Cleaning Before New Equipment Startup | Chemical Cleaning Operating Procedures

I. Importance of Pre-Startup Cleaning for New Equipment

Newly installed industrial equipment inevitably retains various contaminants from the manufacturing, transportation, and installation processes. Equipment piping inner walls may contain mill scale, weld oxide slag, rust-preventive oils and greases, sand, rust, and other mechanical impurities. If these residues are not thoroughly removed, they can cause serious consequences once the equipment is put into operation.

First, impurities can block valves, instruments, and small-bore piping, causing control system malfunction. Second, rust and mill scale can detach under high-temperature, high-pressure conditions and travel with the process medium into heat exchanger tube bundles, reducing heat exchange efficiency or even blocking pipes. More seriously, oils and organic matter carbonize and decompose at high temperatures, contaminating process media and affecting product quality. For catalytic reaction systems, residual impurities can also cause catalyst poisoning and deactivation.

According to chemical industry statistics, the failure rate of newly built units that did not undergo standardized cleaning before startup is over 40% higher than that of units that received thorough cleaning. Therefore, chemical cleaning before the startup of 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 to clarify the materials, 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 formulations, process parameters (temperature, time, flow rate, etc.), waste liquid treatment plan, and safety emergency procedures.

Temporary Facility Setup: Build a temporary cleaning circulation system according to the cleaning plan, including cleaning pump station, solution preparation tank, heating device, connecting pipes, valves, and instrumentation. The cleaning pump flow rate and head should meet the velocity requirements at the most unfavorable point in the system.

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

III. Standard Cleaning Steps

Step 1: Water Flushing. Flush the entire system with industrial water at high flow rates, with velocity not less than 1.5 m/s, until the effluent is clear and transparent with no visible impurities. The purpose of water flushing is to remove loose weld slag, sand, and other mechanical impurities from the piping. Water flushing should be performed in sections to prevent impurity accumulation in low points.

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

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%–8% hydrochloric acid with 0.3%–0.5% urotropine corrosion inhibitor, circulated at 40–50°C for 4–6 hours. For systems containing stainless steel, use 5%–8% nitric acid or 3%–5% citric acid to avoid chloride-induced intergranular corrosion. During acid cleaning, sample every 30 minutes to test iron ion concentration and acid concentration; stabilization of iron ion concentration indicates the cleaning endpoint.

Step 4: Rinsing. After draining the acid cleaning solution, rinse with fresh water to pH 5–6. Then rinse with 0.2%–0.5% citric acid solution to remove residual iron ions and prevent secondary rusting. Rinse temperature: 60–70°C, circulation time: 1–2 hours.

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

Step 6: Drying and Protection. After draining the passivation solution, purge the system with dry compressed air or nitrogen to remove residual moisture. Perform hot air drying if necessary to ensure the system interior is dry and clean. After drying, immediately seal all openings to prevent re-entry of atmospheric moisture and impurities.

IV. Acceptance Standards

After chemical cleaning is completed, strict acceptance inspection must be performed according to national standards and industry specifications. Key acceptance indicators include:

1. Surface Cleanliness: Cleaned surfaces shall be free of mill scale, rust, weld slag, oil, and other residues. A clean white filter paper wipe shall show no rust stains or oil marks.

2. Passive Film Quality: The passive film shall be uniform and dense. In a copper sulfate spot test, the discoloration time on carbon steel surfaces shall be no less than 10 seconds.

3. Corrosion Rate: The corrosion rate measured 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 final water flush effluent shall have suspended solids content below 50 mg/L and iron ion content below 1 mg/L. The pH value shall be between 6 and 9.

V. Common Problems and Solutions

During actual cleaning operations, issues such as cleaning dead zones, air locks, and localized corrosion may be encountered. For complex piping systems, segmented cleaning should be used to ensure effective cleaning of each pipe section. For areas with dead zones, alternating immersion and circulation methods can be applied. If corrosion inhibitor concentration is found to be decreasing, timely replenishment is essential to ensure equipment safety.

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