By Randy C. Turner, Technical Director, Swan Analytical USA
In recent years, there has been increased pressure to reduce start-up times for all units, particularly combined cycle units. Most of the combined cycles in the late 1990’s were designed to be base loaded due to the low price of natural gas at the time. Increased demand drove the price of natural gas to near $15 per million BTU’s. That, coupled with seasonal demand fluctuations and decreased demand due to the economic recession, has resulted in frequent cycling and start-ups. Combined cycle power stations are more frequently operated in cycling mode. In some cases, this could mean more than 250 start-up and shut-down cycles per year. Combined cycle units designed today have reduced the start-up time by as much as 60%.
Given the substantial fuel cost, environmental issues, and economic impact, any shortening of the time between start-up and reaching full load is of paramount importance. Cation conductivity is still the decisive measurement for the start-up of the steam turbine and the focus is now how fast the cation conductivity results are available and how reliable they are.
The time necessary to obtain correct and representative analysis values is greatly influenced by the design of the sampling and monitoring system. A well designed sampling and monitoring system, coupled with the small staff at a combined cycle plant, can provide significant economic rewards.