20.2 Mechanical Integrity, Reliability, and Failure Mechanisms

20.2 Mechanical Integrity, Reliability, and Failure Mechanisms

The NCEES specification lists Process equipment and reliability (e.g., testing, maintenance, mechanical integrity, failure mechanisms) under Operation and Maintenance. Section 16.2 covered the electrochemistry of corrosion. This section covers the program that finds damage before it causes a loss of containment, and the arithmetic that decides how often to look.

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1. The Mechanical Integrity Element

Under (1910.119(j)), mechanical integrity applies to:

• Pressure vessels and storage tanks
• Piping systems, including components such as valves
• Relief and vent systems and devices
• Emergency shutdown systems
• Controls, including monitoring devices and sensors, alarms, and interlocks
• Pumps

The program must include written procedures, trained maintenance personnel, inspection and testing at frequencies consistent with manufacturer recommendations and good engineering practice, documentation of every inspection, correction of deficiencies before further use (or in a safe and timely manner with interim measures), and quality assurance that new equipment is fabricated and installed to specification.

RAGAGEP — recognized and generally accepted good engineering practice — is the benchmark. The standards a chemical engineer is expected to name:

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2. Corrosion Rate, Remaining Life, and Inspection Interval

Thickness is monitored at fixed thickness measurement locations (TMLs) so that successive readings are comparable.

$\text{Long-term rate} = \frac{t_{initial} - t_{actual}}{\text{years between them}} \qquad \text{Short-term rate} = \frac{t_{previous} - t_{actual}}{\text{years between them}}$

The greater of the two governs. A component that corroded at (0.05\text{ mm/yr}) for fifteen years and then at (0.60\text{ mm/yr}) since the last inspection has a long-term average that looks harmless; using it would set an interval that lets the component fail. The short-term rate is what detects a process upset, a change in feedstock, or a failed corrosion inhibitor.

$\text{Remaining life} = \frac{t_{actual} - t_{required}}{\text{corrosion rate}}$

where (t_{required}) is the minimum thickness for the design pressure and temperature (Section 16.1), excluding the corrosion allowance — the corrosion allowance is what is being consumed, not what must be retained.

The interval rule. Under API 510 for vessels and API 570 for piping, the internal or on-stream thickness inspection interval is the lesser of one-half the remaining life or 10 years (shorter for high-consequence piping classes). External visual inspection is typically on a 5-year cycle.

Worked example. A vessel course has (t_{nominal} = 0.500\text{ in}), (t_{required} = 0.300\text{ in}), current measured thickness (0.380\text{ in}), and a governing corrosion rate of (0.008\text{ in/yr}).

$\text{Remaining life} = \frac{0.380 - 0.300}{0.008} = \frac{0.080}{0.008} = 10.0\text{ years}$

$\text{Interval} = \min\left( \tfrac{1}{2}(10.0),\ 10 \right) = 5.0\text{ years}$

The half-life rule exists so that the component is examined at least once more while enough wall remains to act on the finding. Inspecting at the full remaining life would mean the next look happens at the moment of retirement thickness.

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3. Damage Mechanisms

Two of these deserve emphasis because they defeat ordinary inspection. CUI is invisible without removing the insulation, which is expensive, so it is inspected on a risk-prioritized basis targeting known water-ingress points: penetrations, supports, damaged jacketing, and lines that cycle through the damp temperature band. Pitting and SCC produce essentially no measurable general wall loss, so a thickness-only inspection program will report a healthy component right up to the through-wall leak; detecting them requires appropriate techniques such as visual examination, dye penetrant, or ultrasonic shear wave.

Risk-based inspection (API 580/581) allocates inspection effort by (\text{risk} = \text{probability of failure} \times \text{consequence of failure}), concentrating resources on the small fraction of components that carry most of the risk rather than inspecting everything on a uniform calendar.

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4. Reliability Mathematics

During the flat middle of the bathtub curve the failure rate (\lambda) is approximately constant, and:

$R(t) = e^{-\lambda t}, \qquad MTBF = \frac{1}{\lambda}$

  Failure rate lambda
     ^
     |\                                        /
     | \  infant                              /  wear-out
     |  \ mortality                          /
     |   \___________________________________/
     |          useful life (constant lambda)
     +----------------------------------------------> time

Availability accounts for how long repairs take, not only how often failures occur:

$A = \frac{MTBF}{MTBF + MTTR}$

A pump with (MTBF = 8{,}000\text{ h}) and (MTTR = 40\text{ h}) has (A = 8{,}000/8{,}040 = 0.995). Cutting (MTTR) to (10\text{ h}) by stocking a spare rotating assembly raises (A) to (0.99875) — often cheaper per point of availability than extending (MTBF).

System architecture. For components in series (all must work), reliabilities multiply:

$R_{sys} = \prod R_i$

For components in parallel (any one suffices), unreliabilities multiply:

$R_{sys} = 1 - \prod (1 - R_i)$

Three parallel pumps each at (R = 0.90), with any one able to carry the duty, give (R_{sys} = 1 - (0.10)^3 = 0.999). The same three in series — as in a train where every unit must run — give (R_{sys} = 0.90^3 = 0.729). This asymmetry is why a long unspared chain of equipment has poor availability even when every individual item is good, and why sparing is applied to the specific items with the lowest (R).

Maintenance strategy follows from consequence:

Time-based preventive maintenance on an item with a constant failure rate is wasted effort: a memoryless failure process is no less likely to fail just after an overhaul than just before one, and the intervention itself introduces infant-mortality risk. Preventive replacement only pays where there is a genuine wear-out mechanism.