Shell-and-Tube Heat Exchanger Cleaning Case Study

1. Project Background

A major chemical group is a large-scale chlor-alkali chemical producer in China with an annual capacity of 600,000 tons. Its core unit — a shell-and-tube heat exchanger (Tag No. E-1203A) — is used to cool high-temperature process medium vinyl chloride monomer (VCM), handling heat transfer between the synthesis unit and the distillation unit, and serves as the critical thermal node for continuous operation of the entire VCM production line. This heat exchanger had been in continuous operation for over 18 months without cleaning maintenance. Although scale inhibitors and biocides were dosed into the circulating cooling water system, the makeup water hardness was high (total hardness consistently maintained at 280–350 mg/L as CaCO₃), and scaling on the shell-side water gradually worsened over prolonged operation.

Operating data showed that within the past 6 months, the outlet temperature had risen from the design value of 45°C to 63°C, approaching the process safety interlock upper limit (68°C), with daily steam costs increasing by approximately CNY 32,000. Once the interlock shutdown was triggered, a single outage loss was estimated to exceed CNY 800,000, creating an urgent need for an online cleaning solution without shutting down.

Danyang Blue Star Cleaning was commissioned in May 2026 to conduct on-site investigation. After scale sample analysis and operating parameter diagnosis, an online chemical cleaning plan was developed. Throughout the entire operation, tube-side VCM flow continued normally — production was never interrupted.

2. Equipment Overview and Scaling Diagnosis

2.1 Heat Exchanger Parameters

2.2 Scale Sample Analysis

Two scale samples were collected from the shell-side blowdown port and manway for XRD diffraction analysis, chemical titration, and SEM morphology observation. The analysis results are as follows:

SEM morphology showed a dense layered scale structure, with CaCO₃ predominantly in calcite crystal form and relatively high hardness (Mohs hardness ~3). Multi-point thickness measurement showed an average scale thickness of approximately 2.8mm, with localized areas (high-temperature tube sheet region) reaching up to 4.5mm. The thermal conductivity of CaCO₃ is only approximately 0.6 W/m·K, far lower than 316L stainless steel at 16.3 W/m·K. The 2.8mm CaCO₃ scale layer thermal resistance is equivalent to adding approximately 4.7mm of insulation on the outer tube wall (calculated using the series thermal resistance model R = δ/λ) — this is the fundamental heat transfer reason why efficiency dropped from the design value to 60%.

The scaling type was determined to be carbonate hard scale as the primary component, containing silicates and organic slime — a composite scale. The SiO₂ content was not high (8.4%), and Sulfamic Acid combined with Surfactant penetration can effectively dissolve it. Organics needed to be removed first through alkaline wash pretreatment. Fe₂O₃ can be simultaneously dissolved under acidic conditions. The scale type was overall suitable for chemical cleaning, but corrosion risk of the acid solution on 316L material needed to be controlled.

3. Cleaning Plan Design

3.1 Cleaning Method Selection

The core constraint of this project was completing the cleaning without shutting down — the tube-side VCM process flow could not be interrupted. Therefore, a shell-side single-sided isolated circulation cleaning plan was adopted: close the shell-side cooling water inlet and outlet valves, drain the shell-side water, connect temporary cleaning piping via the shell-side inlet/outlet flanges, and perform chemical cleaning exclusively on the shell-side water. The tube side maintained normal VCM flow and process operation, during which the VCM outlet temperature could be monitored in real time as an online indicator for judging cleaning progress.

Acid selection needed to focus on the corrosion resistance characteristics of 316L stainless steel. 316L contains Mo (2.0%–3.0%), exhibiting superior pitting resistance compared to 304 in oxidizing acids and Cl⁻-containing environments, but HCl cannot be used — Cl⁻ has an extremely strong ability to penetrate the passive film under acidic conditions, and even 316L faces pitting and stress corrosion cracking risks. Sulfamic Acid contains no halide ions, has an extremely low corrosion rate on stainless steel (≤0.5 g/m²·h at ambient temperature), and has strong CaCO₃ dissolution capability (dissolution reaction produces soluble calcium sulfamate), making it the preferred primary acid agent for chemical cleaning of 316L stainless steel heat exchangers.

3.2 Cleaning Formulation

Formulation design points: Sulfamic Acid combined with Citric Acid — the former rapidly dissolves the bulk carbonate scale, while the latter chelates Fe³⁺ released during dissolution via carboxyl groups, preventing Fe³⁺ hydrolysis that would generate Fe(OH)₃ secondary precipitation and block tube gaps. The corrosion inhibition system adopts a BTA + Sodium Molybdate + Urotropine ternary combination — BTA chemically adsorbs to form film, Sodium Molybdate provides anodic passivation, and Urotropine adds physical adsorption assist; together they synergistically keep the corrosion rate of 316L in the acidic medium below the safety threshold.

