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Chemical Clenaning of Process Equipment & Piping

Deposits that causes fouling in equipment and piping need to be removed time to time to maintain operational efficiency…

 Introduction: Typical process equipment and piping get  both organic and inorganic substances deposited on them, coming from the fluid being processed such as water, crude oils, petroleum products, lubricating oils, chemicals etc. etc.  These deposits or sometime called fouling need to be removed time to time to prevent damage and maintain efficiency of operation.  Chemical cleaning is the use of chemicals to dissolve or loosen deposits from process equipment and piping. Chemical cleaning is a process which primarily uses chemical solutions to remove fouling from inside plant and equipment.

The chemical cleaning is performed in many industries in the commissioning of new plant facilities and during operational turnarounds of existing plants. The use and purpose of chemical cleaning varies in commissioning vs. operational scenarios.

Pre-commissioning chemical cleaning: Critical plant and equipment requiring higher levels of cleanliness are often chemically cleaned. Critical equipment is equipment whose operating conditions require higher levels of cleanliness due to operating temperature (steam systems) or systems whose continued integrity requires higher levels or cleanliness (lubrication systems) or systems whose product purity is dependent on a clean system (oxygen, food grade and paint production systems).

The purpose of this cleaning is to remove manufacturing and construction contaminants that would be detrimental to that equipment’s operation. The cleaning may be undertaken for degreasing of preservatives and organics and the removal of mill scale and rust from the systems.

Operational plant chemical cleaning: Operating plant and equipment is chemically cleaned for several reasons:

1. Removing deposits that cause reduced productivity of the equipment.
2. Removing deposits that cause overheating and corrosion of the operating equipment
3. Removing vapors and deposits that cause environmental and safety issues when opening equipment for inspection and maintenance.

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In case, the fouling is allowed to continue, catastrophic failure of plant can occur resulting in complete and extended loss of use. This is the main concern with fouling in steam generator plants such as those in power station boilers. As the magnetite scale grows with service time it increases the thermal resistance of the generator tube to the imposed heat flux. As the magnetite gets thicker that can result in the metal temperature of the generator tube exceeding the yield temperature for the given stress conditions. Tube failure results and hundreds of thousands of dollars per day of power generation capacity is lost in downtime. Increasing fouling levels decrease efficiency by changing process conditions from those designed. Some examples are reductions in heat transfer and general corrosion which result in partial or complete loss of equipment output.

For newly constructed piping; chemical cleaning are employed solely for the removal of dirt, paint, metal chips, filings, flux, slag, weld spatter, scale, rust, or other types of contamination that are easily seen should be visually inspected with the unaided eye.

Fouling of Equipment: Deposits that cause fouling accumulate in equipment and piping and impede heat transfer or fluid flow or cause product contamination. Deposits may be organic, inorganic, or a mixture of the two. Scales are crystalline deposits that precipitate in a system.

Organic deposits:
Hydrocarbons
Polymers
Coke formed in various process streams
 Slime and algae found in cooling water circuits
Animal and fats and oils

Inorganic deposits:
Carbonates
Oxides
Hydroxides of Ca, Mg, Fe, Mn, Cu and Zn
Haematite Fe2O3 and geothite FeOOH
Phosphates of Ca, Mg and Fe, e.g.
Hydroxyapatite (Ca3(PO4)2.Ca(OH)2)
Sulphides of Fe, Cu and Zn
Sulphates of Ca and Mg
Silicates (usually complex)

Sludge and corrosion productsThe fouling can exist in a range of forms from light, soft, easily removable "sludge" types to hard, complex, layered "scale" types such as magnetite (Fe3O4) coatings in boiler tubes.

Temporary rust preventatives and corrosion resistant "Japan Black" pipe fitting coatings (a mixture of carbon black, resins and solvents) become "foulings" in the sense that they must be removed prior to commissioning plant. Often such coatings are designed to be chemically resistant, which poses additional problems.

Principal sources of deposits: There are four principal sources of deposits: water-side, fire-side, process-side, and preoperational.

Water-side deposits: Water-side deposits are of many types. Hardness (calcium and magnesium) based deposits and iron oxide are the most common water-side deposits and often affect boilers and cooling systems. Process and oil leaks can foul boilers and cooling systems. Biofouling, mud, and debris are often found in cooling systems. Treatment chemicals, if not properly controlled, can add to deposits and scales. Silica can form hard, adherent deposits in boilers, steam turbines, and cooling systems.

Fire-side deposits: Fire-side deposits can be extremely corrosive. Slags from burning oil and wastes can corrode boiler equipment if they become moist. Fly ash deposits can accumulate in coal-fired boilers. Gas-fired boilers are generally clean. Some compounds that are burned in incinerators or waste heat boiler scan seriously corrode or erode boiler tubes.

Process-side deposits: There are many types of process-side deposits. Organic residues, tars, and coke are common in the petroleum and petrochemical industries. Iron oxide and sulfides are often present in these organic deposits. Sulfate deposits are common in H2SO4 plants. Iron-copper and nickel containing deposits often occur in HF plants. Organic deposits may develop through the polymerization of leaking gases or from the decomposition of process constituents. In some cases, organics help to bond inorganic deposits, such as iron oxides or sulfides. Some process-side deposits are pyrophoric when exposed to air or oxygen. The most common is iron sulfide, which is likely to be found in natural-gas and petroleum-refining processes or when
aqueous solutions of hydrogen sulfide (H2S) are dried in the absence of air.

heat exchanger fouling

Heat transfer efficiency? Low…

Preoperational deposits: Preoperational deposits are formed during the fabrication and erection of process equipment and piping. In addition to mill scale residues, metal surfaces become coated with dirt, oil, grease, weld spatter, pipe-threading compound, protective shop coatings, and corrosion products. Highly alloyed materials, such as stainless steels, nickel-base alloys, reactive metals, or high temperature alloys, may become contaminated with iron from tooling; zinc, cadium, and aluminum from scaffolding; and zinc, sulfur, and chlorine in certain making materials. These elements can cause corrosion or embrittlement.

 

 

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 Identification of deposits: To select an effective cleaning procedure, the deposit must be characterized, or identified. The sample should represent the deposit in the most critical fouling area. For exchangers and boiler, this is the highest heat transfer section. Expediency should not dictate the location of the sample.

A cleaning procedure should not be based on a sample of loose deposit from a non-critical area, because the sample at this location may not be representative. When removed by scraping, the samples should be as intact as possible. They should be removed to the base metal, taking care not to introduce any metallic chips from the blade or substrate.

Thickness, density, porosity, type (homogeneous or layered), and color should be noted. Many analytical techniques are used to characterized deposit samples. Typical methods include x-ray diffraction, optical emission spectroscopy, and x-ray spectrometry.

 

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Calcium carbonate (Calcite) scale sample collected from heat exchanger tubes

 

       
 
YM..
Yousuf Memon
Plant & Equipment Integrity Professional, has been working for national & international upstream, midstream and downstream for several years.  At present, he is CEO of Pakistan based NDT & Plant inspection company, Reliance Swift Veritas (RSV), Karachi.

 

 

 

 

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