Stainless Steel Water Tank Testing

Stainless steel water tank inspection mainly conducts scientific testing and analysis on the appearance and dimension testing, weld quality testing, material composition verification, corrosion resistance testing, cleanliness and hygiene testing and full-water hydrostatic 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: Whether the tank bodies, manholes, ladders and pipe nozzles have the defects such as the scratches, dents, deformations and rust is visually checked. The tools such as the tapes, levels and ultrasonic thickness gauges are used to precisely measure the key dimensions such as the lengths, widths, heights and plate thicknesses of the tanks, ensuring that they meet the design drawings and installation requirements.

Weld quality testing: This is the core of ensuring the structural integrity and sealing of the tanks. The testing includes the weld surface forming (such as the undercuts, weld overfills and blowholes), weld widths and reinforcement heights, as well as whether the internal defects such as the lack of fusion, lack of penetration and cracks exist. It is the basis of the subsequent non-destructive flaw detection.

Material composition verification: The portable spectrometers are used for the on-site fast composition analysis of the tank plates and welding materials, verifying whether the contents of the key alloying elements such as the chromium (Cr), nickel (Ni) and molybdenum (Mo) meet the agreed designation standards such as 304 and 316L, eliminating the substitution of the inferior materials.

Corrosion resistance testing: Usually, the nitric acid method (ASTM A262 Practice C) or sulfuric acid-copper sulfate method (GB/T 4334.5) is adopted for the intergranular corrosion tendency testing, evaluating the stability of the stainless steel material in the specific corrosive media, predicting its corrosion resistance in the long-term use.

Cleanliness and hygiene testing: For the tanks storing the drinking water, whether the inner walls are clean, without the oil stains, welding slag and foreign matters, needs to be checked. When necessary, the water samples can be taken for the physical and chemical and microbial index testing (such as the total colony counts and total coliforms), ensuring the stored water safety.

Full-water hydrostatic testing: The verification testing that must be carried out after the installation of the tanks. The tanks are completely filled with the water and left standing for 24-48 hours, observing whether the tank bodies have the obvious deformations, leakage or settlement, verifying their structural strengths and overall sealing properties.

Testing Scope

Stainless steel plate raw materials: The testing is carried out before the plates are warehoused or cut, including the material certificate document verification, dimension tolerances, surface finishes (such as 2B, BA and No.4), as well as whether the scratches, indentations and colour differences exist, controlling the quality from the sources.

Welding processes and post-welding finished products: Covering the whole welding processes, including the welder qualification review, welding procedure qualification, groove preparation, interlayer cleaning, as well as the weld appearances, dimensions and non-destructive testing of the post-welding finished products, ensuring that the welding qualities are controlled.

Internal tension bars and support structures: The specifications, arrangement spacings and welding firmness of the internal vertical and horizontal tension bars (usually the stainless steel angle steels or channel steels) are checked. The flatness, anti-rust treatment and connection with the foundations of the support structures (such as the channel steel bases) are also the testing emphases.

Ancillary fittings and interfaces: Covering the manholes, internal and external ladders, vents, overflow pipes, drain pipes, and inlet and outlet pipe flanges, etc. Whether their materials match the tank bodies, whether the installation positions are correct, whether the opening reinforcements are compliant and whether the flange sealing surfaces are flat are tested.

Insulation layers and protective shells (for the insulated water tanks): The densities, thicknesses, filling uniformity and fire protection grades of the insulation materials (such as the polyurethane foams and rock wool) are checked. The fixing methods, sealing and appearances of the external protective shells (mostly the stainless steel or color steel plates) are also the testing contents.

Before delivery and after the on-site installation: The testing is divided into two main stages. Before the delivery, most of the item inspections are completed in the factories; after the on-site installation, the full-water testing, sealing re-inspection after the connection with the system pipelines and final cleanliness confirmation are emphatically carried out.

Testing Methods

Visual testing (VT): The most basic and widely used method. The testing personnel systematically check all the visible surface defects under the sufficient lighting with the naked eyes or with the help of the magnifying glasses and endoscopes, etc., according to the standard provisions, and record their natures, positions and dimensions.

