Guidelines for chemical cleaning process of industrial pipelines

This standard is applicable to the pipeline construction requiring chemical cleaning specified in the design documents of industrial metal pipeline engineering. This standard is not applicable to the construction of water washing and oil cleaning in industrial metal pipeline engineering.

Normative references

The clauses in the following documents become the provisions of this standard by reference to this standard. For dated references, the subsequent amendment list (excluding the contents of corrigenda) or the revised version of the cited documents are not applicable to the standard. However, parties who have reached an agreement according to this part are encouraged to study whether the latest version of these documents can be used. For undated references, the latest edition is applicable to this standard GB 50235 code for construction and acceptance of industrial metallic piping engineering SH/T 3517 technical specification for construction technology of petrochemical steel pipeline engineering

Construction preparation

Material preparation

All chemical cleaning chemicals used for chemical cleaning of industrial pipelines must have factory quality certificate, and their chemical composition and characteristics must meet the requirements of relevant standards. Stainless steel or plastic products should be used for pipes, pipe fittings and metal materials used for temporary connection on site.Containers used for on-site storage of cleaning chemicals shall be stainless steel or plastic products.

Equipment

Equipment: acid resistant pump, stainless steel water tank, water pump, air compressor, black light lamp for cleaning quality inspection, yx-125 portable fluorescent detector (power 125w), etc.The tools for soaking and scrubbing include plastic bucket, basin, stainless steel brush, brush, rubber gloves, masks and other necessary labor protection articles.

Operating conditions

The pressure test has been completed and the process handover has been handled.

Construction technology

The construction procedure is shown in Figure 1.

• Installation of temporary pipelines (when cyclic acid washing is required);
• In principle, the diameter of the main pipe is less than 100 mm, and the diameter of the branch pipe is the same as that of the formal pipe diameter;
• Arrange the emptying line and condensate drainage line;
• Temporary supports shall be added for temporary piping according to actual needs.

Pre cleaning

When there are obvious oil spots on the inner surface of the pipeline, no matter what acid pickling method is adopted, necessary pre oil removal treatment shall be carried out before pickling. Generally, 5% sodium carbonate solution is used to remove oil stains or carbon tetrachloride (or trichloroethylene) is used for degreasing treatment. At the same time, the weld quality shall be checked. If there is large weld bead and welding slag, it shall be treated.

Table 1 formula of pickling and passivation solution for carbon steel pipe and low alloy steel pipe

Table 2 formula of pickling and passivation solution for stainless steel pipe and acid resistant steel pipe

Picklin

Tank leaching method or system circulation method can be used for acid pickling. The acid pickling solution shall be prepared in accordance with the prescribed formula in sequence and evenly stirred. When the formula of pickling solution, neutralization solution and passivation solution is not specified in the design, it shall be prepared and used according to the provisions in Table 1 and table 2.In the system circulation method, acid pump is used to fill acid solution into the pipe while discharging air. After the air is exhausted, the operation temperature and duration of acid pickling should be determined according to the removal of rust spots. Generally, it lasts for 6-8 hours, and the temperature of pickling solution is about 70 ℃.

During pickling, the composition of pickling solution shall be analyzed regularly and new solution shall be added in time. When the rust removal effect is obviously reduced, it shall be replaced.

Neutralization cleaning

After acid washing, connect the neutralizing liquid rubber tube, and use the pump to neutralize the residual acid in the pipe dynamically. Check with PH test paper at the discharge outlet, and stop neutralization when the solution is neutral or weakly alkaline.

Passivation

After the acid pickling and neutralization are qualified, connect the passivation solution rubber tube, start the pump to fill the passivation liquid into the pipe, and exhaust the air while filling, so that the passivation liquid circulates in the pipe for half an hour, and then it is discharged after being static for another 2 hours.

Water flushing

After flushing the system with clean water, turn on the circulating water pump.

Drying: after washing with water, purge the circulation system with dry gas immediately until the residual liquid in the pipeline is completely drained.

After the water flushing is qualified, dry oil-free air or nitrogen shall be used to thoroughly dry the inner wall of the pipe. If the inner wall of carbon steel pipe is silver gray, it is considered as qualified.

After the purging and drying of the system, the temporary pipeline should be removed in a short time, and the removed pipe accessories should be restored. If the time from commissioning and start-up is long, after the pipeline system is recovered, the system shall be filled with nitrogen of 0.07MPa to seal the pipeline system.

For small pipe sections, pipe fittings, etc., can be cleaned by immersion method. During cleaning, the pipe sections and fittings are directly put into each solution tank. The steps are the same as that of circulating cleaning. After pickling, neutralization, passivation, water washing and drying, they are assembled, and nitrogen filling protection is carried out as required.

Quality standard

The inner surface of the pipe or the surface of the cleaning parts shall be free of rust, oxidation spots, impurities, scale and metallic luster.

The pipeline after pickling shall be inspected by visual inspection, and the inner wall of pipeline shall be regarded as qualified with metallic luster.

After neutralization, the PH value of the solution should be 9-9.5.

The quality of passivation is checked by blue dot test. Use one drop of test solution on the passivation surface, and the blue spots appear within 15min, less than 8 points are qualified. See Table 3.

Table 3 formula of blue dot test solution

Construction precautions

In order to obtain good cleaning quality and make the chemical cleaning solution produce the best erosion effect on the inner wall of the pipe at a certain flow rate, the large lift and large flow corrosion resistant pump should be selected to make the solution cycle clean the whole loop pipeline under a certain pressure.

During acid pickling, it is very important to empty the pipe, so that there is no air bag in the pipe, otherwise the rust can not be washed, which will seriously affect the pickling quality.

The concentration of pickling solution and passivation solution should be checked regularly, and the specified concentration should be kept at all times, otherwise the treatment effect will be affected.In particular, it is forbidden to mix the passivation liquid with nitrogen dioxide, which will affect the health of personnel.

Passivation shall be carried out immediately after pickling, and sealing shall be carried out immediately after passivation, and intermediate manufacturing shall not be stopped to avoid reoxidation in the pipe.Finished product protection.

All pipes and accessories that are qualified for cleaning shall not be damaged. If the pipeline system cannot be reset in time, it must be sealed with plastic cloth.

After reset, when nitrogen sealing is required, nitrogen shall be filled into the system in time, and the air in the system shall be replaced. More than two pressure gauges shall be set for each system to monitor the nitrogen seal pressure. In order to ensure the effect of nitrogen sealing, special personnel should be set up for regular inspection. When the nitrogen pressure in the system is lower than 0.05Mpa, nitrogen should be replenished in time, and the nitrogen pressure should be kept at 0.07-0.1Mpa.

Source: China Industrial Pipelines Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)