Why do Seamless Steel Pipes Fail Hydrostatic Tests?

Failure in hydrostatic tests of seamless steel pipes & tubes typically manifests as cracking (bursting) or leakage, usually related to defects in the pipe itself, manufacturing issues, or improper test operation.

1. Pipe Material or Manufacturing Defects – The Main Cause

Local defects in the pipe prevent it from withstanding the test pressure. These defects mainly originate from different stages of steel production.

1) Continuous Casting Billet Stage

Raw material defects are the main problem. If the steel billet used to roll the pipe itself has problems, it is difficult to compensate for them in subsequent processes. Major defects include:

● Surface Cracks: Tiny cracks on the surface of the steel billet may propagate during subsequent processing.
● Non-metallic Inclusions: The presence of protective slag, large particle inclusions (such as sulfur and calcium compounds), etc., in the steel can become the starting point for cracks.

● Compositional Segregation: Such as central segregation or surface carbonization, leading to uneven material properties.

2) Rolling and Heat Treatment Stage

During the process of turning steel billets into steel pipes, inappropriate processes can directly create defects:

● Surface scratches and folds: Scratches or metal folds (internal fold defects) generated during rolling significantly reduce the effective wall thickness of the steel pipe and create stress concentration at the defect location. This is a common cause of axial bursting during hydrostatic testing.
● Dimensional defects: Uneven wall thickness or localized thinning makes the weakest part of the steel pipe unable to withstand pressure.
● Heat treatment problems: Surface cracks during quenching, or improper heat treatment of seamless pipes leading to abnormal microstructure (such as coarse grains, segregation bands), and low yield strength, will all cause quality defects in the steel pipe.

3) Pipe Processing and Inspection Stage

This is the final checkpoint before leaving the factory; any negligence can lead to defective products entering the market:

● Poor precision in pipe end thread processing: The root of the thread becomes a new stress concentration point, easily causing cracking to begin there.
● Non-destructive testing omissions: Although hydrostatic testing is conducted, factories typically use eddy current or ultrasonic testing for flaw detection. If these testing steps are missed, defective steel pipes may be mistakenly judged as qualified, ultimately revealing problems during on-site pressure testing.

4) Joint or Installation Issues – Applicable to Installed Pipelines

Joint leakage: Incorrect installation of the socket rubber ring, poor welding quality, or substandard top plate or support structure can cause thrust to separate the joint.
Ground settlement impact: Inadequate backfilling of pipes crossing roads can cause flexible joints to exceed their turning angle limits or rigid joints to crack.
Incomplete air purging: Air inside the pipe can cause pressure gauge pointer fluctuations, slow pressure rise, or instability due to gas compressibility.

5) Testing Equipment or Operational Issues

Testing pump malfunction: Incorrect rotation direction, insufficient head/flow rate, preventing the achievement of test pressure.
Uncalibrated pressure gauge: Long-term lack of calibration leads to reading errors and misinterpretation of test results.
Insufficient soaking time: The pipe wall and interface materials did not fully absorb water, resulting in an abnormal pressure drop during pressure testing due to water absorption.

Other causes:
a. Improper pipe transportation or storage: Causes impacts, pinholes, or micro-cracks, reducing sealing performance.

b. Excessive test pressure: Exceeding the design working pressure by 1.5 to 2 times (especially for precision pipes) may directly lead to rupture.

To improve the success rate of hydrostatic testing, the following should be done:

● Strictly control raw materials and production processes to reduce internal defects;
● Standardize installation operations to ensure interface sealing and sufficient venting;
● Regularly calibrate the pressure testing equipment and perform tests at the standard pressure (usually 1.25 to 1.5 times the working pressure) and pressure holding time (generally ≥10 minutes).

FAQ:

Q1: Is a hydrostatic test mandatory for seamless steel pipes before they leave the factory?

A: Whether a hydrostatic test is mandatory for seamless steel pipes before they leave the factory depends on the pipe's end use and the product standards applied.

For most seamless steel pipes used for transporting fluids or bearing pressure (such as fluid pipes, petroleum cracking pipes, and high-pressure fertilizer pipes), the hydrostatic test is a mandatory item that must be performed on a pipe-by-pipe basis during factory inspection. However, for steel pipes used only as structural components (such as scaffolding and mechanical parts), a hydrostatic test is usually not required for each pipe.

Q2: Why do some steel pipes pass factory inspection but fail the pressure test on site?

A: This usually points to two reasons:

Missing non-destructive testing: Factories typically perform eddy current or ultrasonic testing before shipment. If these tests are neglected (i.e., "missed"), steel pipes with minor defects may be mistakenly judged as qualified and ultimately discovered during the hydrostatic test on site.
Different testing conditions: The test pressure, medium, or environment at the construction site may differ from that in the factory, potentially triggering problems not apparent at the factory.

Q3: Besides bursting, what other problems might occur during hydrostatic testing?

A: Yes, leakage is another possibility. This is usually not an instantaneous brittle burst, but rather due to gradual penetration of the pipe wall. For example, in some stainless steel pipes, if "peeling" defects exist on the inner surface, crevice corrosion may occur under certain water qualities. The corrosion pits gradually deepen, eventually penetrating the pipe wall, leading to water leakage during the pressure test.

Q4: How should a hydrostatic test be analyzed after failure?

A: If a failure occurs, it is recommended to conduct a systematic analysis using the following steps:

On-site inspection: Observe the macroscopic characteristics of the burst or leak, recording its location and morphology.
Sampling analysis: Take samples from the failure site for chemical composition analysis to check material compliance.
Metallographic examination: Observe the metallographic structure and inclusions near the defect under a microscope.
Fracture analysis: The fracture morphology is observed using a scanning electron microscope to determine whether it is brittle fracture, ductile fracture, or fatigue fracture.
Process tracing: Based on the analysis results, the steelmaking, rolling, and heat treatment process records of this batch of steel pipes are traced backwards to identify the root cause.

Read more: Hydrostatic Test of Seamless Steel Pipes or Seam Pipe and Seamless Pipe