A cool way to heat pipe
December 1, 2009
Energy Steel & Supply Co., Lapeer, Mich., a fabricator that manufactures components for nuclear power plants, used to spend quite a bit of time on pre- and postweld heat treatment. After seeing a demonstration of an induction heating system, the company made the switch and enjoys the benefits of a faster, safer way to deliver heat to the components it manufactures.
When it comes to pre- and postweld heat treatment, many fabricators see it as a choice between resistance heating with ceramic heating elements or flame heating with weed burners. However, they have another option: noncontact heating with an induction heating system.
One such fabricator is Energy Steel & Supply Co., Lapeer, Mich.
Michael Mitchell founded Energy Steel in 1982 as a broker of raw steel. In 1996, with its increased knowledge of ASME codes and in response to an increased need for nuclear-qualified suppliers, the company decided to focus its efforts in this area. By 1998 the company had expanded its capabilities and evolved into a value-added supplier, having the capability to weld to nuclear code and supply machined products.
Under the ownership and leadership of Lisa Rice since 2003, the company has continued to grow and expand its capabilities. The company is ASME-accredited, and it holds the N, NPT, NS, and U Code Symbol Stamps and Certificates of Authorization. It is also accredited by the National Board of Boilers and Pressure Vessel Inspectors and has been awarded the R Code Symbol Stamp and Certificate of Authorization.
Many nuclear power plants have been in service for decades—of the 104 nuclear reactors in operation in the U.S., more than half are more than 30 years old—and the components needed to repair them are no longer available from the original suppliers. This puts fabricators such as Energy Steel & Supply Co. in a good position.
"The nuclear power plants that were built 30 years ago don't have spare parts lying on the shelf," said Patrick Siwa, fabrication department foreman, Energy Steel & Supply Co. "When they need a replacement part, it needs to be fabricated from scratch," he said.
Expedited work is the key. In the nuclear power industry, most maintenance is performed when a reactor is offline and the power utility must purchase electricity from another supplier. In other words, every minute of a scheduled outage is costly because the utility is purchasing power rather than generating it. Still, despite the pressure to complete the work as quickly as possible, the nature of this industry leaves no room for error, no sacrificing quality for speed, and no cutting corners. Energy Steel has to supply good parts quickly, and induction heating helps it do this.
An induction heating system induces heat electromagnetically in a metal part rather than generating heat and transferring it through conduction. Induction coils create a high-frequency magnetic field; the magnetic field creates eddy currents inside the part, exciting the part's molecules and generating heat. The coils do not need direct contact with the part. In fact, they can be up to 2 inches away from the material and still induce heat effectively. And, because heating occurs slightly below the metal surface, little heat is wasted.
Because induction heating uses electromagnetism to induce an electrical current in the part, it eliminates the two chief hazards associated with resistance and flame heating: hot components and open flame (see Figure 1). It provides heat consistently and generates data for quality assurance. Finally, one of its biggest advantages is speed: It brings the part to temperature up to four times quicker than other methods.
John Kelly (left) and Dale Nurmi show the plywood form they used to hold the induction coils for a recent piping project. The pipe rotated inside the form while it was being clad internally by a submerged arc welding machine. Because the coils never got hot, the plywood did not present a safety hazard.
A demonstration of Miller Electric's ProHeat™ prompted Siwa to take a close look at this option.
"The person who came to demonstrate it wrapped the blanket around the pipe and brought it up to temperature in 15 minutes, something that would have taken flame torches an hour to do," he said. "Once the unit was switched off, the coils were cool enough to be removed by hand."
Heating Before and After Welding. The induction system is useful for heating a part before, during, and after the welding process. Because it has built-in thermocouples for temperature control and monitoring, it applies heat consistently and provides a data record of the part's temperature down to the minute.
Before switching to induction heating, Energy Steel used weed burners, wands with open flames fueled by propylene. A typical part required three workers.
"Two people with flame torches would hold the preheat temperature while the other guy welds," Siwa said. "Then the welder would back away and the other guys would keep it warm. Depending on the workpiece's size and thickness, preheating could take two hours," he said.
When the welding was finished, Energy Steel used to send the components—and a QA technician—to a heat-treating facility. The QA person would connect a data recorder before the heat-treating process would begin.
"The biggest problem you have with these ovens is a thermocouple could break as it rolls in and you don't know it until it's too late," said John "Jay" Kelly, lead welder. "We don't have that problem [now]. It's all done here out in the open."
Still, proper and complete documentation is critical.
"We have to supply charts to prove we got a part up to the temperature specified in the procedure and that we held it for so many hours," said Siwa. The induction system does this automatically. A digital recorder records time versus temperature from the thermocouples. Data is encrypted and stored on USB flash drives for transfer to a computer for further analysis, storage, printing, or sending by e-mail.
"It collects data on a memory card, which I can then download to my computer and share with others," Siwa said. "I can show minute-by-minute temperatures; I can do it every five minutes, every 10 minutes, every hour, depending on what is required, and then e-mail it to our customers."
To ensure this accuracy, the unit uses a patent-pending input that uses six thermocouples, two for monitoring and four for control. The controller reads the control thermocouples and bases the heating on the hottest thermocouple and the cooling on the coolest thermocouple. This ensures that heating and cooling rates are not violated during the procedure. Separate displays indicate the temperature of each thermocouple (see Figure 2).
Holding Steady. Because induction heating relies on coil wrapped around the workpiece, it tends to heat the part evenly. Also, it simplifies maintaining the temperature after it is reached.
"That's important because we have to hold a certain temperature for a certain amount of time," said Kelly. "If we miss it by a few minutes, we have to do the heating all over again. We would have had to notify the customer that they were going to lose five hours on their part or whatever. So, by getting it right the first time, it saves life on the part, and it saves us 12 to 24 hours depending on how long the hold time is. Plus, our parts are built with a specific amount of time for postweld heat treating. If we exceed that time, we have to start over with a new part."
Maintaining the temperature is beneficial to the part's quality.
"We don't want the parts to repeatedly cool and heat because there's a chance of stress cracking," Siwa said.
"Many of the nuclear suppliers entered the industry at the same time," said Waylon Waters, the company's sales manager. "We've added capabilities and have expanded our business in ways that others have not. The added value of fabricating an entire component through material supply, machining, and ASME Code Welding is very special in the nuclear power industry. In addition to that, we have always been able to accommodate expedited work," he said.
"We get accuracy with this product, not only from a quality assurance perspective, but it allows us to be more accurate with our bids and timing for estimates," Waters said. "It helps in every aspect of our scheduling."