2010

Using Light to Reverse-Engineer a Steam Turbine

A turbine manufacturer wanted to get more power out of hundreds of turbines that were originally built and installed in the 1950s. The basic idea was to use the original outer casing but upgrade the internal components such as the blades and diaphragm. All of the components of the turbine needed to be reverse-engineered so that computer aided design (CAD) models could be created and used as the basis to analyze and optimize the turbine design. The parts ranged from small components to the case, which is 11 feet long, 6 feet wide, and 8 feet tall, and weighs 30,000 pounds.

altThe traditional approach to reverse-engineering, a coordinate measuring machine (CMM), would have required an estimated six months to capture all of the points required to fully define the turbine’s geometry.

Instead, the turbine manufacturer contacted NVision’s Engineering Service Division. Using a non-contact 3D optical scanner called the MAXOS and a HandHeld 3D portable scanner, NVision’s technicians were able to completely reverse-engineer the turbine in only six weeks at a substantially lower cost than the turbine manufacturer had budgeted for the project. The resulting CAD geometry was used to perform a computational fluid dynamics (CFD) simulation whose results helped dramatically improve the performance of the turbine. The geometry was also used as the basis for designing the new internal components of the turbine. Measuring the critical blade geometry to high levels of accuracy made it possible for the turbine manufacturer to perform simulations that helped to redesign the blades and diaphragms to substantially improve the energy efficiency of the hundreds of existing turbines.

CFD technology gives engineers the opportunity to understand how flow affects the performance of turbine blades and quickly and inexpensively evaluate alternative geometries by determining their impact on energy efficiency. In order to run CFD simulation it’s essential to have a CAD model that accurately depicts the as-built turbine geometry. The problem is, nearly all of the turbines that are prime candidates for Steam turbine interior components. design upgrades were designed without a CAD model, so reverse engineering is an essential first step to improving the turbine blade design.

Size Matters

altThe turbine rotor in this application measures 11 feet in length and 6 feet in diameter. The turbine was designed using manual drafting and the drawings were no longer available. Even if they had been available they would not be much help since the design had changed considerably over the years. In the past, the turbine manufacturer could have used one of two primary methods to perform reverse-engineering. The simplest would be to use height gauges and other manual measuring instruments to measure discrete points on the surface of the parts in question. The problem is that a technician can get only critical dimensions such as the location of hole centers, the diameters of holes, and wall thicknesses. In addition, this approach is time-consuming and only as accurate as the person taking the measurements.

The alternative would have been to use a CMM, which has a probe that must physically touch the part. The primary limitations of a CMM are that the operator must manually move the steering system to track each point to be measured and the device can only capture one point at a time. But to accurately model the geometry of a complex 3D contour, like those found on many turbine parts, you need millions of points, sometimes many millions, to get the geometry exactly right.

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