When the Slurry Wear Workbook Stops Being Reproducible
Compute a short-term rate from the two most recent readings and a long-term rate from the baseline, and let the more conservative one govern. In a concentrator the long-term rate is usually the useless one: slurry erosion scales with velocity cubed, so a throughput increase or a coarser grind invalidates every year of history before it.
A spreadsheet stops working in a concentrator at the point where nobody can say which two readings produced a governing rate. That is a reproducibility failure, not a formula failure, and it usually surfaces during a shutdown scope review when a 6 o'clock cell on a tailings elbow shows a wall gain of 3 millimetres. The spool was rotated 120 degrees eighteen months earlier, as the maintenance plan requires. The workbook has no field for that, so the reading was typed into the same cell, and the short-term rate went negative while the long-term rate quietly halved. Meanwhile a rubber-lined section three hundred metres downstream has reported 0.000 for six years and is two months from a breach that will take the substrate out in a single campaign. Both problems are the same problem: the calculation has no record of the events that reset it, and no way to say that a rate is not meaningful.
Source: Written against API 579-1/ASME FFS-1 Part 4 for general metal loss and Part 5 for local metal loss, API 570 and API 653 applied as voluntary technical method where no jurisdiction requires them, ASME Section VIII Division 1 for autoclaves and acid plant vessels, ASME Section V for ultrasonic examination, ASTM G75 slurry abrasivity and ASTM G76 for erosion testing, MSHA 30 CFR Parts 56 and 57, and the Global Industry Standard on Tailings Management.
| Event | What happens physically | What a fixed-cell spreadsheet records | What the calculation should do |
|---|---|---|---|
| Spool rotated 120 degrees on the maintenance plan | The worn invert moves to the 10 o'clock position and fresh wall arrives at 6 o'clock | A sudden 3 mm thickness gain in the 6 o'clock cell | Reset the baseline on that spool, retain prior readings, exclude them from the rate |
| Grind coarsened after a mill or cyclone change | Particle impact energy rises; erosive wear steps up non-linearly | One long-term rate averaging two different wear regimes | Short-term rate governs from the change date, with the change visible on the trend |
| Throughput raised 15 percent | Velocity rises 15 percent; erosive wear rises roughly 40 to 65 percent | No change until the next reading, then an unexplained jump | Change date recorded so the step is attributable rather than mysterious |
| Rubber lining breaches at an elbow extrados | Substrate goes from zero loss to rapid loss within one campaign | Six years of 0.000 rates followed by a wall-out | Liner condition tracked separately; substrate rate reported as not yet meaningful |
| Spool replaced during a shutdown | Wall returns to nominal on that component only | Negative rate, or the historical row is simply overwritten | New component identity and baseline, old history preserved and traceable |
The job: three editors, one workbook, and no memory
The workbook worked when one reliability engineer owned it. It has since acquired a second engineer, a contract inspection coordinator and a planner who filters it during shutdown scoping. It lives on a shared drive. Somebody has pasted values over the governing-rate formula for forty rows because the calculation looked wrong that week. Somebody else inserted a column for the 2024 readings, which shifted the references in the short-term rate formula for the rows below the insertion point. Dates in one sheet are text and sort alphabetically. Two tabs hold the same tailings line with different minimum thicknesses.
None of that is incompetence. It is the predictable end state of a tool with no concurrency control, no audit trail and no schema, applied to an asset base with thousands of measurement locations across kilometres of slurry piping, thickener and leach tank shells, cyclone clusters, launders, autoclave and acid plant vessels. The workbook did not fail suddenly; it stopped being reproducible, and nobody noticed the day it happened.
The moment it becomes visible is usually a challenge to one number. An area superintendent asks why a spool is on the shutdown list, and the answer requires knowing which readings produced the rate. The engineer opens the sheet and cannot say. That is the search that leads here, and it is a governance problem first and a calculation problem second.
Erosion-corrosion does not behave like the model behind a corrosion rate
A corrosion rate assumes that a wall thins because of a chemical process acting over an area, at a pace that is stable enough to extrapolate. Slurry wear does not satisfy either half of that. It is mechanical removal by particle impact and sliding contact, modulated by whatever corrosion the process water contributes, and it is intensely localised. In a bend, material is removed from the extrados over an arc of a few degrees. Downstream of a partially closed valve, wear concentrates in a plume that may not intersect any established measurement location.
The velocity dependence is what breaks extrapolation. Erosive wear in slurry service scales with velocity raised to a power typically between 2.5 and 3.5. A fifteen percent throughput increase therefore raises wear by roughly forty to sixty-five percent, not fifteen. A concentrator that debottlenecks a circuit has changed the wear rate of every line in it by a factor no historical trend can anticipate. The same applies to a grind change that coarsens the particle size distribution, or to an ore body transition that raises abrasivity, which is measurable by the ASTM G75 Miller Number and rarely tracked alongside the thickness data.
