High-Speed Rail Welding

High-Speed Rail Welding

Touch the Sky at 4611 Meters High: Mass Flow Control for High-Speed Rail Welding
Today, “Alicats in the Wild” takes us deep into the Chinese interior, where a novel welding method is revolutionizing the speed and quality of high-speed railway construction. This story comes courtesy of Shanghai Hua Xie, one of Alicat’s distributors for China.
High in the Qinghai-Tibetan Plateau of western China lies the Sky Road, the highest railway in the world. At altitudes as high as 4611 meters, a remarkable gas pressurized welding machine, manufactured by Chengdu Da Cheng Communication Tech Company, Ltd., uses precision mass flow control and pneumatics to pressure weld each rail section on site. This innovative technology ensures very smooth and secure rail to rail transitions, which is especially ideal for meeting the stringent requirements of China’s flourishing high-speed rail industry.
Pneumatic Gas Pressurized Welding for Railways
 
The GPW-1200 gas pressurized welding machine being assembled at Chengdu Da Cheng.
The Qinghai-Tibet Railway, called the Sky Road by locals, is constructed with high-carbon steel rails whose great strength and hardness require welding temperatures as high as 1300°C. In contrast to slow and dangerous traditional thermite welding processes, Da Cheng’s GPW-1200 gas pressurized welding machine yields higher quality welds in only 20 minutes. To accomplish this, the machine itself grasps both rails and hydraulically forces them together to ensure precise alignment during the welding process. Then the GPW heats the rail joint to 1300￢°C with an acetylene-oxygen flame mixed at an exact ratio of 1:1. After 6 minutes, the welding flame extinguishes, and pressure is maintained on the fusing rail ends as they cool. The GPW then polishes and verifies the alignment and smoothness of the new rail joint. A dedicated engineering train carries the GPW from joint to joint every 500 meters, making the entire process very efficient.
 
Da Cheng’s GPW-1200 railway pneumatic pressure welding machine in operation in Tibet.
The quality of these railway welds rests upon the stable control of the acetylene-oxygen welding flame and the precise maintenance of the mixed gas composition at a 1:1 ratio. In its original design, the GPW used rotameters to monitor the flow of the gases to the welding flame. However, because rotameters measure volumetric flow, which changes as atmospheric pressure and temperature change, Da Cheng found that flow readings became inaccurate as the altitudes of the GPW’s operating sites increased. On account of this, Da Cheng turned to thermal mass flow control. However, thermal mass flow controllers from multiple manufacturers all displayed inconsistent flow control that yielded unstable welding flames and poor quality welds. In addition, the 10-minute-long warm-up times of these thermal mass flow controllers slowed down the welding process significantly.
Fast Mass Flow Control for Railway Welding
 
Alicat high-flow mass flow controllers provide stable control for acetylene and oxygen welding gases.
It was at this point that Chengdu Da Cheng turned to Shanghai Hua Xie Automation Equipment Company, Ltd., one of Alicat Scientific’s distributors for China. Hua Xie recommended the installation of Alicat’s high-flow mass flow controllers (MCR Series), which utilize laminar differential pressure-based measurement technology and employ near-frictionless rolamite proportional control valves. With control speeds of less than 100 ms, these mass flow controllers yielded precision control of the gas flow rates and therefore stable welding flames. Because Alicat mass flow controllers also measure pressure and temperature in real time, they provided highly accurate mass flow readings regardless of the working altitude or ambient temperature of the GPW’s work site. With no thermal element and thus no warm-up time, these mass flow controllers improved the GPW’s efficiency, reduced Da Cheng’s scrap metal rate and shortened the overall construction time by 30%. Furthermore, Alicat’s many on-board gas calibrations means any controller can be made to control acetylene or oxygen without the need for recalibration, which simplifies Da Cheng’s stocking requirements.
 
Alicat mass flow controllers mounted on Da Cheng’s GPW-1200′s engineering train in Tibet.
With more than 20 GPW-1200 gas pressurized welding machines ordered in just the past few months, Chengdu Da Cheng Communication Tech Company is poised to revolutionize the way high-speed rail systems are built and maintained, here in China and around the world. In the future, these systems will be built more rapidly and with greater quality on account of Da Cheng’s extraordinary GPW and the fastest mass flow controllers in the world.