1. Introduction
Floating head heat exchangers are one of the most common shell-and-tube heat exchanger types in petrochemical, refining, and coal chemical industries operating under high-temperature and high-pressure conditions. Their core structural feature is that one end of the tube bundle is fixed to the tubesheet while the other end expands and contracts freely through a floating head cover, effectively compensating for thermal expansion differences between the shell side and tube side and preventing tubesheet cracking due to thermal stress. However, the floating head structure also brings special characteristics to overhaul cleaning — the tube bundle can be extracted as a whole, which facilitates thorough cleaning, but the cleaning and reassembly precision requirements for the floating head sealing surfaces are much higher than for fixed-tubesheet heat exchangers. According to industry statistics, floating head heat exchangers account for over 40% of heat exchange equipment in petrochemical plants, and their operating efficiency directly affects unit energy consumption and production capacity. Overhaul cleaning is the core means of restoring floating head heat exchanger performance — rationally selecting a combination of chemical cleaning and physical cleaning can achieve thorough descaling without damaging the base material. This article systematically describes the complete overhaul process for floating head heat exchangers, from shutdown and disassembly, tube bundle extraction, shell-side high-pressure water jet cleaning, and tube-side chemical cleaning, to reassembly and pressure testing, providing actionable technical solutions based on field construction experience.
2. Scaling Mechanism Analysis of Floating Head Heat Exchangers
2.1 Shell-Side Scaling Characteristics
The shell-side medium of floating head heat exchangers is typically a process fluid (such as crude oil, residual oil, circulating water), flowing in crossflow between baffles, with deposition prone to form in low-velocity zones. Common scale types include: CaCO₃ and Mg(OH)₂ mixed water scale (circulating water side), coking scale and polymer scale (slurry oil/residual oil side), and corrosion products Fe₃O₄ and Fe₂O₃. A distinctive feature of shell-side scaling is its non-uniform distribution along the baffle cut direction, with the scaling rate at the inlet side often being 2–3 times that at the outlet side, directly related to local velocity reduction and temperature gradient changes.
2.2 Tube-Side Scaling Characteristics
The tube-side medium flows in a turbulent state inside the heat exchanger tubes, with scaling dominated by chemical deposition. The scale layer is typically thinner but denser than on the shell side. Typical tube-side scale includes silicate scale (when cooling water contains SiO₂), CaSO₄ hard scale (high-sulfate water quality), and organic film layers from process medium polymerization. Tube-side cleaning of floating head heat exchangers is relatively convenient — after bundle extraction, individual heat exchanger tubes can be directly unblocked, which is one of its overhaul advantages.
2.3 Key Parameters
| Parameter | Normal Range | Cleaning Threshold |
|---|---|---|
| Overall Heat Transfer Coefficient K | 300–800 W/(m²·K) | Drop > 30% of original value |
| Tube-Side Pressure Drop | Design value ±10% | Increase > 50% of design value |
| Shell-Side Pressure Drop | Design value ±15% | Increase > 50% of design value |
| Scale Layer Thickness | < 0.3 mm | > 1.0 mm |
| Outlet Temperature Difference | Meets process requirements | Deviation from design > 15°C |
3. Complete Overhaul Cleaning Process
3.1 Shutdown and Disassembly
Overhaul cleaning begins with system shutdown. Disassembly operations may only proceed after confirming that shell-side and tube-side media have been drained, the system has been depressurized to atmospheric pressure, and nitrogen purging has been completed. Disassembly sequence: Remove floating head end outer cover flange bolts → Remove fixed tubesheet end flange bolts → Use a dedicated bundle extractor to pull the entire tube bundle from the shell. During extraction, the tube bundle must be kept moving horizontally to avoid scraping damage between baffles and the shell inner wall. Immediately after bundle extraction, perform a visual inspection of the floating head sealing surfaces, recording gasket groove corrosion and deformation conditions.
3.2 Shell-Side Cleaning
After tube bundle extraction, the shell inner wall is exposed. High-pressure water jet cleaning is the preferred method (pressure 40–70 MPa, flow rate 60–100 L/min). The water jet advances section by section along the baffle gaps, focusing on removing baffle root deposits and shell bottom sludge. For areas with heavy oil scale adhesion, pre-treat by circulating alkaline cleaning agent (5–8% NaOH + 0.5% Surfactant, 60–70°C) for 4–6 hours, then flush with high-pressure water — this significantly improves descaling efficiency.
3.3 Tube Bundle Cleaning
Tube-Side Chemical Cleaning: After bundle extraction, establish a temporary cleaning loop — connect the circulation pump with the fixed tubesheet side as inlet and the floating head side as outlet. The cleaning solution formulation is determined based on scale sample analysis; a typical formulation is as follows:
| Component | Concentration | Function |
|---|---|---|
| Sulfamic Acid | 5–8% | Dissolves CaCO₃/CaSO₄ scale |
| Citric Acid | 2–3% | Complexes Fe³⁺/Fe²⁺, removes iron scale |
| BTA | 0.3–0.5% | Copper tube/copper alloy corrosion protection |
| Urotropine | 0.2–0.3% | Carbon steel pickling corrosion inhibitor |
| Nonionic Surfactant | 0.1–0.2% | Penetration and wetting, enhanced oil removal |
Cleaning solution temperature controlled at 50–65°C, circulation velocity 0.5–1.0 m/s, circulation time 4–8 hours. Cleaning endpoint determination: two consecutive samples showing acid concentration change < 0.1% and Fe³⁺ concentration no longer rising. After chemical cleaning, displace with fresh water to pH ≈ 7, then circulate 0.5% Na₃PO₄ solution for 30 minutes for passivation treatment to prevent flash rust.
