What is a tee fitting?

A tee is a type of pipe fitting or connector used to join pipes. It is primarily used to change the direction of fluid flow, typically where a branch line connects to a main pipeline.

A tee is a chemical piping component featuring three openings—either one inlet and two outlets, or two inlets and one outlet. They come in T-shapes and Y-shapes, with either equal-diameter or reducing-diameter ports, and are used at junctions where three identical or different pipelines converge.

Classification of Tees

Classification by pipe diameter:

Equal tee: A tee where the main pipe and the branch pipe have the same diameter.

Reducing tee: A tee where the branch pipe diameter is smaller than the main pipe diameter.

Classification by connection method:

Butt-weld tee: A tee fitting that is welded after being butt-joined to the connecting pipes.

Socket-weld tee: A tee fitting where the connecting pipes are inserted into the ends of the tee and then welded or fused.

Compression (ferrule) tee: A tee fitting featuring compression fittings at each end; the pipe is inserted and then secured by tightening the nut.

Threaded tee: A tee fitting that connects via threads.

Classification by material: Includes stainless steel tees, carbon steel tees, copper tees, PVC tees, etc.

Classification by branch configuration:

Straight tee: A tee fitting where the branch pipe is perpendicular to the main pipe.

Lateral (angled) tee: A tee fitting where the branch pipe meets the main pipe at an angle.

Equal Tee

The equal tee is a specific type of tee fitting in which all three pipe openings have the same diameter.

Equal tees are widely used in the construction and maintenance of projects across industries such as petrochemicals, oil and natural gas, liquefied gas, fertilizers, power plants (including nuclear), shipbuilding, papermaking, pharmaceuticals, food processing, and urban infrastructure. Industrial applications demand high pressure ratings for these fittings—capable of withstanding up to 600 kg/cm²—whereas residential water pipes operate at much lower pressures, typically around 16 kg/cm². Since tees are used across various industries, they can be classified into several categories based on different criteria:

By material: Carbon steel, cast steel, alloy steel, stainless steel, copper, aluminum alloy, plastic, PVC, PPC, etc.

By manufacturing method: Extruded, pressed, forged, cast, etc.

By manufacturing standard: National Standard (GB), Electric Power Standard, Water Industry Standard, ANSI (US), DIN (German), JIS (Japanese), GOST (Russian), etc.

An equal tee (or straight tee) has the same diameter at all three ends. Designation example: "T3" indicates an equal tee with an outer diameter of 3 inches.

Common materials for equal tees include 10#, 20#, A3, Q235A, 20g, 20G, 16Mn, ASTM A234, ASTM A105, ASTM A403, etc.

The outer diameter range for equal tees is 2.5″–60″; sizes from 26″ to 60″ are typically welded tees. Wall thickness ranges from 28 mm to 60 mm.

Pressure ratings include: Sch5s, Sch10s, Sch10, Sch20, Sch30, Sch40s, STD, Sch40, Sch60, Sch80s, XS, Sch80, Sch100, Sch120, Sch140, Sch160, and XXS.

Reducing Tee

A tee where the branch outlet has a different diameter from the other two ports is called a reducing tee; one where all ends have the same diameter is called an equal tee. Reducing tees are widely used in the construction and maintenance of projects across industries such as petrochemicals, oil and gas, liquefied gas, fertilizers, power plants, nuclear power, shipbuilding, papermaking, pharmaceuticals, food processing, and urban infrastructure. Industrial applications require high pressure ratings for these fittings—up to 600 kg/cm²—whereas residential water pipes typically operate at lower pressures, generally around 16 kg/cm².

Designation example for reducing tees: "T4×4×3.5" indicates a reducing tee with a 4-inch run diameter and a 3.5-inch branch diameter. Reducing tees are typically made from materials such as 10#, 20#, A3, Q235A, 20g, 20G, 16Mn, ASTM A234, ASTM A105, and ASTM A403.

The outer diameter range for reducing tees is 2.5″ to 60″; sizes from 26″ to 60″ are manufactured as welded tees. Wall thicknesses range from 28 mm to 60 mm.

Wall thickness schedules include: Sch5s, Sch10s, Sch10, Sch20, Sch30, Sch40s, STD, Sch40, Sch60, Sch80s, XS, Sch80, Sch100, Sch120, Sch140, Sch160, and XXS.

Tees are used across various industries and can be classified into the following categories based on different criteria:

1. By manufacturing method: mandrel-formed, pressed, forged, cast, etc.

2. By manufacturing standard: Chinese National Standard (GB), Electric Power Standard, Water Industry Standard, American Standard (ANSI/ASME), German Standard (DIN), Japanese Standard (JIS), Russian Standard (GOST), etc.

