Following is a precise chemical cleaning procedure that will ensure the internal and external cleanliness and protection of the pipes before the piping process unit is put into service.
Scope of work covers all the horizontal and vertical pipelines through all the areas in the project and buried underground lines in the infrastructure area as per the specifications and approved shop drawings.
Acid cleaning of pipes to be carried out after completion and approval of fabrication/erection works including pneumatic testing, and the system should be tested and fitted with all the required blinds, vents, and drain points.
Chemical cleaning will be done to remove:
- the storage and assembly rusting,
- protective greases and oils,
- mill scale,
- welding slag,
- dust, dirt, scales
- protective coatings from pipes
- other atmospheric or contact erection residue.
Before delivering the pipes to the construction project site, rusted areas will be cleaned manually by applying emery paper, soft clean cloth & rust removing material such as clear varnish as recommended by the supplier and as applicable for different pipe sizes.
Blasting and painting will be done on the external surface of pipes as per the submitted method statement to remove oil, grease, dirt, rust, scale and any protective coatings from pipes.
Health & Safety Requirements
All the personnel should be knowledgeable of the hazards involved and adequate safety guidelines shall be made available to all parties involved.
The cleaning area should be closed off to unauthorized personnel. Necessary warning signs are posted.
Cleaning agents and chemicals should be transported and stored in closed containers, drum, or tanks etc.
The storage area will be barricaded by bunting, warning signs will be posted. MSDS sheets should be available and displayed at the storage area.
Client safety department/ authority should be informed and consent to be taken for the temporary storage of cleaning and flushing chemicals at site.
Protective clothing such as goggles, gloves, aprons, boots, helmets, etc. should be worn by all the personnel involved in cleaning and flushing works.
During the chemical cleaning operation, clean running water should be available to wash away splashed hazardous chemicals.
All piping and equipment used for chemical cleaning should be pressure tested prior to being used.
As certain that, no damage to personal could occur at chemical cleaning area. Install the safety ropes around the chemical cleaning area and the placard of danger or signboard.
Communication system between conductor and operator should be arranged.
The work area should be well lighted for around- the- clock operation and should be clear of obstruction.
Control valves and all instruments, such as orifice plates, level instruments, gauge glasses, thermo-wells, and any other instruments shall be protected from cleaning solution by any means of substitution by spool assembly, valve closure, blocking off and/or removal from the line as applicable.
Check that all temporary piping and equipment are properly connected to the system.
Prior to commence the cleaning sequence all the permanent and temporary pipe shall be pneumatic tested with a maximum working pressure and two hours holding time.
All vents and overflows should be directed to safe locations, away from possible contact with personnel.
Waste or used chemicals should be safely disposed off in accordance with the environmental protection law and local municipality regulations and directions.
The minimum Personal Protective Equipment (PPE) on site is:
- Hard hat
- Safety boots
- High-visibility vest
- Gloves
- Goggles/Glasses
Necessary Plant and Equipment
The following temporary facilities should be provided for acid cleaning.
- Circulation pumps: The pump should be suitable for chemicals use and its capacity must cover the requirements of system process
- Temporary piping, hoses, and flanges.
- Circulation tanks or mixing tanks
- Chemical analyzer and measuring devices.
- Chemicals
- Waste water tank (capacity 20m3)
- Flushing and water tank
- Electric power (440v/50hz/250kw& 220v/50hz 20kw)
- Potable Water
General Requirements
Physical residues like welding rods, bolts, nuts, brooms, spades, helmets, boots, sleeping fitters etc. have to be removed mechanically before starting the chemical cleaning activity.
The piping system will be identified as per the given pipe schedules, 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 for temporary hoses and mixing tanks etc.
Total volume is the actual value for the pipelines plus the temporary equipment & hoses volume will be the working volume on which calculation will be based (i.e.chemical quantity and timing etc.)
The cleaning is done by circulation of the acid solution.
Cleaning solution will be suitably heated, 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 system wise as appropriate and also depending on erection schedule (completeness of the systems).
Acid Cleaning Procedure
Acid cleaning and flushing for the carbon steel pipelines is done with the following sequence:
- Water flushing
- Acid cleaning
- Water rinsing
- Passivation
- Restoration
- Preservation
For cleaning media is to be used citric acid 3%, ammonia Bifluoride 1%, inhibitor 0.1%, liquid ammonia 0.25%, sodium nitrite 0.3%.
Temporary Provisions
All MOV (motor operated valve) must be ready for operation.
Valves must be clearly identified and labeled after checking the correct valve position according to the valve list.
Vents lines must be connected 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 to be ready for operation.
Sufficient light in all working areas to be arranged.
Water Flushing
Before starting the flushing, regulate all section valves in operating condition.
Fill the circulation water tanks with potable water.
Start the circulation pump and fill the water to the cleaning section.
Check the line during water circulating and if any leakages are found, it should be rectified.
Stop the water flushing, if the water quality is found good, then follow up with next step.
Acid Cleaning
Before starting the acid cleaning, check and line up the system as per the flow diagram.
Add the acid as per the required/calculated quantity ( Citric Acid, Ammonia Bifluoride, Acid Inhibitor etc.)
During the acid cleaning, check the free iron concentration and PH at regular intervals of 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.
After each step is performed, additional water flushing must be performed to meet design, which is agreed with acid cleaning.
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 solution shall be prepared after achieving low concentrations.
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 solution 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.
Visual check is done, there should not be any mild scale or rust, any mild scale or rust shall be considered reason for rejection of whole process.
Please note that any protective coatings cannot be removed by chemical cleaning.
Color of passive layer is shade of grey.
After completion of chemical cleaning and restoration, CS pipes will be preserved / filled with nitrogen gas in order to avoid further rusting.
Chemical of acid phase to be neutralized using caustic soda or lime and then transported to the waste collection pits.
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