Plastic-lined Composite Steel Pipe Testing

Plastic-lined composite steel pipe inspection mainly conducts scientific testing and analysis on the appearance and dimension testing, lining layer thickness testing, bonding strength, corrosion resistance testing, negative pressure resistance testing and cold and hot cycling resistance testing, 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

Appearance and dimension testing: Checking the continuity, smoothness and whether there are the defects such as the bulges, cracks and pinholes of the inner plastic lining layers (such as the PP, PE and PVC) of the plastic-lined composite steel pipes. At the same time, the dimensions of the bonding parts between the steel pipe substrates and lining layers are measured, including the total wall thicknesses, single layer thicknesses, lengths, outer diameters and ovality, ensuring that they meet the product standards and engineering design drawing requirements.

Lining layer thickness testing: The dedicated wall thickness gauges (such as the ultrasonic thickness gauges) are used to carry out the measurements at the multiple cross-sections and in the different circumferential directions of the pipe bodies. This item aims to verify the uniformity of the plastic lining layers. Their thicknesses must reach the minimum values specified by the designs, to ensure the sufficient corrosion-resistant barrier thicknesses and mechanical protection capabilities. It is the key indicator of evaluating the product qualities.

Bonding strength (flattening) testing: The pipe section specimens are cut, and slowly flattened to the specified distances (usually 2/3 or 3/4 of the pipe diameters) on the dedicated pressure testing machines. After the testing, whether the peeling, delamination or cracks occur between the steel and plastic layers is checked. This testing directly evaluates the bonding firmness between the plastic lining layers and metal substrates, and is the core testing of preventing the lining layer detachment during the use.

Corrosion resistance testing: Evaluated through the various chemical immersion testing or electrochemical testing. The specimens are immersed in the acid, alkali and salt solutions of the specific concentrations (such as the hydrochloric acid, sodium hydroxide and sodium chloride). After the specified temperatures and times, the appearance changes, weight changes of the lining layers and whether the solutions permeate are tested, to verify their chemical stability to the conveyed media.

Negative pressure resistance testing: Simulating the internal negative pressure states that may be generated under the working conditions such as the suction or sudden temperature drops of the pipelines. The specified negative pressure values are applied to the specimens and held for a certain time. After the testing, whether the lining layers have the collapse, peeling or permanent deformation is checked, thereby evaluating the capability of the lining layers to resist the external atmospheric pressures and their structural stability.

Cold and hot cycling resistance testing: The specimens are alternately immersed in the high-temperature water (such as 95℃) and low-temperature water (such as 20℃ or lower) for the multiple cycles. This testing aims to accelerate the simulation of the temperature alternating stresses caused by the seasonal or process changes, testing the bonding interface fatigue resistance between the lining layers and steel substrates caused by the differences of the thermal expansion coefficients.

Hydraulic testing: The hydrostatic pressures higher than the nominal pressures by a certain multiple (such as 1.5 times) are applied to the whole or lengthened plastic-lined composite steel pipes, and held for the specified time. Whether there are the leakage, fracture or lining layer bulging at the pipe bodies and connections is checked. This is the factory delivery mandatory inspection item of verifying their short-term pressure-bearing strengths and sealing integrity.

Testing Scope

Raw material incoming inspection: Covering the material certificates of the steel pipe substrates (such as the chemical composition and mechanical property reports of the Q235 and 20# steel), dimension tolerances and surface treatment qualities (such as the shot blasting derusting grade Sa2.5). The verification of the models, melt flow indices, densities, thermal stability and hygienic properties (when used for the water supply pipes) of the plastic powders or particles, controlling the qualities from the sources.

Production process online inspection: The monitoring implemented during the lining processes (such as the rotational molding, powder coating and lining). Including the real-time recording of the process parameters such as the preheating temperatures, plasticizing temperatures and cooling rates, as well as the visual fast inspection of the initially formed lining layers, ensuring the process stability and the timely finding and correction of the early defects.

Finished product factory delivery inspection: The comprehensive inspection carried out on each batch of the completed plastic-lined composite steel pipes. Usually the sampling inspection, and the items include the appearance dimensions, lining layer thicknesses, bonding strengths (flattening) and hydraulic testing. This scope is the last quality gate before the product release to the market, ensuring that the delivered products meet the basic requirements of the standards.

Type testing and certification inspection: Carried out when the new products are finalized, the material/process major changes occur, or the product certifications (such as the drinking water hygienic licenses and fire protection certifications) are carried out. The testing scope is the widest, needing to cover all the items specified in the standards, including the long-term property testing such as the cold and hot cycling resistance and long-term hydrostatic strength testing, to comprehensively evaluate the comprehensive properties of the products.

Engineering site arrival acceptance: After the plastic-lined composite steel pipes are transported to the construction sites, the verification carried out by the construction parties or supervision parties according to the purchase contracts and technical agreements. Mainly including the verification of the quality certificate documents, inspection of the packaging and markings, and sampling inspection of the appearances and dimensions. When necessary, the samples can be sent to the third-party testing institutions for the re-verification of the key items.

Periodic inspection and evaluation of the in-service pipes: For the plastic-lined pipe systems already put into operation, the periodic or post-fault inspection carried out according to their service years, medium characteristics and safety grades. The testing scope may include the wall thickness corrosion measurement, lining layer endoscopic inspection and bonding state evaluation, providing the basis for the maintenance, repair or replacement of the pipelines.

