Thermal Oil System Coking Cleanup: Oil and Coke Sampling, Solvent Circulation and the Three Pre-Start-Up Checks

Abstract: After long service at high temperature, thermal oil oxidises and cracks, and coke builds up on the furnace-tube wall and the heat-transfer surfaces: heat transfer falls away, the oil temperature starts to swing and the tube wall can overheat locally. Coke severity cannot be judged from oil colour alone. This article follows the site sequence: how the oil and coke samples are taken and what is analysed, how the system is drained and isolated, how solvent circulation runs with dispersant and filtration, how far the oil flush has to go, and the three pre-start-up checks - internal inspection, operating data against the pre-cleaning baseline, and a complete document set. Unit-specific figures are marked as pending confirmation.

1. Judge the coke first: oil colour proves nothing on its own

Thermal oil oxidises and cracks in service, and the products deposit on the furnace-tube wall and the heat-transfer surfaces. There are two kinds of deposit. Soft sludge and gum are dissolved and carried away by solvent; hard carbonised coke has to be swollen, loosened and thinned by circulation shear over time. The chemistry and the circulation duration are not on the same scale.

Two samples come first. The oil sample is taken while the system runs, at the circulation pump discharge and the return header, never from the expansion tank, which is a cool buffer and does not represent conditions in the tube; the laboratory reports carbon residue, acid value, kinematic viscosity and flash point. The coke sample is scraped from the hot section of the tube, the bends and the header, and classified as soft sludge or hard carbonised layer ([TO BE CONFIRMED: oil and coke analysis report]).

A dark colour misleads in both directions: dark oil may only mean additive depletion, and oil that still looks clear can already carry a rising carbon residue from high-temperature condensation. Coking also varies along one system, with the hot section and the bends usually worst, so sampling covers those points and each sample is logged with its tag position ([TO BE CONFIRMED: sampling record by position]). No chemical type, dose or circulation duration is quoted until the laboratory report is in hand.

2. Shutdown and isolation: state the system condition before cleaning

The unit is shut down, cooled and depressurised, and the oil charge is drained to a designated container with the volume and disposal route recorded ([TO BE CONFIRMED: oil volume and disposal record]). Low-point drains, tube bends and the tail of the return header hold oil that dilutes the cleaning solution and pulls the dissolution rate down, so each is drained and checked before dosing.

The cleaning loop is connected as a closed circuit: mixing tank, circulation pump, filter and temporary piping are positioned in advance, and pump flow and head are sized on total tube length, parallel branches and filter resistance ([TO BE CONFIRMED: pump flow and head]). The expansion tank and the high-level tank are isolated from the loop, and relief valves, pressure gauges and level instruments are removed or protected for the duration of the run.

A clean-water trial circulation comes next: temporary joints, flanges and valves are checked for leaks, the level is confirmed stable and the pump discharge pressure sits inside the planned range before any chemical is added. The no-load flow is recorded and becomes the reference for judging filter blockage later ([TO BE CONFIRMED: trial circulation flow record]).

3. Circulation: dissolution, dispersion and filtration work together

The main wash is a solvent-type cleaner circulated through the system, with a dispersant that keeps loosened coke particles suspended so they do not settle again in low-velocity sections. The chemical type, the dose and the circulation temperature come from the coke analysis and a bench dissolution test, not from trial and error on site; the bench record is filed ([TO BE CONFIRMED: bench dissolution test record]) before the job is scaled up.

The main wash runs as a closed circuit with the solution passing all furnace tubes and heat-transfer equipment in the designed direction; the direction is switched once during the run so the hot section, the bends and the header dead corners all see fresh solution. A filter catches the loosened coke, and the element is cleaned or replaced on differential pressure and flow trend ([TO BE CONFIRMED: filter element record]). Fast blockage usually means dissolution is working, so the frequency is recorded before it is read as a fault.

Samples are taken on a schedule and checked for colour, suspended solids and viscosity ([TO BE CONFIRMED: monitoring interval]). The end point is not a clock: it is reached when impurities stop rising, the tube inlet-to-outlet temperature difference settles and the filter differential pressure stops climbing. Solution temperature is held inside the range the chemical requires ([TO BE CONFIRMED: solution temperature range]) and the heat-up and hold stages are attended throughout.

4. Flush and replacement: residue left behind contaminates the new oil

The spent solution is drained to a designated container and consigned to a licensed hazardous-waste route, with volume and route recorded ([TO BE CONFIRMED: waste volume and disposal record]). It never goes to a rainwater gully or the sewer.

