High-pressure flange: pressure classes and materials 

What is a high-pressure flange and why the pressure class is more important than the steel grade

A high-pressure flange is not just a connecting part, but a critical safety element of a pipeline system operating at pressures of 10 MPa and above. In our engineering practice, we have observed cases where choosing a material based on the “more expensive” principle without taking into account the pressure class led to depressurization of the unit after just three months of operation under cyclic loads. The key mistake buyers make is focusing on the steel grade (for example, 09G2S or AISI 316) while ignoring the real pressure rating (PN/Class), which determines the geometry and weight of the product. For systems with operating pressures above 16 bar, standard flanges are not suitable; this requires solutions that comply with ASME B16.5 Class 300-2500 or GOST 33259-2015 standards for pressures up to 20 MPa.

The correct selection of a high-pressure flange begins with an analysis not of the chemical composition, but of the temperature and pressure characteristics of the medium. If your system operates at temperatures above 400°C, the yield strength of the steel drops and the PN16 flange may deform even at rated pressure. We recommend immediately screening out suppliers who cannot provide leak test certificates at extreme temperatures. Below we will take a closer look at how pressure classes relate to materials and what hidden risks exist when importing these components from Asia.

Pressure classes: PN to ASME Class and conversion traps

Understanding pressure classes is the foundation for safe operation. In international practice, there are two main designation systems: European (PN - Pressure Nominal) and American (Class - lbs). An error in their comparison costs companies millions of rubles in losses annually. Many purchasing managers mistakenly believe that PN16 is approximately equal to Class 150 and PN40 is approximately equal to Class 300. This is a dangerous oversimplification. Actual compliance depends on the ambient temperature. At room temperature (20°C) the Class 150 flange actually withstands about 20 bar, which is close to PN20. However, when heated to 400°C, the permissible pressure for Class 150 drops to 8-9 bar, while PN16 maintains its rating to higher temperatures due to a different method of calculating safety factors.

For high-pressure applications where operating pressures exceed 100 bar (10 MPa), we use ASME Class 600, 900, 1500 and 2500. The Class 2500 flange can withstand enormous loads, but its weight is 3-4 times that of a low-pressure equivalent of the same diameter. In one of our refinery modernization projects, replacing Class 900 flanges with Class 600 flanges (due to a specification error) resulted in a complete line shutdown after two weeks. The sealing surface simply could not hold the gasket during water hammer. Therefore, rule number one: never change the pressure class “one step lower” without recalculating the stresses in the bolted connection.

The Russian standard GOST 33259-2015 “Flanges of fittings, connecting parts and pipelines for pressures up to 20 MPa” is harmonized with international standards, but has its own characteristics in the dimensions of the sealing surfaces. If you order a high-pressure flange according to GOST, make sure that the mating flange on the imported equipment has a compatible projection geometry (lug-valley or tongue-groove). Incompatibility even by 0.5 mm will make installation impossible without the use of adapters, which themselves become a weak link in the high-pressure line. Always request a drawing with tolerances before paying your invoice.

The choice of pressure class also dictates the requirements for fasteners. For Class 150 flanges, grade 8.8 bolts are often sufficient, but for Class 900 and above, alloy steel studs (such as ASTM A193 Gr.B7) with a strength grade of at least 10.9 or 12.9 are required. The use of ordinary ferrous metal is unacceptable here - the bolts will simply stretch under load and the connection will lose its tightness. We have seen cases where saving 5% on fastener costs resulted in 100% of the product being lost due to an accident. Check the fastener specification with every high-pressure flange order.

Materials for extreme conditions: carbon, alloy and stainless steels

The choice of material for a high-pressure flange is determined by the aggressiveness of the environment and temperature conditions, and not just by the project budget. Carbon steels such as ASTM A105 (similar to St20 or 20GL) are the standard for temperatures from -29°C to +425°C. This is the most common option for medium pressure oil and gas pipelines. However, if the temperature drops below -29°C, carbon steel becomes brittle. In our practice, there was a case of destruction of a flange made of A105 steel at a temperature of -35°C during the winter start-up of an installation in Siberia. For cryogenic applications, normalized steel or low-alloy grades such as ASTM A350 LF2, which maintain toughness at low temperatures, are a must.

