External spiral patterns on fired heater tubes?

[jonesey]

We have a fired heater open for inspection and have observed spiral patterns on the outside of the 5Cr heater tubes in the radiant zone. The patterns can be seen with the naked eye, but cannot be felt. The tubes are of seamless construction. These patterns have not been seen at previous inspections. The tubes have previously been pigged. Has anyone an possible explanation to the cause of these spirals ?

 

[metengr]

If these are circumferential surface indications (they have some depth), they could be the start of thermal fatigue damage. Can you describe in more detail the fired heater? Fuel type? How long the tubes have been in service?

 

[ApC2Kp]

jonesey,

Seamless tube and pipe rolled by Mannesman(?) machines have a helical or spiral pattern left in the cylinder that could be noticably thinner wall (manufacturing spec limit -12.5%) at the pinch points of the rolls. The helical pitch of spiral might depend on the tube diameter / wall thickness. Is the pitch of the observed spirals more or less than 1 spiral per tube diameter?

The thicker wall area of tubes could run at somewhat hotter temperature than the thinner wall areas of tubes. Try a thickness measurement of different portions of the tubes, and see if the thickness / thinness correlates to spiral patterns. If the thickness measurements confirm the spiral pattern, there could be something connected with fatigue comment from metengr. If the previous pigging operation did not completely clean the inner diameter, but left some residue on the I.D. at helical spiral (groove), then the thinner wall portions could run at hotter temperture from the added fouling heat transfer resistance of residues in the helical spiral internal groove.

 

[jonesey]

The possibility of thermal fatigue is already being investigated by performing replica of the tube external surfaces. We will receive results back in the coming days.
The external spiral patterns have a pitch approximately equal to the pipe diameter of 5" NB.

 

[Jake1234]

I have a weil mclain boiler with a honeywell L8124a,c aquastat and a taco 3 zone control for a boiler install, i am connecting a solar hot water heating system and need to find a aquastat for the solar tank to control the boiler when not needing,and the boiler to run only when solar temp is to low to heat the house?.