Inspection of Pressure Vessels

A pressure vessel is the general term for closed-volume engineering equipment in which the pressure of the contained fluid (steam, air, gas or liquid) is significantly higher than atmospheric pressure; steam and hot water boilers, air receivers, compressor tanks, hydrophore and expansion vessels, autoclaves and LPG/gas storage vessels all fall into this class. Throughout their service life this equipment is exposed to the combined effect of internal pressure, temperature cycling, corrosion, erosion and material fatigue; thinning of the wall, crack formation in weld seams or degradation of the mechanical properties of the material creates the risk of a sudden and destructive rupture. Periodic inspection is the systematic engineering activity that keeps this risk within acceptable limits by verifying the mechanical integrity of the vessel at defined intervals, using competent personnel and appropriate inspection techniques.

The inspection process begins with a review of the design and manufacturing documentation of the vessel (manufacturer's certificate, material certificates, calculation report, previous inspection records). The equipment is then taken out of service, depressurised and safely isolated from energy and fluid sources so that the external examination can be carried out; visual testing covers surface corrosion, deformation, traces of leakage, corrosion under insulation, fasteners and support structures. Where access is possible, an internal examination reviews internal surface corrosion, pitting, scaling and weld areas. Suspect areas are then examined in greater detail using non-destructive testing (NDT) methods.

The principal verification tool in pressure vessel inspection is the hydrostatic test. In this test the vessel is completely filled with water and the air inside is vented; the pressure is raised gradually with a calibrated test pump to 1.5 times the operating pressure unless the standards state otherwise, and is held for the specified period while pressure drop, permanent deformation and leakage are observed. Because water is incompressible, it limits the released energy in the event of a rupture, making the test safer than an air or gas test. Ultrasonic thickness gauges are used to measure wall thinning; the measured values are compared with the manufacturing thickness and the calculated minimum allowable thickness. Safety devices such as the safety valve, pressure gauge and level indicator are also separately checked for correct function.

The depth of the inspection varies with the type of vessel and the hazard of the fluid it contains. In steam boilers, the tube areas and fire-side regions exposed to thermal stress and overheating are of particular concern; in air receivers, internal corrosion in the lower areas where condensate accumulates; and in LPG and gas vessels, the integrity of weld seams and stress corrosion cracking. Whether the corrosion allowance has been consumed is determined by comparing the measured thicknesses with the design calculations; where required, surface cracks are investigated by magnetic particle or penetrant testing and volumetric discontinuities by ultrasonic or radiographic testing.

At the end of the inspection the findings are recorded, together with the operating pressure and temperature of the vessel and its manufacturing and measurement data, in a traceable inspection report. The acceptance criteria are the absence of permanent deformation and leakage in the hydrostatic test, measured wall thickness remaining above the minimum allowable value, and the correct function of the safety devices. The report states whether the equipment can be used safely until the next inspection date and identifies any situations requiring repair/reinforcement or restriction of operating conditions; for the employer it means both the fulfilment of a legal obligation and the documentation of operational safety. TÜV AUSTRIA SILA KALİTE performs the periodic inspection of pressure vessels as an accredited Type A inspection body under TS EN ISO/IEC 17020, within the framework of the relevant regulations and standards.