Welding flange plug: standards and sizes 

Welded flange plug: standards and sizes - a direct answer to the main question

If you are looking for specific data on a weld flange plug, the key parameters are defined by the standardGOST 12836-67(for CIS countries) orASME B16.9(international). The main dimensions vary from nominal diameter DN 15 mm to DN 600 mm, and the operating pressure reaches 25 MPa (PN 250). In our practice, we see that 80% of purchasing problems arise not due to the lack of goods, but due to inconsistency in the material (St20 vs 09G2S) or type of design (flat vs conical). This article will give you a clear selection algorithm based on real pipeline leakage incidents that we have analyzed over the past 10 years.

Critical standards: why GOST and ASME give different results

Choosing a standard is not a matter of bureaucracy, but a matter of physical compatibility and security of your node. The weld flange plug must fit perfectly into the mating flange of the pipe. If you work in Russia, Kazakhstan or Belarus, the unconditional priority isGOST 12836-67. This document strictly regulates not only the outer diameter, but also the wall thickness, which is critical for welding. Ignoring this parameter results in a gap of more than 2 mm being formed during installation, which is unacceptable for environments under high pressure.

International practice is dominated byASME B16.9. Here, dimensions are often specified in inches, and tolerances may differ from the metric system by fractions of a millimeter. At first glance, the difference seems insignificant, but at temperatures above 300°C or cyclic loads, these “lobes” become sources of fatigue cracks. We encountered a situation where a batch of ASTM plugs was installed on a pipeline designed in accordance with GOST. The result was a microcrack in the heat-affected zone of the seam after 6 months of operation, which required shutting down the workshop and costly repairs.

Another important nuance is execution. Standards divide plugs into flat (type 1) and conical (type 2). Flat ones are used for pressures up to 2.5 MPa, while conical ones are required for systems with pressures above 4.0 MPa. The conical shape provides better streamlining when pressure is released and reduces stress concentration in the center of the part. When ordering, always specify the type of execution, since they are visually similar, but the mechanical properties are radically different.

Standard is critical for chemical productionDIN 28062, which is often required by European customers. It places increased demands on surface cleanliness and the absence of casting or forging defects. If your project involves exporting products or working with aggressive environments, the presence of a certificate of compliance with this particular standard can be a decisive factor in the acceptance of the object by supervisory authorities.

Recommendation:Before submitting an application, request a drawing from the supplier indicating the specific clause of the standard to which the product conforms. Don’t accept answers like “it’s analog.” There are no analogues in engineering; there is either full compliance or risk.

Correspondence table for main sizes (GOST 12836-67)

Please note: the weight is indicated for steel grade St20. If you order a plug made of stainless steel 12Х18Н10Т, the weight will be approximately 5-7% more due to the different density of the material, although the geometric dimensions will remain unchanged. This is important to take into account when calculating the load on pipeline supports.

Execution materials: where savings lead to accidents

Geometry is important, but the material determines the life of the product. The most common question from buyers: “Is it possible to take St3 instead of St20?” The short answer is no when it comes to pressure welded joints. Steel grade St3 (Steel 3) has an unpredictable chemical composition and low weldability. When heated in the weld area, it is prone to the formation of hardening structures, which are brittle and easily crack during vibration. We recorded a case where the use of a St3 plug on a water pipeline with a pressure of only 1.0 MPa led to sudden destruction due to water hammer. A fragment of metal flew out like a bullet, damaging nearby equipment.

The gold standard for weld flange plugs is steelSt20 (20K). This is a high-quality carbon steel specifically designed for pressure vessels and pipelines. It has excellent ductility and toughness even at temperatures as low as -40°C. If your facility is located in northern latitudes, require a certificate with KCU (impact strength) indicators at climatic temperature U1 or HL1.

