Steam Pipeline Testing

Steam pipeline inspection mainly conducts scientific testing and analysis on the wall thickness measurement, weld quality testing, corrosion condition testing, geometric dimension and deformation testing, hardness and metallographic analysis, and leakage 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

Wall thickness measurement: The ultrasonic thickness gauges are used to measure the key parts of the steam pipelines. It is the basic testing item of evaluating the wall thinning conditions caused by the corrosion and erosion. The vulnerable areas such as the elbows, tees, reducers and the bottoms of the straight pipe sections are emphatically tested. Through the comparison with the original design wall thicknesses or the previous testing data, the corrosion rates are calculated and the remaining service lives are evaluated.

Weld quality testing: The non-destructive testing is carried out on the connection parts such as the butt welds and fillet welds of the steam pipelines, mainly checking whether there are the defects such as the cracks, lack of fusion, lack of penetration, slag inclusions and blowholes. The welds are the areas prone to the stress concentration and fatigue damage, and their qualities are directly related to the structural integrity of the pipeline systems, especially under the high-temperature and high-pressure working conditions.

Corrosion condition testing: Comprehensively evaluating the corrosion types and degrees of the inner and outer surfaces of the pipelines. The internal corrosion mainly includes the oxygen corrosion, acidic condensate corrosion and erosion; the external corrosion mainly checks the corrosion under insulation (CUI). The testing means include the visual inspection, ultrasonic scanning, corrosion coupon analysis and corrosion product analysis, to determine the corrosion mechanisms.

Geometric dimension and deformation testing: Checking the straightness, roundness and ovality of the pipelines, as well as whether there are the local indentations, bulges or overall bending deformations. These geometric defects may be caused by the foundation settlement, external loads, thermal stresses or manufacturing defects, which will affect the medium flow, increase the local stresses, and even lead to the instability failures.

Hardness and metallographic analysis: The samples are taken at the parts such as the weld heat-affected zones and the outer sides of the bent pipes for the hardness testing and metallographic structure observation. This testing item aims to evaluate whether the materials have deteriorated after the long-term high-temperature service, such as the pearlite spheroidization, graphitization, creep damage or temper embrittlement phenomena, providing the basis for the material deterioration grading.

Leakage testing: The methods such as the acoustic emission testing, infrared thermography, soap solution leak detection or helium mass spectrometer leak detection are adopted to check whether there are the steam leaks at the parts such as the pipe bodies, flange connections and valve packing boxes. The micro leaks not only cause the energy losses, but the high-temperature and high-pressure steam carried by them may also pose the direct threats to the personnel and equipment safety.

Stress state and support system testing: Evaluating whether the stress distributions of the pipeline systems are within the allowable ranges, and checking the operating states of the pipe supports and hangers. Including the checks of the load-displacements of the spring supports and hangers, jamming of the constant-force supports and hangers, damage or falling-off of the rigid supports and hangers, ensuring that the thermal displacements of the pipelines are effectively restrained and compensated, avoiding the excessive secondary stresses.

Testing Scope

Main steam pipelines and reheat steam pipelines: This is the pipeline part with the highest pressures and temperatures in the power plant boiler systems, usually made of the alloy steels. The testing scope covers all the pipe sections from the boiler outlets to the turbine inlets, including the main pipelines, drain pipes and bypass pipes, emphatically focusing on their high-temperature creep damage and oxide scale shedding conditions.

Industrial heating steam pipe networks: Widely applied to the process steam and heating systems of the industries such as the petrochemical, textile and food. The testing scope usually starts from the boiler rooms or heat source outlets, along the overhead, trench or directly buried laying paths, until the inlets of the various steam-using equipment, emphatically testing the internal corrosion caused by the condensate and the insulation failure.

Pressure pipeline components: Including the tees, elbows, reducers, pipe caps, flanges, valves and expansion joints in the steam pipeline systems. These components have the complex geometric shapes and obvious stress concentrations, and are the emphases and difficulties of the testing. The combined use of the multiple testing technologies is needed to carry out the full-coverage inspections.

High-temperature and high-pressure parts and stress concentration areas: The testing scope must prioritize the coverage of the high-temperature sections with the steam temperatures exceeding 450℃, high-pressure sections with the working pressures higher than 4.0MPa, as well as the stress concentration areas such as the welds, opening reinforcement areas and structural discontinuities. These areas are the high-risk points of the material deterioration and defect initiation.

Pipe sections with the abnormal operation histories: For the pipe sections that have experienced the overtemperature, overpressure operation, water hammer, severe vibration or have the recorded defect histories, they should be included in the key testing scope. The testing frequencies and testing item depths need to be increased, to monitor the defect growth trends and the further changes of the material properties.

Safety valves and discharge systems: Checking the safety valves, relief valves and their discharge pipelines connected to the steam pipelines. Ensuring the accurate lifting pressures of the safety valves, and the firm fixation and no blockage of the discharge pipelines, to ensure the unblocked pressure relief paths under the overpressure conditions. This is the last barrier of the pressure pipeline safety protection.

Testing Methods

Ultrasonic testing (UT): It is the core non-destructive testing method of the steam pipeline inspection. The pulse-echo ultrasonic method is used to detect the internal defects of the welds; the ultrasonic thickness measurement method is used to measure the wall thicknesses; the TOFD (time of flight diffraction) or phased array ultrasonic testing (PAUT) is used for the precise imaging and quantitative analysis of the key welds, especially suitable for the coarse-grain materials and thick-wall pipe testing.