When it comes to temperatures above 450°C or corrosive environments (acids, alkalis, hydrogen sulfide), carbon steel is excluded. This is where chrome-molybdenum alloy steels, such as ASTM A182 F11 or F22, come into play. These materials contain chromium and molybdenum, which improve heat resistance and creep resistance. A flange made of F22 steel can operate at temperatures up to 550-580°C under high pressure, while ordinary steel will already begin to “leak”. The cost of such flanges is 40-60% higher, but this is the only technically sound solution for boiler plants and cracking processes.

Stainless steels (Austenitic Stainless Steels), such as ASTM A182 F304/L and F316/L, are used not so much for high pressure, but for corrosion resistance. It is important to understand the difference between F304 and F304L. The "L" stands for Low Carbon. When welding high-pressure components, conventional F304 is susceptible to intergranular corrosion in the weld area, which can lead to pressure cracks. We always recommend using "L" grades for pressure welded structures, no matter how inert the environment may seem. For environments with high chloride content (sea water, some chemical processes), F316L steel with added molybdenum is required, which resists pitting corrosion.

Particular attention should be paid to the quality of casting and forging. The high-pressure flange must be forged rather than cast, except on special alloys where forging is not possible. Forging refines the grain of the metal, eliminates internal voids and improves mechanical properties. Cast flanges (labeled CF8M instead of F316) have lower stress ratings and are often used for low pressure applications only. Check the markings on the flange end: the presence of an "F" in front of the grade number (for example, F316) usually indicates forged material. If you see markings like “WCB” or “CF8” without indicating the manufacturing method, check with the supplier about the production technology, since for pressures above 10 MPa, casting is often unacceptable without additional x-ray control.

These stringent material requirements directly impact the selection of an OEM supplier. For example,Wuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd.specializes in the design and production of complex units where high-pressure flanges are integrated into heat exchangers and reactors. The company produces equipment from a wide range of materials - from carbon and alloy steels to titanium, C46400 marine brass and N06625 nickel alloys. Their products, including ASME high-pressure heat exchangers and tube bundles, are PED and ASME certified to ensure they meet stringent corrosion resistance and extreme pressure requirements. This approach, when the manufacturer controls the quality of not only the flanges themselves, but also the entire assembly, minimizes the risks of material incompatibility in the aggressive environments of the oil refining and chemical industries.

Types of sealing surfaces and their effect on tightness

The geometry of the sealing face (Face Type) plays a decisive role in the ability of a flange connection to withstand high pressure. For low- and medium-pressure systems, the RF (Raised Face) design is widely used. However, for pressures above 100 bar (Class 600 and above) or for toxic media, the RF design is often not reliable enough. In these cases, engineers switch to RTJ (Ring Type Joint) or MFM (Male-Female Face) designs. In our experience of servicing compressor stations, 90% of leaks on high-pressure lines occurred precisely on the RF flanges due to squeezing out of the gasket during pressure pulsations.

The RTJ version is the gold standard for ultra-high pressures (up to 250 bar and above). Here, tightness is ensured not by compression of the soft gasket, but by plastic deformation of the metal ring (oval or octagonal cross-section) that fits into the groove on the flange. A metal ring (usually mild steel or coated stainless steel) fills in any imperfections in the groove, creating a monolithic joint. The advantage of RTJ is that the flange body itself is not in contact, the load is transferred only to the ring. This allows the connection to be disassembled and reassembled multiple times without losing seal if the rings are replaced. The disadvantage is the high cost of the rings and the requirement for precision grooving.

The MFM (Thorn-Groove) and TG (Tongue-Groove - Groove-Groove) versions occupy an intermediate position. They are ideal for high pressure applications where the use of soft gaskets is not possible, but the use of metal RTJ rings is excessive in cost or installation complexity. In these designs, the gasket is held mechanically inside the groove, which prevents its extrusion under the influence of internal pressure. We recommend using MFM for flanges up to DN300 diameter and pressures from PN100 to PN160. An important nuance: during installation, it is necessary to strictly control the parallelism of the planes, since misalignment will lead to damage to the thin walls of the tenon or groove.

