Zhuojin Pipe Fitting Co., LtdPressure Control and Sealing Methods in Three-Way Tee Hydraulic Bulging
Literature Overview
This paper, authored by Li Zhijie, Feng Jun, and Hu Weimin from the 725th Research Institute of China Shipbuilding Industry Corporation, was published in the journal "Pipe Technology and Equipment" in 2000. The work addresses two critical engineering challenges in the hydraulic bulging process for manufacturing three-way tee fittings: the sealing of pipe ends during the bulging operation and the precise control of bulging pressure. The authors give particular emphasis to the implementation of a stepped overflow method for pressure regulation, which represents a practical engineering innovation for achieving uniform wall thickness in the formed tee geometry.
Core Technical Content
Hydraulic bulging of tee fittings is a solid-forming process in which a seamless pipe blank is placed inside a contoured die, and internal hydraulic pressure is applied to force the pipe material to conform to the die cavity, thereby creating the branch opening characteristic of a tee. The process eliminates the need for cutting, welding, or heavy machining, and offers advantages in material utilization, mechanical property preservation, and production efficiency. However, two fundamental problems must be solved before successful bulging can occur: reliable end sealing to contain the hydraulic fluid at high pressure, and accurate pressure control to ensure uniform material flow into the die cavity.
End Sealing Methods
The end sealing problem arises because the hydraulic pressure, which can reach several hundred megapascals, must be contained within the pipe blank while the material deforms against the die. The authors describe several sealing approaches:
| Sealing Method | Description | Applicable Diameter Range | Key Consideration |
|---|---|---|---|
| Plug sealing with elastic seals | Steel plugs with O-rings or polyurethane seals inserted at both pipe ends | DN50 to DN400 | Must withstand radial pressure without extrusion |
| Mechanical clamp with seal | End caps bolted or clamped to the pipe with gasket sealing | DN400 and above | Requires precise bore preparation |
| Integral weld-seal | Pipe ends welded shut with thin-wall caps before bulging | Small diameter pipes | Adds heat-affected zone concerns |
| Hydraulic clamp with internal pressure | External hydraulic rams compress seal rings against pipe bore | Large diameter, high-pressure applications | Requires dedicated tooling |
The choice of sealing method depends on the pipe diameter, material grade, bulging pressure level, and the required production rate. For carbon steel tees with wall thicknesses between 8 and 25 mm, the plug-and-elastic-seal method is the most commonly employed in industrial practice because it balances cost, reliability, and throughput.
Stepped Overflow for Pressure Control
The stepped overflow technique is the central innovation discussed in this paper. In hydraulic bulging, the pressure required to form the tee varies significantly along the length of the pipe blank because the material must flow into a branching geometry where the cross-sectional area changes abruptly. A single constant pressure cannot accommodate this variation, leading to either over-thinning at the branch intersection or incomplete forming at the pipe ends.
The stepped overflow method introduces a pressure-regulating mechanism at the overflow port that allows the system pressure to be adjusted in discrete steps during the forming cycle. The principle is as follows: the hydraulic system is equipped with a bypass or overflow line fitted with adjustable restriction orifice plates. By sequentially opening or closing these restrictions during the bulging stroke, the effective pressure applied to the pipe blank is modulated in a stepwise fashion. The early stage of forming, where the pipe wall first contacts the die at the branch opening, requires higher pressure to initiate material flow. As the material progresses and fills the die cavity, the required pressure decreases, and the overflow is progressively opened to reduce the effective forming pressure.
This approach offers several practical advantages:
- It avoids the need for complex, real-time feedback-controlled pressure systems that are expensive and prone to failure in industrial environments.
- It provides repeatable pressure profiles that can be optimized offline and applied consistently in production.
- It reduces the risk of over-pressurization and burst failure of the pipe blank, which is a significant safety concern at pressures exceeding 300 MPa.
The authors note that the number of pressure steps and the magnitude of each step must be determined through trial forming and finite element simulation. Typically, three to five pressure steps are sufficient for standard tee geometries with branch angles of 90 degrees.
Engineering Practice Integration
From a manufacturing standpoint, the stepped overflow method has been adopted in several Chinese pipe fitting production lines for carbon steel and low-alloy steel tees conforming to ASME B16.9 and GB/T 12459. The method is particularly effective for tees with wall thickness ratios (D/t) between 20 and 40, where the material has sufficient ductility to form without cracking but also requires careful pressure management to avoid excessive thinning.
In practice, the following process parameters are critical:
- Forming temperature: Room temperature for carbon steel; 200 to 400 degrees Celsius for low-alloy steels such as 15CrMo or 12Cr1MoV.
- Bulging pressure: Typically 200 to 450 MPa depending on material strength and geometry.
- Forming speed: Controlled by the hydraulic pump flow rate, typically 1 to 5 mm/s of die punch travel.
- Die clearance: 0.5 to 1.5 percent of the pipe wall thickness, accounting for springback.
A common defect encountered when the stepped overflow is not properly calibrated is localized wall thinning at the branch root, where the material experiences the highest strain. This can lead to failure of the hydrostatic test or reduced fatigue life in service. The countermeasure is to increase the number of pressure steps and extend the dwell time at the initial high-pressure stage to allow more uniform material distribution.
Study Insights
This paper, while published in 2000, addresses fundamental engineering problems that remain relevant in modern hydraulic bulging practice. The stepped overflow concept is essentially a simplified form of pressure trajectory control, and its practical implementation demonstrates the value of straightforward mechanical solutions over complex electronic control systems in high-pressure forming environments. For engineers working on tee fitting production, this paper provides a clear framework for understanding how pressure modulation can be achieved with simple hydraulic hardware, and it underscores the importance of sealing reliability as a prerequisite for safe and successful bulging operations.