Heat Exchanger Cleaning Method Statement: Scope, Sequence and Handover

Abstract: The cleaning route for a heat exchanger is rarely the hard part; turning it into a method statement the site can actually work to is. Where these documents fail, it is usually on four unconfirmed items: no deposit sample before the chemical system is chosen, no confirmed tube and shell material before dosing, no agreed outage window with the operating team, and no waste route. Any one of those left vague means rework once the job starts, or a programme stopped dead by a disposal note. This article follows the order in which a method statement is actually built: the four pre-conditions and how to establish them, the six work stages with a backward schedule, field control points for isolation and corrosion coupons, confined space and waste segregation requirements, and the acceptance items and handover documents under GB/T 25146-2010, closing with an anonymised case from four shell-and-tube exchangers.

1. Why Cleaning Method Statements Fail on Site

The chemistry itself is well established: carbonate scale goes to a circulating chemical wash, sulphate hard scale and blocked tubes go to high-pressure water jetting, oily deposits get an alkaline stage before the acid stage. The weak point is the document stage, where the author holds nothing but a plant equipment list - no deposit sample, no confirmed material, no outage window - and copies a generic template to close the file.

Three kinds of blockage follow. First, isolation is left vague: the exchanger sits on a two-level frame, the inlet and outlet valves pass, chemical reaches the process system, and the job stops to fit spectacle blinds a day late. Second, the material is never checked: the bundle is stainless steel but the statement doses a chloride-bearing acid, and the coupon trial before the main wash sends the whole chemical order back for replacement. Third, no waste receiver is named: the wash finishes, the neutralising sump is full, no tanker is booked, and re-assembly waits two more days.

What these three share is a quantity that was never confirmed. A method statement is not a description of a process; it is the place where the uncertain items are eliminated on paper - how much deposit, which material, how many days available, where the waste goes. Leave a condition unconfirmed and the site will pay for it in rework.

2. Four Items to Settle Before Work Starts

These four pre-conditions decide how every later stage can be written. If any one is unresolved the statement can only be worded ambiguously, and ambiguous wording constrains nobody during construction.

Sampling is not skippable. The tube side and the shell side of the same exchanger frequently carry two different deposits: circulating water leaves carbonate scale under slime on the tube side, while the shell side may hold coking, oil or salt crystallisation from the process medium. Sample only one and the statement misses half the duty; the extra stage is then improvised on site, and both programme and chemical quantities have to be recalculated.

Material confirmation has to reach individual components. Tubes, tubesheet, baffles, shell and flange gaskets can all differ, and stainless steel with its chloride ceiling is a pass-or-fail item, while copper alloys require a copper inhibitor in the formulation. Putting these into a material compatibility table in the statement does far more than a single sentence telling the crew to mind the material.

3. Six Work Stages and the Backward Schedule

The stages are sequential by nature: a stage done badly cancels the effect of the one after it. The classic case is a rushed rinse and displacement - spent liquor stays in the bundle, the passivation film never forms properly, and rust returns within a fortnight of start-up.

Build the schedule from both ends towards the middle, not outwards from the middle. The two ends do not compress: cooldown and drain-down normally take 6-8 h, and re-assembly, pressure testing and insulation take the best part of a day. Only the middle is negotiable, so put a start and finish time against every stage and the crew can see which one is running late.

Where several exchangers are in scope, the statement has to say whether they run in parallel or one after another. A single circulation pump serves one wash circuit, but two circuits can be rotated; on four DN800 exchangers, running two in parallel over two batches gives roughly 2 days of cleaning and 4 days including isolation and re-assembly, half the time of working through them one by one. That arrangement has to be written down, because the default on site is one at a time.

4. Field Control Points: Isolation, Coupons, Monitoring

Isolation is the first line keeping risk out of the wider system. Blinds are numbered, tagged and registered, with a check on fitting and another on removal; inlet and outlet valves on both sides of the exchanger are locked off and tagged, never trusted closed. Where an exchanger has a bypass or a spare line, the bypass valve is checked for passing and blinded as well if necessary.