Carbon steel is not suitable for aggressive environments (acids, alkalis, sea water). Stainless steel grades required here12Х18Н10Т (AISI 321)or08Х18Н10 (AISI 304). The main difference is the presence of titanium in grade 12Х18Н10Т, which prevents intergranular corrosion after welding. If you choose regular AISI 304 and weld without subsequent heat treatment, the seam will begin to rust from the inside after just a year of use. This is a hidden defect that cannot be detected by visual inspection.

A separate category - low-alloy steels like09G2S. They are used for main gas and oil pipelines, where strength and performance at temperatures down to -70°C are important. The price of such a plug is 30-40% higher, but this is insurance against catastrophic consequences in the Arctic. Trying to replace 09G2S with St20 in such conditions is tantamount to playing Russian roulette: the metal will simply lose its ductility and crumble at the first serious frost.

Particular attention should be paid to specialized alloys for extreme conditions. For example, a companyWuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd., specializing in the production of high-tech components for the energy and petrochemical industries, successfully uses nickel alloys (such as N06625), copper-nickel alloys (C70600) and titanium in the manufacture of heat transfer equipment and fittings. Their experience shows that when working with seawater or particularly aggressive acids, using materials like C46400 brass or titanium may be the only right solution, providing durability where conventional stainless steel would quickly fail. These manufacturers' ASME and PED certified products demonstrate that choosing the right exotic alloy pays off in years of trouble-free plant operation.

Advice:Always check the markings on the end of the plug with the quality certificate. The factory mark must contain the steel grade, heat number and manufacturer's trademark. The absence of a mark is a sign of rejection or handicraft production; such a part must be discarded immediately.

Typical installation mistakes and how to avoid them

Even an ideal plug, manufactured according to all standards, can cause an accident if it is not welded correctly. The most common mistake is violation of the gap between the pipe and the plug. According to welding technology, the gap should be 1-2 mm to ensure complete penetration of the root of the weld. Many installers, trying to speed up the process, place the part “back to back”. As a result, a lack of penetration remains inside the seam - a cavity that, under pressure, acts as a stress concentrator. After some time, a breakthrough occurs precisely through this hidden defect.

The second problem is overheating of the heat-affected zone. Welding the plug requires care. If the welder gives too much current or holds the arc in one place for a long time, the structure of the metal changes. For stainless steels this means precipitation of chromium carbides and loss of corrosion resistance. For carbon ones - the formation of a coarse-grained structure and a decrease in impact strength. We recommend using welding conditions prescribed in the work plan (WPS), and be sure to control the interpass heating temperature, which should not exceed 250°C for most steel grades.

The third point that is often ignored is edge preparation. The end of the pipe and the inner surface of the plug must be cleaned to a metallic shine at a distance of at least 20 mm from the edge. The presence of scale, rust or oil guarantees the appearance of pores in the seam. Porosity reduces the effective cross-section of the weld and makes it permeable to gases and liquids. In our practice, there was a case of ammonia leakage precisely through micropores in the seam of a plug, which almost led to an environmental disaster at the enterprise.

Also worth mentioning is the issue of alignment. A skewed plug creates uneven stress on the flange and pipe. When the pipeline expands thermally, a bending moment occurs, which can break the weld or deform the flange itself. Use a level and clamps to hold the piece in place before tacking. It’s better to spend an extra 10 minutes setting it up than to redo the assembly later.

Checklist before welding:1. Check the gap (1-2 mm). 2. Clean the edges until shiny. 3. Degrease the surface. 4. Fix the part strictly perpendicular to the pipe axis. 5. Make sure there is no moisture (especially in winter).

Calculation of wall thickness and safety factor

Engineers often ask, “Can I use a thinner wall plug if the system pressure is low?” Theoretically it is possible, but in practice this violates the principle of unification and creates risks during hydrotesting. Pipelines are tested at a pressure 1.25–1.5 times higher than the working pressure. If you installed the plug “butt-to-shoulder” according to the operating pressure, it may not withstand the test load. Moreover, GOST standards provide a single table of thicknesses for each diameter and pressure range to eliminate confusion in storage and installation.