Tee Manufacturing Process

1. Hydraulic Bulging

Hydraulic bulging is a forming process in which the branch outlet of the tee is expanded through the axial compensation of the metal material. The process utilizes a specialized hydraulic press to inject liquid into a tubular blank with a diameter equal to that of the tee. Two horizontal side cylinders of the press move synchronously toward the center to compress the blank. As the blank is compressed, its volume decreases, causing the internal liquid pressure to rise. When the pressure reaches the level required to form the branch outlet, the metal material flows along the mold cavity—driven by the combined action of the side cylinders and the internal liquid pressure—to bulge out the branch.

The hydraulic bulging process allows for single-step forming and offers high production efficiency; the wall thickness of both the main run and the shoulder of the tee increases during the process.

Due to the high equipment tonnage required for the hydraulic bulging of seamless tees, this process is currently primarily used in China for manufacturing standard-wall-thickness tees with diameters smaller than DN400. Suitable forming materials include low-carbon steel, low-alloy steel, and stainless steel—all of which exhibit a relatively low tendency for work hardening—as well as non-ferrous metals such as copper, aluminum, and titanium.

2. Hot-Press Forming

Hot-press forming of a tee involves flattening a tubular blank (with a diameter larger than that of the finished tee) to approximately the tee's diameter and cutting a hole at the location where the branch pipe is to be drawn. The blank is heated and placed into a forming die, with a drawing punch inserted inside to form the branch. Under pressure, the blank undergoes radial compression; during this process, the metal flows toward the branch area and is shaped into the branch pipe by the drawing action of the punch. The entire process relies on the radial compression of the blank and the drawing of the branch section. Unlike hydraulic bulging, the metal for the branch pipe in hot-press forming is supplied through the radial movement of the blank; hence, this is also known as the radial compensation process.

Because the tee is pressed after heating, the equipment tonnage required for forming is reduced. Hot-press forming is compatible with a wide range of materials, including low-carbon steel, alloy steel, and stainless steel; this process is particularly preferred for large-diameter tees and those with thicker walls.

Causes and Solutions for Angular Deviation During Tee Manufacturing

(I) Tee fabrication is prone to issues such as angular deviation, uneven surfaces, unequal diagonal measurements at the pipe ends, and loose or imperfect seams. The primary causes include:

1) Inaccurate squaring when laying out the straight sheet components for the inner and outer arcs. Failure to account for material thickness results in the outer arc piece being too short and the inner arc piece too long after assembly, thereby affecting the angle.

2) Layout errors when marking the two curved flat sheet components.

3) Formation of small holes at the seam where the outer arc is bent and joined.

4) Unequal widths in the combined-angle seams, leading to uneven stress distribution at the joints.

(II) Key preventive and corrective measures include:

1) Using a calibrated square to ensure accuracy when squaring and cutting materials. When marking out the side walls of the tee, use dimensions R1 and R2; add allowances for bending and seam locking (folding) to the width to ensure a proper fit during installation.

The developed lengths for the outer and inner arcs of the elbow are 1.57R1 and 1.57R2, respectively; ensure accuracy during the arc-rolling process.

2) After cutting the two large flat pieces, strictly square off both ends of the sheets. Align the two curved flat sheets to check for profile mismatch and adjust them within the permissible tolerances.

3) Small holes appearing at the bent corners of the tee's outer arc may be repaired via tin soldering for stainless steel plates.

4) When forming seams manually, ensure the width is correct to maintain the tee's overall dimensions, and apply force evenly during the locking process.

Factory Inspection of Tees

The chemical composition of the tees shall be verified against the material specifications; if special requirements exist, re-inspection may be conducted in accordance with relevant standards. Arbitration testing methods shall comply with the relevant standards in the GB/T 223 series.

The tee surfaces must be free from cracks, porosity, undercuts, and slag inclusions. Both inner and outer surfaces must be smooth and free from defects such as scratches exceeding the negative tolerance of the wall thickness, dents, or straightening marks. Cut ends must be free from burrs. Visual inspection of the stainless steel tee's appearance shall be conducted under natural or artificial lighting (a 5x magnifying glass may be used).

Dimensional inspection shall be performed using measuring tools of appropriate precision.

Factory Inspection: Tees shall be submitted for acceptance in batches. Each batch must consist of fittings of the same grade, size, and manufacturing process. Batch sizes are limited to 400 pieces for nominal diameters of 25 mm or less, and 200 pieces for nominal diameters greater than 25 mm.