Corrosion coupons are the item most often trimmed from a statement and the one that hurts most at acceptance. At least 2 coupons go into every exchanger, one at the cleaning liquor inlet and one at the return end, and both ends have to come in under the limit for the job to pass. Coupon material matches the bundle and shell, mounted on a dedicated bracket with insulating wire rather than pressed against the exchanger wall, where flow velocity approaches zero and the measured corrosion rate reads low and will not stand up.

Monitoring runs at a fixed interval, written into the statement together with the record format. Log pH, turbidity, iron concentration and temperature every 30 min during the wash; a sudden rise in iron means the inhibitor film has broken down or the surface is over-cleaning, and the response is to cut acid strength or add inhibitor immediately rather than to keep running the clock. On stainless steel bundles the chloride ceiling of 25 mg/L in the cleaning liquor is watched throughout, with water changed or the chemical system adjusted the moment it is exceeded.

5. Safety and Environmental Requirements in the Document

Safety risk on heat exchanger cleaning concentrates in two kinds of work: confined space and chemical handling. Once the end covers are off, the channel head, the shell and the on-site mixing tank are all confined spaces - isolate connected pipework, force ventilate, test oxygen and hydrogen sulphide, work to a permit system and keep an attendant outside. Test readings belong on the permit, not in a verbal handover.

Chemical management is written as acid and alkaline stored separately under a named custodian. Dilution follows the rule of adding alkali to water, with a wash point, eyewash and emergency supplies next to the mixing area; safety data sheets are posted on site and crews wear goggles, acid-resistant gloves and a face shield. Spent liquor is segregated: acid and alkaline streams are collected separately, neutralised to pH 6-9 and sent to the plant treatment system or to a licensed contractor, while oil-bearing waste is collected on its own so that emulsified oil never reaches the biological stage.

The statement should also state the spent liquor volume per clean and how it is collected. Estimating at 1.5 times system volume covers rinse water as well: one DN800 exchanger holds about 3 m3 on the tube side, so four of them produce roughly 12 m3, and sump capacity, neutralising reagent and tanker bookings all follow from that number. Disposal and recovery receipts form part of the handover documents.

6. Acceptance and Handover Documents

Acceptance runs on two tracks: cleaning quality and operating performance. Quality is judged against the scale removal, corrosion rate and passivation film criteria of GB/T 25146-2010, with scale removal measured by weighing and visual inspection together and evidence kept from both sides. Performance is judged from the exchanger itself - inlet and outlet temperatures, pressure drop and process-side temperature against the pre-shutdown baseline. A bright-looking bundle is not acceptance evidence on its own.

The handover list belongs in the statement from the start, so documents are collected as work proceeds instead of being chased at acceptance: method statement and process card, chemical certificates and safety data sheets, coupon corrosion monitoring records, process logs covering pH, turbidity, iron and time, rinse water reports, passivation records, pressure test records and waste disposal receipts.

7. Case Study: Four Shell-and-Tube Exchangers at a Chemical Plant

In June 2026 Blue Star Cleaning cleaned four fixed tubesheet exchangers at a chemical plant in East China, working inside a five-day maintenance shutdown. The units were DN800 with carbon steel shells and stainless steel tube bundles, tube side on circulating water; before the shutdown the tube-side inlet and outlet temperature difference had fallen from 12 C to 6 C and pressure drop had risen by about 40%. Two deposit samples were taken at the document stage: grey-white hard scale under a thin slime layer on the tube side, 0.5-0.9 mm thick, and coking with an oil film on the shell side. The route was set accordingly - chemical descaling on the tube side, alkaline degreasing followed by acid on the shell side.

