Tube Removal and Stabilization
In the U.S. steam generator (SG) service market, tube sample removal has become a rare event due to advances in eddy current and in situ pressure testing technologies, diminishing the readiness and current experience of some service providers. BWXT has continued to provide tube removal services in virtually every outage season, having successfully removed more than 100 SG and pre-heater tubes in the past 4 years.
BWXT has mated their proven tube removal tooling to the Zetec ZR-100 robot and to a variety of other manipulators. Our system is designed to remove all tube sizes from 1/2 inch to 7/8 inch, removing any length of tubing up to the straight leg tangent point. Custom engineering solutions are available for removing U-bend tube samples should the need arise.
The tube removal process includes several process steps that must be performed with critical sequencing and tolerances. The initial step of our process is the Tungsten Inert Gas (TIG) shrinking of the tubeto-tubesheet expansion joints (where applicable), which is performed with our standard plug removal TIG system. For hard rolled and fully expanded tubes the shrinking process is necessary to relieve the radial stress between the tube and tubesheet. The TIG process is carefully monitored for power and travel so that only designated areas in the tube are shrunk. For some tubes with minimal tubesheet contact, shrinking is not required. Machining of the tube-to-tubesheet weld must be aligned properly to ensure a proper fi t-up for the subsequent welded tubesheet plug. So that the tubesheet hole is not mis-drilled, B&W uses a unique centering cutter that protects the ID of the original tubesheet hole.
Unique to BWXT's tube removal process is the use of Metal Disintegration Machining (MDM) to sever the target tube at the upper cut location. MDM is far superior to other tube severing methods such as the use of high speed routers and mechanical blade cutters, providing reliable, readily verifi ed tube cuts in 3 to 5 minutes under tightly controlled parameters.
Once the upper tube cut is performed and verified, a computer controlled hydraulic gripper/jack assembly is used to pull the tube through the tubesheet. Predetermined pull force limits are used so that the tube is not stretched during removal. For tubes with special interest defects, the pulling force is adjusted to protect the sample. And, defects are gripped both above and below, to further provide integrity of the area(s) of interest. During removal, sections of tube are cut as they exit the tubesheet. If desired, a video probe can be fed through the tube to view each support plate region as the tube passes through.
BWXT also provides full laboratory analysis of tube samples through our Lynchburg Technology Center.
Key Features and Benefits:
- Tubes can be removed at any length up to the tangent of the U-bend
- Unique MDM tube cutting provides a superior tube severing method
- A video probe can be inserted into the tube as it is being pulled to provide inspection of each support plate region as the tube passes through
- Custom design cutter removes tube-to-tubesheet weld, protects the tubesheet hole ID and preps for a welded plug in one step
- Tooling can remove tubes from 1/2 inch OD to 7/8 inch OD