The choice of gasket is inextricably linked to the type of surface. For RF, Spiral Wound Gaskets with a centering ring are used. For high-pressure applications, the correct choice of filler and lining material is critical. Graphite filler is good for high temperatures, but suffers from oxidation at very high temperatures in the presence of air. PTFE (Teflon) is chemically inert, but has a high creep coefficient, which requires regular tightening of bolts. In systems with pressures above 150 bar, we strongly recommend the use of gaskets with an internal and external stainless steel centering ring to prevent the filler from being destroyed by water hammer.

Manufacturing defects and quality control: what to look for upon acceptance

The market is flooded with flanges that look perfect on the outside, but have hidden defects in the metal structure. The most common deception is using remelted scrap instead of virgin steel or violating the heat treatment regime. It is impossible to visually distinguish such a flange, but under load it behaves unpredictably. One of our clients was faced with a situation where a batch of 09G2S steel flanges crumbled during hydraulic testing. Laboratory analysis showed that the manufacturer skimped on quenching and tempering, which resulted in the metal's hardness being below normal and the grain structure being coarse.

When accepting high-pressure flanges, please request a Type 3.1 material certificate according to EN 10204. This document confirms that a particular batch has passed all the necessary tests and the results are recorded. Certificate 2.2 or the manufacturer's declaration of conformity is not sufficient for pressures above 10 MPa. Certificate 3.1 must indicate the results of chemical analysis (all elements, including harmful impurities of sulfur and phosphorus), the results of mechanical tests (yield strength, tensile strength, elongation) and data on impact strength at operating temperature.

Pay attention to the markings. According to ASME and GOST standards, each high-pressure flange must be clearly marked, including: manufacturer's name, material grade, pressure class, size, heat number and heat treatment. The absence of any of these elements is a red flag. Particularly important is the Heat Number, which allows you to trace the history of the metal from the steel shop to the finished product. If the seller cannot match the number on the flange to the number on the certificate, refuse the purchase. The risk of getting counterfeit goods is too great.

We also recommend random ultrasonic testing (UT) of the flange body, especially in the hub-to-ring transition area. This is a place where stress is concentrated. Cracks that occur during forging or heat treatment are often hidden there. For critical objects (nuclear power plants, main gas pipelines), hardness control over the entire cross-section is mandatory. Too high a hardness indicates a risk of brittle fracture; too low a hardness indicates insufficient strength. The optimal hardness range for carbon steels is usually 140-190 HB, for alloy steels it is higher, depending on the grade.

Logistics and import certification: EAC, GOST and customs risks

Import of high-pressure flanges into the EAEU countries requires strict compliance with the technical regulations of the Customs Union (TR CU 032/2013 “On the safety of equipment operating under excess pressure”). Without EAC marking and a valid certificate of conformity, customs clearance is not possible, and the operation of such equipment is prohibited by law. The process of obtaining a certificate can take from 2 weeks to 2 months depending on the certification scheme. For serial deliveries, it is more profitable to issue a certificate for a batch or serial production than to receive a one-time conclusion for each shipment.

When ordering flanges from China or other Asian countries, make sure that the manufacturer is familiar with the requirements of GOST and TR CU. Many Asian factories produce products strictly according to ASME or DIN, the dimensions of which may differ from GOST. For example, the center-to-center distance of the holes (bolt circle diameter) for DIN and GOST flanges of the same nominal diameter and pressure may differ by several millimeters, which will make installation impossible without altering the mating part. We recommend including in the contract a clause stating that the geometry complies with Russian standards or providing the manufacturer with detailed drawings with tolerances.

Packaging and preservation play an important role during long sea transportation. High-pressure flanges often have machined sealing surfaces that will rust within two weeks in a damp container. Request the supplier to apply an anti-corrosion compound and package each flange individually in polyethylene film with silica gel and then in a wooden box. Wooden packaging must undergo phytosanitary treatment and have the appropriate stamp (IPPC), otherwise the cargo may be turned away at customs. In our practice, there were cases when a flange of ideal quality arrived with a corroded seal mirror due to cheap packaging, which required expensive re-grooving or replacement.

