PREN Calculator (Pitting Resistance Equivalent, ASME BPE) — Sample Report

1 · Design inputs

A 316L hygienic distribution loop. The mill certificate reports 17.0 % Cr, 2.10 % Mo and 0.050 % N; no tungsten. Stainless/duplex form.

2 · Method

stainless / duplex: PREN = %Cr + 3.3(%Mo + 0.5·%W) + 16·%N

nickel alloys: PREN = %Cr + 1.5(%Mo + %W + %Nb)

The calculator applies one of two closed forms according to the alloy family you select, then places the result on a band scale. For stainless steels and duplexes, chromium enters at unit weight, molybdenum at 3.3, tungsten at half the molybdenum weight, and nitrogen at 16 — the nitrogen coefficient is large because nitrogen is a potent and cheap pitting inhibitor in austenitic and duplex structures, which is why nitrogen-alloyed grades outrank their nitrogen-free equivalents on the same chromium and molybdenum. For nickel-base alloys the form changes: chromium still enters at unit weight, but molybdenum, tungsten and niobium share a single 1.5 coefficient, so the same nominal molybdenum content produces a very different index depending on which form is applied. Selecting the wrong family is the most common way to get a wrong answer here, and it is not a small error. The index is then classified against two thresholds — higher-alloy and super-duplex/super-austenitic — both of which arrive as defaulted inputs rather than hard-coded constants, so a specification that draws its bands elsewhere can be honoured without a software change. Every element is optional except chromium and molybdenum; omitted elements are taken as zero, which is the conservative reading of a mill certificate that does not report them.

3 · Calculation

1. Molybdenum term: 3.3 × (2.1 + 0.5 × 0) = 6.93.
2. Nitrogen term: 16 × 0.05 = 0.80.
3. PREN = 17 + 6.93 + 0.80 = 24.73.
4. Below the higher-alloy threshold → band: standard grade.

4 · Results COMPUTED

5 · Verification statement

The result values above are locked as fixture pre-pren.json, case “316L austenitic”, tolerance 0.001, in the calc-core continuous verification gate (see https://pipingtoolset.com/trust/validation.html). Allowable-stress and factor data are user-supplied inputs; citations: ASME BPE — App. F-3, PRE/PREN forms and the descriptive classification bands (thresholds are defaulted inputs).