Heating P91 boiler pipe
Induction heating gives contractor faster, more consistent results
In the power piping industry, turnaround time on a boiler pipe project typically is from 20 to 36 weeks. But J.F. Ahern Co. (JFA), Fond du Lac, Wis., a company ranked as one of the Midwest's top 10 mechanical contractors according to the May 2002 Contractor magazine, isn't typical. Neither were the results JFA achieved when it switched to induction technology for pipe preheating.
|Insulating blankets are an integral part of induction heating equipment. After the pipe is heated to the proper temperature, the induction coil is removed and the pipe is welded. <|
"Our turnaround time averages eight to 12 weeks, and that includes pipe made from P91 chrome-moly steel," said Tom Krenn, plant superintendent, Pipe Fabrication Division. "Using induction heating on P91 [has] reduced our preheating time from two hours or more to 30 to 60 minutes. It reduces hydrogen bake-out time too. That's money to the bottom line."
By the end of this decade, 300 to 400 new power plants will be built. Much of the pipe that carries high-pressure steam to the turbines now is made from a newer-grade chromium-molybdenum alloy, 9Cr-1Mo-V (P91) steel. The new grade offers a much better strength-to-weight ratio than the old standby, 21¼ Cr-1Mo (P22) steel.
"A 26-inch-diameter main steam line made from P91 can have a wall thickness of 21¼ in. With P22, you'd be looking at a product almost 5 in. thick," noted Krenn. "It [P91] holds up better too. These parts are wear items. Would you want to replace something every five years or spend the money up-front and have it last 15 years? As a result, engineers are specifying P91 to get longer life out of power houses."
JFA recently switched from direct heating methods-propane flame or resistance heating with ceramic pads-to induction heating, an indirect method that heats pipe from within. Krenn evaluated all three heating methods before selecting one for use on P91, a particularly difficult metal to fabricate.
Krenn's heating selection criteria included quality, documentation, productivity, safety, and consumables cost. Quality relates to the heating method's ability to heat the pipe consistently and uniformly throughout its wall thickness, so that the ID and OD temperatures both are within specifications. In addition, he needed a system that would heat the pipe to ±25 degrees of the customer's specification.
Associated with quality is documentation, which many customers demand, that the temperature specifications were met. Productivity is based on reducing the heating cycle time, which in turn reduces overall welding time. Reducing welding time is a key component of increasing productivity, considering that labor accounts for approximately 85 percent of a weldment's cost.
Safety concerns include minimizing potential hazards caused by heating elements and hot pipe and eliminating airborne insulation particulates.
Consumables cost includes the costs for insulation, gas, and electricity.
|After wrapping induction coils around a P91 pipe, pipefitter Charlie Yadon connects a thermocouple to the induction heating power system.|
Controlling Weld Parameters
"A weld is only as good as your control. If you don't weld in a controlled environment, you're going to have problems," Krenn said. "With P91, one key is to maintain the proper preheat temperature, interpass temperature, and hydrogen bake-out temperature."
Preheating, typically to 400 to 550 degrees F, drives off moisture, thereby reducing the hydrogen content of the metal. Hydrogen embrittlement can lead to cold cracking of the finished weld. For this reason, maintaining interpass temperatures (the temperature between welding passes) and a hydrogen bake-out of the finished weld are also required by various codes.
"If you don't weld in a controlled environment, you're going to have problems. With P91, one key is to maintain the proper preheat temperature, interpass temperature, and hydrogen bake-out temperature."
Depending on the application, codes that apply include ASME B31.1 Power Piping, ASME B31.3 Chemical Plant and Petroleum Refinery Piping, and ASME Section I Power Boiler Parts. These codes also require postweld heat treatment, but JFA subcontracts this activity to an outside firm with an oven that can heat three or four 14,000-pound pipes at once.
Reducing Time to Temperature
With more of his work involving P91, and the work demanding strict quality control, Krenn sought to improve his heating methods. Like most fabricators, JFA previously used propane flame heat. However, Krenn said, "Most of our P91 customers do not want it. They do not trust flame heat to heat the pipe uniformly throughout its thickness, maintain interpass temperatures, or minimize the heat-affected zone. With flame, you're always taking a temperature stick—a crayonlike stick that melts when it reaches a specified temperature—and checking the temperature, adding more heat, watching for the heat to drop, adding more heat, and so forth. Flame heat can be too inconsistent to ensure proper quality with P91."
