Graphite Heat Exchanger Maintenance and Overhaul Procedure

In chlor-alkali and hydrogen chloride synthesis units, graphite heat exchangers are used for cooling hydrochloric acid, cooling wet chlorine gas, and recovering heat from sulfuric acid dilution; the media involved are typically high-temperature, strong acids. In block-hole graphite heat exchangers, intersecting channels are drilled into a solid block of graphite, while shell-and-tube graphite heat exchangers consist of a tube bundle made up of graphite tubes and tube sheets. The maintenance logic for both types of structures revolves around disassembly, cleaning, inspection, repair, reassembly, and pressure testing, though the specific operational steps differ. A graphite heat exchanger overhaul is not merely a simple cleaning process; rather, it is a comprehensive procedure that includes re-measuring dimensions, evaluating tube blockages, and repairing sealing surfaces—which constitutes the core of graphite heat exchanger repair. Actual records from European chlor-alkali plants show that the overhaul interval for graphite heat exchangers is typically 2.5 to 4 years, depending on the anolyte temperature in the electrolysis cells and the amount of acid mist entrained. When selecting an electrolyzer manufacturer, it is essential to clarify the outlet chlorine gas temperature, acid mist content, and salt content in the caustic solution at full load. These parameters directly determine whether a graphite heat exchanger requires a complete overhaul every 3 or 5 years. Although there are slight differences in the design loads of Japanese and German electrolysis cells, graphite block channels corrode more rapidly in plants with poor acid mist control; therefore, the original manufacturer’s drawings must be verified before every graphite exchanger overhaul.

Maintenance Procedures for Block-Type and Shell-and-Tube-Type Graphite Heat Exchangers

A major overhaul of a block-type graphite heat exchanger begins with the dismantling of the graphite block. First, remove the upper and lower heads, then loosen the shell connection bolts to pull or push the steel shell off the graphite block as a single unit, and hoist the graphite block out to the maintenance area. For cleaning, prioritize high-pressure water jetting of the process and cooling channels; for stubborn scale, use acid washing, but strictly control the acid concentration and temperature. After cleaning, perform visual inspections, dye penetrant testing, and ultrasonic testing, focusing on cracks and penetrations at channel intersections. Failed individual channels cannot be replaced separately; instead, perform channel plugging on a small number of channels using acid-resistant graphite putty and graphite plugs. If cracks affect the entire pressure-bearing block or if the proportion of plugged channels exceeds the manufacturer’s specifications, proceed directly with a complete block replacement. Sealing surfaces are ground flat, and the graphite block-to-shell gasket is replaced. 

During reassembly, components are tightened in the reverse order of disassembly, followed by hydrostatic and air tightness tests—this constitutes the complete closed-loop process required for a comprehensive graphite heat exchanger repair. Major overhauls of shell-and-tube graphite heat exchangers begin with graphite bundle pulling. After removing the heads, use specialized lifting equipment or a core-pulling machine to slowly extract the entire bundle of graphite tubes from the shell. Due to the high brittleness of graphite tubes, they must not be pried out in sections. After completing the graphite bundle pulling, the extraction force should be recorded; if it is abnormally high, this indicates scaling in the shell or jammed baffles. After extraction, perform graphite bundle hydro-jetting on both the interior and exterior of the tubes, and clean the shell side separately. 

During inspection, perform wall thickness measurements, dye penetrant testing, and eddy current testing on each graphite tube individually. Use a ball-passing test to determine blockages. If feasible, perform graphite tube plugging on individual damaged tubes by pressing a conical graphite plug into the tube end; if there are multiple damaged tubes or the tubesheet sealing surface is damaged, proceed directly with graphite bundle replacement, which is more reliable than replacing tubes one by one. The focus of tubesheet seal repair is on the graphite tube-to-tubesheet seal, using the manufacturer’s recommended adhesive or compressed graphite sealing rings. 

During reinstallation, the graphite bundle must be reinserted into the shell slowly to avoid impacting the baffles. Finally, pressure testing for the graphite heat exchanger overhaul of both types of equipment must be conducted according to the same standards; requirements must not be lowered simply because shell-and-tube heat exchangers have fewer potential leak points, otherwise the graphite heat exchanger repair will likely require rework.

Comparison Summary Table for Two Types of Graphite Heat Exchangers

The following table lists the terminology for the same maintenance procedures in both types of structures side by side to facilitate cross-referencing between on-site work orders and English documentation. The most common source of confusion in “Graphite Exchanger Overhaul” is the interchangeable use of terms for “block-and-hole” and “shell-and-tube” configurations, which leads to ordering the wrong spare parts. On-site, “Graphite Block Exchanger Dismantling” is often simply referred to as “block hoisting,” and “Graphite Bundle Pulling” is called “core extraction”; however, any graphite heat exchanger repair work order must use the full English terminology.