Delivery time is another critical factor. The production of forged high-pressure flanges is a lengthy process. It includes heating the workpiece, forging, machining, drilling, heat treating and finishing. The standard batch production time is 45-60 days. Если поставщик обещает отгрузку через 2 недели, это повод насторожиться: скорее всего, он предлагает складские остатки неизвестного происхождения или продукцию, изготовленную с нарушением технологии (например, без полноценной термообработки). Планируйте закупки заранее, учитывая производственный цикл, чтобы не срывать сроки монтажа объекта.

Frequently Asked Questions

Какой класс давления выбрать для системы с рабочим давлением 80 бар?

Для рабочего давления 80 бар (8 МПа) минимально допустимым классом является PN100 или ASME Class 600. Выбор Class 300 (который теоретически выдерживает около 50 бар при высоких температурах) будет рискованным и нарушит нормы запаса прочности. Мы рекомендуем брать Class 600, так как это обеспечит необходимый запас на гидроудары и возможные скачки давления в системе. Всегда выбирайте класс, превышающий рабочее давление минимум на 20-30%.

Можно ли использовать фланцы из нержавеющей стали 304 для сероводородной среды?

Нет, стандартная нержавеющая сталь AISI 304 (08Х18Н10) не подходит для сред, содержащих сероводород (H2S), особенно под высоким давлением. Она подвержена сульфидному коррозионному растрескиванию под напряжением. Для таких условий необходимо использовать специальные марки сталей, прошедшие тесты NACE MR0175 / ISO 15156, например, дуплексные стали (2205) или аустенитные стали с низким содержанием углерода и специальной термообработкой. Использование обычной “нержавейки” приведет к внезапному хрупкому разрушению.

В чем разница между фланцем ГОСТ и DIN, можно ли их стыковать?

Стыковать фланцы ГОСТ и DIN напрямую категорически не рекомендуется, даже если номинальное давление и диаметр совпадают. Основные отличия заключаются в диаметре болтового круга (расстояние между центрами отверстий) и количестве отверстий. Например, у фланца DN100 PN16 по ГОСТ и DIN эти параметры могут отличаться, что не позволит совместить отверстия для болтов. Кроме того, могут различаться размеры уплотнительной поверхности. Используйте переходные адаптеры только в крайних случаях и после согласования с главным инженером проекта.

Как часто нужно менять прокладки на высоконапорных фланцах?

Периодичность замены прокладок не регламентируется жестким сроком, а зависит от условий эксплуатации. Однако, согласно лучшим практикам, прокладки на фланцах классов выше PN100 следует заменять при каждом демонтаже соединения. Повторное использование спирально-навитых или металлических прокладок недопустимо, так как они уже получили остаточную деформацию и не обеспечат герметичность при повторной затяжке. Для систем с вибрацией или термоциклированием рекомендуем проводить контрольную подтяжку болтов через 24 часа после запуска и далее по графику ТО.

Conclusion and recommendations for choosing a supplier

Выбор высоконапорного фланца — это баланс между стоимостью, безопасностью и долговечностью. Экономия на классе давления, материале или качестве обработки уплотнительной поверхности всегда приводит к многократным потерям в будущем из-за аварийных остановок и ремонтов. Надежный поставщик должен не просто продать деталь, а предоставить полный пакет технической документации, включая сертификаты 3.1, паспорта качества и гарантии соответствия ТР ТС. Мы работаем только с проверенными кузнечными заводами, которые контролируют весь цикл производства от плавки стали до финальной упаковки.

Если вы планируете закупку партии высоконапорных фланцев для ответственного объекта, не рискуйте качеством. Свяжитесь с нами сегодня для получения консультации по подбору материалов и классов давления под ваши конкретные условия эксплуатации. Наши инженеры помогут составить спецификацию, исключающую ошибки совместимости, и организуют поставку сертифицированной продукции в кратчайшие сроки.Посмотреть каталог высоконапорных фланцевили запросить коммерческое предложение с учетом вашей специфики.