PRES 2 common mistakes
PRES 2 · Understand the traps and how to avoid them.
Treating the digester as a single uniform vessel when selecting materials, instead of separating the liquid-phase, vapor-phase (gas/headspace), and splash-zone exposure zones and assigning a different alloy or lining to each.
Candidates memorize one 'digester alloy' per process type (kraft, sulfite, prehydrolysis) and apply it to the whole shell, missing that the vapor space and the liquor-line interface are typically the most aggressive zones and often require different materials or protection than the submerged shell.
What to do instead
For every materials question, first sketch the vessel with three labeled zones (vapor/headspace, splash and liquor line, fully submerged), then assign the dominant corrodent (e.g., H2S/mercaptans, chloride, thiosulfate, acidity) and the candidate alloy or lining to each zone before answering.
Choosing a stainless steel or nickel alloy for a pulp-mill stream without checking chloride concentration, temperature, and the presence of oxidizing species against the alloy's pitting and stress-corrosion-cracking resistance.
Candidates focus on general corrosion resistance and overlook localized attack; chloride SCC of austenitic stainless in warm pulp streams and pitting under deposits are frequent field failures that exam items target directly.
What to do instead
Use a two-step screen: first general corrosion rate against the stream's chemistry, then localized attack (chloride level vs. temperature, PREN or critical pitting temperature, and SCC susceptibility). Reject any alloy that fails the localized screen even if its general rate is acceptable.
Confusing design pressure, MAWP, and relief set pressure, and sizing a relief device using the design pressure or the operating pressure rather than the correct allowable accumulation basis.
These three values are close in normal operation but diverge in upset scenarios; candidates default to the operating or design pressure because those numbers appear first in the problem statement, and then size the orifice against the wrong driving force.
What to do instead
Always identify the governing code basis first (e.g., ASME Section VIII Division 1 UG-125 through UG-137 accumulation limits), set the relief device at or below MAWP, and compute the required capacity at the maximum relieving pressure including allowable accumulation, not at design or operating pressure.
Applying a generic pressure-vessel code rule to a fired or pulp-digester component without checking whether the specific code section, service restriction, or joint-efficiency and corrosion-allowance rule actually governs that component.
Candidates learn one code path and reuse it everywhere; digester shells, continuous digesters, and their nozzles and attachments may fall under different code sections, and joint efficiency, corrosion allowance, and post-weld heat-treatment exemptions change with thickness, material, and service.
What to do instead
Before calculating, state the applicable code section, the material specification, the joint category and efficiency, the corrosion allowance, and whether PWHT is required; then confirm each assumption against the code clause cited in the question.
Selecting a non-destructive examination method by habit (usually radiography or dye penetrant) instead of matching the method to the defect type, geometry, material, and access constraints.
Candidates associate 'inspection' with one familiar technique and forget that volumetric versus surface methods, magnetic versus non-magnetic materials, and thick versus thin sections each exclude certain techniques; this produces answers that cannot detect the defect described.
What to do instead
Classify the defect first (surface-breaking vs. internal, planar vs. volumetric, in ferromagnetic vs. austenitic material), then eliminate methods that cannot detect it or cannot access the geometry, and only then choose among the remaining qualified techniques.
Treating a remaining-life or fitness-for-service assessment as a single thickness measurement, ignoring corrosion rate trending, local metal loss, and the required future corrosion allowance.
Candidates compute remaining life from one thickness reading divided by a nominal rate, which ignores scatter in inspection data, localized thinning, and the code-mandated corrosion allowance that must be reserved for the next inspection interval.
What to do instead
Use at least two inspection epochs to establish a rate, apply the shortest-interval or most-conservative rate where data scatter, subtract the required future corrosion allowance, and state the assessment standard and acceptance criteria used for local thin areas.
Answering maintenance and repair questions with a field-repair action without checking whether the repair requires code qualification, welding procedure qualification, or post-repair examination and pressure testing.
Candidates think in operational terms (weld it, patch it, replace the section) and skip the quality-assurance layer that the exam tests: repair procedures, welder qualification, NDE after repair, and re-rating of the component.
What to do instead
For any repair scenario, list in order: applicable code repair clause, approved welding procedure and welder qualification, material traceability, post-repair NDE, pressure test or alternative, and any re-rating or documentation update before returning to service.
Misapplying the risk-assessment logic by ranking consequences and likelihood independently and then combining them without checking the risk matrix definition or the layer-of-protection credit already claimed.
Candidates treat risk ranking as arithmetic intuition and double-count independent protection layers or ignore the defined likelihood and consequence scales, producing a risk category that contradicts the facility's own matrix.
What to do instead
State the likelihood and consequence scales and the matrix cell explicitly, credit each independent protection layer only once and only if it meets independence and audit requirements, and verify the resulting risk category against the matrix before selecting safeguards.
Confusing continuous and batch digester process logic when answering equipment or process questions, applying batch temperature-time profiles or chip-column behavior to a continuous system or vice versa.
The two systems share vocabulary (cooking, impregnation, blow) but differ in residence-time distribution, liquor-to-wood ratio control, and equipment layout; candidates pattern-match on the shared words and select an answer valid only for the other system.
What to do instead
Before answering, identify the digester type from the stem, then reason within that type's residence-time and liquor-flow model; if the stem is ambiguous, note which assumptions differ between batch and continuous and choose the answer consistent with the stated equipment.
Ignoring the fiber-line stage context (digester, blow tank, washing, screening, bleaching, or recovery interface) and answering a corrosion, erosion, or materials question with a rule that applies to a different stage of the process.
The pulp mill is a chain of stages with very different chemistries and solids loadings; candidates recall a material or failure mode from one stage (e.g., bleaching) and apply it to another (e.g., black liquor or washing), where the dominant degradation mechanism is different.
What to do instead
Anchor every question to the named process stage, list that stage's dominant chemistry, temperature, and solids or velocity conditions, and only then select the material or failure mechanism; discard any answer whose justification depends on a different stage's conditions.