Radiographic testing (RT): Mainly using the X-rays or gamma rays to transmit and expose the pipeline girth welds. Through the films or digital imaging systems, the internal volumetric defects (such as the blowholes and slag inclusions) and planar defects (such as the lack of fusion and lack of penetration) are observed. For the occasions where the double-sided contact ultrasonic testing cannot be carried out, the RT has the irreplaceable advantages.

Magnetic particle testing (MT) and penetrant testing (PT): Used to check the surface and near-surface open defects of the ferromagnetic material (MT) or non-ferromagnetic material (PT) steam pipelines, such as the cracks and laps. The MT has the good detection capabilities for the near-surface defects, while the PT is more suitable for the external surface inspection of the non-ferromagnetic pipelines such as the austenitic stainless steel.

Eddy current testing (ET): Suitable for the fast screening of the surface and near-surface defects of the conductive material steam pipelines, especially used to check the heat exchanger tube bundles and small-diameter pipelines. For the in-service pipelines, the remote field eddy current technology can also be adopted to detect the internal wall corrosion and wall thinning of the carbon steel pipelines.

Acoustic emission testing (AE): A dynamic and overall testing method. During the pressurization or operation of the steam pipelines, the elastic waves released from the defects are monitored through the transducers arranged on the pipeline surfaces, thereby locating and evaluating the active defects (such as the crack growth). It is often used for the pressure testing monitoring and online monitoring.

Macro inspection and hardness testing: The macro inspection is the basis. Through the visual inspection, endoscopes and measuring tools, the macro defects and geometric dimensions of the inner and outer surfaces of the pipelines are checked. For the hardness testing, the Leeb or Brinell hardness testers are used to quickly evaluate the material hardnesses on-site, indirectly judging whether the material strengths and structures have changed.

Metallographic inspection and creep damage evaluation: Belonging to the destructive sampling inspection. The replicas or micro samples are taken from the pipelines, and the metallographic specimens are prepared in the laboratories. Through the optical or electron microscopes, the structure changes are observed, and according to the relevant standards (such as DL/T 884 and ASTM E1181), the damages such as the pearlite spheroidization, graphitization and creep cavities are graded.

Testing Standards

Domestic Standards:

1. DL/T 884, Technical guide for metallographic inspection and evaluation of thermal power plants

2. GB/T 231.1-2018, Metallic materials — Brinell hardness test — Part 1: Test method

3. GB/T 17394, Metallic materials — Leeb hardness test

International Standards:

1. ASTM E1181, Standard test methods for characterizing duplex grain sizes

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

Digital ultrasonic flaw detector and thickness gauge: The modern ultrasonic testing equipment generally has the functions such as the A-scan display, digital storage, automatic gain and DAC/TCG curve production. The high-precision digital thickness gauges have the resolutions reaching 0.01mm, and are equipped with the high-temperature probes, which can directly measure at the certain temperatures. They are the necessary tools of the wall thickness general survey.

Phased array ultrasonic testing system: This system electronically controls the ultrasonic beams emitted by the array probes for the focusing and deflection, realizing the sector or linear scanning without moving the probes, and generating the intuitive C-scan or S-scan images. It is especially suitable for the efficient and high-precision testing of the complex geometric shape parts (such as the welds and elbows).

X-ray digital imaging system (DR/CR): The DR uses the flat panel detectors for the real-time imaging, and the CR uses the imaging plates for the recording and reading after the exposure. Compared with the traditional film radiography, the digital imaging systems have the high efficiencies, environmental friendliness and convenient image storage and transmission, and can carry out the image enhancement and quantitative analysis through the software, gradually becoming the mainstream.

Endoscope and pipeline crawling robot: The industrial video endoscopes are used to check the areas that cannot be directly observed by the human eyes, such as the pipe interiors and the backs of the valves. For the large-diameter and long-distance steam pipelines, the pipeline crawling robots equipped with the camera, ultrasonic or laser ranging modules can be adopted for the automated internal inspection, obtaining the comprehensive internal state data.

Acoustic emission testing system: Consisting of the multiple high-sensitivity piezoelectric transducers, preamplifiers, data acquisition cards and dedicated analysis software. The system can collect, process and analyse the acoustic emission signals in real time, locate the sound sources, and evaluate the activity and severity of the defects through the analysis of the signal parameters (such as the amplitudes, counts and energies).

Metallographic microscope and hardness tester: The key equipment of the laboratory analysis. The stereomicroscopes are used for the low-power observation, and the optical metallographic microscopes are used for the high-power structure analysis. The portable Leeb hardness testers are used for the fast on-site testing, while the more precise Brinell and Vickers hardness testers are used for the hardness calibration of the sampled specimens in the laboratories.

Infrared thermal imager and laser vibrometer: The infrared thermal imagers are used for the non-contact measurement of the temperature distributions of the pipeline surfaces, which can find the insulation layer defects, internal blockage or leakage points. The laser vibrometers are used to precisely measure the vibration amplitudes and frequencies of the pipelines in the operating states, assisting in the analysis of the problems such as the pipeline support and hanger states and fluid-induced vibration.