Valve Selection in Distillation Units
8 min read
Overview of Distillation Units
Distillation units, commonly referred to as atmospheric and vacuum distillation units, are essential in the initial processing of crude oil. These units separate crude oil into various fractions such as gasoline, kerosene, diesel, wax oil, and residual oil through distillation, which utilizes the differing boiling points of the substances in crude oil. Given the numerous components in crude oil and the small differences in their boiling points, complete separation is challenging. However, precise separation is not required for crude oil processing; it's sufficient to separate the crude into different fractions within specific boiling point ranges for further processing in secondary units. Modern crude oil distillation units operate under both atmospheric and vacuum conditions to increase yield and prevent decomposition at high temperatures. The efficiency of these distillation units is critical to the overall productivity of the refinery.
Summary of Valve Applications
Valve Specifications in Atmospheric and Vacuum Distillation Units
- General Valve Applications:
- Valves used in these units are typically rated below CLASS 600 and DN800, with special cases reaching up to DN2000 for torch lines and vacuum tower top lines.
- For temperatures below 220℃, valves are usually made of carbon steel. For temperatures above 220℃, the material selection depends on the sulfur and naphthenic acid content in the medium, which may require carbon steel, alloy steel (WC9,C5), or stainless steel (316L).
- Crude Oil Pipeline System:
- Valve Type Selection:
- Crude oil is neither highly flammable nor explosive, and its hydrogen sulfide content is low. Hence, the toxicity of hydrogen sulfide is not a concern.
- The operating pressure and temperature in crude oil pipelines are relatively low, so there are no special requirements for the valves.
- Gate valves and globe valves are typically chosen for their reliability and cost-effectiveness. Ball valves may be used in specific cases to prevent clogging due to viscous media.
- For large-diameter and high-temperature applications, the economic feasibility of using ball valves must be considered.
- Valve Structure:
- Given the medium and low-pressure conditions, bolted bonnet valves are adequate to ensure sealing.
- Packing Selection:
- For high-temperature applications, stainless steel and flexible graphite braided packing should be used to avoid issues with non-metallic packing materials.
- Operation Method:
- For large diameter gate valves (CLASS ≤ 300, DN ≥ 400 and CLASS = 600, DN ≥ 350), a gear mechanism is recommended.
- For butterfly valves of any pressure level with DN ≥ 200, a worm gear mechanism is recommended.
- For ball valves of any pressure level with DN ≥ 200, a worm gear mechanism is recommended.
- Valve Type Selection:
- Top Oil and Gas Condensation System of the Three Towers:
- Pipes at the top of the towers contain hazardous substances like hydrogen sulfide and hydrochloric acid. Corrosion-resistant materials and low-leakage valves are required.
- Due to the large diameter of the pipes, butterfly valves are preferred over gate valves to reduce weight and structural dimensions, thereby easing operational difficulty.
- The vacuum is established through a vacuum extractor on the tower top oil and gas pipeline, so the valve's pressure drop and tightness significantly impact the vacuum's establishment. Ball valves with low leakage measures are recommended here.
- Bottom Pipeline System of the Three Towers:
- The bottom medium is viscous and at high temperatures (240-395°C), with post-pump pressures also high (1.5-3.0 MPa), often accompanied by high-temperature sulfur or naphthenic acid corrosion.
- Reliable valves such as gate valves and globe valves are preferred. For materials prone to high viscosity and clogging, ball valves may be used.
- For strong naphthenic acid corrosion, full-port ball valves are recommended to reduce flow speed and turbulence, thus mitigating corrosion.
- Vent Torch Lines:
- These lines have low pressure and temperature but large diameters, making gate valves and butterfly valves suitable for the main pipe shut-off.
- The inlet and outlet valves of the safety valves can be gate or butterfly valves depending on the diameter, with shut-off valves designed to be sealed open (CSO) or locked open (LO).
Conclusion
Efficient operation of atmospheric and vacuum distillation units is crucial to the overall productivity of a refinery. The appropriate selection of valves—considering factors such as material, structure, and operation method—ensures reliability and cost-effectiveness in processing crude oil. By carefully selecting valves suited to the specific conditions of various systems within the distillation units, refineries can maintain optimal operation, minimize maintenance issues, and enhance overall safety and efficiency.
FAQs
- What is the purpose of using both atmospheric and vacuum distillation in crude oil processing?
- Atmospheric distillation separates crude oil into fractions at different boiling points under normal pressure, while vacuum distillation further processes the heavier fractions at lower pressures to avoid thermal decomposition.
- Why is material selection important for valves in high-temperature applications?
- Material selection is crucial because high temperatures can cause certain materials to degrade or react with the medium, leading to corrosion or failure of the valve.
- How do gear mechanisms aid in the operation of large-diameter valves?
- Gear mechanisms reduce the effort required to operate large-diameter valves, making them easier to open and close and ensuring precise control.
- What are the advantages of using butterfly valves in large-diameter, low-pressure systems?
- Butterfly valves are lighter and more compact than gate valves, reducing the weight and structural load on the pipeline while also being easier to operate.
- How do full-port ball valves help mitigate naphthenic acid corrosion?
- Full-port ball valves reduce the flow speed and turbulence of the medium, which in turn reduces the erosive and corrosive effects of naphthenic acid on the valve's internal surfaces.