Abstract: Acceptance of a cleaned heat exchanger rests on three things matching: the deposit type, the right limit, and the evidence left behind. Removal and cleaning rates are graded by deposit type - carbonate, rust and oil deposits at no less than 95%, sulphate, silicate and other deposits at no less than 85%. Corrosion rate and corrosion amount are graded by material - carbon steel no more than 5 g/(m2·h) on site, stainless steel and copper alloys no more than 1.5 g/(m2·h), with an amount cap of 80 g/m2 and 20 g/m2 respectively. The passivation film is judged by the red-dot or blue-dot test. This article sets out the GB/T 25146-2010 criteria, the sampling evidence and the mistakes that most often turn a clean job into a rework.
1. Three things decide acceptance: deposit type, limit, evidence
A quantitative deposit sample fixes the deposit type. The type sets the removal and cleaning rate thresholds; the material sets the corrosion rate and corrosion amount thresholds; and the coupons plus analytical records fix the process in writing. Skip the type and apply a flat 95% removal rate to everything, and the acceptance report will not match the clause it quotes - which is exactly how jobs end up back on site.
| Deposit type | Composition threshold | Field identification |
|---|---|---|
| Carbonate | Carbonate as CaCO3 ≥60% | Heavy bubbling in hydrochloric acid, little residue |
| Sulphate | Sulphate as CaSO4 ≥40% | Slow dissolving, white precipitate with barium chloride |
| Silicate | Silicate as SiO2 ≥20% | Insoluble in acid, mostly dissolves with hydrofluoric acid |
| Rust scale | Iron oxide ≥80% | Slow dissolution, yellow-green solution |
| Oil deposit | Oil content ≥5% | Ground sample in ether gives a yellow-green solution |
Field identification is a rough screen only; the quantitative result decides. Get the type wrong and both thresholds shift by one grade - treating sulphate scale as carbonate means demanding 95% from a system the standard grades at 85%, and the only way to reach it is to over-clean, which raises corrosion instead of lowering it.
2. Removal rate and cleaning rate: thresholds depend on deposit type
Removal rate measures how much of the original deposit has gone; cleaning rate measures how clean the surface is left. Both are read from the same grade of the same table, and both must be met at the same sampling points.
| Deposit type | Removal rate N % | Cleaning rate B % |
|---|---|---|
| Carbonate | ≥95 | ≥95 |
| Sulphate | ≥85 | ≥85 |
| Silicate | ≥85 | ≥85 |
| Rust scale | ≥95 | ≥95 |
| Oil deposit | ≥95 | ≥95 |
| Other (carbon, polymer, process deposit) | ≥85 | ≥85 |
Sampling points decide whether the number means anything. On a shell-and-tube unit, take points at both tube-sheet ends, the upper and lower shell, and behind the baffles, because sludge and hard scale collect where velocity is lowest. Plate and spiral units have to be opened along the whole channel, not judged from the ports. Keep the point map and photographs with the report so a rework has something to compare against.
3. Corrosion rate and corrosion amount are two different limits
Corrosion rate K is a speed in g/(m2·h); corrosion amount A is a total in g/m2. The first shows how aggressive the cleaning solution is at any moment, the second shows the metal lost across the whole cleaning cycle. The standard separates laboratory verification from on-site measurement: the laboratory result decides whether the recipe is usable, the site result decides whether this particular job ran out of control.
| Material | Lab g/(m2·h) | On site g/(m2·h) | Amount g/m2 |
|---|---|---|---|
| Carbon steel | ≤2 | ≤5 | ≤80 |
| Stainless steel | ≤1 | ≤1.5 | ≤20 |
| Copper | ≤1 | ≤1.5 | ≤20 |
| Copper alloy | ≤1 | ≤1.5 | ≤20 |
| Aluminium alloy | ≤1 | ≤1.5 | ≤20 |
Coupons of the same material as the equipment go into the cleaning loop and stay there for the whole wash, then are weighed and converted. Tube bundle, tube sheet and heads are often different materials, so the coupon follows the most sensitive one; on mixed stainless and copper units, hold the job to 1.5 g/(m2·h). A contract clause reading "corrosion rate not above 6 g/(m2·h) for all materials" has no basis in the standard. Inhibition efficiency is assessed by the definition and formula in clause 3.3 with the acid-inhibition test result - the standard sets no "99% or better" figure.
