New construction facilities often undergo a cleaning process prior to start-up. Piping and vessels may contain a variety of debris, scale, oxides, and oil that must be removed if the plant is to run effectively.
To continue to operate at peak efficiency, process systems, vessels, and pipelines must be kept as clean as possible. Some of the normal by-products of operating processes create deposits ranging from scales to sludge, precipitates, and even metal deposits. These types of build-up foul the vessels and piping and can significantly reduce system efficiency, eventually causing plugging and failure of the unit.
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.
Pre-operational chemical cleaning:
Performed to remove any foreign material remaining from the construction activities either on the pipe or system fabrication. Major considerations in the pre-operational phase include mill scale, corrosion products, weld scale, oil, grease, sand, dirt, temporary protective coatings, and other construction debris.
Post-operational cleaning:
Performed for a number of reasons, including reduced heat transfer, reduced flow, safety (e.g., H2S, pyrophoric iron, LELs, ammonia, etc.), reduced surface area (e.g., catalyst), access to full inspection, and more. The type and frequency of post-operational cleaning varies with system design, operating requirements, and history of operation and fluid/water treatment.
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.). The system is filled with a chemical cleaning solution and drained after a period of time. If necessary, this can be repeated several times 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 the removal of metallic scale and corrosion products. It is usually applied in a three-stage operation: degreasing, metal collection, and passivation. This method has become a last resort option due to a variety of reasons, including environmental, safety, and disposal issues.
Alkaline Scale Removal
Alkaline scale removal is used for the removal of organic deposit only. Like the fill and soak method, it is also used if circulation is not feasible. However, increased agitation can improve the results. Steam sparging is a common form of agitation in this method.
This method involves filling a system with a chemical cleaning solution and circulating it with a pump. This is the most common method used for chemical cleaning in the industry. It is important to keep the fluid velocity in a certain range to avoid corrosion. Furthermore, the concentration and temperature of the cleaning solution have to be monitored during the entire operation. As with 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 a three-stage operation—degreasing, metal collection, and passivation. Despite the separate cleaning stages, citric acid and EDTA are used in a single batch solution, significantly reducing the volume of waste generated during the process.
Mineral Acid Cleaning – Pickling
Mineral acid cleaning is very similar to chelating agent pickling. It is still used often due to its low cost. It is more commonly used on piping systems than on expensive system vessels. In addition to being economical, the mineral acids are able 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, above all else, organic matter. If incorporated with filtration, it can be used for debris removal from a system during the pre-operational cleaning. Some of the most 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 type of solvent used for solvent cleaning 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 is achieved at minimum expense and risk to the system. If the system volume is 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 cleaning 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. A hot mixture of chemicals is added near the top of the vessel and cascades down through the trays, dissolving 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 patterns can be used in two-phase flow, three of the most common being bubbly, slug, and annular flow. Each pattern requires special engineering design to reduce the risk.
Small diameter/volume piping
Small diameter piping can be effectively cleaned by applying a slug flow pattern. Typically, water and air are used in specific amounts 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, especially if the piping was not designed for any “conventional” cleaning method.
The slug flow method is specially designed for pipeline cleaning. It applies a slug of liquid chemicals sent 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 has to be paid to the velocity and volume calculations during the pre-engineering phase.
This method is designed for fast and efficient—and therefore economical—process plant cleaning in a single step. The term “process plant” refers to large process vessels, reactors, exchangers, and interconnecting piping. This is an upgraded steaming process where chemicals are injected into the steam stream. Although diverse, this method is used more often for post-operational cleaning to remove H2S, benzene, LELs, pyrophoric iron, mercaptans, and ammonia.
Degassing and Decontamination Cleaning
Degassing is a chemical cleaning process that eliminates dangerous gaseous elements inside petro-chemical processing equipment. To improve the degassing process, it is recommended that decontamination (solvent circulation) precede degassing to reduce the source of contamination (e.g., sludge, heavy deposits, etc.). Using a specially designed series of chemicals reduces and/or eliminates any risk for the maintenance shut down during hot work. This method reduces both the amount of waste and human exposure to dangerous substances during cleaning and maintenance work.
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 can cause 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. It is appropriate for systems characterized by a large ratio of volume to surface area. This would offer the same or very similar results with a 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 the volume to surface area ratio is high. Instead of filling the entire system with costly chemicals, 360° rotating head nozzle(s) are 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 nozzle cleaning method are:
Large volume vessel cleaning
Tank chemical cleaning is a specific process that has to be designed on a case-to-case basis. Based on the nature of the deposit, a proper solvent has to be chosen, which can minimize manpower and waste material. The chosen solvent is then circulated with an external pump, establishing several circulation loops. This method has several advantages to the conventional tank cleaning practice: it is a cost effective method, reduces cleaning time, generates minimal to no waste (over 95% recovery of hydrocarbons), requires no crew entry, and presents minimized environmental risk.
