In the installation and renovation of chemical pipelines and equipment, flanges serve as the core fittings for pipeline connections and equipment docking. The rationality of their selection directly determines the sealing, safety, and service life of the entire device. Many chemical engineering, procurement, and technical personnel often encounter problems such as confusion in flange standards, incorrect selection of models, mixed systems leading to installation rework, and potential leakage hazards. Shanghai Hongguan Stainless Steel provides high-end flanges and related supporting products, with complete materials that can be customized.

This article is based on the current official standards in China, combined with practical scenarios in chemical engineering sites, to systematically explain how to select chemical flanges, what standards correspond to different working conditions, pressure levels, sealing surfaces, and flange types. It is a universal selection manual suitable for new construction projects, renovation and maintenance, and equipment matching. Shanghai Hongguan Stainless Steel provides high-end flanges and related supporting products, with complete materials that can be customized.

1、 The mainstream standard system for domestic chemical flanges (identifying the three core standards)

At present, the domestic chemical industry has eliminated old JB/T machine standard flanges, and new construction and renovation projects uniformly adopt the three major systems of chemical industry standards, national standards, and pressure vessel specific standards. The division of labor among the three is clear and cannot be mixed.

1. Chemical pipeline special flange (preferred for engineering)

Core standard: HG/T 20592-20614-2009 (PN European System)

This is the universal preferred standard for process pipelines in domestic fine chemical, coal chemical, and chemical industrial parks. The entire set of standards includes specifications for flange size, type, sealing surface, gasket, bolt fastener selection, with a complete system and strong adaptability, suitable for the vast majority of domestic valves, pump bodies, and process pipelines.

2. National standard universal pipe flange (applicable to public works)

Core standard: GB/T 9112-9124-2010

The national standard flange is completely interchangeable with HG/T 20592 in size and can be used interchangeably on site. It is mainly used for public engineering pipelines such as water supply and drainage, circulating water, sewage, ventilation, etc. in the factory area, and can be used for ordinary non high risk pipelines.

3. Pressure vessel special flange (equipment exclusive)

Core standard: NB/T 47020-47027-2012

This standard is a specialized flange for storage tanks, reactors, towers, heat exchangers, equipment manholes, and equipment pipe openings. It is strictly prohibited to mix it with pipeline flanges, and the size, bolt holes, and pressure parameters are completely different from those of pipeline flanges. It is an exclusive standard for chemical equipment matching.

4. Eliminate old standards

JB/T 79-86 old mechanical flanges are only allowed to be used for maintenance in old factory areas, and are prohibited from use in all new and renovated chemical projects.

2、 Step by step selection method for chemical flanges (exclusive to domestic standards)

The selection of formal chemical engineering follows the order of "setting standards first, then pressure, then sealing, and finally structure", with zero errors and adaptability to all working conditions.

Step 1: Lock the flange standard according to the usage position

-Process pipelines, valves, and pump inlet and outlet pipelines: HG/T 20592 chemical pipe flanges are preferred

-Equipment interfaces for storage tanks, reactors, towers, and heat exchangers: NB/T 47020 pressure vessel flanges shall be uniformly selected

-Circulating water, sewage, rainwater, and ventilation common pipelines: GB/T 9112 national standard flanges can be selected

Step 2: Match PN nominal pressure rating according to pressure and temperature

Domestic chemical flanges uniformly adopt PN pressure rating (unit: bar), and the pressure bearing capacity of flanges decreases with increasing temperature. High temperature and high-risk working conditions require a safety margin to be reserved upwards.

-PN6, PN10: Suitable for normal pressure and low pressure conditions, suitable for sewage, rainwater, normal pressure exhaust gas, and normal temperature clean water pipelines

-PN16: The most universal grade for chemical engineering, suitable for circulating water, room temperature acid-base, ordinary solvents, and low-pressure gas conventional process pipelines

-PN25, PN40: Suitable for medium pressure steam, methanol, ethanol, oil, and combustible medium pipelines under medium pressure conditions

-PN63, PN100 and above: Suitable for high-pressure and high-temperature working conditions, suitable for high-pressure steam, high-pressure process gas, strong corrosion, and high-pressure reaction pipelines

Important rule: When the medium temperature exceeds 150 ℃, regardless of the working pressure, the pressure level must be increased by one level.

