Stainless Steel Gate Valve Testing

Stainless steel gate valve inspection mainly conducts scientific testing and analysis on the material composition analysis, dimension precision and geometric tolerance testing, shell strength and sealing performance testing, opening and closing torque and operation flexibility testing, surface quality and non-destructive flaw detection, and corrosion resistance evaluation, 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

Material composition analysis: Through the methods such as the spectroscopic analysis and chemical titration, the chemical compositions of the stainless steel materials of the key components such as the valve bodies, gates and stems are precisely determined, such as the contents of the elements such as the chromium (Cr), nickel (Ni), molybdenum (Mo) and carbon (C), ensuring that they meet the designation standards (such as 304, 316 and 316L). It is the basis of verifying the corrosion resistance of the valves.

Dimension precision and geometric tolerance testing: The tools such as the callipers, micrometers and coordinate measuring machines are used to measure the structural lengths, connection flange dimensions, bore sizes, stem diameters and sealing face widths of the valves, and detect their geometric tolerances such as the roundness, flatness and perpendicularity, ensuring that the valves can be accurately installed and matched with the pipeline systems.

Shell strength and sealing performance testing: The hydrostatic or pneumatic strength testing is carried out on the pressure-bearing components such as the valve bodies and bonnets according to the standards, usually applying 1.5 times the nominal pressure (PN) or design pressure, to verify that there is no visible leakage or permanent structural deformation. The sealing testing is carried out on the seats and stem packing, detecting the internal leakage and external leakage under 1.1 times the nominal pressure.

Opening and closing torque and operation flexibility testing: The torque testers are used to measure the operating torques required by the valves at the fully open and fully closed positions. This testing aims to evaluate the matching states between the stems and packing, and between the gates and seats. The excessively large torques may indicate the overtightened packing, component wear or improper assembly, affecting the service lives and operation safety of the valves.

Surface quality and non-destructive flaw detection: Whether the inner and outer surfaces of the valves have the defects such as the cracks, slag inclusions, blowholes and scratches is visually checked. For the key pressure-bearing welds and stress concentration areas, the non-destructive flaw detection methods such as the liquid penetrant testing (PT) or ultrasonic testing (UT) are used to find the micro defects on the surfaces and near surfaces, preventing the potential failure risks.

Corrosion resistance evaluation: In addition to the material analysis, the accelerated corrosion methods such as the salt spray testing and intergranular corrosion testing (such as the sulfuric acid-copper sulfate method) may also be used to simulate the harsh working conditions, evaluating the pitting resistance, crevice corrosion resistance and stress corrosion cracking resistance of the stainless steel valves in the specific corrosive media (such as the chloride ion environments).

Testing Scope

Castings and forged blanks: The testing covers the castings or forgings at the initial stage of the valve manufacturing, including the blanks such as the valve bodies, bonnets and gates. The internal shrinkage cavities, porosity, cracks and other casting defects, as well as the metallographic structures such as the grain sizes and non-metallic inclusions are emphatically checked, ensuring that the raw material qualities meet the subsequent machining and pressure-bearing requirements.

Machined semi-finished products and finished products: The components completing the key machining processes are tested, such as the flange sealing faces, stem threads and seat sealing faces. The testing at this stage ensures that the dimensions, smoothness and geometric precision of the various components meet the drawings, and is the key link of guaranteeing the valve assembly quality and interchangeability.

Assembled integral valves: The final inspection is carried out on the complete valves completing all the assembly processes. This is the most comprehensive testing stage, covering all the above performance tests (strength, sealing and torque, etc.), and checking the nameplate information, coating quality and packaging protection, ensuring that the factory-delivered products meet the technical agreements and standard requirements.

Periodic inspection and condition evaluation of the in-service valves: The testing scope extends to the valves already in use. Through the periodic inspection, the wear, corrosion and leakage conditions after the operation are evaluated, and the stem wear amounts and sealing face damage degrees are measured, providing the basis for the maintenance, replacement or life prediction of the valves. It is the important content of the preventive maintenance.

Additional items for the specific application environments: For the gate valves used in the extreme or special environments such as the food, pharmaceutical, nuclear power, deep sea and high-temperature high-pressure occasions, the testing scope needs to be expanded. For example, the cleanliness, radioactive substance residue, ultra-low temperature impact performance and high-temperature creep performance are tested, to meet the industry-specific regulations and standards.

Valve actuators and accessories: For the gate valves equipped with the electric, pneumatic or hydraulic actuators, the testing scope should include the performances of the actuators, such as the motor insulation, torque output, stroke control and signal feedback precision, as well as the functional reliability of the accessories such as the limit switches, solenoid valves and position indicators.

Testing Methods

Chemical analysis method and spectroscopic method: The inductively coupled plasma optical emission spectrometry (ICP-OES) or spark direct-reading spectrometry is used for the fast composition screening, supplemented by the carbon-sulfur analysers for the determination of the carbon and sulfur contents. For the arbitration or high-precision requirements, the classical wet chemical analysis methods such as the gravimetric method and titration method are used, ensuring the reliability and accuracy of the material composition data.

