Chemical Cleaning will be done to remove the storage and assembly rusting, Protective greases and oils, Mill scale, Welding slag, Dust, Dirt, Scale and most protective coatings from pipes and other atmospheric or contact erection residue. After completion of chemical cleaning and restoration, CS pipes will be preserved / filled with nitrogen gas in order to avoid further rusting.
Physical residues like welding rods, bolts, nuts, brooms, spades, helmets, boots, sleeping fitters, and etc. have to be removed mechanically prior chemical cleaning.
The items will be identified as per the given pipes, and compared to Piping & Instrumentation Diagram (P&ID). Volume will be calculated as per actual dimensions and the given Isometric drawings. Extra volume will be added since we have to add the volume for our temporary hoses and mixing tanks. Total volume is the actual value for the pipelines plus the temporary equipment & hoses volume will be the working volume which our calculation will be based ( Chemical amounts, Timing …..etc)
The cleaning is done by circulation of the acid solution. The solution will be suitably heated and diluted chemical water is circulated through concerned part of piping system for proper mixing. Heat source is suitably selected according to site availability. The cleaning will generally be carried out for system-wise as appropriate and also depend on erection schedule (completeness of the systems).
Acid Cleaning Procedure
Acid cleaning for the carbon steel pipelines is done with the following sequence:
- Water flushing
- Acid cleaning
- Water rinsing
For cleaning media is to be used citric acid 3%, ammonia Bifluoride 1%, inhibitor 0.1%, liquid ammonia 0.25%, sodium nitrite 0.3%.
All MOV (motor operated valve) must be ready for operation. All valves must be clearly identified and labeled after checking the correct valve position according to the valve list.
Vents lines must be conducted to the ground, if not definitely done so then at by hoses to avoid damage to the insulation and enable proper venting.
All temporary valves must be installed near the connecting point of the line to avoid dead ends.
After completion of the temporary piping, whole system has to be filled up and a tightness test has to be executed.
The internal parts of all control valves have to be removed and replaced by spool pieces. Wastewater collecting tank must be empty, discharge pumps ready for operation. Sufficient light in all working areas to be arranged.
After each step is performed, additional water flushing must be performed to meet design, which is agreed with acid cleaning.
During the acid cleaning, check the free iron concentration and PH at regular intervals at one hour sampling. If the Fe ion concentration and the PH level are maintained constantly over, three times the acid cleaning works will be finished.
Drain Out of Acid Cleaned Solution: After completion of the acid circulation, the acid cleaning solution will be drained out in the temporary waste tanks.
Water Rinsing: After finishing the drain of each section completely the water rinsing with 0.1 Citric acid works to be done same as water flushing, circulate the rinsed water for at least two hours.
Neutralizing and Passivation: Neutralization and Passivation shall be prepared after low concentration. For neutralization , the recommended density of liquid ammonia is monitored to reach the PH value to 8.2 to 9.5. When the PH value reaches 8.2, 0.3% of sodium nitrite, which is passive chemical, is being put gradually and continues to circulate. It is necessary to circulate for 2 or 3 hours to maintain the nitrite to constant, and to coat the film of passive on the surface of the system inside.
After Passivation, all passive chemicals remaining in the system should be drained to temporary wastewater tanks through the temporary drain line. All drain valves should fully open during draining in order for the chemicals not to remain inside the system. After all the process of chemical cleaning, it is recommended to inspect the surface of the pipes.