4. Passivation film and surface cleanliness
Carbon steel surfaces must carry a complete passivation film: red-dot test, no red dot earlier than 5 s. Stainless steel is judged by the blue-dot test, no more than 8 blue dots within 10 min. There must be no secondary flash rust, no copper plating and no sign of over-pickling before passivation, and if the film is incomplete the passivation is repeated rather than left to form in service.
Surfaces in contact with oxidising media also have to pass an oil check: no oil trace on a filter paper wipe, no fluorescence under UV, a camphor ball that keeps spinning, or carbon tetrachloride oil content no more than 125 mg/m2 - any one of the four is acceptable. Residual oil breaks the continuity of the film and drives under-deposit corrosion in the first weeks after start-up.
5. Chloride and ferric iron: process limits count as acceptance too
Where stainless steel is present, chloride in the cleaning solution must stay at or below 25 mg/L, and that is controlled when the solution is made up and topped up, not sampled afterwards. When ferric iron exceeds 1000 mg/L during acid cleaning, add a reducing agent or complexing agent, or replace the solution. Both belong in the daily log with sampling time, point, value and the action taken; an entry reading "process normal" proves nothing in a quality dispute.
6. What the acceptance report must contain
- Pre-cleaning deposit photographs, deposit weight and quantitative type analysis
- Corrosion rate and corrosion amount converted from same-material coupons
- Process log: temperature, time, solution analysis and make-up records
- Chloride and ferric iron sampling times with the actions taken
- Rinse, neutralisation and passivation test records (red-dot or blue-dot)
- Acceptance point map, images and both-party signatures
The completion-acceptance chapter of the standard is procedural: quality acceptance, works acceptance and document archiving. It sets no heat-transfer efficiency index. Any efficiency gain therefore belongs in the report as a typical-case experience figure, never as a contractual acceptance criterion; the real criteria are removal rate, cleaning rate, corrosion rate, corrosion amount and passivation film.
7. Common acceptance mistakes
- One 95% removal rate demanded regardless of deposit type
- Corrosion amount of 20 g/m2 written into the contract as a rate
- Only carbon steel coupons used on a copper tube exchanger
- Judging by how bright the tubes look, with no point map or images
- No passivation test, then reworking flash rust after start-up
- Chloride recorded as "within limit" with no original value
- Ferric iron over the limit and the solution kept circulating
- Efficiency gain written in as a hard acceptance index
8. FAQ
How is the removal rate judged after heat exchanger cleaning?
By deposit type against Table 3 of GB/T 25146-2010. Carbonate, rust and oil deposits must reach 95%, sulphate, silicate and process deposits 85%, with the same figure for the cleaning rate; sample both tube-sheet ends and behind the baffles.
What corrosion rate limit applies to a carbon steel exchanger?
5 g/(m2·h) measured on site, 2 g/(m2·h) from laboratory verification, with a corrosion amount cap of 80 g/m2. The three figures are recorded separately, because a contract that names only one of them gives the crew nothing to compare against.
Can a copper tube exchanger be acid cleaned without an inhibitor?
No. Copper and copper alloys are limited to 1.5 g/(m2·h) on site and 1 g/(m2·h) in the laboratory, with a 20 g/m2 amount cap. Without an inhibitor, or without an inhibition test first, wall thinning and dezincification are almost impossible to find afterwards; fit copper coupons, weigh them and stop the wash if the limit is passed.
How is the passivation film accepted?
Carbon steel by the red-dot test, no dot earlier than 5 s; stainless steel by the blue-dot test, no more than 8 dots in 10 min. There must be no flash rust or copper plating before the test, and an incomplete film means re-passivation.
What if chloride or ferric iron goes over the limit?
Hold chloride at 25 mg/L, controlled at make-up time on any unit containing stainless steel; when ferric iron passes 1000 mg/L, add a reducing or complexing agent or change the solution. Both need sampling times and original values on file.
The criteria above are taken from GB/T 25146-2010, Acceptance specification for industrial equipment chemical cleaning quality, Tables 1, 2 and 3 and clauses 5.2.8, 5.2.9, 5.2.10, 5.2.11, 5.3.3.1, 5.3.4.1 and 5.3.4.2; jetting re-cleaning and job safety follow GB/T 26148-2025, Safety specification for high-pressure water jetting.
Heat exchanger cleaning · acceptance and evidence
Deposit typing | Limit review | Coupon and test records
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