Severe Bowing and sagging of Heater Tubes 1

[DBreyer]

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.

 

[0707]

Make ultrasonic thickness measurements of your tubes, see if you have creep growth, make hardness testes and micographs, try to have extra supports for your tubes. With the resulting values make a estimation of heater tubes remaining life according API 579. Make decoquification to your tubes and hidraulic test before start up, from your description and age of your tubes it is also important for you to pondere the up date of your tubes metalurgie to A335 P9.

Good luck

 

[GD2]

OP,
You are lucky that you are still running the furnace with 57 years old tubes that is typically designed for 100,000 hrs (12 years) per API 530.
No doubt, there will be issues. I guess you have a 'Mechanical Integrity Program' build for the tubes.
There could be so many reasons for what you have noticed on the tubes. The good news is that you operate the tubes at 50% of the design pressure.
Following are few pointers/guidelines:
1. Do you have any retirement thickness (which is determined by dead loads and corrosion) set for the tubes? API 579 don't provide this but usually US refiners set it at 0.10" - 0.13".
2. Excessive bow or sag is usually limited to 1 x Diameter. You are above this. This puts excessive stress on return bends which is a concern. Replace the tubes that exceeds the above limitation and install appropriate hangers at missing locations.
3. It's difficult to pinpoint if the bowing is due to creep damage, high Tube Material Temperature (TMT) or mechanical changes such as decarburization or graphitization.
4. Looks you operate the heater (400 -500C) at the creep regime which is usually 427C for the A335 T5. If it is, the tubes are governed by the creep rupture design.
5. You should evaluate the tubes for both elastic design (t-min)and creep design to estimate remaining life. WRC 541 provides creep data and easy solution. API 530 had adopted data from WRC 541.
6. Keep in mind that you are operating the tubes that has exceeded @ 5 times it's design life. The simplest way will be to replace the tubes as and when you get an opportunity, starting with the bad ones.


GDD
Canada

 

[0707]

"Static equipment and piping are designed for a certain design life, often 25 years. During this prolonged use, the material may deteriorate even when the system is used correctly as a result of:

Creep due to prolonged loading under high temperature
Corrosion and erosion due to the process medium
fatigue due to cyclic operation or vibrations
When the design life has been reached, a re-assessment is required of its remaining service time, especially when design parameters may effectuate creep, fatigue or corrosion.

During the design life, damage may also occur due to occasional events such as impacts, overheating, severe wind (hurricane) loadings, human error such as forgetting to remove a temporary stop, and more. Both the effects of prolonged use and the occasional damages can be assessed by analysis conform to the Fitness For Service (FFS) code API 579. Dynaflow Research Group performs this type of assessments conform API 579 at all assessment levels, up to the most detailed Level 3."

 

[Krausen]

If you can smart pig these tubes to check for wall thicknesses & ovality/creep bulging, that will help a bunch.

Furnace tubes can take a great deal of sagging before they fail an API579 FFS Level 3. However, if the tubes are creep bulged (radial growth on ID and/or OD) above 3% or so of their nominal diameter, you will have a more difficult time passing an API579 FFS on them.

Have you done a simple visual flashlight/torch check on these tubes yet to check for any signs of creep bulging? Simply shine a light down each tube OD to ensure there is no subtle "wavy" pattern on tube ODs. This can be a quick, simple visual check to get an idea on any creep bulging issues.

 

[EdStainless]

While my first thought was creep, I wonder if something has changed that is limiting thermal expansion growth.
How old are the newest tubes in the unit?
Wondering if things were altered during a partial replacement.
Start the work on a full rebuild.
It sounds like you have a number of issues to address.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed