This article discusses procedures for chemical cleaning of Carbon Steel and Stainless Steel piping.
Chemical Cleaning of Carbon Steel Piping
- Static cleaning: The static method is useful where only short runs of pipe are concerned and consist merely of filling the pipe, or immersing short sections in a tank, and leaving it to stand for approximately 10 hours.
- Circulation cleaning: The circulation method is employed where long awkward pipe runs are encountered and requires a reservoir, a small circulation pump capable of handling mildly corrosive liquids and some lengths of flexible hosing of suitable diameter. The various piping items are bolted up to form a closed loop and the solution is pumped through the loop and recirculated on the reservoir. This method is much more efficient than the static method due to the constant movement of the solution through the pipes. It also allows several systems, i.e. the lube and seal oil piping from three identical compressors, to be cleaned at the same time.
Chemical Cleaning of Stainless Steel Piping
- Degreasing: If degreasing is thought to be necessary, proceed and followed by rinsing with either demineralized or good quality potable water.
- Cold pickling at ambient temperature: In a 200 liter plastic drum, charge 100 liter of demineralized (or high quality) water.
- Add the following chemicals and mix thoroughly.
- Nitric acid (HNO3), from a solution of 65% wt concentration, 310 g/l.
- Hydroflouric acid (HFL), from a solution of 40% wt concentration, 37 g/l.
- Charge the solution to the piping. For static cleaning leave to stand for three hours, for circulation, cleaning two hours should be adequate.
- Dump the cleaning solution paying careful attention to effluent disposal.
- Carry out a final rinse, again with demineralized water. Thoroughly drain the piping and air blow dry using instrument air.
- Reassemble piping and carry out oil flushing, etc. as required.
Other cleaning methods for Carbon Steel piping:
- Cold degreasing: Cold degreasing is carried out only if pipes have been delivered prefabricated or prefabricated on site and then greased or are thought to contain an oil based preservative. The following degreasing solutions often used (12 hours duration):
- Sodium carbonate Na2 CO3: 3 g/l
- Sodium tripolyphosphate P3 O15 Na5: 2 g/l and trisodium phosphate Na3 PO4: 1 g/l, in either case a suitable proprietary wetting agent may be used
- Rinsing: With clean potable or preferably de-mineralized water (in any case, even if no degreasing has taken place). Water pH minimum 7.0.
- Cold pickling at ambient temperature - Chemical composition of the pickling solution:
- Hydrochloric acid (HC1): 200 g/l (from a solution 33% wt concentration specific gravity = 1.165)
- Ammonium bifluoride NH4 F2: 10 g/l
- Inhibitor Rodine 213 or Armohib 28.2 g/l (1% of HCl content) or equivalent
- Neutralization/Passivation at ambient temperature: This consists of circulating (or charging to static lines) a solution of 1% of citric acid in water, preferably de-mineralized but at least potable, with an inhibitor type Armohib 25, for two hours, followed by:
- Neutralization: add ammonia in order to obtain pH 9 to 9.5
- Passivation: add Na NO2 up to 10 g/l and circulate for three hours (five hours in static piping)
- Effluent disposal: All effluents containing acids, alkalines or proprietary chemical compounds must be disposed of so as not to cause environmental damage. In certain countries local regulations will apply. In any event the effluents should always be neutralized (pH 7.0) before disposal. The disposal of effluents containing wetting agents may influence the operation of oily water separators and such like equipment, and advice should always be sought prior to disposal.
- Final rinsing: Carry out a final rinsing. Thoroughly drain the piping and dry with instrument air. Carry out a visual check of each piping item for cleanliness. Pipewalls should have a matte grey appearance.
- Re-assembly and oil flushing: Reinstall each piping item using new gaskets according to the drawings or specifications. For lube and seal oil systems, check the cleanliness of the oil reservoir(s). If suspect, manual cleaning using a suitable detergent is required. The system may now be filled with the correct grade of lube oil, and commissioned.