The loop is then flushed with new oil or a dedicated flushing oil that carries out the residual cleaner and suspended coke particles, with the flush volume recorded ([TO BE CONFIRMED: flush oil volume]). The flush continues until the sample passes: clear oil, no suspended solids, and carbon residue and acid value back close to new-oil levels ([TO BE CONFIRMED: flush acceptance limits]). The expansion tank, the high-level tank and the low-point drains are drained and inspected one by one.

Charging fresh oil on top of an incomplete flush accelerates oxidation of the new charge: the acid value rebounds within weeks and the wash has to be repeated in the next cycle.

5. Acceptance and start-up: oil samples, temperature difference, records

Acceptance rests on three items. First, internal inspection: manholes are opened or a borescope is run to confirm the coke has been removed from the tube wall, with any residual area recorded and the locally re-cleaned sections listed ([TO BE CONFIRMED: re-cleaned sections]). Second, the operating comparison: at the same load after start-up, the tube inlet-to-outlet temperature difference, the oil flow and the furnace temperature are compared with the pre-cleaning baseline ([TO BE CONFIRMED: pre-cleaning baseline]; [TO BE CONFIRMED: post-start-up record]); no improvement figure is stated without that comparison, and none is claimed here. Third, the documents: analysis reports, mixing records, monitoring sheets, waste disposal records and the acceptance sheet filed together.

Start-up follows a slow heat-up curve ([TO BE CONFIRMED: start-up curve]) with oil temperature, oil pressure and tube-wall temperature watched throughout; anything abnormal is investigated at reduced temperature rather than run through. The first operating cycle is patrolled more frequently and the temperature and pressure trend is logged as the reference for the next cleaning interval.

For comparison, GB/T 34352-2017, the Chinese guideline for cleaning organic heat carrier boilers and systems, sets system-level criteria: more than 90% of the visible surface cleaned, and a sample taken within 10 days of charging new oil showing carbon residue no more than 0.3%, acid value no more than 0.3 mgKOH/g and low-boiling matter no more than 5%. Those are system-level figures; the figures for a specific unit still come from that unit's own analysis reports and operating records.

FAQ

The oil sample shows a high carbon residue but the tube wall carries little coke - should the heater be cleaned

Separate oil degradation from tube-wall coke first. A high carbon residue says the oil has aged, while the cleaning target is the coke layer on the tube wall; open a manhole or run a borescope, and read the result together with the tube temperature difference and furnace temperature trend. Where the oil itself is badly degraded, an oil change or on-line reclaim comes first, and thin coke is no reason for a separate shutdown.

The filter blocks quickly during the main wash - was the dose too high

Usually not. Fast blockage means a large volume of coke is being loosened and the wash is working; clean or replace the element, log the blockage frequency, and compare the flow with the no-load figure from the trial circulation. If a fresh element still blocks within minutes, check the circulation velocity and the dose against the bench test before adding anything or raising the temperature.

Two circulation passes are done and the borescope still shows hard coke at a bend - what now

A bend is a low-velocity corner where the solution exchanges slowly, so hard carbonised coke there dissolves slowly as well. Extend the circulation locally or switch the direction and re-clean that section, and write the extra duration into the monitoring sheet. Coke that still will not move is measured and marked, and whether to switch to mechanical cleaning is agreed by both sides instead of adding more chemical passes.

The flush sample looks clear - can the new oil be charged straight away

Not on appearance alone. Sample again for carbon residue and acid value, confirm they are close to new-oil levels, check there is no residual solvent smell, and drain the expansion and high-level tanks. Against GB/T 34352-2017 the sample taken within 10 days of charging should show carbon residue no more than 0.3% and acid value no more than 0.3 mgKOH/g; if it does not, the flush continues.

Why must the expansion tank and high-level tank be isolated during cleaning

Because both are hot, near-atmospheric spaces: solution migrating into them dilutes the main circuit and can carry solvent vapour into a hot space. The isolation valves are closed and tagged, blinds are fitted where needed, and valve position is re-checked before dosing and again during the hold.

Related reading

• Thermal oil boiler and pipeline chemical cleaning
• Coke removal in thermal oil boilers
• Thermal oil boiler and pipeline cleaning
• All cleaning articles

Analytical items, the cleaning route and the acceptance criteria follow GB/T 34352-2017, Guideline for cleaning organic heat carrier boilers and systems; the figures for a specific unit come from that unit’s own analysis reports and operating records.

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