March 15, 2007

Focus on O&M (March 2007)

STEAM TURBINES

Tighter tolerances in retrofits

Getting the most out of a turbine requires reducing clearances to the minimum possible. But tolerances cannot be so tight that rubbing occurs as turbines move through thermal gradients and critical speeds. Some gencos are finding that they can significantly boost their plant's output and efficiency by retrofitting some of its systems with lighter components with tighter tolerances.

"Turbine retrofits are fairly inexpensive, and they can raise a unit's output and power and efficiency considerably, so electric utilities are increasingly using them to increase revenue," says Peter Tavis, quality control inspector for Edison ESI, a subsidiary of Southern California Edison.

One reason why retrofits aren't costly is that the existing casing and other components remain in place. But the casing also creates a constraint. Whereas a manufacturer of new rotors and blades can optimize their dimensions, units designed for retrofits must fit into the existing shell. Even a small gap between components can make a huge difference in performance. For example, interstage steam losses around the seals of a shaft or a blade tip can take a big toll on a turbine's heat rate.

How much of a heat-rate penalty can be avoided by good retrofit choices? TurboCare Inc. (www.turbocare.com) explains that during normal turbine operation, retractable packing rings can ride closer to the shaft—without rubbing—than conventional rings. Based on its use of retractable rings in more than 500 turbine retrofits, the company has found that they lower a unit's heat rate by about 2% while boosting output by up to 3%.

Brush seals, which ride even closer to the shaft than retractable seals, are another good choice. TurboCare says that in field tests, its brush seals reduced by a factor of five the steam flow around a new labyrinth seal with a nominal 7-mil (0.007-inch) gap. When a brush seal replaced a worn labyrinth seal whose gap had grown to 20 mils, the reduction was even greater—a factor of 17.
 

What's under the hood?

For a retrofit to be successful, existing components must be accurately measured to ensure that the new parts fit and work better than the ones they are replacing. TurboCare explains that when it machines aftermarket parts, it doesn't just replicate specs of original equipment manufacturer (OEM) parts, for three reasons. One is that OEMs guard those specs as intellectual property and typically won't release drawings or data files either to a customer or a third party hired to do repairs.

Rich Olivier, TurboCare's manager of thermodynamic design, recalls how this policy complicated a retrofit of an impulse steam turbine at a geothermal plant. "Since [the OEM wouldn't release] the drawings, we had to open up the machine and take measurements out in the field so that the new design would fit into the existing casing," he says. "We have some structured stages that we could have applied, but they wouldn't necessarily fit in the envelope. So we designed some new airfoil shapes from scratch."

The second reason TurboCare doesn't rely on OEM information is that it may not reflect reality. For example, when project manager Joel Saenz opened up a 350-MW steam turbine to install a new rotor, he found that its eighth stage was displaced (Figure 1). It turned out that the "as-built" figures provided for the stage were just design specs, not actual measurements.

1. Out of shape. The deflection of this eighth-stage vane arc segment had to be measured before the assembly could be reengineered. Courtesy: TurboCare
 

"I looked at the readings from 1985 when the turbine was built, and every one was perfect. None was off by even a ten-thousandth of an inch," Saenz says. "No one has ever built a perfect machine. But based on those readings, you might conclude that this OEM had."

The third reason for taking measurements—both in the field and in the shop—prior to a retrofit is that wear and tear alters equipment clearances over time. Before designing new parts to raise turbine performance, you must quantify the condition of existing parts with great precision.