DECLINING PRESSURE METHOD FOR BOILER STORAGE AND BOILER CLEANLINESS ASSESSMENT BY UNTRASONIC TECHNIQUE

摘要

During the 1980’s a West Coast Utility had concern about the lack of boiler corrosion protection during idle periods when cycling for economics that led to searching for better ways to protect the boiler waterside and still meet the needs of the load dispatch control center. With system load requirements dictating short periods of shutdown for some units a challenging dilemma for the boiler water chemistry program developed due to the reluctance on the part of those involved with operations to call the economic shutdowns “storage outages”. The duration of the economic shutdowns often varied between being overnight to 40 days and seasonally could be as long as 6 months. Wet storage treatment was not considered until the boiler had been off-line for seven days. The declining pressure method of storage utilizes the residual boiler temperature and pressure of ~200 psi to thermally mix a low level storage dosage that is injected with the existing boiler water chemical feed system. The effectiveness of the boiler storage program has been evaluated over the past 20 years when determining the need to chemically clean the boiler, reference cleaning history can be found in Table 1. One of the tools was the attempted use of the ultrasonic technique to measure the water wall oxide thickness in high heat areas. Confirmation by tube specimen deposit analyses yielded mixed results although the ultrasonic testing did prove to be helpful in selecting the tube with the highest potential for the most deposition. A recent study found that the Kyushu UT method could provide accurate oxide thickness measurements of tube oxides.