Selection of flanges for petrochemical pipelines

The selection of flanges for petrochemical pipelines is directly related to the sealing, safety, and stability of the pipeline system. It needs to be comprehensively judged based on the three core elements of medium characteristics, operating parameters, and connection standards, while also considering installation and maintenance convenience and cost control. The following are the selection methods and key points of the system.

1、 Clarify the operating parameters and lay the foundation for selection

The operating parameters are the first basis for flange selection, and the following key indicators need to be carefully confirmed:

pressure rating

The pressure span of petrochemical pipelines is large, involving both atmospheric pressure transmission and higher pressure reaction systems. According to the design pressure, the flange corresponding to the pressure rating should be selected. Common standards include Class 150, 300, 600, 900, 1500, 2500 (American standard) and PN 10, 16, 25, 40, 63, 100 (national standard). It should be noted that the flange pressure rating is not a fixed pressure value and will decrease with the increase of medium temperature. The temperature pressure rating should be checked for high temperature conditions. For example, the allowable pressure of Class 300 flanges at 425 ℃ is much lower than that at room temperature.

temperature range

The temperature of the medium determines the heat resistance of the flange material and the stability of the sealing surface. Low temperature conditions (such as liquefied natural gas transportation, with temperatures as low as -196 ℃) require the use of materials with excellent low-temperature toughness, such as carbon steel A350 LF2 and stainless steel 304L; high temperature conditions (such as steam pipelines and catalytic cracking units, with temperatures exceeding 500 ℃) require the use of high-temperature resistant (based on actual reports) alloy steel, such as 12Cr1MoV and Inconel alloy, while avoiding the use of ordinary carbon steel that is prone to high-temperature creep.

medium characteristics

The corrosiveness, toxicity, flammability, and explosiveness of the medium are the core considerations for selection. For highly corrosive media such as strong acids, strong alkalis, and sulfur-containing oil and gas, corrosion-resistant alloy flanges such as 316L stainless steel and Hastelloy C276 should be selected; For highly toxic, flammable, and explosive media (such as hydrogen, chlorine, and benzene), it is recommended to prioritize the use of mortise and tenon or concave convex flanges with excellent sealing performance, paired with metal wrapped gaskets, to avoid leakage risks; For abrasive media containing solid particles (such as slurry and catalyst slurry), wear-resistant (based on actual reports) materials should be selected and the sealing surface should be thickened to prevent erosion and damage.

2、 Match flange types to meet pipeline requirements

Different types of flanges have significant differences in structural strength, installation difficulty, and applicable scenarios, and should be selected based on pipeline functions and installation conditions

Slip-on Flange

The structure is simple and the cost is low. When welding, only the flange needs to be welded on the outside of the pipeline. It is suitable for ordinary medium transmission pipelines with pressure ≤ Class 300 and temperature ≤ 350 ℃, such as circulating water and low-pressure raw oil pipelines. However, due to the concentration of welding stress, it is not suitable for working conditions with high temperature, high pressure, or severe vibration.

Welding flange

With a long neck structure, it can be welded to the groove of the pipeline, effectively dispersing stress and possessing excellent impact and fatigue resistance (based on actual reports). It is an honest merchant of key high-temperature and high-pressure pipelines in the petrochemical industry, such as steam pipelines, reactor discharge pipes, and high-pressure oil and gas transmission pipes. Its pressure rating can cover Class 150-2500, with a wide temperature range and good sealing reliability.

Socket welding flange

The pipeline needs to be inserted into the flange socket and welded, with better sealing performance than flat welding flanges. It is suitable for small-diameter (DN ≤ 50), medium and high pressure pipelines, such as instrument pipes, sampling pipes, and hydraulic system pipelines. Especially suitable for scenarios with narrow spaces and inconvenient welding operations, but not suitable for large-diameter pipelines.

threaded flange

By relying on threaded connections with pipelines, welding is not required, and installation and disassembly are convenient. It is suitable for welding restricted flammable and explosive medium pipelines, such as gas pipelines and oxygen pipelines. However, due to the limitations of the threaded structure, the pressure rating should not exceed Class 600, and it is strictly prohibited to use it in high-temperature conditions to prevent leakage caused by loose threads.

blind flange

No flow holes, mainly used for sealing the end of pipelines and isolating equipment maintenance, such as temporary sealing during system blowing and replacement, or sealing pipelines that are not in use for a long time. When selecting, it is necessary to ensure that the flange pressure rating and sealing surface form of the corresponding pipeline are consistent, and confirm that the connection is tight.