General Precautions for Graphite Heat Exchanger Overhauls

Whether block-and-hole or shell-and-tube types, the greatest risks during a graphite heat exchanger overhaul are shock loads and localized stress. Graphite is resistant to compression but not to tension; therefore, wide nylon slings or specialized supports must be used for lifting, and it is strictly prohibited to directly secure the graphite body with steel wire ropes. The selection of sealing materials is more critical than one might imagine; in environments involving hydrochloric acid or wet chlorine, fluorinated elastomers or flexible graphite composite gaskets must be used—ordinary rubber gaskets cannot be substituted. Hydrostatic testing should be conducted at 1.3 to 1.5 times the original design pressure. The pressure must be increased slowly, and the pressure must be maintained for at least 30 minutes. Desalinated water or low-chlorine water must be used for the test. 

Euro Chlor technical documents indicate that the overhaul cycle for graphite heat exchangers used in wet chlorine gas cooling should be assessed based on acid mist content and free chlorine concentration, rather than solely on operating hours. Both Mersen and SGL maintenance manuals mention that gas tightness tests following a graphite heat exchanger overhaul are more critical than hydrostatic tests; they recommend first checking for micro-leaks using nitrogen at a low pressure of 0.2 MPa before increasing the pressure to the test pressure. When performing graphite heat exchanger repairs, the anti-corrosion coating on the inner wall of the shell and the sacrificial anodes must also be inspected, as shell-side corrosion is often overlooked. Flange bolts should be tightened symmetrically in three stages according to the manufacturer’s specifications; over-tightening can cause the graphite flanges to crack. Field experience shows that the most common causes of overhaul failure are damage to the graphite tubes caused by prying during core removal, incorrect selection of gaskets, and residual water not fully drained after pressure testing, leading to freeze-induced cracking in winter. When selecting an electrolytic cell manufacturer, prioritize those that can provide complete operating limits for graphite heat exchangers and recommended overhaul intervals; avoid sales representatives who only provide power consumption data and sidestep issues related to acid mist and scaling.

Detailed Maintenance Procedures for Graphite Heat Exchangers

Each of the following steps is a controlled action in the Graphite Exchanger Overhaul process and must not be skipped. The table below lists the key English terms for both block-and-hole and shell-and-tube types side by side, making it convenient for direct use when creating work orders and spare parts requests. (For replaced parts, you can contact a precious metal recycler to have them tested for precious metals.)

Before beginning Phase 0 of the Graphite Exchanger Overhaul, a work permit must be obtained and lockout/tagout procedures must be implemented. After depressurizing the system, drain the medium and purge with nitrogen until combustible and toxic gas detection tests pass. When performing the Graphite Exchanger Overhaul, Phase 1 involves disassembling the structure: for block-and-hole type units, first remove the end caps, then push out the shell, and hoist out the graphite blocks; The key to Graphite Block Exchanger Dismantling is to record the original gasket thickness and the relative positions of the shell and graphite blocks; otherwise, misalignment may occur during reassembly. For shell-and-tube type Graphite Bundle Pulling, the tube bundle must be inspected for adhesion to the shell before pulling the core; if necessary, soak in water or use steam heating to loosen it. Stage 2 cleaning uses high-pressure water jets, with pressure typically controlled between 500 and 800 bar; steel nozzles must not be used in graphite channels. Stage 3 inspection must include dimensional measurements and surface crack detection; for shell-and-tube types, each graphite tube must be numbered. During the repair stage of a graphite exchanger overhaul, decisions depend on the number of leaks and pressure-bearing risks; for block-and-hole types, if more than 5% of the channels are blocked or cracks span multiple holes, the entire unit must be replaced; for shell-and-tube types, if more than 10% of the tubes are blocked or the tube sheet sealing surface is damaged, the entire tube bundle must be replaced. During Stage 5 (reassembly), the greatest concern is hard contact between graphite blocks or tube bundles and the shell; positioning guide rods must be used. During Stage 6 of the graphite heat exchanger overhaul, pressure testing must be conducted separately for hydrostatic and air tightness; air tightness testing alone cannot substitute for hydrostatic testing. 

After completing the graphite heat exchanger repair, update the maintenance records and archive the tube plugging locations, serial numbers of replaced parts, and pressure test curves. The success of a graphite heat exchanger overhaul is ultimately determined not by how clean the equipment’s surface appears, but by the absence of leaks at all sealing points within 48 hours after heating up and by pressure drops remaining within the design range. After completing the graphite heat exchanger repair, update the maintenance record and file the locations of clogged tubes, serial numbers of replaced parts, and pressure test curves. The success of a graphite heat exchanger overhaul is ultimately determined not by how clean the equipment’s surface is, but by whether all seal points remain leak-free for 48 hours after heating up and whether the pressure drop remains within the design range. When selecting an electrolytic cell manufacturer, I require them to provide records of graphite heat exchanger overhauls from similar installations in Europe or North America that have been in continuous operation for at least three years. I pay particular attention to the efficiency of the acid mist scrubber and changes in the acidity of the anolyte, as these indicators reveal whether the electrolytic cell is compatible with downstream equipment—and this is the fundamental factor in extending the graphite heat exchanger overhaul cycle.