There is also a regime change hiding in the hydraulics. Above the deposition velocity the solids stay suspended and wear concentrates at bends and impingement points. Below it a settled bed forms and wear moves to the invert, and the pipe can be thinning at 6 o'clock while every CML at 12 and 3 o'clock reads healthy. A throughput reduction is therefore not automatically good news, and a rate computed from CMLs in the wrong place is a rate for a location that is not failing.
Spool rotation: the arithmetic trap no oil and gas page warns you about
Slurry pipeline maintenance plans commonly call for spools to be rotated on a schedule, typically by 120 degrees, so that wear is distributed around the circumference instead of concentrating in one place until the spool is scrap. This roughly triples usable life and is entirely standard practice. It is also, from the point of view of a thickness database, a component substitution that leaves the component in place.
Consider a CML defined by line, station and clock position. Before rotation it sits on the worn invert and reads 8.2 mm against a 12.7 mm original wall. The spool is rotated. The same CML, defined the same way, now sits on wall that was at 2 o'clock and reads 12.1 mm. The short-term rate for that interval is strongly negative. The long-term rate, computed from the original baseline to 12.1 mm, has just fallen to near zero. If the workbook has no field for the rotation, the two people who knew about it will remember for a year and then will not.
There are two defensible fixes and one indefensible one. The defensible fixes are to define the CML in a frame attached to the spool, so that a rotation carries the location with the metal, or to record the rotation as a dated reset event that starts a new baseline while preserving prior readings. The indefensible fix, and the common one, is to overwrite the historical readings so the trend looks smooth. That destroys the only record of how fast the spool wore before rotation, which is the number the planner needs in order to predict when it will need rotating again.
Why the short-term rate governs in a plant that is still changing
In stable service the long-term rate is the more reliable of the two because it averages out measurement noise. Mining operations are frequently not in stable service. Throughput ramps over the first years of a mine, expansions add mills and thicken slurry, ore hardness and abrasivity vary as the pit advances through zones, and process changes at the front end propagate into every downstream line. The long-term rate carries all of those regimes averaged together, weighted by how long each lasted, which is not how a forward projection should be weighted.
This is exactly the situation the governing-rate rule was written for. Taking the higher of short-term and long-term means the calculation follows the plant when the plant steps up, without requiring anyone to decide in advance which regime to trust. What the rule needs to work is that the short-term rate is computed over a genuinely recent interval and is not swamped by noise, which puts a floor on inspection frequency and a ceiling on acceptable measurement scatter.
The complementary requirement is that operational changes are in the data. A dated marker for a throughput change, a grind change, a liner material change or an ore zone transition costs one row and transforms the interpretation of a trend. Without it, a step in the wear rate looks like a measurement problem and gets investigated as one; with it, the engineer knows on sight why the two rates diverged and which one to act on.
Lined and non-metallic assets report zero right up to failure
A large fraction of a concentrator's wetted surface is not bare steel. Tailings and process lines are rubber lined or HDPE. Leach and CIL tanks may be lined or coated. Autoclave interiors are brick lined with a membrane. SX settlers are lined concrete or FRP. Acid plant equipment uses specialised alloys and anodically protected coolers. For all of these, the thickness of the pressure-retaining or structurally significant substrate is not the quantity that determines when the asset fails.
This creates a specific and dangerous artefact in a rate-based system. The substrate loses nothing while the lining is intact, so the computed rate is zero and the remaining life is effectively infinite. The asset sorts to the bottom of every risk-ranked list. When the lining breaches — at an elbow extrados, at a flange, at a repair patch, at a fastener penetration — the substrate is exposed to the full erosive duty at that point and can go through-wall inside a single campaign. The rate history offers no warning at all, because there was nothing to measure.
The correct handling is to model the lining as the primary condition indicator, inspected by its own methods, and to mark the substrate rate as not yet meaningful rather than zero. A system that cannot express "no measurable loss, and that is expected" as distinct from "zero measured loss, therefore healthy" will produce a risk ranking that inverts reality on the most numerous assets on the site.
Averaging: Part 4, Part 5, and the assessment a spreadsheet cannot support
API 579-1/ASME FFS-1 provides two routes for metal loss. Part 4 covers general metal loss and permits thickness averaging over a defined length, on the premise that the loss is broadly uniform and the averaged value fairly represents the section. Part 5 covers local metal loss and evaluates a flaw using a critical thickness profile and a remaining strength factor, accounting for the fact that a short, deep groove behaves differently from a long, shallow one.
Slurry erosion produces local metal loss. Applying Part 4 averaging to an erosion groove blends the thin section with sound material on either side and returns an averaged thickness that is higher than the metal actually present at the deepest point. The assessment then passes when a Part 5 evaluation of the same scar would not. This is not an edge case in a concentrator; it is the normal geometry of the damage.
The practical obstacle is data structure. A Part 5 assessment needs a thickness profile along and across the flaw, not a single number per location. A spreadsheet row holding one thickness per CML per date physically cannot store a profile, so the assessment defaults to whatever Part 4 permits, and the engineer either does the Part 5 work in a separate document that never links back or does not do it at all. Any system being evaluated should be asked directly whether it can hold a scan profile against a location and carry it into an assessment, because that capability determines which code route is available.
