Tube bowing or sagging is normally caused by one of two conditions - inadequate tube supports or broken supports between tube spans or thermal bowing caused by start-up conditions where trapped condensate is causing a thermal mismatch in metal temperature and resulting in a bow in the unsupported tube length.
I have a 9 superheater pipes in the horizontal with 8 bends. The bottom superheater tube has sagged 1.25in. 6 others have sagging of less deflection. Is that excessive sagging?
Take a look at the picture I have attached. I would say from 10 to 12 feet length. On the bottom tube you can see the measuring tape straight across and the superheater tube sagging.
Yes, I reviewed the picture. The sagging is not an issue currently. I would monitor tube sag it during future inspections and determine if it is getting worse or has stablized. If it is getting worse over time, this could be creep deformation.
Do you think this is typically what you would expect for a boiler that was put in service in late 1987 and has an expectancy to last 30 years before it has to be recommissioned? Would this be a normal sag you would expect to see?
It could also be due to overheating from a number of reasons. Sagging could also trap liquid which could be a problem upon restarting the boiler. There should not be any sag even after 25 years of service unless some past event caused this condition. Talk to your boiler inspector for an opinion. What type of a boiler is this?
