DBreyer
Materials
- May 16, 2014
- 62
Hi all,
upon entering a an atmospheric crude distillation fired heater in a turnaround we are faced with severely deformed heater tubes. Most likely due to coke buildup
and associated high tube metal temperatures during a 1 year time span in the previous run (we did a pig clean during the run to rectify the high temperatures but no visual inspection).
The heater is build with horizontal tubes. Normal tube metal temperatur is between 400-500°C. Design limit 560°C. Design presure 20barg (but oparation pressure well below 10barg) Tube material is ASTM A335 T5.
A huge number of tubes , especially the 57 years old T5 tubes in the middle and lower section of the radiant section are partially without support (hangers) over
axial distances exceeding the recommended 35 x OD or 6 m (API 560). Due to bad experiences with vibrating tubes in the past with unsupported
axial lengths > 6 m on a CDU heater we are really concerned to run in similar problems.
Additionally we found tubes that are pressed on the refractory wall of the furnace.
In a small number of tubes we found excessive external wear due to fretting of the tube hangers in the tube material -> we will due intelligent pigging soon. The locations found can then be repaired with weld build up or new tube inserts.
Some Hangers in the roof section loose also their function as support , additionally tubes in the T5 radiant section are heavily tensed-up, sometimes with the hangers sometimes
with the refractory wall. There are some T5 tubes with a vertical deviation of approx. 1,2 -1,3 x OD from their center line and a larger number of tubes with deviation around 1 x OD.
Due to the deformation and the large number of tensed-up tubes the anticipated additional stress for the coils seems to be significant and we are afraid that during start-up of the heater
hindered thermal expansion of the tubes may happen or jumping out of supports.
Preliminary results of sample hardness measurements as well as sample replicas of the tubes don’t show bad results in terms of the values as for the microstructural picture.
Any ideas on how to proceed?
-Leave the tubes as they are, as the thermal (secondary) stresses will be mostly relaxed?
- Cut some tubes and see how the bundle will react and settle? (how will we get a new straight section in between the bend ends?)
- Adjust hangers so the bend tubes are properly supported again and prevent them from jumping into the firebox
- Replace everything (not really feasable due to the large number of tubes required)?
Thank you for your ideas.
upon entering a an atmospheric crude distillation fired heater in a turnaround we are faced with severely deformed heater tubes. Most likely due to coke buildup
and associated high tube metal temperatures during a 1 year time span in the previous run (we did a pig clean during the run to rectify the high temperatures but no visual inspection).
The heater is build with horizontal tubes. Normal tube metal temperatur is between 400-500°C. Design limit 560°C. Design presure 20barg (but oparation pressure well below 10barg) Tube material is ASTM A335 T5.
A huge number of tubes , especially the 57 years old T5 tubes in the middle and lower section of the radiant section are partially without support (hangers) over
axial distances exceeding the recommended 35 x OD or 6 m (API 560). Due to bad experiences with vibrating tubes in the past with unsupported
axial lengths > 6 m on a CDU heater we are really concerned to run in similar problems.
Additionally we found tubes that are pressed on the refractory wall of the furnace.
In a small number of tubes we found excessive external wear due to fretting of the tube hangers in the tube material -> we will due intelligent pigging soon. The locations found can then be repaired with weld build up or new tube inserts.
Some Hangers in the roof section loose also their function as support , additionally tubes in the T5 radiant section are heavily tensed-up, sometimes with the hangers sometimes
with the refractory wall. There are some T5 tubes with a vertical deviation of approx. 1,2 -1,3 x OD from their center line and a larger number of tubes with deviation around 1 x OD.
Due to the deformation and the large number of tensed-up tubes the anticipated additional stress for the coils seems to be significant and we are afraid that during start-up of the heater
hindered thermal expansion of the tubes may happen or jumping out of supports.
Preliminary results of sample hardness measurements as well as sample replicas of the tubes don’t show bad results in terms of the values as for the microstructural picture.
Any ideas on how to proceed?
-Leave the tubes as they are, as the thermal (secondary) stresses will be mostly relaxed?
- Cut some tubes and see how the bundle will react and settle? (how will we get a new straight section in between the bend ends?)
- Adjust hangers so the bend tubes are properly supported again and prevent them from jumping into the firebox
- Replace everything (not really feasable due to the large number of tubes required)?
Thank you for your ideas.