Leak-Free Valves take the Pressure for Posco

Korea-based Posco is one of the world's largest steelmakers, processing about 30 million tons of steel annually or enough to make about 75,000 automobiles a day. The giant steel works which is located on the east coast of Korea, consists of 14 Combined Heat and Power Plants (CHP) at the Pohang site. The site replaced two problematic bypass systems with state-of-the-art CCI VLB bypass valves featuring combined pressure control and desuperheating. The installation also included auxiliary water-control valves.

Bypass valves are designed to remain closed during normal operation. In a CHP they allow steam to flow safely when the turbine is intentionally shut down or trips. A leaking bypass system causes great variation in the pressure and temperature of the steam. These variations can damage the boiler and also cause fatigue to the piping, which can take months to repair. Such leaks, exceeding the tolerable percentage of maximum capacity in both the high-pressure and low-pressure loops, resulted in the problems caused at Posco.

At the Pohang site, in one of the plants, a leaky bypass system consisting of separate pressure control and desuperheating, in the high-pressure loop caused a large variation in the temperature and pressure of steam to the boiler’s cold reheat line. The other similar leaky bypass system in the low-pressure loop, caused not only temperature variation to the condenser but also a 38 mm (1.5”) buckling in the 800 mm (31”) piping downstream of the pressure control valve, close to the desuperheater location. It was deduced that the buckling resulted from thermal stress associated with poor desuperheating.

CCI Valve Doctors analyzed the problematic systems and installed state-of-the-art VLB bypass valve technology. The excellent bypass valve design with the smooth VLB body, customized for each application, provides great resistance to fatigue and a steam cooling spray-water system that protects from sensitivity to thermal stress.

All valve body penetrations are circular to prevent asymmetrical stress patterns. Spray water atomization is excellent avoiding hitting heavy metal walls and other components sensitive to thermal stress. Years of experience suggest that bypass valves combining pressure control and desuperheating provide the best steam conditioning and consequently optimum system performance.

As a result of the installation, Chui Ki Cho, the Utility Electrical Manager in the electrical maintenance section at Posco states: 'We have experienced neither leakage nor variation of any sort from either of the valves. The valves have functioned perfectly. We are very happy with them."

“During the one year they have been in operation, there have been 3 planned shutdowns and 1 turbine trip. Steam flow takeover has been immediate after the trips, and flow to the turbine after startup has also been very good. The valves function very well....”

Published in SOLUTIONS Spring 2006