Shell-Side High-Pressure Water Jet Cleaning: Scale between baffles on the outside of the tube bundle is removed using high-pressure water jet cleaning. A rotating nozzle advances section by section along the tube bundle axis at 40–60 MPa pressure, with the nozzle maintained at a distance of 200–300 mm from the tube bundle surface. For stubborn scale at baffle roots and the back of the tubesheet, use a rigid lance with a 0° forward nozzle for point-to-point removal.
3.4 Tube Bundle Pass-Through Inspection
After cleaning is complete, inspect each tube individually for pass-through rate. Use nylon balls with a diameter 2–3 mm smaller than the tube inner diameter, propelled by compressed air through each tube. Mark tubes that fail and clear them with a flexible shaft mechanical drill. The pass-through qualification rate must reach 100% — any blocked heat exchanger tube can cause flow maldistribution and localized overheating during operation. Additionally, perform sample wall thickness inspection on heat exchanger tubes (ultrasonic thickness gauge); tubes with wall thickness reduction > 30% should be plugged or replaced.
3.5 Reassembly and Pressure Testing
Reassembly is the most critical step in floating head heat exchanger overhaul. Key operational points include: the floating head gasket groove must be thoroughly cleaned and inspected for radial scratches; select spiral-wound gaskets or serrated composite gaskets (according to design pressure and temperature class); tighten floating head cover bolts using a hydraulic torque wrench in a cross-symmetric pattern in 3 stages — 50%, 80%, 100% of design torque. After reassembly, perform a shell-side hydrostatic test (1.25× design pressure, hold for 30 minutes), confirming no leakage at the floating head sealing surface and tubesheet joints before placing into service.
4. Engineering Case Study
Equipment Parameters: A floating head heat exchanger in a petrochemical enterprise's FCC unit, model BES1200-2.5-400-6/25-4, heat transfer area 400 m², shell-side medium: slurry oil (inlet 280°C), tube-side: circulating cooling water, carbon steel shell + #10 steel tube bundle. After 3 years of operation, the heat transfer coefficient dropped from the design value of 520 W/(m²·K) to 310 W/(m²·K), and shell-side pressure drop increased to 2.1 times the design value.
Scale Sample Analysis: Shell-side disassembly inspection revealed heavy deposits of coking oil scale + CaCO₃ mixed scale between baffles, scale layer thickness 2.5–4.0 mm, particularly severe at the bottom. Tube-side was mainly CaCO₃ water scale + minor rust, scale layer 0.8–1.5 mm.
Cleaning Plan: Shell side first treated with 5% NaOH + 0.3% Surfactant solution circulated and soaked for 6 hours to soften oil scale, then cleaned section by section with 55 MPa high-pressure water jet, taking 12 man-hours to complete. Tube side cleaned with 6% Sulfamic Acid + 2% Citric Acid + 0.3% BTA + 0.25% Urotropine formulation circulated for 6 hours, followed by passivation treatment. Tube bundle pass-through rate: 83% before cleaning, 100% after cleaning.
Cleaning Results:
| Indicator | Before Cleaning | After Cleaning | Recovery Rate |
|---|---|---|---|
| Heat Transfer Coefficient K | 310 W/(m²·K) | 498 W/(m²·K) | 95.8% |
| Shell-Side Pressure Drop | 0.32 MPa | 0.16 MPa | Restored to design value |
| Tube-Side Pressure Drop | 0.18 MPa | 0.10 MPa | Restored to design value |
After commissioning, the unit's processing capacity recovered to design load, with annual steam cost savings of approximately CNY 280,000.
5. Summary and Recommendations
Floating head heat exchangers, with the structural advantage of extractable tube bundles, enable complete separation treatment of tube and shell sides during overhaul cleaning, yielding significantly better cleaning quality than online cleaning of fixed-tubesheet heat exchangers. The core control points of overhaul cleaning are: thorough system isolation and purging before disassembly, guided protection during tube bundle extraction, coordinated use of shell-side high-pressure water jet and tube-side chemical cleaning, and precision assembly of the floating head sealing surfaces during reassembly.
O&M recommendations: It is recommended that floating head heat exchangers in petrochemical plants undergo preventive overhaul cleaning every 2–3 years, rather than waiting for significant heat transfer efficiency decline or abnormal pressure differentials before passive overhaul. For operating conditions with high solids content and fast scaling rates, periodic shell-side backflushing can be supplemented during daily operation to slow scale accumulation. For floating head heat exchangers with austenitic stainless steel tube bundles, Cl⁻ content must be strictly controlled below 50 ppm during cleaning to prevent intergranular corrosion. In the first month after overhaul and recommissioning, inspection frequency should be increased, focusing on floating head end sealing surface temperature changes and shell-side outlet pressure drop trends to ensure sealing reliability.