Whether onsite or off-site, the methods for cleaning shell and tube heat exchangers can vary. Refining and petrochemical operators will agree that high standards must be employed with each method. Choosing the right method can make the difference between smooth operations and unforeseen equipment shutdowns.
This three-part series explores the various methods, benefits, challenges and solutions to efficiently and effectively cleaning heat exchangers, both onsite and off-site. In Part I, we will discuss the importance and key benefits of optimal cleaning and whether heat exchangers should be cleaned onsite or off-site.
"Many refineries and petrochemical plants plan periodic cleaning of heat exchangers during turnaround operations. Typically, heat exchangers are cleaned every one to three years, depending on the type, size, and service."
Importance of Cleaning
Over the operational lifespan of most heat-exchange equipment, various chemicals or solids (known as foulants) will “fall out” or be extracted from feedstock solutions due to changes in temperature. These solids can adhere to the walls of the equipment’s tubes, pipes, and inside valves and flanges, causing the equipment to foul. Over time these foulants will compound, dramatically changing the flow-rates of fluid through the exchanger, causing a loss of efficiency and functionality, affecting throughput and heat transfer, and increasing the risk of unsafe operations.
To avoid such results, operators must periodically remove or “purge” the buildup of chemicals and residue that coat the tubes before the contaminants reduce or stop heat transfer from coils to water. Most methods are referred to as “descaling” the tubes, because the expelled sediment or contaminants appear as little flakes resembling fish scales.
Many refineries and petrochemical plants plan periodic cleaning of heat exchangers during turnaround operations. Typically, heat exchangers are cleaned every one to three years, depending on the type, size, and service. Keeping heat exchangers free of buildup from sediment, hard water deposits, chemicals, and solids via thorough and proper cleaning improves operational efficiency, and extends the exchanger lifecycle. Conversely, heat exchangers with runtimes stretched beyond suggested cleaning dates can experience reduced effectiveness, decreased lifespan, and in worst-case scenarios, fail during operations.
Effective heat exchanger cleaning is also important in ensuring that the equipment can pass an Eddy Current (EC) inspection test and/or Internal Rotational Inspection Systems (IRIS) test. Essentially, these tests involve probes that are introduced into the tubes to measure wall thickness. If tubes are not effectively cleaned, test results can fluctuate and lead to false readings. Inaccurate test results will also lead to equipment inefficiencies, failures, or equipment runs beyond the lifecycle safety horizon.
In the past, most heat exchangers were cleaned by personnel while on site at the refinery or petrochemical complex. The benefit to onsite cleaning is that during a turnaround, maintenance personnel and inspectors know where each piece of equipment is located in the plant, that it is available for maintenance, and can readily monitor progress and performance.
During onsite cleaning, operators must ensure that trained personnel and equipment are available to disconnect and remove the parts requiring cleaning. For example, in a pull-through type of shell and tube heat exchanger, the bundle is first extracted from the shell. On site at the refinery, this work requires a crane operator and personnel to unbolt various parts of the exchanger, as well as heavy equipment such as bundle extractors. This process can be time-consuming, expensive, and requires knowledgeable technicians to perform the work. After removal, the heat exchanger bundle is transported to the slab area for cleaning. At that point, one or two personnel are required for cleaning, depending upon the exchanger configuration. During this phase, personnel will feed a high-pressure hose, calculated to a specific water pressure, to shoot water through the tubes. After cleaning, the tubes are usually inspected and transported back to the work area and are reinserted into the shells using the bundle extractor. Typically, three to four properly trained technicians are required to re-connect and bolt together the system.
A challenge for onsite cleaning is that heat exchanger cleaning work performed onsite can take longer to perform then sending the exchanger to a third-party cleaning-service provider because the main method of cleaning exchangers on site is with the use of high-pressure water. Therefore, wastewater treatment is required and the contaminated run-off water must be captured and treated.
Many industrial complexes are not well-equipped to safely contain wastewater when the cleaning takes place in crowded and complex processing-equipment areas. On site, the cleaning is usually performed on a concrete slab where the runoff water is not as easily contained. Because there are few, if any, ground facilities in place to capture and treat contaminated water, the water capture, cleanup, and disposal can be time-consuming for facility operators and can pose hazardous waste disposal risks. Onsite cleaning challenges also include increased plant congestion, a higher exposure risk for accidents, injuries and fatigue due to operating hand-held equipment, and the associated aerosol drift. These safety issues will be discussed in further detail in Part III of this series.
Another challenge for operators who schedule onsite heat exchanger cleaning is to schedule and permit inspectors to inspect the exchangers after cleaning. Typically, this process involves obtaining a Transportation Worker Identification Credential (TWIC) card or Safety Council Certification (SCC) document, both of which require extensive time and documentation.
More frequently, petrochemical and refining operators are finding that the benefits of outsourcing heat exchangers to a third-party exchanger-cleaning supplier outweighs the reluctance of transporting equipment, as often times cleaning the exchangers off site can take less time to perform than mitigating complications involved with cleaning the equipment in-house.
When outsourcing the heat exchanger cleaning process third-party service providers typically send a truck to pick up the heat exchanger. The entire heat exchanger is transported to the off-site facility with no disassembly required from the client. At the off-site facility, the heat exchanger is disassembled and cleaned. While several methods of cleaning are available, the most common involves the use of high-pressure water. In contrast to onsite cleaning, personnel perform this type of cleaning by utilizing overhead cranes and running hydraulic equipment remotely, which is safer than physically holding and manipulating a high-pressure hose. After cleaning and inspection, the unit is then reassembled. An adaptation to the off-site cleaning method involves the extraction of the bundle by the refining or petrochemical operators, making it ready for transport to the third-party facility for cleaning.
Off-site cleaning service providers will also schedule and manage the tube inspection services which do not require TWIC card or SCC documents for inspectors to enter and work at the location. After off-site cleaning, the heat exchanger is returned to the refinery or petrochemical plant as a packed unit ready for reinstallation.
Additionally, off-site service providers generally have better equipment than that which is available at the refining or petrochemical complex. For example, water treatment systems are safer, well regulated and less costly at off-site cleaning facilities, which are structurally and procedurally prepared to handle this complicated aspect of the cleaning process.
Overall, third-party off-site heat exchanger facilities built specifically for the purpose of heat exchanger maintenance tend to be better equipped with specialists on hand should any mechanical anomaly be discovered. Additionally, these facilities will include a wider variety of cleaning options and the use of chemicals and high-temperature ovens. These alternative types of cleaning are not typically cost-effective, feasible, or even available at refining and petrochemical facilities. However they are still necessary for the removal of certain foulants.
When choosing an off-site or onsite heat exchanger cleaning company, refinery and petrochemical managers should ensure that the service company employs API-certified inspectors and an experienced, well-trained staff who are ASME-certified and experienced to work on all aspects of the heat exchanger, whether the work involves cleaning, fabricating, welding, repair, bolting, or additional attention. Regardless of the method, it is imperative to maintain exchangers regularly to maximize output and machine lifespan, and for the avoidance of unforeseen operational malfunction.
This article appeared in the January/February 2015 issue of Inspectioneering Journal