Insulated Steel Pipe Testing

Insulated steel pipe inspection mainly conducts scientific testing and analysis on the steel pipe dimension and appearance testing, polyurethane foam insulation layer property testing, outer protective pipe property testing, bonding property testing, overall performance testing of the insulated pipe and non-destructive flaw detection, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Steel pipe dimension and appearance testing: Checks whether the geometric dimensions such as the outer diameters, wall thicknesses, ovality and lengths of the steel pipe meet the design drawings or standard specifications. At the same time, the steel pipe surfaces are checked with the naked eyes or with the aid of the auxiliary tools for the defects such as the cracks, folds, lamination, scratches, pits, rust and anti-corrosion layer damages, ensuring the appearance quality of the base pipe.

Polyurethane foam insulation layer property testing: This is the core testing item, mainly evaluating the physical and mechanical properties of the insulation layer. Including the density testing (reflecting the compactness of the foam structure), compressive strength testing (measuring its structural stability under the long-term compression), thermal conductivity testing (directly determining the advantages and disadvantages of the insulation effects) and closed cell content testing (affecting the water absorption and long-term insulation performance).

Outer protective pipe property testing: The outer protective pipe (usually the high-density polyethylene HDPE) is the protective layer of the insulation structure. The testing items include the density, tensile yield strength, elongation at break, environmental stress cracking resistance and carbon black content. These indicators are related to the mechanical strength, impact resistance, aging resistance and resistance to the ultraviolet rays of the outer protective pipe.

Bonding property testing: Evaluates the bonding strengths between the insulation layer (polyurethane foam) and the steel pipe outer wall, as well as between it and the outer protective pipe inner wall. The good bonding can ensure that the three-layer structure (steel pipe-insulation layer-outer protective pipe) forms a whole, preventing the insulation failure or thermal bridge effects caused by the delamination. Usually evaluated quantitatively through the peel testing.

Overall performance testing of the insulated pipe: The comprehensive tests simulating the actual working conditions, including the high-temperature compressive creep property testing (evaluating the deformation conditions of the insulation layer under the long-term high temperature and pressure), long-term temperature resistance testing and shear resistance testing. These tests can more realistically reflect the reliability of the insulated pipe in the long-term operation.

Non-destructive flaw detection: The non-destructive testing is carried out on the steel pipe itself to find the potential internal and near-surface defects. The common methods include the ultrasonic flaw detection (UT, for detecting the internal defects and thickness measurement) and eddy current flaw detection (ET, mainly used to detect the surface and near-surface defects), ensuring the pressure-bearing safety of the conveying pipeline.

Testing Scope

Prefabricated directly buried insulated steel pipes: This is the most important testing object, including the "steel pipe-polyurethane foam-HDPE outer protective pipe" three-layer structure pipes used in the projects such as the urban central heating, petrochemical and refrigeration. The testing covers the whole process from the incoming raw materials to the ex-factory finished products, ensuring that they meet the technical requirements of the directly buried laying.

Anti-corrosion and insulation composite steel pipes: Inside the insulation layer, the steel pipe outer wall may be first treated with the anti-corrosion (such as the epoxy powder and 3PE anti-corrosion), and then the insulation is implemented. The testing scope extends to the thicknesses, adhesion and electrical insulation of the anti-corrosion layers, as well as the overall properties after the combination of the anti-corrosion layers and insulation layers.

Steel-jacketed steel steam insulated pipes: The composite structure pipes for conveying the high-temperature steam, usually the "working steel pipe-insulation layer-outer protective steel pipe" structures or the structures with the sliding supports. The testing scope particularly focuses on the high-temperature resistance, the effectiveness of the sliding structures between the inner and outer steel pipes, and the anti-corrosion properties of the outer protective steel pipes.

Insulated pipe fittings and pipe joints: Including the prefabricated insulated pipe fittings such as the elbows, tees and fixed joints, as well as the field joints and connection areas during the on-site installation. These areas are the weak links of the insulation system, and the continuity, sealing of the insulation layers and the connection reliability of the outer protective layers need to be emphatically tested.

Insulation materials and outer protective pipe raw materials: The source control testing of the raw materials constituting the insulated pipes. For example, the ratios and properties of the polyurethane combination polyether and isocyanate, the melt indices and densities of the HDPE pellets, and the material certificates and mechanical property reports of the steel pipes are the basis for ensuring the quality of the finished products.

In-service insulated pipeline evaluation: The excavation sampling testing or trenchless testing is carried out on the insulated pipelines that have been in operation for many years, evaluating the insulation performance degradation, corrosion conditions and structural integrity, providing the scientific basis for the maintenance, repair or replacement of the pipe network.

Testing Methods

Dimension measurement method: The standard measuring tools such as the callipers, micrometers, π tapes and thickness gauges are used for the multi-point measurement of the diameters and wall thicknesses of the steel pipe; the steel tape measures or laser range finders are used to measure the lengths. The appearance inspection is usually carried out under the sufficient natural light or illumination light, and the auxiliary tools such as the weld inspection gauges and magnifying glasses are used when necessary.