Liquid hydrostatic testing method: This is the most commonly used strength and sealing testing method. One end of the valve is sealed, the cavity is filled with the water, the pressure is gradually increased to the specified value and held for a certain time (usually not less than 3 minutes), and whether the pressure gauges are stable and the shells have the leakage or sweating phenomena is observed. The test water is usually required to be clean, with the chloride ion contents below the specific limits to prevent the corrosion.

Gas medium testing method: For the valves designed for the gas media or unsuitable for the water testing, the compressed air or inert gases can be used for the testing. The gas testing has the higher risks and needs to strictly comply with the safety regulations, usually using the water immersion method or soap solution smearing method to check the leakages. This method has the high sensitivity, often used for the low-pressure sealing testing and micro leakage detection.

Torque measurement method: The valve is fixed on the test bench, and the calibrated torque wrenches or dynamic torque transducers are used to simulate the manual or actuator operations. The valve is slowly closed clockwise to the fully closed position, recording the maximum closing torque; then reversely opened to the fully open position, recording the maximum opening torque. Compared with the design values, the reasons for the differences are analysed.

Non-destructive testing method cluster: The different methods are selected according to the testing objectives. The liquid penetrant testing (PT) is used to find the surface open defects; the magnetic particle testing (MT) is suitable for the surface and near-surface defects of the ferromagnetic materials; the ultrasonic testing (UT) is used to detect the internal defects and measure the wall thicknesses; the radiographic testing (RT) can provide the planar images of the defects, often used for the weld quality evaluation.

Simulated working condition accelerated testing method: To evaluate the long-term performance, the accelerated life testing can be adopted. For example, thousands of cyclic opening and closing operations are carried out on the test benches to simulate the years of use, and then the sealing and torque testing are carried out again, to verify the durability and reliability of the valves. The corrosion testing uses the equipment such as the salt spray chambers and autoclaves to simulate the harsh environments.

Testing Standards

Domestic Standards:

1. GB/T 12234, Steel gate valves with flanged or butt-welding ends for general purpose

2. GB/T 13927, Industrial valves — Pressure testing

3. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

4. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

5. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

1. ISO 5208, Industrial valves — Pressure testing of metallic valves

2. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

3. ISO 6508-1, Metallic materials — Rockwell hardness test — Part 1: Test method

4. ISO 6507-1, Metallic materials — Vickers hardness test — Part 1: Test method

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

Spectroscopic analyser and carbon-sulfur analyser: The direct-reading spectrometers can complete the quantitative analysis of the multiple elements in the stainless steel samples within seconds, and are the necessary equipment of the incoming material inspection. The carbon-sulfur analysers are specially used for the precise determination of the contents of the two elements carbon and sulfur which have the significant effects on the stainless steel properties (especially the corrosion resistance and weldability).

Mechanical property testing machine and hardness tester: The universal material testing machines are used to determine the tensile strengths, yield strengths and elongations of the materials such as the stems and fasteners. The Brinell, Rockwell or Vickers hardness testers are used to detect the hardnesses of the parts such as the valve bodies and sealing faces, ensuring that the material has the sufficient strengths and wear resistance to withstand the medium pressures and friction.

Pressure testing device: Including the electric test pumps or pneumatic booster pumps, precision pressure gauges or pressure transducers, safety protection devices and testing fixtures. The system can carry out the pressurizing, pressure holding and pressure releasing according to the preset procedures, with the data recording functions. It is the core equipment of carrying out the shell strength testing and sealing performance testing.

Precision dimension measuring instruments: The vernier callipers, outer diameter micrometers and inside diameter dial gauges are used for the routine dimension measurement. For the complex contours and geometric tolerances, the coordinate measuring machines (CMM) need to be used. Through the probe contact with the workpieces, the computer systems precisely calculate the three-dimensional coordinate data, realizing the high-precision and digitalized geometric quantity detection.

Non-destructive testing equipment series: The penetrant testing kits (cleaners, penetrants and developers), magnetic particle flaw detectors (including the yokes and coils), ultrasonic flaw detectors (with the probes of the different frequencies and angles), X-ray or γ-ray flaw detectors and imaging systems. These equipment need to be operated and film-read by the personnel holding the corresponding qualification certificates.

Torque tester and valve life test bench: The digital torque transducers and torque wrench calibrators are used for the precise measurement of the operating torques. The dedicated valve life test benches can programmatically control the automatic cyclic opening and closing of the valves, and monitor the parameters such as the torques, strokes and leakage rates in real time, used for the reliability verification and product quality comparative analysis.

Metallographic microscope and corrosion testing equipment: The metallographic microscopes are used to observe the micro-structures of the material, judging the heat treatment effects, grain size grades and whether the abnormal structures exist. The salt spray test chambers, constant temperature and humidity chambers, high-temperature high-pressure reactors, etc. are used to simulate the various corrosion environments, carrying out the accelerated evaluation of the corrosion resistance of the valves or their specimens.