Rubber-lined Steel Pipe Testing

Rubber-lined steel pipe inspection mainly conducts scientific testing and analysis on the appearance and dimension inspection, rubber lining layer thickness testing, rubber lining layer hardness testing, rubber lining layer bonding strength testing, electric spark pinhole testing and corrosion resistance verification, 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 inspection: Checks whether there are the defects such as the bubbles, cracks, pinholes, delaminations, wrinkles and embedded foreign matters on the inner and outer surfaces of the rubber-lined steel pipe. At the same time, the geometric dimensions of the steel pipe need to be measured, including the pipe diameter, wall thickness, length, ovality and rubber lining layer thickness, ensuring that they meet the requirements of the design drawings and purchase technical agreements.

Rubber lining layer thickness testing: The non-destructive magnetic thickness gauges or ultrasonic thickness gauges are used to measure the multiple representative points on the pipe body. The test results need to ensure that the rubber lining layer thicknesses are uniform, and not lower than the minimum thickness values specified in the standards or contracts. This is the basis for guaranteeing the corrosion resistance and mechanical strength of the rubber-lined steel pipe.

Rubber lining layer hardness testing: The Shore A type or Shore D type hardness testers are adopted, and in accordance with the standards such as GB/T 531.1 or ASTM D2240, the hardness values are measured at the multiple points on the rubber lining layer surface. The hardness is an important indicator measuring the vulcanization degree and physical properties of the rubber material, and its value should be within the technical parameter ranges provided by the rubber manufacturers.

Rubber lining layer bonding strength testing: Evaluates the bonding firmness between the rubber lining and the steel pipe substrate. The peel or tensile testing is usually carried out with a tensile testing machine, determining the peel forces per unit width. The unqualified bonding strength will cause the blistering or falling off of the lining during the use, and it is the key testing item affecting the service life.

Electric spark pinhole testing: A high-voltage electric spark detector is used for the 100% scanning of the rubber lining layer. When the probe passes through the insulating defects such as the pinholes and cracks, the high voltage will break down the air to produce the sparks and alarms. This is the most direct and effective non-destructive method of detecting the continuity and micro-pore defects of the rubber lining layer, and the voltage settings need to be determined according to the rubber lining layer thicknesses.

Corrosion resistance verification: The chemical stability of the rubber lining layer in the specific media (such as the acid, alkali and salt solutions) is evaluated through the laboratory immersion testing. The testing items include the mass change rates, volume change rates, hardness changes and appearance changes, thereby judging whether the selected rubber material is applicable to the expected working condition environments.

Testing Scope

Ex-factory inspection of the newly manufactured rubber-lined steel pipes: Covering all the newly produced rubber-lined steel pipes, including the pipe fittings such as the straight pipes, elbows, tees and reducers. Before leaving the factory, the comprehensive testing must be completed, ensuring the qualified product quality and issuing the corresponding test reports. This is the primary link of the quality control.

Arrival acceptance inspection before the installation: The re-inspection that needs to be carried out by the user units or supervision parties after the rubber-lined steel pipes arrive at the construction sites. The qualified certificate documents are emphatically checked, and the sampling inspections of the appearance, dimensions and electric spark are carried out, preventing the damages caused during the transportation and storage.

Periodic in-service inspection and monitoring: For the rubber-lined pipeline systems already in operation, the periodic inspection plans are formulated according to their use environments, medium characteristics and operating pressures. The aging, wear, swelling and local falling off conditions of the rubber lining layers are mainly inspected, evaluating their remaining service lives.

Quality confirmation after the maintenance or replacement: For the pipelines after the local repair or overall replacement of the linings, the comprehensive testing must be re-carried out, especially the bonding strengths, thicknesses and electric spark testing of the repair areas and their surroundings, ensuring that the maintenance quality reaches the original design standards.

Special testing under the specific working conditions: For the rubber-lined pipelines conveying the high-temperature, high-pressure, strongly abrasive or strongly corrosive media, the targeted testing items need to be added. For example, the properties after the thermal aging need to be tested under the high-temperature working conditions; the wear resistance needs to be tested under the abrasive working conditions.

Lined pipes applicable to the different rubber materials: The testing scope covers the steel pipes lined with the various elastomer materials such as the natural rubber (NR), butyl rubber (IIR), chloroprene rubber (CR), ethylene-propylene rubber (EPDM), nitrile rubber (NBR) and fluororubber (FKM).