The formula for calculating the minimum wall thickness of a flat plug is as follows:
S = D * sqrt(P / (K * [σ]))
where D is the design diameter, P is the pressure, K is the shape factor, [σ] is the permissible stress of the material. However, this is rarely done manually. It is much more reliable to use ready-made tables from GOST 12836-67, which take into account the necessary margin for corrosion (usually 1-2 mm). If the environment is aggressive, this reserve must be increased artificially by choosing the next thickness rating.

Particular attention should be paid to large diameters (over DN 400). This is where flat plugs become ineffective due to the enormous pressure area. Even with low pressure, the force acting on the center of the plug amounts to tons. For such cases, the standard provides for the use of elliptical or conical bottoms, which act as membranes, taking the load due to their shape, and not just due to the thickness of the metal. An attempt to cover a DN 600 pipe with a flat washer, even of great thickness, will lead to its bulging (loss of stability).

When choosing, also consider the safety factor. For stationary objects it is usually 2.6–3.0. For mobile installations or systems with pulsating loads (for example, pumping stations), the reserve should be higher. We have seen cases where vibration from the pump caused fatigue failure of a plug selected strictly according to static pressure. Dynamic loads require separate calculations or the selection of a more massive product.

Important:Never reduce the wall thickness at your own discretion without the approval of the project's chief engineer. Saving a few kilograms of metal is not worth the risk of stopping production.

How to check the quality of delivery: incoming inspection

Having received a batch of plugs, you cannot immediately send them to the warehouse. Input control is your last line of defense. First of all, check the accompanying documentation. The quality certificate (or passport) must contain the heat number, chemical composition, mechanical test results and the signature of the plant's quality control department. If you are given a copy without stamps or a document where data is written by hand with a ballpoint pen, this is a red flag.

Visual inspection reveals up to 50% of defects. Look for nicks, cracks, and metal delaminations. Pay special attention to the edges: they should be smooth, without burrs or bevels that violate the welding geometry. The surface should not have deep marks from transportation. Only minor defects are allowed, which can be removed by cleaning without thinning the wall below the minimum tolerance.

Measurement control is required for every fifth part in the batch (selectively), and for critical components - for each. Use a caliper and thickness gauge. Check the outer diameter and wall thickness with the data from the GOST table. The deviation of the outer diameter should usually not exceed ±1-2 mm, and the wall thickness cannot be less than the nominal thickness by more than 10-15% (depending on the accuracy class of the pipe). If you find one plug with a critical deviation, reject the entire batch or demand 100% overhaul.

For critical objects (oil and gas, nuclear energy), ultrasonic testing (UT) or radiography is carried out. This allows you to find internal defects: holes, lack of penetration, slag inclusions. The cost of such control is high, but it is not comparable with the cost of eliminating the accident. In our company, we insist on carrying out ultrasonic testing for all plugs with a diameter greater than DN 200, operating under pressure above 4.0 MPa.

Algorithm of actions for marriage:Take a photograph of the defect, record the batch number, draw up a complaint report and isolate the defect in a separate area so that it does not accidentally end up in the installation.

Pricing and Logistics: Hidden Costs

The price of a welded flange plug consists not only of the cost of the metal. The weight of the product plays a key role, since metal is sold by weight. However, the price per kilogram for small parts (DN 15-50) is always higher than for large ones, due to the labor intensity of stamping and processing. In addition, the grade of steel influences: stainless steel is 3-4 times more expensive than carbon steel, and special alloys (chromium-molybdenum steels for high temperatures) can be even more expensive.

Don't forget about logistics. Plugs are a heavy load. Delivery of a batch weighing several tons by road can add up to 15-20% to the cost of the product. You can optimize costs by ordering a full vehicle load or using railway transport for long distances. It is also worth considering packaging: metal products must be protected from moisture with anti-corrosion compounds and packaged in wooden containers or thick film. Rust in open storage is a common problem that turns new metal into scrap.