The schedule was built backwards as 3 days of cleaning plus 2 days of re-assembly and testing. Day one covered isolation, blinding and pre-rinsing, with the four exchangers running two at a time in two batches; the tube side was circulated with a Citric Acid system and inhibitor at 40-50 C for 5 h with chloride held at or below 25 mg/L throughout, two same-material coupons per exchanger, and corrosion rate monitored under clause 5.2.11 of GB/T 25146-2010, while the shell side took 3 h of alkaline degreasing before the acid stage. Day two covered rinse, displacement and passivation; day three the go-through check and pressure test. Spent liquor was segregated into acid and alkaline streams, neutralised and handed to the plant treatment system with disposal receipts included in the handover file.

Acceptance results: 96% scale removal on the tube side; coupon corrosion rate 0.62 g/(m2.h) for carbon steel and 0.18 g/(m2.h) for stainless steel; ferroxyl test 12 s and 2 blue spots within 10 min, all inside the standard limits; and the go-through check found no residual blockage or tube-end damage. After start-up the tube-side temperature difference returned from 6 C to 11 C and pressure drop fell by about 35%, and the plant then wrote the sampling and coupon requirements of the job into its annual maintenance template. Where on-line cleaning is the better fit, or how such work is priced, see the notes on on-line non-stop chemical cleaning and the industrial equipment cleaning cost guide.

8. Frequently Asked Questions

How is the cleaning schedule for a heat exchanger job built backwards?

Fix the two ends first, because neither compresses. Cooldown and drain-down take 6-8 h, while re-assembly, pressure testing and insulation normally consume a full day. Only the middle belongs to you: on four DN800 fixed tubesheet exchangers, one cycle of descaling runs 4-6 h, rinse displacement 2 h and passivation 3 h, so two units in parallel over two batches gives about 2 days of cleaning and 4 days including isolation and re-assembly. Put start and finish times for every stage on a time line.

Why does the waste route need its own section in the method statement?

Acid and alkaline spent liquors cannot share a collection tank. Mixed together during neutralisation the pH rebounds repeatedly and will not sit inside 6-9. Estimate the volume at 1.5 times system volume: one DN800 exchanger holds about 3 m3 on the tube side, so four of them generate roughly 12 m3, and the sump, neutralising chemicals and tanker have to be booked in advance. Name the discharge point, the reagent quantity and the receiving facility in the document, or a single disposal note can hold the job up for two days.

How many corrosion coupons go in, and where?

At least 2 per exchanger, one at the cleaning liquor inlet and one at the return end, and both ends have to come in under the limit for the job to pass. Coupon material matches the tube bundle and shell; carbon steel coupons are judged by weight loss under Table 2 of GB/T 25146-2010 and weighed to 0.1 mg before and after. Mount them on a dedicated bracket with insulating wire rather than against the exchanger wall, because flow velocity at the wall approaches zero and the reading comes out low.

How do you measure scale removal so the figure stands up?

Use weighing and visual inspection together, and keep evidence from both. Take 3-5 representative tubes, record the deposit thickness before cleaning, say 0.5-0.9 mm, then check each tube for residual deposit afterwards; confirm the tube bores are clear with a go-through check on every tube. Visual acceptance requires exposed bare metal over at least 95% of the surface, with no fewer than 10 sampling points photographed, numbered and signed off with the plant at handover.

Water jetting or manual rodding - which goes into the method statement?

Both, as a sequence rather than a choice. Once the covers are open, run 30-50 MPa water jetting tube by tube to strip hard scale and silt, then clear the odd blocked tube by flexible-shaft rodding by hand; writing only the open-cover manual route stretches the programme, and 800 tubes across four exchangers take more than 3 days by hand. Pressure follows the material: stainless steel and thin-wall copper are held at 20-30 MPa while carbon steel bundles can take 50 MPa, with the nozzle centred before the lance advances.

All cleaning processes and acceptance criteria in this article follow GB/T 25146-2010 Quality acceptance specifications of chemical cleaning for industrial equipment.

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