How a modified ferrule, installed in a refinery SRU boiler in 2005, lowered peak heat flux at the tube inlet.
The hottest spot in the boiler is the first inches of each tube.
The ferrule decides how hot it gets.
This paper describes an improved tube and ferrule design that was installed in the waste heat boiler of a sulfur recovery unit in November 2005 to prevent film boiling at the tube inlets. The ferrule shape was changed so that the hot process gas enters the tube with less turbulence, which lowers the peak heat flux just past the tubesheet. A US refinery that developed a patented ferrule with Blasch in the same year later reported longer boiler tube life from lower peak heat flux.
The boiler is sized for average heat flux, but the tube fails where the gas enters, so the inlet geometry matters more than the tube count.
In a waste heat boiler, water boils on the outside of the tubes while hot process gas flows inside. As long as bubbles form and leave the wall, heat moves well and the tube stays close to water temperature. If the local heat flux gets too high, a vapor film forms on the tube wall, insulates it, and the metal temperature rises sharply. That is film boiling.
In an SRU boiler the gas arrives from the Claus reaction furnace at its highest temperature. The first section of each tube, just behind the tubesheet, sees the highest heat flux.
The ferrule protects the tube inlet and the tubesheet from the hot gas. According to the published summary of the 2017 presentation by the refinery, turbulence from eddies at the ferrule outlet raised the heat flux sharply in the tube behind it. The patented modification to the Blasch ferrule smoothed the transition, reduced the peak heat flux and led to longer boiler tube life. The summary gives no figures.
Ferrule shape also affects pressure drop. A Blasch laboratory study found that a sharp edged inlet and outlet cost over 35% more total pressure drop than a tapered inlet with a chamfered outlet, measured with air at room temperature as a comparison.
If tubes fail near the tubesheet while the rest of the bundle is sound, look at the inlet: heat flux, ferrule length and the transition from ceramic to steel. More on the ferrule types on round and hex ferrules, on the boiler itself on SRU waste heat boiler, and on the wider thermal section in Reliability in Recovery.
A local heat flux above what nucleate boiling can carry away. The vapor film insulates the tube and the metal overheats.
By shaping how the gas enters and leaves the ferrule. Less turbulence at the outlet means a lower heat flux peak in the tube behind it.
Yes. It was installed in an SRU waste heat boiler in November 2005. A patented ferrule developed with a US refinery that year was presented with operating results in 2017.
Send tube size, pitch, gas inlet temperature and where the tubes fail. A Blasch engineer looks at the ferrule and the inlet.