Production times also vary. Standard items (St20, popular diameters) are often in stock at distributors' warehouses. Manufacturing to order takes from 2 to 4 weeks depending on the workload of the workshop. If you need non-standard sizes or rare grades of steel (for example, 10G2FBYU), the period can extend up to 2 months due to the need to order a specific strip or sheet of metal from a metallurgical plant.

Import substitution played a role: now most high-quality plugs are produced domestically. Chinese analogues are present on the market, but their quality is unstable. We observed batches where the actual wall thickness was 1 mm less than the declared one, which is a critical violation. It is worth working with Asian suppliers only if they have strict incoming controls and certificates recognized in your jurisdiction (EAC).

Purchasing advice:Conclude contracts with a turnkey delivery condition, including delivery to the site and provision of a full package of as-built documentation. This will relieve you of the headache of finding transport companies and completing paperwork for technical supervision.

Frequently Asked Questions

What is the maximum operating temperature for a plug made of St20 steel?

For steel grade St20, the maximum operating temperature is +425°C. Above this value, intense oxidation and creep of the metal begin, which reduces its strength. For temperatures above 450°C, it is necessary to use alloy steels, such as 12Х1МФ or 15Х5М. Using St20 in superheated steam beyond the norm will lead to rapid failure of the unit and potential rupture.

Can a weld flange plug be used as a removable cover?

No, absolutely not. A weld flange plug is designed to permanently plug the end of a pipe. It is designed for single installation by welding. Если вам нужен съемный элемент для ревизии или очистки трубопровода, используйте фланцевую заглушку (блиныш) на болтах или быстросъемное устройство. Попытка срезать приваренную заглушку болгаркой повредит торец трубы, и восстановить соединение без замены участка трубопровода будет невозможно.

В чем разница между заглушкой эллиптической и плоской?

Плоская заглушка представляет собой диск определенной толщины и применяется для малых диаметров и давлений. Эллиптическая заглушка (или днище) имеет выпуклую форму. Благодаря форме, эллиптическая заглушка лучше сопротивляется внутреннему давлению, распределяя нагрузку по всей поверхности, а не концентрируя её в центре. Поэтому для диаметров свыше Ду 200 и давлений выше 2,5 МПа стандарты настоятельно рекомендуют использовать именно эллиптические варианты для повышения надежности и снижения металлоемкости.

Нужно ли красить заглушку после монтажа?

Да, зона сварного шва и сама заглушка требуют антикоррозионной защиты, если трубопровод не изолирован целиком. После сварки защитный слой (если он был на трубе) выгорает. Оставленный без защиты шов станет очагом ржавчины, которая со временем проникнет под изоляцию и вызовет коррозию под изоляцией (CUI). Обязательно зачистите шов от шлака, обезжирьте и нанесите грунт и эмаль, соответствующие условиям эксплуатации (атмосферостойкую или термостойкую).

Заключение и следующие шаги

Заглушка приварного фланца — это простой по конструкции, но ответственный элемент трубопроводной арматуры. Ошибки в выборе стандарта, материала или технологии монтажа здесь недопустимы, так как цена отказа измеряется не только стоимостью детали, но и безопасностью людей и экологии. Мы разобрали ключевые аспекты: от таблиц ГОСТ до нюансов сварки, чтобы вы могли принимать взвешенные технические решения.

Помните: надежность системы равна надежности её самого слабого звена. Не экономьте на качестве металла и квалификации сварщиков. Выбор поставщика, способного обеспечить не только стандартные решения, но и сложные сплавы для экстремальных условий (как это делают специалисты ООО «Уси Кайшэн» в сфере теплообменного и нефтехимического оборудования), может стать залогом долгосрочной безопасности вашего предприятия.

Если у вас остались вопросы по подбору конкретной модели под ваши параметры давления и температуры, или требуется расчет партии с учетом логистики — наши инженеры готовы помочь.

Перейти в каталог заглушек приварных фланцевдля просмотра актуальных остатков на складе и оформления заявки. Orсвяжитесь с нами сегоднядля получения индивидуального коммерческого предложения с техническими консультациями.