Optimizing steam temperature reduces energy costs and increases operational reliability. In an 18-minute webcast, 3 Steps to Better Steam Temperature Control, Emerson’s Douglas Morris shares how advanced steam control for boilers and heat recovery steam generators (HRSGs) improves performance. Doug shares how to effectively validate critical steam measurements, ensure optimal steam attemperation, and improve control methodologies for these assets.
After introductory remarks describing the webinar environment, he opens showing a typical operating profile for a combined-cycle plant. A key challenge for electrical power producers is to maintain consistent superheat and reheat temperature under widely varying energy supply and load conditions. A typical plant has about 250 starts per year—2 cold starts, 48 warm starts and 200 hot starts.
Although combined-cycle plants are designed to be flexible to address load swings from renewable energy sources, fast and frequent startups and shutdowns have an impact on unit performance and efficiency.
The more these units are cycled, the greater the risk of unplanned downtime from tube leaks and other pressure part failures. Replacement costs are also greater with expediting charges when a unit goes offline unexpectedly.
Doug notes that the root cause of most forced outages is HRSG damage caused by corrosion and fatigue. These conditions result from thermally induced cyclic stresses caused by improper attemperator control.
Doug describes a typical steam attemperator loop. Feedwater is injected into the main steam header before it goes into the superheat section. The factors which must be optimally measured and controlled include the spray water temperature, initial and final steam superheat temperature, spray water pressure, and steam velocity. Installation factors which must be considered for optimal control include the pipeline size, downstream straight pipe length, and steam pipe liner.
The superheated steam must be controlled not to exceed the material temperature limits in the turbine and HRSG. Three critical areas require focus: the attemperation itself (spray nozzles, associated control & isolation valves, etc.), accurate and repeatable temperature measurements, and PID loop control—which is great for steady state conditions but less so for transient conditions.
Spray control valve have historically been sized for steady state conditions but must be re-examined and re-sized for use in transient operating conditions. Spray valves should also have higher turndowns for the spray nozzles to perform more complete mixing. Ideal water spray covers most of the pipe area while not contacting it and causing water impingement on the pipe. Nozzle maintenance is also critical for optimal spray. They need to be inspected on a regular basis and changed in all desuperheaters every 18-30 months.
Watch the webinar for Doug’s guidance on effective temperature measurement and loop control strategies to increase overall operational performance and reliability. On the webinar page you can download a whitepaper detailing these 3 steps to better temperature control or consult with a power industry automation expert. More information is also available
The Power Generation section on Emerson.com has more information on solutions to help with safe, efficient, reliable and secure operations.