Heat Exchanger Fouling Removal: Choosing Between Chemical Cleaning, High-pressure Water Jetting and On-line Cleaning

Abstract: Once a heat exchanger in a chemical plant fouls, the boundary between chemical cleaning, high-pressure water jetting and on-line cleaning is set by the deposit type, the material, the shutdown window and the site conditions - not by which method is "stronger". The wrong choice rarely costs only money; it costs tube wall thickness through over-pickling, copper plating or a bundle that thins out. Written for the plant equipment supervisor, this article gives the applicability boundary, the prohibited conditions and the combined route for each of the three processes, with a comparison table. Acceptance follows GB/T 25146-2010: removal and cleaning rates by deposit type, corrosion rate, corrosion amount and the passive film judged by field measurement, the ferroxyl red-spot test and the blue-spot test.

1. The Short Answer: Which Deposit, Which Equipment, Which Route

The three processes are not substitutes for each other; each has its own entry condition. Heat transfer surfaces carrying carbonate scale, rust or oil foulant go to chemical cleaning, where the liquid dissolves or complexes the deposit and carries it out. Polymer, coke, hard sulphate shells and already blocked tubes go to high-pressure water jetting, which opens the passage by breaking and stripping the deposit. Systems with a thin deposit, no room for a shutdown and a controlled material and leak status go to on-line cleaning.

Four filters decide it. First, the deposit type - take a sample and run the qualitative identification, because the deposit decides the cleaning medium and also the removal-rate band. Second, material and wall thickness - carbon steel, stainless steel, copper alloys, aluminium and thin-wall bundles have very different corrosion allowances, and a thin-wall bundle will not survive a long acid circulation. Third, the shutdown window - a shutdown is what makes end-cover removal, bundle withdrawal, jetting and mechanical rodding possible at all. Fourth, waste and site conditions - whether the site can neutralise, collect and consign the waste decides whether a chemical route can be discharged at all. If any one filter crosses a prohibited line, change the route; do not push it through with more inhibitor, more pressure or more time.

2. Process One: Chemical Cleaning (Circulation or Soaking)

Chemical cleaning starts from the deposit. Carbonate scale dissolves largely in dilute hydrochloric acid with strong gas evolution and little residue; rust is mostly iron oxide, dissolving slowly in dilute hydrochloric acid to a yellow-green liquor; oil foulant is broken first with an alkaline or solvent wash, after which the medium is chosen by the deposit underneath the film. In chemical cleaning these three deposits reach a removal rate band of not less than 95%; sulphate scale, silicate scale and the "other" group such as carbonaceous and polymer deposits sit in the not less than 85% band. Never promise both bands as one number (deposit classes in Table 1 and the removal and cleaning rate bands in Table 3 of GB/T 25146-2010).

The material sets both the medium and the prohibited line. Where stainless steel is present, chloride in the cleaning fluid may not exceed 25 mg/L (clause 5.2.9), and that single line puts an HCl system out of the running; the route has to be a chloride-free acid or a chelant. Copper and copper alloys are intolerant of ammonia, so a high-ammonia chelant system needs a dedicated copper inhibitor and the surface has to be checked under 5.2.8 for absence of copper plating. A thin-wall bundle has the smallest thickness margin and is held to the field measurement column of Table 2 as well: carbon steel no more than 5 g/(m²·h) and 80 g/m², stainless steel and copper tightened to 1.5 g/(m²·h) and 20 g/m². Both limits have to be met, and the longer the job runs the likelier the cumulative amount is the one that binds: for carbon steel held at the 5 g/(m²·h) ceiling, 16 h of cleaning already brings the amount to 80 g/m², and about 13 h for stainless steel and copper.

Two process controls apply. When Fe³⁺ in the pickling liquor exceeds 1000 mg/L, add a reducing or complexing agent, or change the liquor outright (5.2.10). Throughout the job, fit a corrosion monitoring coupon of the same material as the equipment (5.2.11), so acceptance rests on coupon data rather than on the colour of the liquor. The inherent weakness of circulation is the dead zone: behind shell-side baffles, at the lower tube sheet and at tube roots, circulation never reaches the surface, so after chemical cleaning these areas need either a water-jet pass and tube rodding, or the equipment should have been put on the jetting route in the first place.

