When a heat exchanger begins to fail or becomes problematic, it may indicate problems ranging from inappropriate mechanical design to unforeseen chemical reactions. Process industry professionals know all to well that damaged or failed heat exchangers can lead to unexpected downtime and corresponding consequences. While a replacement heat exchanger may seem like the only choice, properly diagnosing and evaluating the root problem should be the first course of action.
“There are multiple factors that you need to measure and evaluate,” says Daniel Bina, president and CEO of American Power Services (APS), Erlanger, Ky., a nationwide provider of heat transfer equipment services such as troubleshooting, repair and complete rebuilds. “It is usually best to keep an open mind about whether you can restore the efficiency of heat transfer equipment and thereby extend its life -- or whether you must spend the capital and downtime to replace an existing unit with a new one.”
Bina says that from his experience, it is often more cost-effective and process efficient to repair or upgrade heat exchangers -- thus eliminating the root cause of failure -- rather than to replace a unit. This is particularly true for the smaller heaters found throughout the process industries. According to Bina, plant managers and engineers should consider several factors to determine the best course of action.
Performance Analysis. The immediate question to ask is, “How old is the equipment?” If your problem heat exchanger is less than 30 years old, then the problem could be due to such factors as water chemistry, cycling, operating controls or conditions that affect dynamics like velocity and vibration. All of these can be diagnosed.
“If performance appears to be off, or if you have a tube failure, a performance analysis can tell you a lot,” Bina says. An analysis can verify the flow through the heat exchanger, the temperatures in and out, and the pressure drop across the heat exchanger’s tubes.
“All of those tests will give you an indication of what is off of normal performance parameters,” Bina says. “If it is a tube failure, which does occur, where is the tube failure located? Is it near an inlet, or near an outlet or a baffle? Where is it in relation to other parts of the heat exchanger?”
Troubleshooting Assistance. If a performance problem is evident, users may be able to solve the problem themselves. Bina suggests that users first phone their service professionals and describe the situation.
“Their service provider should be willing to advise the user about troubleshooting the problem: what specific things they can do to diagnose and pinpoint the damage, and possibly how to repair it themselves,” he says.
“There are several types of testing methods available to pinpoint the problem,” Bina adds, “such as eddy current, ultrasonic, hydrostatic and magnetic particle testing, depending on what the situation requires.”
Among the many types of testing, eddy current testing can tell you the most regarding the overall condition of a heat exchanger. This nondestructive test involves sending a test probe down a heat exchanger tube. The probe determines where the defect is located along the length of the tube. It also determines the type of defect, measures the severity of the defect, and its probable cause.
Pulling a Tube Sample. In many instances, it is necessary to cut and pull a tube sample. This is used to diagnose the exact failure area to determine the root cause of failure such as chemistry, a failed weld, pitting or stress cracking of tubes. Pulling a tube sample also allows the technician to determine the percentage of tube wall loss. Thorough analysis will indicate whether impending tube failures are imminent and, if they are, over what period of time they will likely happen.
“Extracting a tube sample can go a long way toward indicating whether a heat exchanger can be repaired -- rather than replaced,” Bina says. “The analysis allows the service provider to determine what repair procedures are required as well as the time that is likely to be involved.”
Simultaneous Test and Repair. The criteria for choosing heat exchanger repair vs. replacement varies from heat exchanger to heat exchanger based specific application conditions. Certainly, downtime can be a deciding factor. In many instances, however, downtime can be nearly eliminated through the service provider’s ability to test and repair simultaneously.
“By doing both at the same time, we often limit downtime to just a few hours,” says Bina. “Of course, the user could call a testing laboratory that will do the testing and give a report with a recommendation as to what repairs might be required. But then the user will have to call in someone else to service or replace the damaged or failed heat exchangers.”
A service provider whose technicians can test and repair the heat exchanger in the same visit can keep downtime to an absolute minimum, Bina says. Of course, some repairs require more time. For example, if testing indicates stress cracking in some heat exchanger tubing, the quickest solution -- or at least a temporary one -- may be to sleeve the inner diameter of the tube to eliminate the leakage as well as containing the cracking problem. Typically, sleeving can be completed within a day or two.
For more information from American Power Services, Erlanger, Ky., call (859) 283-1230 or visit www.1aps.com.