Testing Methods

Visual and dimension measurement method: The appearance inspection is carried out under the natural light or standard light sources, and with the help of the endoscopes, the inner walls of the pipes can be observed. The dimension measurement uses the tools meeting the precision requirements such as the callipers, micrometers, Pi tapes and ultrasonic thickness gauges. According to the measurement positions and point numbers specified by the standards, the results take the averages or minimum values for the determination.

Flattening testing method: According to GB/T 241 or the relevant product standards (such as CJ/T 136). The pipe ring specimens with the lengths not less than 50mm are cut and placed between the two parallel platens of the testing machines, and the loads are applied at the specified rates until the platen spacing reaches the standard requirements. After the unloading, the specimens are cut open for the direct observation or the tool probing of the bonding interface states.

Chemical immersion testing method: According to the expected conveyed media, the test solution concentrations, temperatures and immersion times are selected (commonly using the 20% sulfuric acid, 40% sodium hydroxide and 10% sodium chloride). After the specimens are completely immersed, they are taken out, washed and dried. Through the comparison of the weight changes of the specimens before and after the immersion, the observation of the lining layer surface morphology changes and whether the solutions change color, the corrosion resistance is evaluated.

Negative pressure testing method: The two ends of a certain length of pipe section are sealed, and one end is connected to the vacuum pumps and pressure gauges. The vacuum pumps are started to reduce the pressures in the pipes to the specified negative pressure values (such as -0.08MPa), and held for the specified time (such as 1 hour). During the pressure holding, whether the pressure gauge readings are stable is observed. After the testing, whether the lining layers have the defects such as the collapse and peeling is checked.

Cold and hot cycling testing method: The specimens are alternately immersed in the high-temperature water tanks and low-temperature water tanks, and held for the specified times at each temperature (such as 1 hour at the high temperature and 1 hour at the low temperature), forming one cycle. Tens to hundreds of cycles are continuously carried out (such as the 30 cycles required by CJ/T 136). After the testing is completed, the flattening testing is immediately carried out, to test whether the bonding strengths are attenuated.

Hydraulic testing method: The water or other suitable liquids are used as the pressurizing media. One end of the pipe is sealed, and the other end is connected to the pressurizing pumps. The pressure is slowly increased to the testing pressure (usually 1.5 times the nominal pressure). The pressure holding time is not less than 5-10 minutes. During and after the testing process, the parts such as the pipe bodies and flange sealing surfaces are carefully checked for the leakage, permanent deformation or abnormal sounds.

Testing Standards

Domestic Standards:

1. CJ/T 136, Plastic-lined composite steel pipes for water supply

2. GB/T 241, Metallic pipes — Hydrostatic test method

3. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

International Standards:

1. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

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

Ultrasonic thickness gauge: Used for the non-destructive measurement of the total wall thicknesses and plastic lining layer thicknesses of the plastic-lined composite steel pipes. The instrument emits the ultrasonic waves through the probes, measuring the time differences reflected at the material interfaces to calculate the thicknesses. The high-precision models optimized for the composite material and thin layer measurement need to be selected, and the standard thickness blocks are regularly used for the calibration.

Universal material testing machine: Used for the mechanical property testing such as the flattening testing and tensile testing (testing the substrates). This equipment can precisely control the loading rates and displacements, and record the load-displacement curves in real time. When carrying out the flattening testing, the dedicated platens with the sufficient widths and parallelism need to be equipped, to ensure that the testing meets the standard requirements.

Hydraulic testing machine (hydrostatic testing machine): The equipment specially used for the pressure-bearing strength and sealing testing of the pipes and pipe fittings. Including the high-pressure pumps, pressure vessels, pressure transducers, safety valves and control systems, which can realize the automatic pressure increase, pressure holding, pressure release and data recording, ensuring the safety and accuracy of the testing process.

High and low temperature cycling test chamber: The dedicated environmental simulation equipment used to carry out the cold and hot cycling resistance testing. It has the liquid tanks or spraying systems inside, which can precisely program and control the temperatures to carry out the fast cycling switching within the set ranges (such as 0℃ to 100℃), and maintain the temperature uniformity, to simulate the harsh temperature alternating environments.

Vacuum pump and negative pressure testing system: Consisting of the vacuum pumps, vacuum gauges (pressure gauges), sealing fixtures, valves and pipelines. Used to generate and maintain the required negative pressure environments in the pipes. The system needs to have the good sealing and stable pumping capabilities, so as to maintain the constant negative pressure values within the specified testing times, thereby accurately evaluating the lining layer negative pressure resistance.

Endoscope and optical measurement equipment: The pipeline endoscopes (industrial video endoscopes) are used to visually inspect the surface conditions of the lining layers in the long-distance pipes, such as the cracks, bulges and contamination points. Combined with the optical measurement software, the dimension quantitative analysis can also be carried out on the inner wall defects. It is the important tool of realizing the internal non-destructive testing.

Chemical analysis and aging testing equipment: Including the constant temperature immersion tanks, analytical balances (precision 0.1mg), pH meters and electrochemical workstations. Used for the chemical corrosion resistance testing and long-term aging property research of the materials. The constant temperature tanks need to provide the stable testing temperature environments, and the analytical balances are used to precisely measure the mass changes of the specimens before and after the immersion.