Jurisdiction, the tailings envelope, and who eventually asks
Mining sites often sit outside the pressure equipment jurisdiction that oil and gas takes for granted. Many tanks and slurry lines are not registered pressure equipment at all, and MSHA under 30 CFR Parts 56 and 57 regulates safety practice rather than fitness for service. That absence of an external inspection mandate is exactly why site standards drift and why a spreadsheet survives far longer than it should.
There are hard exceptions. Pressure oxidation autoclaves and acid plant vessels are ASME Section VIII Division 1 equipment with real jurisdictional oversight, and boilers likewise. More significantly, since the Global Industry Standard on Tailings Management, tailings delivery pipelines sit inside a governed risk envelope with a named Engineer of Record and a defined review cadence. A tailings line leak is not a maintenance event; it is a reportable environmental release inside a facility whose integrity is under formal external review.
That reframes the record-keeping question. The engineer of record and the corporate tailings governance process will eventually ask for the basis of a remaining life on a tailings line, and the acceptable answer is a traceable calculation, not a workbook that three people edit. Sites that adopt API 570 and API 653 methods voluntarily, without any jurisdiction requiring them, generally do so for this reason: the methods provide a defensible structure where the regulator does not supply one.
Evaluating a corrosion rate module for a concentrator
Start with reproducibility, because that is the failure you are actually fixing. Take a governing rate from the current workbook and ask the candidate system to show, for the equivalent location, the two readings behind the short-term rate with dates, technician, instrument and calibration reference, the baseline and its source, the exposure interval, the minimum thickness in force and its revision history, and the procedure under which the rate was computed. If any of those is absent or editable without a trace, you have bought a nicer spreadsheet.
Then test the mining-specific behaviours deliberately. Can you record a spool rotation as a dated event and see the baseline reset while prior readings remain visible? Can you define a CML in a frame attached to the spool rather than the line station? Can you mark an asset as lined and have the substrate rate reported as not meaningful rather than zero? Can you store a scan profile against a location so a Part 5 local metal loss assessment is possible? Can you record a throughput, grind or ore zone change as a dated marker that appears on the trend?
Finally, test the field and the audit ends. Data capture in a concentrator happens on scaffold, in wet conditions, often with no network, so offline capture with later reconciliation is not a luxury. At the other end, ask for the export that goes to the tailings engineer of record or the jurisdictional inspector for the autoclave, and check whether it is a report the system generates or a document somebody assembles by hand. The second answer means the workbook survives, and if the workbook survives, so does the problem.
Why does a corrosion rate model fit slurry service badly?
Because the model assumes loss that is roughly uniform and roughly linear in time. Slurry wear is neither. It is concentrated at bend extrados, tee blind ends, downstream of throttling valves, and along the invert where a settled bed forms below the deposition velocity. It scales with velocity to a power near three, with solids concentration, and with particle size and hardness. A rate expressed as millimetres per year hides all of that in one number.
What does spool rotation do to a thickness trend?
It invalidates it, unless the coordinate frame is tied to the spool rather than the line. Rotating a spool 120 degrees moves the worn invert to a new clock position and brings unworn wall to the old one. A CML defined as "6 o'clock at station 14+20" now measures different metal. The trend shows a wall gain, the short-term rate goes negative, and the long-term rate drops by roughly the amount of wear that was rotated away.
Why does a rubber-lined line report a corrosion rate of zero right up to failure?
Because until the lining breaches, the steel substrate is not in contact with the slurry and genuinely loses nothing. The thickness trend is flat and the computed rate is zero, which reads as excellent condition. When the lining fails locally, the substrate is exposed to full erosive duty at that spot and can be through-wall in weeks. A substrate rate on a lined asset is not a low rate; it is an absent measurement, and it should be presented that way.
When is API 579 Part 4 the wrong assessment for an erosion scar?
When the metal loss is local rather than general, which describes most slurry wear. Part 4 permits thickness averaging over a defined length, and averaging across an erosion groove blends the thin section with sound wall on either side, producing a non-conservative remaining thickness. Part 5 handles local metal loss with a critical thickness profile and a remaining strength factor. A spreadsheet that carries only a single averaged thickness per location cannot support the Part 5 route at all.
What can an audit ask for that a shared workbook cannot produce?
The provenance of a specific number. Which two readings produced this governing rate, on which dates, taken by whom, with which instrument and calibration block, under which procedure revision, against which minimum thickness, and who last changed that minimum thickness and why. A workbook can hold all of those as columns and none of them as a guarantee, because any editor can overwrite any cell and nothing records that it happened.
Is API 510, 570 or 653 inspector training part of this offer?
No. This page concerns software for computing and governing wall loss rates on mining assets. Atlantis NDT provides NDT training to ASNT SNT-TC-1A and ISO 9712 across UT, RT, MT, PT, ET, VT, PAUT and TOFD, ASNT Level III consulting, inspection management and reporting software, digital twins, 3D laser scanning and report validation. API inspector certification is run by the American Petroleum Institute and sites requiring it engage that programme directly.
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