Foam property testing method: The density is calculated by measuring the masses and volumes of the specimens according to the GB/T 6343 or ISO 845 standards. The compressive strength is determined by applying the pressures to the specimens on an electronic universal testing machine until the deformation according to the GB/T 8813 or ISO 844 standards. The thermal conductivity is determined with the guarded hot plate method according to the GB/T 10294 or ISO 8301 standards.

Outer protective pipe property testing method: The tensile properties are tested with the dumbbell-shaped specimens on the tensile testing machines according to the GB/T 1040 or ISO 527 standards. The environmental stress cracking resistance is tested according to the GB/T 1842 or ASTM D1693 standards, immersing the notched specimens in the specific surface active agents and observing the cracking ratios within the specified time.

Bonding strength testing method: Usually carried out with the peel testing method according to the standards such as the relevant product standards. The specimens of the specific sizes are prepared, and the insulation layers or outer protective layers are peeled from the steel pipes at the constant rates. The force values in the peeling process are recorded through the transducers, and the average peel strengths are calculated.

Non-destructive testing method: The ultrasonic flaw detection is carried out according to the GB/T 5777 or ASTM E213 standards, using the reflection and transmission characteristics of the ultrasonic waves at the defects for the detection. The eddy current flaw detection is carried out according to the GB/T 7735 standard, using the electromagnetic induction principle to detect the surface and near-surface defects of the conductive materials. For the corrosion of the steel pipes under the insulation layers, the advanced testing technologies such as the pulsed eddy current or guided waves are sometimes adopted.

Overall thermal property simulation testing: According to the standards such as the relevant product standards, the "heat loss method" or "surface temperature method" can be adopted for the evaluation of the overall insulation performance. A complete section of the insulated pipe is placed in the environment with the specific temperature differences, and the heat losses or surface temperatures in the stable state are measured, deriving its overall thermal conductivity properties.

Testing Standards

Domestic Standards:

1. GB/T 29047, Prefabricated directly buried insulating pipes with high density polyethylene outer pipe and rigid polyurethane foam for heating supply

2. GB/T 6343, Cellular plastics and rubbers — Determination of apparent density

3. GB/T 8813, Rigid cellular plastics — Determination of compression properties

4. GB/T 10294, Thermal insulation — Determination of steady-state thermal resistance and related properties — Guarded hot plate apparatus

5. GB/T 1040.2, Plastics — Determination of tensile properties — Part 2: Test conditions for moulding and extrusion plastics

6. GB/T 1842, Plastics — Polyethylene environmental stress cracking resistance determination

7. GB/T 5777, Seamless steel pipe and tubing methods for ultrasonic testing

International Standards:

1. ISO 845, Cellular plastics and rubbers — Determination of apparent density

2. ISO 844, Rigid cellular plastics — Determination of compression properties

3. ISO 8301, Thermal insulation — Determination of steady-state thermal resistance and related properties — Heat flow meter apparatus

4. ISO 527-2, Plastics — Determination of tensile properties — Part 2: Test conditions for moulding and extrusion plastics

5. ASTM D1693, Standard test method for environmental stress-cracking of ethylene plastics

6. ASTM E213, Standard practice for ultrasonic examination of metal pipe and tubing

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Geometric dimension measuring equipment: Including the digital display/vernier callipers, outer diameter micrometers, ultrasonic thickness gauges, π tapes, steel tape measures, laser range finders and weld inspection gauges. The high-precision measurement is the basis for ensuring the pipe installation butt joint and the uniform insulation layer thicknesses.

Material mechanical property testing machine: The electronic universal testing machine is the core equipment. Equipped with the different fixtures and transducers, it can complete the multiple mechanical property tests such as the foam compressive strengths, outer protective pipe tensile strengths and bonding peel strengths. The data acquisition systems can precisely record the stress-strain curves.

Thermal property testing instruments: The guarded hot plate thermal conductivity tester is the key equipment for accurately measuring the thermal conductivity of the insulation materials. In addition, the differential scanning calorimeters can be used to analyse the temperature resistance of the insulation materials, and the heat deflection/vicat softening temperature testers are used to evaluate the heat resistance of the outer protective pipes.

Chemical and analytical instruments: The carbon black content testers (thermogravimetric method) are used to determine the dispersed carbon black contents in the HDPE outer protective pipes. The Fourier transform infrared spectrometers can be used for the qualitative identification and purity analysis of the raw materials (such as the polyether and isocyanate).

Non-destructive testing equipment: The digital ultrasonic flaw detectors used with the probes of the various angles are used for the defect detection of the steel pipe welds and base materials. The eddy current flaw detectors are used for the fast screening of the steel pipe surface qualities. For the composite structures, the dedicated pulsed eddy current testing instruments are sometimes needed to evaluate the corrosion conditions of the steel pipes under the insulation layers.

Environmental simulation and durability equipment: The high and low temperature alternating damp heat test chambers are used to simulate the long-term properties of the insulated pipes in the various temperature and humidity environments. The pressure testing machines (hydrostatic/pneumatic) are used for the pressure-bearing strength and sealing testing of the steel pipes. The long-term thermal stability and creep resistance testing need the dedicated constant-temperature constant-pressure loading devices.