Plastics processing industry professionals know all too well that damaged or failed heat exchangers can lead to unexpected downtime and the consequences interrupted production can bring. Heat exchangers are used in process lines and power systems for the plastics, for temperature control of the melt. 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/CEO of American Power Services (APS), a nationwide provider of heat transfer equipment services that include troubleshooting, repairing, and complete rebuilding projects. "Essentially, 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's usually 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, particularly for the smaller heaters found throughout the plastic processing industry. According to Bina, there are several criteria that plant managers and engineers should consider when determining the best course of action:
1. Performance analysis: The immediate question to ask is: If your problem heat exchanger is less than 30 years old, then the problems could be due to such factors as water chemistry, cycling, operating controls, and a variety of factors that could affect dynamics such as velocity and vibration, all of which 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. "You can verify the flow through the heat exchanger, and also the temperatures going into and out of the unit, as well as the pressure drop across the heat exchanger's tubs," Bina explains. "Those tests will give you an indication of what is outside the performance parameters. 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?"
2. 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. "For instance, our organization has half a dozen people always available to provide that type of service, which can save users considerable time and costs."
3. Professional testing methods: If the problem is difficult to diagnose, or the severity requires comprehensive analysis, the user will normally call in the service provider to perform sophisticated testing so that appropriate repair recommendations can be provided. "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 and measures the severity of the defect and its probable cause.
4. Pulling a tube sample: In many instances it is necessary to cut and pull a tube sample out and diagnose the exact failure area to determine the root cause of failure, whether it's chemistry, a failed weld, pitting or stress cracking of tubes, or the percentage of tube wall loss. Thorough analysis will also indicate whether impending tube failures are imminent, and over what period of time that will likely happen.
In high-pressure heat exchanger applications, pulling a tube sample by cutting inside the tube can be very difficult. For that reason, APS developed its own tool, a unique plasma ID tube cutter that can penetrate heavy-duty materials such as 0.120-wall stainless steel, making it possible to cut anywhere along the length of the tube.
"Extracting a tube sample can go a long way toward indicating whether a heat exchanger can be repaired rather than replaced, and what repair procedures are required as well as the time that is likely to be involved," Bina says.
5. Simultaneous test and repair: The criteria for choosing heat exchanger repair vs. replacement varies from heat exchanger to heat exchanger based on a multitude of criteria. Downtime can certainly be a deciding factor. However, in many instances, downtime can be virtually eliminated through the service provider's abilities to test and repair simultaneously.
"In many instances, the same people doing the testing can do the repair work," says Bina. "By doing both at the same time, we often limit downtime to just a few hours. Of course, the user could call a testing laboratory to do the testing and provide you with 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. So, you've kept your downtime to an absolute minimum by using a service that has the capability of testing and repairing the problems right away."
Bina points out that some repairs require slightly more time. For example, if testing indicates stress cracking in heat exchanger tubing, the quickest solutions-or at least a temporary one-may be to sleeve the ID tube in order to eliminate the leakage as well as containing the cracking problem. Sleeving can typically be done within a day or two, thereby diminishing downtime and consequential problems.
6. Rebuild vs. replace: In cases of more extensive damage to heat exchangers or other tubular heat transfer equipment, the service provider's recommendation to rebuild or replace the unit may hinge on their capabilities or inclination to sell new equipment. Yet, the choice to replace can have a huge impact on both replacement and downtime costs. In some cases replacement is unavoidable, yet the months of waiting for a new unit can be tremendously expensive, particularly in process operations. In one such instance, a process plant lost both redundant heat exchangers.
"The whole process was shut down," says Bina. "The plant couldn't get a replacement heat exchanger quickly, given the number of components that had to be fabricated and assembled. "Fortunately, by using materials that were already in stock, we were able to design a heat exchanger that was delivered within six weeks of receiving the purchase order. The customer saved a significant amount of money by getting their processing line back online months ahead of the time that would have been otherwise required."—Edited by Clare Goldsberry