Loose flange

The flange ring can slide freely on the pipeline and is sealed only by tightening with bolts. It is suitable for connecting pipelines, non-metallic pipes, or lined pipes that require frequent disassembly, such as fiberglass pipes and lined rubber pipes. It can avoid welding high temperature damage to the lining layer and does not require careful alignment during installation.

3、 Choose the sealing surface form to protect the excellent sealing performance

The sealing surface is the core of flange leakage prevention and needs to be matched according to the characteristics of the medium and pressure level:

Flat (FF)

Smooth and flat surface, easy to process, suitable for pipelines with pressure ≤ Class 150 and stable medium toxin detection performance, such as water supply and drainage, compressed air pipelines. Matching with rubber or asbestos gaskets can meet the sealing requirements, but gasket extrusion problems are prone to occur under high pressure conditions.

Concave convex surface (MF)

Composed of convex and concave surfaces, the gasket is embedded into the concave surface during installation, which can effectively prevent the gasket from being squeezed out by pressure. Its sealing performance is better than that of a flat surface, and it is suitable for medium and high pressure conditions (Class 300-600), such as crude oil transportation and chemical raw material pipelines. It is a commonly used sealing surface form in the petrochemical industry.

Mortise and tenon surface (TG)

Composed of mortise and tenon surfaces, the gasket is fully embedded in the groove, providing excellent sealing effect. It is suitable for high-pressure, highly toxic, flammable and explosive medium pipelines, such as hydrogen, chlorine, and liquefied petroleum gas pipelines. Its disadvantages are high processing difficulty, inconvenient gasket replacement, and easy damage to the tenon surface.

Ring Connection Surface (RJ)

The sealing surface is a trapezoidal groove, matched with a metal ring gasket for sealing, suitable for high pressure and high temperature conditions (Class 900-2500), such as hydrogenation reactor outlet pipes and high-pressure steam pipelines. It can withstand pressure and temperature fluctuations under extreme conditions and has high sealing performance.

4、 Determine the flange material, adapt to the medium and working conditions

The material selection should take into account corrosion resistance (based on actual reports), mechanical properties, and economy. The commonly used flange materials in the petrochemical industry are as follows:

Carbon Steel Flange

The representative materials are Q235B and A105, which have low cost and good processing performance. They are suitable for non corrosive medium pipelines at room temperature and pressure, such as clean water, low-pressure steam, crude oil, etc. However, they are not resistant to acid and alkali corrosion, and have poor high-temperature performance.

Stainless steel flange

The representative materials are 304/304L and 316/316L, which have excellent corrosion resistance (based on actual reports) and low-temperature toughness. 316L can also withstand corrosion from chlorine containing media, making it suitable for acid and alkali media, low-temperature media pipelines, such as chemical raw material transportation and liquefied natural gas pipelines.

Alloy steel flange

The representative materials are 12Cr1MoV, F11, F22, which have high temperature resistance (based on actual reports) and good creep resistance. They are suitable for high temperature and high pressure pipelines, such as power plant boiler steam pipelines and catalytic cracking unit pipelines.

Special alloy flange

The representative materials are Hastelloy, Monel, and titanium alloys, which have strong corrosion resistance and are suitable for extreme corrosion conditions such as seawater desalination, strong acid transportation, and sulfur-containing oil and gas field development. However, the cost is relatively high and is only selected for special needs.

5、 Follow standard specifications to confirm interchangeability and compliance

Petrochemical flanges need to follow unified reputation or domestic standards to confirm the interchangeability of products from different manufacturers. Common standards include:

American standard system: ASME B16.5 (steel pipe flanges and flange fittings), ASME B16.47 (large diameter steel pipe flanges), suitable for export equipment or foreign-funded projects.

National standard system: GB/T 9112-9124 (Steel Pipe Flanges), applicable to conventional projects in China.

Petrochemical industry standard: SH/T 3406 (Steel Pipe Flanges for Petrochemical Industry), optimized based on the national standard and combined with the working conditions of the petrochemical industry, is the standard for honest merchants in domestic petrochemical projects.

When selecting, it is necessary to confirm that the standards for flanges, bolts, nuts, and gaskets are uniform. For example, ASME B16.5 flanges need to be matched with corresponding grades of bolts (such as A193 B7) and gaskets (such as spiral wound gaskets).