Equipment inspection interval
The due date of your next inspection, calculated from your measurements then capped by the regulation that applies to this equipment.
Everything is calculated in your browser. The result is a ceiling, not an authorisation: the order of 20 November 2017 requires the inspection to take place 'as often as necessary' and to shorten these intervals where the condition of the equipment warrants it.
The equipment
What you have measured
What your knowledge is worth
What the calculation does not see
Where the ceilings come from
The 2, 4 and 10-year intervals come from the order of 20 November 2017 on the in-service monitoring of pressure equipment, articles 15 and 18. Those for storage tanks come from the order of 3 October 2010, article 29. Those for lifting equipment from the order of 1 March 2004, article 23. The extensions to 5, 6 and 7 years assume an inspection plan drawn up in accordance with an approved professional guide and validated by an authorised body.
For pressure piping and for equipment outside the scope of the pressure regulation, no regulation sets an interval. The tool says so instead of displaying a figure that does not exist.
How the interval is calculated
Remaining life equals the remaining margin divided by the degradation rate. It is then multiplied by a fraction, half in the usual case, and by a confidence factor between 0.5 and 1 that reflects the quality of what you know about the equipment: number of previous inspections, quality of the data and methods, results obtained, repairs carried out.
The interval used is the shorter of this calculation and the applicable ceiling.
What this tool does not calculate
It deals only with thinning. Localised corrosion, cracking, fatigue and creep cannot be deduced from a thickness, and the tool flags this rather than producing a misleading figure.
It calculates no sanitary risk. A crack of two tenths of a millimetre on a drying tower is mechanically insignificant and can be sanitarily major. The link runs through the criticality of the defect with respect to the product, not through the thickness margin.
It says nothing about the extent or the location of the inspections. An inspection on the right date in the wrong place detects nothing.
The full reasoning is set out in the article on inspection intervals, with the references of the regulations cited.
Definition
The inspection interval is the time between two regulatory checks of an equipment. For pressure equipment, the French order of 20 November 2017 sets maximum deadlines: periodic inspection monitors fitness for service, while periodic requalification re-attests that fitness under the control of an approved body. These ceilings must be shortened when the condition of the equipment warrants it.
Formula
Calculated deadline = (current thickness - minimum allowable thickness) / corrosion rate. The result, the remaining life, is then multiplied by a confidence factor between 0.5 and 1 depending on data quality, and finally capped by the applicable regulatory interval. Variables: thicknesses in mm, corrosion rate in mm per year.
How to use this tool
- First check the applicable regulatory ceiling: for pressure equipment, the order of 20 November 2017 sets the periodic inspection and requalification (ten years as a rule, six years if the fluid is toxic or corrosive, three years for certain aggressive fluids).
- Compute the remaining life section by section, from dated thickness readings and the most penalising corrosion rate.
- Apply a confidence factor between 0.5 and 1 depending on the number of previous inspections, the data quality and the equipment history.
- Keep the shorter of the calculated interval and the regulatory ceiling, and tighten further if a localised mechanism or a doubt about integrity requires it.
- Document the rationale: without usable history, no interval extension is defensible in an audit.
Worked example
A vessel has a current thickness of 12 mm and a minimum allowable thickness of 8 mm. The corrosion rate used is 0.25 mm per year. The remaining life is (12 - 8) / 0.25 = 16 years. With a confidence factor of 0.5, the deadline drops to 8 years. As the regulatory requalification ceiling is ten years, the shorter value is kept: 8 years.
References
Frequently asked questions
Which safety factor should be used?
The tool does not decide it: it comes from your inspection plan and the site's policy. A factor of 2 amounts to scheduling the inspection at half the calculated time, which leaves room for measurement uncertainty and for an acceleration of the degradation.
Why is the result sometimes capped?
Because a regulatory or contractual interval overrides the calculation. If the calculation allows eight years and the interval imposes four, the due date is four years: a calculation never replaces an obligation.
What should be done if the recent rate is higher than the background rate?
Take the more unfavourable one for the due date, and treat the discrepancy as a signal: a change of product, of temperature, of regime, or a measurement point that has moved. An unexplained acceleration is met with a tighter interval; it is not averaged out.
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