3. Process Two: High-pressure Water Jetting

Water jetting starts from "is there a passage" and "is the deposit hard". Where tubes are already blocked, or the deposit is a hard shell or carries polymer and coke, a chemical route has too little contact area and a full day of circulation will not open it. A rigid lance, or a flexible lance with a self-propelled nozzle, rodding each tube, followed by high-pressure water to flush the stripped residue out, is the most common route in chemical plants. It introduces no cleaning chemical, so there is no spent chemical liquor to handle, which is a clear advantage where the site has no neutralisation or collection capability.

The price is that the equipment must be opened: end covers off, and where necessary the bundle withdrawn or the plates of a plate exchanger separated, so the schedule is tied to the shutdown window. Pressure level and nozzle type are set by deposit hardness and tube bore, and thin-wall bundles and copper tubes need controlled pressure and jet angle to avoid over-cleaning and wall thinning. Job safety follows GB/T 26148-2025, Safety Specification for High-pressure Water Jet Cleaning Operations. The clauses that matter most at a heat exchanger: before a flexible lance clears a blocked tube, pre-flush the tube ends for 100 mm to 200 mm (6.9.4.2); the nozzle safety device sits no more than 3 cm from the pipe end face (6.9.6); on a U-tube exchanger the outlet-side tube ends must be physically shielded so that jetting does not damage the same side (6.9.10); work inside a vessel uses a three-dimensional rotating nozzle with all feed control outside the vessel (6.11). Machines are selected under GB/T 26135-2020, High-pressure Cleaning Machines, covering the rated pressure range of 10 MPa to 300 MPa.

4. Process Three: On-line Cleaning (No Shutdown)

On-line cleaning starts from "we cannot stop". The cleaning medium is circulated through the system, or a hydraulic method keeps scouring the heat transfer surface while the unit runs. It suits systems where the deposit is thin, no tube is blocked, the equipment does not leak, the material tolerates the medium and the site can monitor the process continuously. It works on soft carbonate deposits; it does essentially nothing to a deposit that has already hardened into a shell or turned polymeric.

Three conditions are all required. The deposit is carbonate-based and its thickness is controllable, so it has not swallowed most of the flow section; the system has a complete circulation loop and on-line monitoring (pH, iron, corrosion coupons or probes); and the cleaning does not disturb downstream processes and can be stopped at any moment. Without all three, on-line cleaning simply hides the risk: incomplete removal, failed corrosion monitoring and dead-zone deposit left in place exactly where it was.

It is not a substitute for shutdown cleaning but the other end of the same timeline. A lightly fouled unit on continuous duty can continue with on-line cleaning, and the shutdown window is then used for one thorough offline clean and passivation that also clears the dead zones and hard shells.

5. The Three Processes Compared

6. Combined Routes: When Two Processes Must Run in Sequence

In real turnarounds the answer is rarely one process, but "jetting to open the passage, chemical cleaning to remove the rest, then flush and passivate". The order matters: open the blocked tubes and hard shells mechanically first, so that the cleaning liquor can reach every tube and every crevice; otherwise the chemical only works on the outer layer. After chemical cleaning, confirm under 5.2.8 that there is no secondary rust, no copper plating and no over-cleaning, and that a complete passive film has formed, then flush with clean water and dry before return to service.

Chemical cleaning before jetting also works, with two cautions. Residue left inside after a chemical clean will form a fresh deposit on the heat transfer surface once the unit is back in service, so it must be flushed out. High-pressure water poses no extra risk to a freshly passivated carbon steel surface, but on stainless steel the passive film should not be given an unnecessary hard impact.

7. Prohibited Conditions

None of the following is relaxed because the schedule or the budget is tight. Where stainless steel is present and the site cannot hold chloride in the cleaning fluid at or below 25 mg/L, an HCl system is prohibited. On a plate exchanger, applying ultra-high-pressure jetting without separating the plate pack will damage the gaskets, so it is prohibited. A bundle that already leaks is not put through on-line or chemical cleaning; leakage is investigated, tubes are plugged or replaced first. A circuit whose tubes are blocked is not put on plain circulation; open the passage first. And "if it does not come clean, run it longer" is prohibited on any unit, because it drives the corrosion amount towards the Table 2 ceiling; change the process instead of extending the time.