Seeing Krenn's concern with quality, Peter Flood, manager of welding products distributor Airtec Inc. of Fond du Lac, arranged a demonstration and trial test of Miller Electric's inverter-based induction heating power system (IHPS). This system uses a rapidly alternating (10- to 20-kilohertz) electromagnetic field in a flexible coil. The magnetic field crosses the part and induces eddy currents within the part, creating heat.
The induction process does not rely on conduction to heat the part, so the coils do not heat up. The source of heat is just under the surface, where 87 percent of the heat is created by eddy currents. This makes the process efficient in its use of energy, which is why it brings parts to the proper temperature quickly and why it requires only a 45-amp, 460-volt circuit.
|Induction heating poses fewer safety risks than other heating methods. The insulating blankets can help protect the welder from contacting hot pipe. Here pipefitter Charlie Yadon leans on the insulating blanket to steady his hands while using gas tungsten arc welding (GTAW) for a root pass.|
Induction heating also provides more uniform heating of thick sections from the inside to outside diameter and at the top, sides, and bottom of the pipe.
"The benefits of induction heating technology quickly attracted Krenn," Flood said. "It's much faster, cleaner, and safer than some other methods. Going from two to four hours to raise a part to temperature to just minutes to temperature requires a shift in mindset."Also, its one-man portable and has a low consumables cost," Flood added.
Typical applications include pipe butt joints; pipe weldolets; pipe-to-valve connections; pipe-to-elbow connections; pipe-to-T connections; and nozzle-to-pipe, nozzle-to-vessel, and nozzle-to-reducer connections. Induction heating is suitable for preheating most pipe configurations from 2.5- to 24-in.-dia. Schedule 40 to 120.
In addition to P91, JFA uses induction heating for P11 and P22 chrome-moly steels.
Induction Heating Setup and Materials
"With induction heating, we're set up in 10 to 15 minutes," Krenn said. "For setup, we weld a thermocouple onto the pipe, wrap a simple blanket around the pipe, and wrap the pipe with the induction coil."
The silica needle mats used with induction heating typically last for 50 operations. "Induction mats don't break down, so you can reuse them. This considerably lowers our monthly consumables cost," Krenn said.
The induction blankets serve two purposes: First, they prevent the joint from losing heat to the atmosphere. Second, they protect the induction coil and the equipment operator from direct contact with the hot pipe.
"Our operators won't burn themselves handling the induction coil or mats," Krenn said. The silica needle mats remain cool enough that operators can rest their arms on them while welding. Airtec offers mats for standard-size joints and can create custom sizes and shapes, such as for a weldolet.
Proof Is in the Hard Copy
Induction heating technology allows a contractor to guarantee a specified temperature and temperature consistency, usually within 5 to 10 degrees.
"We can produce a hard-copy graph and download all our heating data onto a disk. We give that disk to a customer's quality control department because it requires documentation. Customers definitely have a lot more confidence because of it," Krenn said.
He added, "We're happy not just because our customers are happy, but because induction improved our productivity considerably. Our turnaround times are among the fastest."The use of chrome-moly alloys is growing quickly. Krenn estimates that five years ago perhaps 10 percent of his work involved chrome-moly steels. Today it comprises more than 50 percent. As the use of chrome-moly grows, induction heating will continue to help heat P91 quickly and uniformly.
Mike Roth is product manager of induction technology for Miller Electric Mfg. Co., 1635 W. Spencer St., Appleton, WI 54912-1079, 800-426-4553, fax 920-735-4168, mroth@MillerWelds.com, www.MillerWelds.com. Miller Electric manufactures and supplies welding equipment and accessories for light to heavy industrial applications.
J.F. Ahern Co., 855 Morris St., P.O. Box 1316, Fond du Lac, WI 54936, 920-921-9020, www.jfahern.com.
Airtec Inc., 659 S. Hickory St., Fond du Lac, WI 54935, 920-923-3577.
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