Valve Selection in Hydroprocessing Units

Introduction

Hydroprocessing units are critical in the crude oil refining process. They not only increase the yield of crude oil but also produce high-quality light fuel oils. Modern refineries consider hydroprocessing units essential. Depending on the type of feedstock and the desired product, hydroprocessing units can be classified into hydrofining units, hydrodesulfurization units, hydrocracking units, and lube oil hydrotreating units. The principles for selecting valves for these units are generally similar. This article focuses on the valve selection for hydrocracking units as a representative case.

Overview of Valve Applications

In medium and low-pressure environments like utility systems, auxiliary production systems, upstream feed systems, and downstream distillation systems, the valve application principles are relatively standard. Here, we highlight some specific valve applications in hydrocracking units.

Valves for Hydroprocessing Systems

1. Hydrogenation Reaction System Valves

The hydrogenation reaction system includes pipelines for high-pressure heat exchangers, feed heaters, reactors, and high-pressure separators. These systems operate under typical high-pressure, hydrogen-rich conditions, with pressures ranging from 18-21 MPa and temperatures from 60-410°C, sometimes containing hydrogen sulfide. The selection principles and technical requirements for valves in these conditions are summarized below:

• Valve Types: In high temperature, high pressure hydrogen-rich environment, the valve should have low resistance and reliable sealing. Due to their low resistance and reliable sealing, gate valves are preferred under these conditions. Shut-off valves have high closing torque and resistance, making them less suitable except in limited applications. Ball valves and plug valves have good flow characteristics, but under high temperature and high pressure, they are expensive and poor reliability. Because of the poor closing ability of butterfly valves under high pressure, it is not recommended to use butterfly valves. For pipes that transition from high pressure to medium and low pressure, Angle valves can be used.
• Check Valves: For high temperature and high pressure hydrogen environments, non-wafer check valves are preferred due to better sealing reliability. For DN50 and above, swing check valves are used, and for DN200 and above, tilt disc check valves are preferred. For DN40 and below, the lift check valve is suitable.
• Valve Structure: For DN50 and above, it is recommended to use a pressure seal valve cap to enhance the seal under high pressure. For DN40 and below, bolt valve covers are used due to size limitations and reduced pressure sealing effect.
• Stem Packing: Low emission valve design is necessary. Improve the processing accuracy of valve stem and packing box, pre-compression packing, using lamp ring, using live load bolts, can effectively reduce leakage.
• Sealing Surface Precision: High-pressure hydrogen environments require high precision sealing surfaces to ensure effective sealing.
• Connection Methods: Welded connections are preferred to mitigate mechanical seal problems. DN50 and above adopt butt welding, DN40 and below adopt socket welding.
• Material Purity: Pressure components require high purity materials to avoid hydrogen-induced damage such as blistering and cracking.
• Resistance to Stress Corrosion Cracking: Materials must comply with NACE MR0103 standards to prevent sulphide stress corrosion cracking (SSCC) and hydrogen-induced cracking (HIC) in wet hydrogen sulfide environments.

2. New Hydrogen System Valves

The new hydrogen production system involves the process piping of the hydrogen compressor. The system uses fresh hydrogen without hydrogen sulfide, with an operating temperature of 60-110 ° C and a pressure of 2.5-21 MPa. Valves must meet the requirements for high pressure hydrogen use and additional requirements for compressor pulsation emissions:

• Straight-Through Flow Path: To minimize vibrations caused by pressure fluctuations, straight-through flow paths like those in ball or plug valves are recommended over globe or angle valves.
• Full Bore Valves: Full bore valves reduce turbulence and associated vibrations.
• Anti-Loosening Bolts: Anti-loosening measures are necessary due to inevitable vibrations in the pipeline system.

3. Carbon Disulfide System Valves

Before the hydrocracking unit is started, the catalyst must be vulcanized using a carbon disulfide system, which operates at room temperature and around 4.0 MPa. The valves in this system must strictly control external and internal leaks. Due to their leak-proof capabilities, diaphragm valves are preferred for pre-pressurization systems, while bellows are used for post-pressurization systems.

4. Emergency Vent System Valves

An emergency exhaust valve is installed at the top of the high-pressure separation system to release pressure in the event of an accident. These valves must meet the requirements of high pressure, hydrogen rich and wet hydrogen sulfide applications, and also need to consider:

• Operation Mechanism: Manual operation with linkage mechanisms is preferred over pneumatic or electric due to reliability concerns during power or air supply failures.
• Heating and Insulation: To prevent the condensation of hydrogen sulfide and subsequent corrosion, valves and upstream pipelines should be heated to maintain the medium above the dew point.

Conclusion

In summary, selecting the right valve for a hydrotreating unit, especially a hydrocracking system, requires careful consideration of the operating environment, including high pressure, high temperature, and the presence of hydrogen and hydrogen sulfide. Key factors include valve type, construction, connection, material purity, and specific vibration and sealing measures. Meeting these stringent requirements ensures the safety, efficiency and service life of the hydrotreating unit.

FAQs

1. What are the main types of valves used in hydrocracking units?
   • Gate valves, swing check valves, and pressure seal bonnet valves are commonly used in high-pressure, hydrogen-rich environments.
2. Why is material purity important for valves in hydroprocessing units?
   • High material purity helps prevent hydrogen-induced damage such as blistering and cracking.
3. What connection methods are preferred for high-pressure hydrogen environments?
   • Butt welding for DN50 and above, and socket welding for DN40 and below, to ensure better sealing reliability.
4. How do you mitigate valve vibration in a new hydrogen system?
   • Using straight-through flow path valves and full bore valves, along with anti-loosening measures for bolts.
5. What additional features are required for emergency vent system valves?
   • Manual operation mechanisms and heating to prevent hydrogen sulfide condensation and corrosion.
6. What are the main types of valves used in hydrocracking units?
   • Gate valves, swing check valves, and pressure seal bonnet valves are commonly used in high-pressure, hydrogen-rich environments.