8. Acceptance and Return to Service

Acceptance does not vary with the process; everything lands on GB/T 25146-2010. Removal and cleaning rates follow the deposit type in Table 3 - carbonate scale, rust and oil foulant not less than 95%, sulphate scale, silicate scale and other deposits not less than 85%. Corrosion rate and corrosion amount are held to the field measurement column of Table 2, with the data coming from coupons of the same material fitted during the job. After cleaning, check under 5.2.8 that there is no secondary rust, no copper plating and no over-cleaning and that a complete passive film has formed; a carbon steel passive film passes the ferroxyl red-spot test at not less than 5 s (5.3.4.1), and a stainless steel film passes the blue-spot test with no more than 8 spots within 10 min (5.3.4.2). The jetting route adds one check: tube-by-tube gauging or ball passage, confirming no residual blockage before reassembly.

Four records are kept: deposit identification and quantification, the process log (medium, temperature, circulation time), coupon corrosion rate and amount, and the removal-rate inspection record. Those four also form the basis for judging the next cleaning interval and route, which is far more reliable than fixing a date by experience.

9. Frequently Asked Questions

Can a blocked tube bundle still be chemically cleaned?

Not by plain circulation - rod the blocked tubes through with water jetting first, then clean chemically. Once a tube is plugged the cleaning liquor cannot enter it, so circulation only burns chemical; on a typical bundle, more than 1 blocked tube already starts to remove that share of the heat transfer area, so the passage has to be restored before removal rates mean anything.

Which of the three processes damages the base metal least?

Water jetting is normally the gentlest, because it introduces no chemical corrosion. In chemical cleaning the metal loss comes from the corrosion rate and the corrosion amount, held by the field measurement column of Table 2 to 5 g/(m²·h) and 80 g/m² for carbon steel, tightened to 1.5 g/(m²·h) and 20 g/m² for stainless steel and copper. On-line cleaning avoids the shutdown and the dismantling, so there is no mechanical damage to the base metal; its risk is failed corrosion monitoring and incomplete removal.

Can on-line cleaning replace a shutdown clean?

No, it only slows the fouling down. On-line cleaning works on thin soft carbonate deposit and typically buys about 3 to 6 months of running time; it does nothing to a hard shell or a polymer deposit, so the turnaround still needs one thorough offline clean finished with passivation.

How soon does a heat exchanger foul again after cleaning?

Water hardness, operating temperature and the process medium decide it, and 6 to 18 months is the common range; the change in temperature difference and terminal difference is a more reliable signal than the calendar. Run the equipment type, water quality and past cleaning records through the Maintenance Cycle Calculator for a suggested interval, then check it against the measured parameters.

Where do the three processes differ in price?

In chemical charge, labour hours and waste disposal - not in some notional "similar scope of work". A chemical clean carries chemical and spent-liquor disposal, jetting carries dismantling and per-tube labour, and on-line cleaning is priced by circulation period. The same exchanger differs widely between plants and duty conditions, so a quotation is prepared on the equipment survey with value added tax shown separately at 6%.

Related Reading

• Cleaning methods by heat exchanger type - process choice by construction
• Shell and tube heat exchanger cleaning - tubes and shell side in practice
• Plate heat exchanger cleaning - separating the pack and protecting the gaskets
• Floating head heat exchanger cleaning - bundle withdrawal and cleaning
• Double pipe heat exchanger on-line cleaning - field practice without a shutdown
• Passivation after cleaning - judging the passive film
• Fouling type identification · Chemical cleaning acceptance standards
• Equipment Scaling Diagnosis Tool · Maintenance Cycle Calculator

The deposit classes, removal rate bands, corrosion rate and corrosion amount limits and acceptance requirements in this article follow GB/T 25146-2010 Acceptance Standard for Chemical Cleaning Quality of Industrial Equipment (deposit classes in Table 1, removal and cleaning rates in Table 3, corrosion rate and corrosion amount in Table 2, chloride limit of 25 mg/L for systems containing stainless steel in clause 5.2.9, passive film judged under 5.3.4). Where a high-pressure water jet is used, the safety of the operation follows GB/T 26148-2025 Safety Specification for High-pressure Water Jet Cleaning Operations, and the machines are selected under GB/T 26135-2020 High-pressure Cleaning Machines.

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