Spiral Tube Quality Testing Methods: NDT, Hydrostatic and Tightness Checks

Spiral welded tube is produced from steel strip formed into a helix and joined by a continuous helical seam, with a longitudinal butt weld where successive coils are joined. Because the seam carries the full hoop stress of the line, quality testing has to cover surface condition, internal defects, pressure tightness and strength. This guide explains the inspection methods used in production, the pressure calculation applied to the hydrostatic test and the sampling rules applied to non-destructive examination of seams.

Visual Inspection of the Weld and Dimensions

Appearance inspection is the simplest and most widely used check, and it remains an important part of finished product release. It detects surface defects and dimensional deviation, and it is the first indication of problems that may extend inside the weld.

Performed with the naked eye, assisted by a weld gauge, a standard template and a magnifier

Covers undercut, overlap, surface porosity, cracks, arc strikes and poor cap profile

Covers dimensional checks including outside diameter, wall thickness, ovality and straightness

Carried out along the full length of the helical seam and the longitudinal butt seam

Any surface irregularity is treated as a possible indicator of a buried defect, so it triggers follow-up examination by a physical method rather than being judged on appearance alone.

Physical and Non-Destructive Testing Methods

Physical methods use a measurable physical phenomenon to reveal internal defects. For spiral tube the standard toolbox is:

Method	Detects	Application
Ultrasonic testing	Lack of fusion, internal cracks, inclusions, laminations	Helical seam, butt seam and plate edges
Radiographic testing	Porosity, slag, incomplete penetration, cracks	Seams and repair welds, film or digital
Penetrant testing	Surface breaking cracks and porosity	Accessible weld surfaces and repairs
Magnetic particle testing	Surface and near-surface defects	Ferromagnetic tube and weld caps

Method selection depends on defect type and orientation. Ultrasonic testing is most effective for planar defects oriented across the beam, while radiography is better for volumetric defects such as gas pores and slag inclusions.

Strength and Tightness Testing

Pressure retaining items are not only checked for leaktightness; they must also demonstrate adequate strength. Two tests are used in practice:

Hydraulic or water pressure test: the standard method, using water as the pressure medium, which limits stored energy and is therefore the safer option

Air pressure test: more sensitive and faster than the water test, and the product does not need to be drained afterwards, which suits items that are difficult to empty. Because stored energy is much higher, formal safety measures are mandatory during the test

Tightness testing is used for welded containers holding liquid or gas. It reveals non-density defects such as penetrating cracks, pores, slag inclusions, incomplete fusion and loose structure. Common methods include the kerosene test, the water carrying test and the water flushing test, chosen according to the medium and the size of the item.

Hydrostatic Test and Pressure Calculation

Every steel pipe is subjected to a hydrostatic test without leakage. The test pressure is calculated from the hoop stress formula, expressed as P = 2ST/D, where P is the test pressure, S is the test stress in MPa, T is the nominal wall thickness and D is the outside diameter.

The test stress is derived from the strip standard, which normally stipulates 60 percent of the minimum yield value of the grade. For a general structural grade with a minimum yield of 235 MPa this gives a test stress of 141 MPa. Pressure holding time depends on diameter:

Outside diameter	Minimum holding time at test pressure
D below 508 mm	Not less than 5 seconds
D of 508 mm and above	Not less than 10 seconds

Pressure and time are both recorded, because a test held for too short a period cannot reveal a slow leak at a pinhole or a partially fused seam.

Non-Destructive Testing of Welds and Sampling

Non-destructive examination is applied to repair welds, to the steel strip butt welds and to the circumferential seams of finished pipe. The extent of examination depends on the service:

Steel spiral welds used for conveying flammable or ordinary fluids: full radiographic or ultrasonic examination of the seam

Spiral welds for water, sewage, air, heating steam and similar ordinary fluids: radiographic or ultrasonic examination on a sampling basis, typically 20 percent of the length

Repair welds: examined over the full repaired length regardless of the base sampling rate

Sampling is only valid when the sample is drawn systematically along the length rather than from convenient positions, and when a defect found in the sample triggers extension of the examination to the surrounding length. Records of examination extent, method, sensitivity level and results should be kept with the test report for the batch, together with the hydrostatic test chart.

FAQ

Q: What is checked during visual inspection of a spiral tube?
Surface condition of the helical and longitudinal seams, dimensional deviation, undercut, overlap, surface porosity and arc strikes, using a gauge, template and magnifier.

Q: Which non-destructive methods are used on spiral welded pipe?
Ultrasonic testing, radiographic testing, penetrant testing and magnetic particle testing, selected according to the defect type and its orientation.

Q: How is hydrostatic test pressure calculated?
From the formula P = 2ST/D, where S is the test stress in MPa, T is the nominal wall thickness and D is the outside diameter.

Q: How long must the test pressure be held?
At least 5 seconds for outside diameter below 508 mm and at least 10 seconds for 508 mm and above.

Q: Why is an air pressure test considered more dangerous?
Compressed gas stores far more energy than water at the same pressure, so a sudden failure is violent and formal safety measures must be observed.

Q: What sampling rate applies to spiral welds for ordinary fluids?
Radiographic or ultrasonic examination on a spot check basis, typically 20 percent, while flammable fluid service requires full examination of the seam.