Step 3: Select the sealing surface form according to the hazard of the medium

Under the same flange standard and pressure rating, the sealing surface directly determines the safety of pipeline sealing, and chemical working conditions are strictly selected according to the medium:

-RF protrusion: The most versatile, suitable for clean water, circulating water, ambient temperature oil products, non-toxic and weakly corrosive media, 90% of PN6-PN40 working conditions are selected

-MFM Concave and Convex Surface: Specially designed for high-risk media, suitable for flammable, toxic, and easily permeable media such as chlorine gas, benzene, liquefied gas, VOC solvents, etc. It can be equipped with limit gaskets to prevent medium leakage and flushing

-TG tenon groove surface: high sealing grade, suitable for vacuum pipelines, highly flammable and toxic media, fully sealed with gaskets to prevent leakage

-RJ ring connection surface: dedicated for high pressure and high temperature, only used for PN63 and above grades, suitable for high temperature hydrogen and ultra-high pressure steam pipelines

Step 4: Select the type of flange structure according to the working conditions

Based on HG/T 20592 standard, different structural flanges are adapted to different installation and working conditions:

-PL plate flat welding flange: low cost, easy installation, only used for PN10/PN16 low-pressure public engineering pipelines, prohibited for use in flammable, explosive, high-pressure, and strong vibration pipelines

-WN neck butt welding flange: the main selection of chemical process pipelines, with strong rigidity, resistance to high temperature and high pressure, and deformation resistance, suitable for all medium and high pressure, hazardous chemical media, and vibration process pipelines

-SO with neck flat welding flange: a compromise design, easy to disassemble and assemble, suitable for process pipelines with moderate pressure and frequent maintenance needs

-TH threaded flange: No welding required, suitable for explosion-proof and fire-resistant areas, small pipe diameter below DN50, prohibited for use in high temperature, high pressure, and strong vibration conditions

-IF integral flange: The equipment and valves come with their own flanges, which are matched with the equipment standard and do not require separate selection

Step 5: Selection Specification for Matching Gaskets and Fasteners

The flange selection must be matched with the same standard accessories, and mixing is strictly prohibited:

-HG/T20592 flange: gaskets comply with HG/T 20606~20609, bolts and nuts comply with HG/T 20610~20613, and overall selection follows HG/T 20614 specifications

-NB/T47020 Pressure Vessel Flanges: Comes with specialized pressure vessel gaskets and fasteners, pipe flange fittings are not allowed

3、 One click selection comparison table for typical scenarios of chemical flanges

To facilitate the rapid implementation of the project, we have compiled standardized selection plans for common chemical scenarios in China

-Factory circulating water and sewage pipe network: HG/T20592/GB/T9112, PN10/PN16, RF sealing, PL flat welding flange

-Process pipelines for normal temperature acid-base and ordinary solvent: HG/T20592, PN16/PN25, RF sealing, WN butt welding flange

-Flammable, explosive and toxic pipelines for methanol, liquid chlorine, aromatic hydrocarbons, etc.: HG/T20592, PN25/PN40, MFM concave convex surface, WN butt welding flange

-Medium and high temperature steam pipelines: HG/T20592, PN40/PN63, RF/RJ seals, WN butt welded flanges

-Explosion proof and no fire zone small diameter pipelines: HG/T20592, PN16 and below, RF seal, TH threaded flange

-Reactor, storage tank, tower equipment pipe openings: NB/T47020, according to equipment design pressure, RF/MFM sealing, pressure vessel special flange

-Frequent maintenance of medium pressure pipelines: HG/T20592, PN16-PN40, RF seal, SO with neck flat welding flange

4、 Taboos and key points for avoiding pitfalls in domestic chemical flange selection

Many engineering leaks and installation rework issues arise from non-standard selection. The following are industry mandatory red lines:

-Mixing of systems is strictly prohibited: HG/T flanges for pipelines and NB/T flanges for equipment have different sizes and bolt hole positions, and must not be mixed or docked

-Eliminate the mixing of old and new standards: old JB/T flanges are prohibited for new projects, and it is recommended to uniformly replace old equipment with HG/T20592 standards for renovation

-Pressure level should be higher than lower: for high temperature, vibration, outdoor low temperature, and high-risk medium conditions, the pressure level must be upgraded upwards

-Strict uniformity of caliber: The nominal diameter of the flange DN must be exactly the same as the DN of the pipeline and valve

-Material adaptation: Corrosion conditions do not require changing standards, only upgrading materials such as 304, 316L, duplex steel, etc. are needed

-Standard exchange rules: HG/T20592 and GB/T9112 can be temporarily exchanged. For long-term projects, it is recommended to unify HG/T20592 throughout the factory and standardize them

5. Summary

The core logic of domestic chemical flange selection: HG/T20592 for pipelines, NB/T47020 for equipment, and GB/T9112 for public works. On this basis, the PN level is selected according to the working pressure and temperature, the sealing surface is matched according to the danger of the medium, the flange structure is selected according to the installation scenario, and corresponding gaskets and bolts are strictly matched to fully meet the requirements of safe, stable, and long-term operation of chemical plants. This article can serve as a regular selection reference standard for chemical design, construction, procurement, and operation personnel.