Improving HRSG Availability through Better Desuperheating

Thermal fatigue in heat recovery steam generator (HRSG) tubes can lead to lost electrical production. Emerson’s Douglas Morris, a member of the Power and Mining industry teams, recaps a presentation he attended by Emerson’s Mark Nord.

Emerson's Douglas MorrisTube leaks in HRSGs are a problem for all combined cycle plant operators. The Generating Availability Data Systems (GADS) from North American Electric Reliability Corporation (NERC) shows that HRSG tube leaks are the number one contributor to lost megawatt production.
One primary cause of these leaks is the thermal fatigue that results from improper desuperheating. Thermal cycling will stretch tubes and eventually lead to cracks and an outage, which, on average, last about 4 days.

Example of stretched tubes

Example of stretched tubes.

It turns out that a modest thing like a nozzle has a lot to do with tube problems. When a nozzle clogs or functions improperly, cooling spray will be too little or too much which both lead to stressed tubes. A simple way to avoid unnecessary fatigue is routine maintenance. The recommendation is to inspect insertion style desuperheaters annually—checking for cracks and missing or loose nozzles.
Example of plugged nozzle

Example of plugged nozzle

Example of good nozzle

Example of good nozzle

A good engineering practice is that, regardless of condition, all nozzles should be changed every 18 to 30 months. In addition to replacing these nozzles, surrounding insulation and piping should be checked, along with upstream strainers.

Back to the impact of these outages—4 days of downtime averages about $2.4M in lost production. Hopefully nozzle inspection just became more important.

You can connect and interact with Doug, Mark and other industry experts in the Power industry track of the Emerson Exchange 365 community.