Testing Methods

Visual inspection and endoscope-assisted inspection: Under the sufficient lighting, the inspectors directly observe the outer surface of the rubber lining layer. For the large-diameter pipes, the internal inspection can be carried out; for the small-diameter or non-directly-viewable parts, the pipeline endoscopes need to be used for the video recording and inspection, ensuring no omissions.

Ultrasonic thickness measurement method: The ultrasonic pulse reflection principle is used to measure the rubber lining layer thicknesses. The probe is coupled on the rubber lining surface, and the instrument displays the round-trip time of the ultrasonic waves in the rubber lining layer, and the thickness is obtained through the calculation. This method has the high precision, applicable to most rubber materials.

Magnetic thickness measurement method: Applicable to the situation where the substrate under the rubber lining layer is the iron and steel. The magnets in the thickness gauge probes generate the magnetic attraction with the steel substrate. The thicker the rubber lining layer, the smaller the magnetic attraction. The thickness is indirectly obtained by measuring the changes of the magnetic attraction, with the simple and fast method.

Peel strength testing method: In accordance with the standards such as HG/T 20678 or the relevant standards, the rubber lining strips of a certain width are cut on the samples, and a tensile machine is used to peel them from the steel substrates at a constant rate (such as 100 mm/min), recording the average force values in the peeling process and calculating the peel strengths per unit width.

High-voltage electric spark testing method: In accordance with the relevant standards, the detection voltages are adjusted according to the rubber lining layer thicknesses (usually 3000-5000 V applied per millimetre of thickness). During the detection, the probe moves at a uniform speed of about 0.1-0.3 m/s, ensuring that the whole rubber lining surface is uniformly scanned, and the absence of the electric spark leakage is qualified.

Laboratory immersion testing method: The standard specimens are cut from the rubber-lined pipes or test plates, immersed in the specified chemical media, and maintained for a certain time (such as 23 ℃ × 168 h) at the specific temperatures. After the expiry, they are taken out, and the changes of their masses, volumes and hardnesses are measured, and whether there are the phenomena such as the swelling, cracking and discoloration on the surfaces is observed.

Testing Standards

Domestic Standards:

1. GB/T 18241.1, Rubber lining — Part 1: Anticorrosion lining for equipment

2. GB/T 531.1, Rubber, vulcanized or thermoplastic — Determination of indentation hardness — Part 1: Durometer method (Shore hardness)

3. HG/T 20678, Code for design of steel vessel lined with rubber

International Standards:

1. ISO 7619-1, Rubber, vulcanized or thermoplastic — Determination of indentation hardness — Part 1: Durometer method (Shore hardness)

2. ASTM D2240, Standard test method for rubber property — Durometer hardness

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: The core equipment for the precise measurement of the rubber lining layer thicknesses. The models suitable for the measurement of the non-metal materials such as the rubber need to be selected, and the probe frequencies are usually 5 MHz. Before the use, the calibration with the standard thickness test blocks is needed, and the good acoustic coupling between the probe and rubber lining surface (using the couplants) is ensured.

High-voltage electric spark detector: The dedicated instrument for detecting the pinholes and cracks of the rubber lining layer. The equipment consists of a high-voltage generator, detection electrode (spring brush or metal ring) and grounding wire. The key parameters are the adjustable output voltage ranges and alarm sensitivities, and the accuracy of its output voltages must be regularly verified.

Shore hardness tester: The portable tool for measuring the hardness of the rubber lining layer. Divided into the type A (for the soft rubber) and type D (for the harder rubber). During the measurement, the presser foot needs to be steadily pressed on the specimen surface, the indenter is perpendicular to the specimen, and the maximum indication is immediately read. Each specimen should be measured at the multiple points and the average values should be taken.

Electronic tensile testing machine: Used for the mechanical property tests such as the peel strengths, tensile strengths and elongation at break of the rubber lining layers. The equipment needs to have the sufficient load ranges and precision, and the fixtures should be able to effectively clamp the specimens without slipping. The testing process is controlled by the computers and the data curves are automatically recorded.

Digital vernier callipers and tape measures: Used to measure the macro dimensions such as the outer diameters and lengths of the rubber-lined steel pipes. The measuring tools with the suitable precision (such as the callipers with the 0.02 mm precision) should be selected. During the measurement, the parallax needs to be eliminated, and the multiple measurements at the different pipe body cross-sections are needed to evaluate the ovality.

Constant-temperature immersion testing device: Including the constant-temperature water baths or ovens, glass or corrosion-resistant containers with the covers, and analytical balances. Used to carry out the medium corrosion resistance testing of the rubber lining materials under the controlled temperature conditions, ensuring that the test environments meet the standard requirements and guaranteeing the comparability and accuracy of the data.