Seamless Three-way Pipe Size Selection Guide: From

# "A Comprehensive Guide to Seamless Tee Pipe Size Selection: From Standard Specifications to Engineering Practice" 

## Abstract 

This article comprehensively analyzes the diameter requirements of seamless three-way fittings, systematically introduces the main standard systems at home and abroad, elaborately expounds the selection principles of diameters under different application scenarios, deeply analyzes the correlation between diameters and pressure grades, as well as material properties, and provides practical selection suggestions and installation precautions. Through this article, readers will master the professional technical knowledge for selecting the diameters of seamless three-way fittings, understand the solutions to common problems, and provide reliable references for pipeline system design and construction. 

## Introduction 

In pipeline systems of industries such as petroleum, chemical engineering, and power generation, seamless three-way fittings are crucial components. The rationality of the selected pipe diameter directly affects the safety and operational efficiency of the entire system. Correct diameter matching not only ensures smooth flow of the medium but also effectively reduces pressure loss and energy consumption. This article will deeply explore the diameter standards, selection methods, and engineering application points of seamless three-way fittings, helping engineering technicians make scientific decisions in design and construction and avoiding system problems caused by improper diameters. 

## 1. Seamless Three-way Pipe Diameter Standard System 

1.1 International General Standards 

The bore specifications of seamless three-way fittings are mainly based on authoritative standards such as ASME B16.9 and MSS SP-75 internationally. The ASME B16.9 standard covers the size range from NPS 1/2 to NPS 48 (DN15 to DN1200), stipulating key parameters such as the wall thickness and the center-to-end dimension of different bore three-way fittings. This standard classifies three-way fittings into two categories: equal-diameter and unequal-diameter. Equal-diameter three-way fittings have the same diameter for the main pipe and the branch pipe, while unequal-diameter three-way fittings typically have a branch pipe diameter that is 1-2 specification grades smaller than the main pipe. MSS SP-75 specifically targets high-strength forged steel welded pipe fittings, and its bore specifications are basically corresponding to the ASME standards, but there are stricter requirements for certain special sizes. The German DIN 2615 standard is widely adopted in the European market, using the DN series for nominal diameters, which is consistent with the ISO standards and is convenient for international engineering projects. 

1.2 Domestic Industry Standards 

The standard system for seamless three-way fittings in China mainly includes GB/T 12459, GB/T 13401 and HG/T 21635, etc. GB/T 12459, "Steel Welded Seamless Pipe Fittings", adopts ASME B16.9 as an equivalent standard and is the most commonly used standard in China's petrochemical industry. Its pipe diameter range covers from DN15 to DN1200. GB/T 13401 is for steel plate welded pipe fittings and is applicable to large-diameter low-pressure pipeline systems. HG/T 21635, "Carbon Steel and Low Alloy Steel Welded Pipe Fittings", is widely used in the chemical industry and has stricter regulations on the pipe diameter tolerance under special working conditions. It is worth noting that the nominal diameter in the national standard is usually represented by DN, which has a certain conversion relationship with the NPS of the American standard. For example, DN50 corresponds to NPS 2, and DN100 corresponds to NPS 4, etc. In actual engineering, the three-way pipe diameter must be selected according to the specified standard system in the design documents to avoid interface mismatch caused by mixing different standards. 

1.3 Special Industry Standards 

In addition to the general standards, certain specialized industries have their own seamless tee diameter specifications. The petroleum and natural gas industry follows the API 5L standard, where the diameter series of pipeline pipe tees is different from that of conventional pipe fittings, and it pays more attention to large-diameter high-pressure applications. The nuclear power sector adopts the RCC-M standard, which has extremely strict tolerance control for the diameter of the tees, typically requiring it to be within ±0.5%. The shipbuilding industry uses the ISO 15590-1 standard, and its diameter series takes into account the compact design requirements of offshore platforms. The food and pharmaceutical industry commonly uses the 3A hygiene standard, where the diameter of the tees is mostly in inch units, facilitating connection with imported equipment. In cross-industry projects, special attention must be paid to the conversion of diameters between different standard systems. In case of necessity, consulting professional pipe fittings suppliers for detailed comparison tables is advisable. 

## 2. Relationship between Bore Size Selection and Pressure Rating 

2.1 Classification of Pressure Levels 

The pressure rating of a seamless three-way fitting is closely related to its diameter. Internationally, the Class classification system specified by ASME B16.5 is commonly adopted, including multiple grades such as Class 150, 300, 600, 900, 1500 and 2500. In China, there is a corresponding PN series, such as PN10, PN16, PN25, etc. For large-diameter three-way fittings (DN300 and above), due to the large force area, the pressure rating is usually lower, mostly Class 150 or PN10; for medium and small-diameter fittings (DN50-DN300), the pressure rating can reach Class 300-600 or PN16-PN40; while for small-diameter high-pressure three-way fittings (DN50 and below), the pressure rating can reach Class 1500 or above or PN100 or above. When choosing, it is necessary to ensure that the pressure rating of the three-way fitting is not lower than the design pressure of the pipeline system, and at the same time, consider the pressure-temperature rating, because the allowable stress of the material decreases with the increase in temperature. 

2.2 Wall Thickness Calculation Principles 

The wall thickness design of the seamless tee directly affects its pressure-bearing capacity and diameter accuracy. The ASME B31.3 specification provides detailed wall thickness calculation formulas, taking into account factors such as design pressure, allowable stress of the material, and welding joint coefficient. For equal-diameter tees, the wall thickness of the main pipe and the branch pipe is usually the same; while for unequal-diameter tees, the wall thickness requirements for the main pipe and the branch pipe need to be calculated separately. In engineering practice, the actual wall thickness of the tee is generally not less than 1.2 times the wall thickness of the connected pipe to ensure sufficient strength margin. It is worth noting that large-diameter tees may require an increase in wall thickness under high-pressure conditions, but this will reduce the actual diameter and increase fluid resistance. Therefore, hydraulic calculation verification is necessary. Professional pipe fittings manufacturers will optimize the wall thickness design through finite element analysis based on the customer-provided operating conditions parameters, ensuring safety while minimizing material usage as much as possible. 

2.3 Matching Principle for Different Diameter Trifurcations 

The matching of diameters for an adapter fitting requires following the scientific principles of fluid mechanics. The ratio of the diameters of the branch pipe to the main pipe (d/D) is usually controlled within the range of 0.3 to 0.8. Exceeding this range either too much or too little will result in significant local resistance losses. Common matching schemes in petrochemical pipelines include: main pipe DN200 with branch pipe DN150 (ratio 0.75), main pipe DN150 with branch pipe DN100 (ratio 0.67), etc. In situations requiring large flow diversion, a "double branch pipe" design can be adopted, where two smaller-diameter branch pipes replace one large branch pipe to maintain better flow characteristics. For systems prone to water hammer, such as steam pipelines, the diameter of the branch pipe should not be less than 50% of the main pipe to avoid pressure shock caused by sudden cutoff. In complex network systems, it is recommended to use professional hydraulic calculation software to simulate the pressure drop and flow distribution of different diameter combinations and select the optimal adapter fitting configuration.