3.3 Construction Procedure

1. System Isolation and Temporary Loop Setup: Close shell-side cooling water inlet/outlet valves, drain stored water. Install DN200 temporary cleaning connections (with pressure gauge, thermometer, sampling valve), connect cleaning pump station (80 m³/h / 40m head) and PE circulation tank (8 m³, with steam heating). System pressure test at 0.6 MPa held for 30 min with zero pressure drop.
2. Water Flush: Inject fresh water to establish circulation, flow rate 60–70 m³/h, flush for 30 min then drain, observe turbidity and suspended solids in discharge.
3. Alkaline Wash Degreasing: Solution: Na₂CO₃ 1.5% + Surfactant 0.1% + Na₃PO₄ 0.3%, 60–70°C circulation for 2 hr. Alkaline wash solution changed from colorless to yellowish-brown turbidity, indicating effective organic stripping. After alkaline wash, flush with fresh water until pH ≤ 8.5.
4. Main Acid Cleaning Stage: Add Sulfamic Acid at 8% initial concentration + Citric Acid + BTA + Sodium Molybdate + Urotropine + Surfactant + Defoamer, circulate at 50–60°C. Sample every 15 min for Ca²⁺ and pH. Ca²⁺ concentration trend: sharp rise to 4200 mg/L at 0.5 hr, maintained high plateau from 0.5–2 hr, gradual decline from 2–4 hr, dropped below 800 mg/L at 5.5 hr with no change over 3 consecutive checks, pH rose from 1.5 to 2.8 and stabilized — endpoint determined. Total acid cleaning approximately 6.5 hr, with two Sulfamic Acid top-ups (15% each at 2 hr and 4 hr).
5. Rinsing: Drain acid solution, high-flow fresh water circulation flush, replace every 10 min. After 3 cycles, pH rose to 5.8, total iron < 5 mg/L — qualified.
6. Passivation Treatment: Solution: NaNO₂ 1.5% + Na₃PO₄ 0.5%, adjust pH to 9.5–10.5 with NaOH, circulate at 40–50°C for 4 hr. Generate γ-Fe₂O₃ + Cr₂O₃ composite passivation film on 316L surface. After passivation, visual inspection showed uniform silver-gray tube sheet with no secondary flash rust.
7. Final Flush and System Restoration: Fresh water flush to neutral pH (6.5–7.5), remove temporary piping, restore original shell-side cooling water inlet/outlet flange connections. Gradually open valves to resume water flow — tube-side VCM flow was never interrupted throughout.

4. Performance Data

4.1 Operating Parameter Comparison

4.2 Corrosion Monitoring

Standard corrosion coupons of the same material as the heat exchanger tubes (316L, 50×25×2 mm, surface roughness Ra ≤ 0.8 μm) were placed in the cleaning loop. After cleaning, the coupons were cleaned with acetone, dried, and weighed on a precision balance (0.1 mg accuracy): corrosion rate was 0.38 g/m²·h, far below the 3.0 g/m²·h upper limit specified in GB/T 25146-2010 Quality Acceptance Specification for Chemical Cleaning of Industrial Equipment, and also better than the company's internal control standard of 1.0 g/m²·h. After cleaning, the coupon surface showed no visible pitting to the naked eye, and SEM at 500× magnification revealed no signs of intergranular corrosion. The BTA + Sodium Molybdate + Urotropine ternary corrosion inhibition system achieved design requirements for protecting 316L stainless steel in Sulfamic Acid medium.

4.3 Economic Benefit Analysis

Direct Energy Savings: After cleaning, daily steam costs decreased by approximately CNY 30,000 (reduced VCM outlet temperature decreased steam consumption in distillation column reboiler). Based on 300 operating days per year, annual steam cost savings are approximately CNY 9 million. Circulating cooling water consumption reduced by 140 m³/h; at CNY 0.3/m³, annual water savings benefit is approximately CNY 300,000.

Avoided Shutdown Loss: Before cleaning, the outlet temperature of 63°C was approaching the 68°C interlock shutdown threshold. According to historical statistics from the chemical group, the probability of unplanned shutdown under similar conditions was approximately 35% per year. A single shutdown loss is approximately CNY 800,000, giving an annual avoided expected loss of approximately CNY 280,000.

Return on Investment: Total cost of this cleaning project was approximately CNY 180,000 (including chemicals, labor, equipment rental, and waste liquid treatment). Combined annualized benefit is approximately CNY 9.58 million (energy savings 9M + water savings 0.3M + avoided shutdown 0.28M), with a payback period of less than 7 days and an input-output ratio exceeding 1:50.

5. Lessons Learned and Recommendations

This project validated the reliability and efficiency of the Sulfamic Acid + Citric Acid composite cleaning system for online cleaning of 316L stainless steel shell-and-tube heat exchangers. The shell-side single-sided isolated circulation cleaning plan successfully met the core constraint of no-shutdown construction, with tube-side VCM production unaffected throughout. The ternary corrosion inhibitor combination (BTA + Sodium Molybdate + Urotropine) kept the 316L corrosion rate below 0.38 g/m²·h, providing a safe formulation and process reference for cleaning similar stainless steel equipment.

Based on experience from this case, the following O&M optimization recommendations are proposed:

• Set the shell-and-tube heat exchanger cleaning interval at 12–14 months to avoid scale accumulation exceeding 3mm, which causes significant heat transfer efficiency decline before passive cleaning becomes necessary;
• Install online hardness monitors and automatic chemical dosing devices in the circulating cooling water system to control makeup water total hardness at ≤200 mg/L (as CaCO₃), slowing the scaling rate at the source;
• Install online fouling thermal resistance monitoring instruments at shell-side inlet/outlet to achieve data-driven precision maintenance decisions through real-time K-value tracking, upgrading from "scheduled cleaning" to "condition-based cleaning."

Equipment cleaning and maintenance is not an isolated event but part of a management loop consisting of water quality management, operating parameter monitoring, and maintenance cycle optimization. A professional cleaning can restore equipment performance, but only sustained preventive management can keep equipment operating stably in the optimal performance range over the long term.