclusion

Valve selection in urea plants is critical due to the demanding conditions of high pressure, corrosive media, and mechanical stresses. Proper material selection and structural design are essential to ensure valve performance and longevity, particularly in preventing stress corrosion cracking, mechanical vibration issues, and crevice corrosion. By adhering to these guidelines, urea production processes can be optimized for safety and efficiency.

We encourage you to visit the SNBV Valve website to gain a deeper understanding of the Urea Service Valve and explore our range of solutions designed for severe service applications across a variety of industries. Our team of experts is prepared to offer personalized consultation and assistance in selecting the most suitable valve solutions for your specific application needs.