The two major categories of chemical cleaning are pre operational chemical cleaning, part of pre-commissioning or commissioning activities, and post operational or maintenance chemical cleaning, part of regular shut down work.
To chose a proper chemical cleaning method several following factors should be considered:The Fill and Soak Method is often used for internal surface cleaning of large volume vessels and pipes where a proper circulation is not feasible, e.g., heat exchangers, vessels, boilers, etc. In this method a system is filled with a chemical cleaning solution and drained after a period of time. If necessary this can be repeated several times or until the equipment is clean. Mineral acids have the capability to react with metal deposits with little or no agitation. This application can be used for pre-operational and/or post-operational (maintenance) cleaning.
Mineral Acid Cleaning – Pickling
Mineral Acid Cleaning is used for metallic scale and corrosion products removal. Usually applied in a three stage operation; degreasing, metal collection, and passivation. Recently this method has been used as a last option due to a variety of reasons, e.g., environmental, safety, disposal issues, etc
Alkaline scale removal
Alkaline scale removal is used for organic deposit removal only. Fill and soak again is used if circulation is not feasible. However, increased agitation can improve the results; therefore, steam sparging is a common way of agitation in this method.
This method involves filling a system with a chemical cleaning solution and circulates it by a pump. This is the most common method used for various chemical cleanings in the industry. It is important to keep the fluid velocity in a certain range to avoid corrosion of the system being cleaned. Furthermore, concentration and temperature of the cleaning solution has to be monitored during the entire operation. The same as any pickling method it is recommended to apply this method in three different stages (degreasing, metal collection, and passivation).
Chelating Agent Cleaning – Pickling
The most common chelating agents used in pickling are citric acid and EDTA. These agents are recommended for both pre operational and post operational cleaning of steam generating systems. Similar to mineral acids but much safer to use, these agents are also applied in three stage operation (degreasing, metal collection, and passivation). Despite the three separate cleaning stages, citric acid and EDTA are used in a single batch solution significantly reducing the volume of waste generated during the cleaning process.
Mineral Acid Cleaning – Pickling
Mineral acid pickling is very similar to chelating agent pickling. It is still commonly used due to its low cost. It is more often used on piping systems rather than on expensive system vessels. Another advantage of this method is the ability of mineral acids to perform cleaning at ambient temperatures when used in higher concentrations.
Degreasing
Degreasing usually refers to an alkaline wash of internal surfaces on the process equipment to remove, first of all, organic matter. If incorporated with filtration it can be used for debris removal from a system during the pre operational cleaning. Commonly used chemicals for degreasing purposes to prevent foaming and/or to improve heat transfer on process equipment include: sodium hydroxide, tri-sodium phosphate, sodium meta-silicate, sodium carbonate and non-ionic surfactants.
Solvent Cleaning
The choice of solvent should be based on laboratory studies of the deposit sample found inside the system. This will help ensure that the expected result of the chemical cleaning are achieved at minimum expense and risk to the system. If the system volume is significantly large a cutter fluid can be used to reduce the cost of a potentially expensive solvent.
When large volume vessels are not designed to sustain full liquid levels the cascading method is the best option. Adding chemicals at the top of the vessel and maintaining the level at the bottom is a common method in tower cleaning.
Tower Cleaning
The cascading method is commonly used for towers with a large number of trays. The hot mixture of chemicals added near the top of the vessel cascades down through the trays and dissolves any deposit on the trays. To improve the cleaning process it is a good practice to agitate the fluid with a gas injected at the bottom of the tower.
Two phase flow cleaning can be applied to reduce the cost and amount of waste generated. Several different patterns can be used in two phase flow. Three most common are bubbly, slug, and annular flow. Each one requires special engineering design to reduce the risk.
Small diameter/volume piping
Small diameter piping can be very effectively cleaned by applying a slug flow pattern. Usually water and air are used in a specific ratio in order to produce a high cleaning force ratio.
Large diameter/volume piping
With proper design, annular flow can be used to clean large diameter piping. This method is cost effective for large diameter piping especially if the piping was not designed for any “conventional” cleaning method.
This method is specially designed for pipeline cleaning. The method applies a slug of liquid chemicals send between two separator pigs. The driving force to move “the slug” through the pipeline can be either a liquid or a gas.
Pipeline Cleaning
Any pipeline regardless of diameter requires a large volume of chemicals to be filled and circulated with. To reduce amount of chemicals and waste a slug flow method can be used to achieve very similar results. Special attention during the pre-engineering phase has to be paid to the velocity and volume calculations.
This method is designed for fast and efficient process plant cleaning in one step. Process plant means large process vessels, reactors, exchangers, and interconnecting piping. This is an upgraded steaming process where chemicals are injected into the steam stream to significantly reduce time and money. Although diverse, the method is much more used for post operational cleaning to remove H2S, benzene, LELs, pyrophoric iron, mercaptans, and ammonia.
Degassing and Decontamination Cleaning
For a safe and efficient start up of steam generating equipment it is recommended to remove any organic matter from the internal surfaces. Construction oil and grease compounds may cause foaming and reduce heat transfer on the tubes which consequently can cause a tube failure. As a prevention measure boiler boil outs are recommended prior to start up activities. This is another alkaline way of cleaning steam generating systems.
Commissioning Boiler/Condenser Cleaning
Commissioning Boiler Cleaning is highly recommended for any steam generating systems. The cleaning is performed when a boiler is filled with a water based chemical solution. The solution is heated either by starting the boiler itself or by using steam from an external boiler. Maintaining the conditions above the boiling point, chemical concentration and oil content are monitored for a certain period of time or until the oil content drops to a predetermined level.
Instead of using a large volume of concentrated chemical solution this method uses a gas mixed with the same concentration of chemical solution. The method is applicable for the systems that are characterized by a large ratio of volume to surface area. This would offer the same or very similar results with significantly reduced amount of chemicals used and waste generated. Based on a particular application the nature of gas can be determined; however, an inert gas is highly recommended for this application. The method can be used for the following applications:
Large volume vessel/pipe cleaning
Equipment/Piping - not designed for liquid weights
Nozzle cleaning is another method used when volume vs. surface area is significantly high. Instead of filling the entire system with costly chemicals, 360° rotating head nozzle(s) may be used to spray the walls of the vessel and keep the volume of chemicals relatively low. This allows flow and temperature of the chemical solution to be kept in the recommended range. The most common applications of the method are:
Large volume vessel cleaning
Storage tank cleaning
Tank chemical cleaning is a specific process that has to be designed for each particular case. Based on the nature or the deposit a proper solvent has to be chosen, which can minimize manpower and waste material. The chosen solvent is circulated with an external pump establishing several circulation loops. This method has several advantages to the conventional tank cleaning practice. This is a cost effective method, reduces cleaning time, generates minimum or no waste (over 95 % recovery of hydrocarbons), no crew entry and minimized environmental risk.
