In Service Inspection of Pressure Vessels: A Mandatory Requirement

18 September 2017

A specific set of inspection procedures are employed when pressure vessels exit the manufacturing phase. They ensure the pressure vessel complies with certain design criteria, as determined by product-relevant commissioning guidelines. However, in service inspections are a little different. They’re mandatory, just like the post manufacturing examination, but they’re evaluating a more dynamic operational scenario. After all, weld seams expand and contract when they’re in service, don’t they?

Evaluating In Service Weld Seams 

Rolled steel sheets are welded to create a geometrically sound cylinder. Internal baffles and spheroidal end segments are next on the welder’s agenda. They’re seamlessly anchored to the overall frame then inspected to ensure they’ll handle the high temperatures and compressed fluids that are stored or processed in the pressure vessel. Just like that old “a chain is only as strong as its weakest link” adage, every seam join and material segment must perform according to its design specs. Unfortunately, those conditions are changing, perhaps they’re even going through a state change. Vapours condense or evaporate, transient temperature spikes hit the inside hard, and the vessel responds accordingly. The walls swell then draw inwards. Weld seams are stressed by this effect, but the in service inspection monitors each and every dimensional shift, no matter how small that expansion effect is during this cyclical heating and cooling event.

Major Repair Maintenance 

Like an industrious product sentry, an in service inspection guards against all types of internal stresses, including those fluid state changes we just covered. Environmental conditions are next, perhaps with a salty spray of sea water causing material fatigue. The briny water is naturally corrosive, so even the finest alloys and weld joints can feel this material oxidizing effect. Lastly, what about major repair work? Pressure vessel integrity alters when an alteration to an existing fluid containment unit is made. An extensive repair project, well, that work could dramatically alter the pressure or temperature handling capabilities of the vessel, so a mandatory in service inspection is an absolutely essential procedure here, with the visually conducted phase accounting for substandard welds, material corrosion, and damaged safety features.

Industriously designed and diligently manufactured pressure vessels must comply with their initial construction specs. Afterwards, however, those design specifications will undergo much dynamic stress, a degree of usage stress that can’t be predicted by the design plant. In order to bridge this design-to-installation employment profile, a mandatory in service inspection strategy must be initiated. In accordance with the guidelines, be they ASME or OSHA mandated, the pressure vessels are inspected while they’re operating so that all internal and external stress factors can be properly monitored.

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