I. The Importance of Pre-Commissioning Cleaning for New Equipment

Newly installed industrial equipment inevitably retains various contaminants from the manufacturing, transportation, and installation processes. Equipment and pipeline inner walls may have residual mill scale, welding oxide slag, anti-rust grease, sand, rust, and other mechanical impurities. If these residues are not thoroughly removed, they will cause serious consequences once the equipment is put into operation.

First, impurities can block valves, instruments, and small-diameter piping, causing control system malfunction. Second, rust and mill scale can detach under high-temperature, high-pressure conditions and flow with the medium into heat exchanger tube bundles, causing heat transfer efficiency decline or even pipeline blockage. More seriously, grease and organic matter will carbonize and decompose at high temperatures, contaminating process media and affecting product quality. For catalytic reaction systems, residual impurities may also cause catalyst poisoning and deactivation.

According to chemical industry statistics, newly built equipment that has not undergone standardized pre-commissioning cleaning has a failure rate over 40% higher than equipment that has been thoroughly cleaned. Therefore, chemical cleaning before commissioning new equipment is an indispensable critical link in the entire engineering construction and commissioning process.

II. Preparatory Work Before Cleaning

Technical Documentation Preparation: Collect equipment drawings, material lists, welding records and other documentation, clearly identifying the material, specifications, and connection methods of each component. Select appropriate cleaning agents and corrosion inhibitor formulations based on material characteristics.

Cleaning Plan Development: Prepare a detailed chemical cleaning plan, including cleaning scope, cleaning process flow, cleaning agent formulation, process parameters (temperature, time, flow rate, etc.), waste liquid treatment plan, and safety emergency response plan.

Temporary Facility Setup: Construct temporary cleaning circulation systems according to the cleaning plan, including cleaning pump stations, solution preparation tanks, heating devices, connecting pipes, valves, and instruments. The cleaning pump flow rate and head must meet the flow velocity requirements at the most unfavorable point of the system.

Safety Measure Implementation: Establish safety isolation zones, equip personal protective equipment (acid-resistant suits, acid-resistant gloves, face shields, safety shoes, etc.), install emergency shower devices, and post safety warning signs. Conduct safety technical briefings for all participating personnel.

III. Standard Cleaning Steps

Step 1: Water Flushing. Perform high-flow flushing of the entire system with industrial water at a flow velocity no less than 1.5 m/s, until the discharge water is clear and transparent with no visible impurities. The purpose of water flushing is to remove loose welding slag, sand, and other mechanical impurities from the pipelines. Water flushing should be performed in sections to prevent impurities from accumulating in low-lying areas.

Step 2: Alkaline Degreasing. Use a mixed solution of 0.5% to 1.5% sodium hydroxide (NaOH), 0.3% to 0.5% trisodium phosphate (Na₃PO₄), and 0.1% to 0.3% surfactant, circulating at 70 to 85°C for 6 to 8 hours. The purpose of alkaline cleaning is to remove anti-rust grease and organic contaminants from equipment inner walls. After alkaline cleaning, drain the waste liquid and flush with clean water to pH 8 to 9.

Step 3: Acid Cleaning for Rust and Scale Removal. Select an appropriate acid cleaning formulation based on equipment material. For carbon steel equipment, commonly use 5% to 8% hydrochloric acid with 0.3% to 0.5% urotropine corrosion inhibitor, circulating at 40 to 50°C for 4 to 6 hours. For systems containing stainless steel, use 5% to 8% nitric acid or 3% to 5% citric acid to avoid chloride ion-induced intergranular corrosion of stainless steel. During acid cleaning, sample and test iron ion concentration and acid concentration every 30 minutes. Stabilization of iron ion concentration indicates the cleaning endpoint.

Step 4: Rinsing. After draining the acid cleaning solution, flush with clean water to pH 5 to 6. Then use 0.2% to 0.5% citric acid solution for rinsing to remove residual iron ions and prevent secondary rusting. Rinsing temperature is controlled at 60 to 70°C, with circulation time of 1 to 2 hours.

Step 5: Passivation Treatment. After rinsing, perform passivation at pH 9 to 10. Use 1% to 2% sodium nitrite (NaNO₂) solution, or 0.5% to 1% trisodium phosphate solution, circulating at 50 to 60°C for 4 to 6 hours for passivation. The passivated metal surface forms a dense protective film that effectively prevents atmospheric corrosion.

Step 6: Drying and Protection. After draining the passivation solution, use dry compressed air or nitrogen to purge the system, removing residual moisture. If necessary, perform hot air drying to ensure the system interior is dry and clean. After drying, immediately seal all openings to prevent atmospheric moisture and impurities from re-entering.

IV. Acceptance Criteria

After chemical cleaning is completed, strict acceptance inspection must be conducted in accordance with national standards and industry specifications. Main acceptance indicators include:

1. Surface Cleanliness: The cleaned surface shall be free of mill scale, rust, welding slag, grease, and other residues. Wiping with clean white filter paper shall show no rust stains or oil marks.

2. Passivation Film Quality: The passivation film shall be uniform and dense. In the copper sulfate spot test, the color change time on the carbon steel surface shall be no less than 10 seconds.

3. Corrosion Rate: The corrosion rate detected by coupon testing during the cleaning process shall not exceed 6 g/(m²·h), exceeding the requirements of national standard GB/T 36542-2018.

4. Residue Analysis: The suspended solids content in the final water flushing discharge shall be less than 50 mg/L, and iron ion content shall be less than 1 mg/L. The pH value shall be between 6 and 9.

V. Common Problems and Solutions

In actual cleaning processes, issues such as cleaning dead zones, air pockets, and localized corrosion may be encountered. For complex piping systems, sectional cleaning should be adopted to ensure effective cleaning of each pipeline section. For areas containing dead zones, a combination of soaking and circulation can be employed. If a decrease in corrosion inhibitor concentration is detected, it should be replenished promptly to ensure equipment safety.

Danyang Blue Star Anti-corrosion Cleaning Co., Ltd. possesses extensive experience in pre-commissioning cleaning for new equipment and can provide full-process services from solution design to on-site execution. Please call for consultation: +86 18952832843.

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