Liquid penetrant testing (PT): Suitable for detecting the open defects on the surfaces of the stainless steel water tank welds and base materials, such as the fine cracks and pinholes. Through the steps of applying the penetrants, cleaning and developing, the defects are displayed as the visible indications, with the high sensitivity and relatively simple operations.

Ultrasonic thickness measurement (UTM): Using the ultrasonic pulse reflection principle to measure the plate thicknesses. Used to verify whether the actual thicknesses of the stainless steel plates meet the nominal values, and the wall thickness reduction conditions caused by the corrosion can be regularly monitored during the use, evaluating the remaining lives.

Weld radiographic testing (RT): The X-ray or γ-ray is used to penetrate the welds, forming the images on the films or digital imaging plates, used to detect the volumetric defects (such as the blowholes and slag inclusions) and some planar defects (such as the lack of penetration) inside the welds. The results are intuitive and can be archived.

Weld ultrasonic testing (UT): The high-frequency sound beams are used for the testing based on the characteristics that the reflections are generated when encountering the defects during the propagation in the welds. It has the high detection rates for the planar defects such as the cracks and lack of fusion, especially suitable for the thick plate welds, and has no radiation hazards to the human body.

Water quality sampling analysis method: According to the relevant specifications such as the Standards for drinking water quality (GB 5749), the sterile sampling bottles are used to collect the water samples from the tank outlet ports or specific sampling points, and the samples are sent to the laboratories for the testing of the chroma, turbidity, residual chlorine, metal ions and microbial indicators.

Testing Standards

Domestic Standards:

1. GB 5749, Standards for drinking water quality

2. GB/T 4334.5, Corrosion of metals and alloys — Corrosion test of stainless steels in sulfuric acid-copper sulfate solution

3. GB/T 3280, Cold rolled stainless steel plate, sheet and strip

4. GB/T 11170, Stainless steel — Determination of multi-element contents — Spark discharge atomic emission spectrometric method (routine method)

International Standards:

1. ASTM A262, Standard practices for detecting susceptibility to intergranular attack in austenitic stainless steels

2. ISO 3651-2, Determination of resistance to intergranular corrosion of stainless steels — Part 2: Ferritic, austenitic and ferritic-austenitic (duplex) stainless steels — Corrosion test in media containing sulfuric acid

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

Spectrometer: Used for the on-site fast and non-destructive analysis of the alloy compositions of the stainless steel materials. The common handheld X-ray fluorescence spectrometers (XRF) can determine the contents of the elements such as Cr, Ni and Mo within seconds, and are the key equipment of the material identification.

Ultrasonic thickness gauge: The portable electronic instrument, equipped with the probes of the multiple frequencies to adapt to the different surface states and thickness ranges. During the use, the measured surfaces need to be ground to obtain the good acoustic coupling, with the readings precise to 0.1mm. It is the necessary tool of verifying the wall thicknesses.

Weld inspection ruler: A multi-functional measuring tool that can measure the multiple dimension parameters of the welds such as the reinforcement heights, widths, misalignment amounts, groove angles and clearances. With the simple structure and convenient use, it is the basic instrument of the quantitative evaluation of the weld appearance dimensions.

Non-destructive testing equipment: Including the radiographic testing machines (with the films or digital imaging systems), ultrasonic flaw detectors (with the angle-beam probes and straight-beam probes) and penetrant testing kits (penetrants, cleaners and developers). These equipment need to be operated by the personnel holding the corresponding qualification certificates.

Precision dimension measurement tools: Such as the large tapes (30m/50m), laser distance meters, frame levels and plumb bobs, etc. Used to accurately measure the overall and local dimensions, installation verticality and levelness of the tanks, ensuring that the geometric precision meets the design and installation requirements.

Pressure and leak testing equipment: For the pressure-bearing stainless steel water tanks, the pressure testing pumps need to be used for the hydrostatic or pneumatic testing, combined with the precision pressure gauges to monitor the pressure changes. For the full-water testing of the atmospheric tanks, whether the leakage occurs is mainly judged through the water level observation and tank outer wall inspection.

Surface roughness tester and gloss meter: Used for the quantitative evaluation of the surface treatment qualities of the stainless steel. The roughness testers measure the arithmetic mean deviation of the surface profiles (Ra values), and the gloss meters measure the light reflection ability of the surfaces, which is particularly important for the tanks with the special surface treatment requirements (such as the mirror surfaces).