7 replies to this topic

[Skyline]

Dear all,

I have a problem with fired heater. It indicates a really high tube skin/tube wall temperature. We are operating on duty around 88% than the design (heat absorbed) yet, the tube skin indication (590 C) is much much higher than the design (470 C). We have 8 TIs, and all of them show same indication (much higher than design). The efficiency (heat absorbed divided by heat release) looks good (around 88-89%) and there's no flame impingement from visual inspection.

Do you have any idea or same experience with me? How do you solve this problem? What causes this? Thank you before.

Best regards,

[mav9rick]

Skyline,
What service is the fired heater in? Gas or liquid HC? If you are getting HC carryover from an upstream vessel or process, there could be potential coke formation in the tubes, leading to non-uniform heat transfer.
This can be catastrophic, I am talking from personal experience.
Karan

[S.AHMAD]

1. What is the heat flux?
2. The problem usually related to deposition of some solid materials - such as coke, mud e.t.c
3. Especially for liquid evaporation case such as crude oil heater, or reboiler furnace.

[Flame]

Hi,

You can check the pressure drop across the heater, if its higher than design then there might be significant coke deposition inside the tubes. You should then decoke the tubes.

If a shutdown is scheduled in near future, check the skin thermocouples assembly on the tubes, rectify any damage.

Alternatively, option of Thermography can be used to verify the skin temperatures readings, provided the flame shape is good / acceptable.

[Skyline]

Dear all, thank you for your responses.

mav9rick: this is a crude charge heater, for CDU service.

S.AHMAD & Flame : I tried to calculate the coke thickness using API 530, and it results in 8-9 mm deposit. But, we did SAD (steam air decoking) about 6 months before. Is it possible that 8-9 mm deposit resulted in just less than 6 months? What could be the cause of such a thick deposit in such a short time?
And also, do you have any suggestion of thermography/thermogun (brand or technique to use it) that can accurately measure the tube skin across the flame? I once measured it using portable infrared thermometer, and it resulted in a very high temperature, almost twice than the reading from thermocouple (TI). My colleagues kept saying the measurement is not accurate since I took it across the flame, but it was the only way to mesure it. Do you have any idea about this?

Thank you

[S.AHMAD]

1. What is the crude BS&W? and the wax content?
2. I have no experience for such short duration of coking. Normally take years.
3. Most probably the root cause is in the crude quality - probably accumulation of wax and sediment at the bottom. If this is so, the pre-heat train performance should shows some degradation (fouling)
4. Check crude tank bottom - probably a lot of sediment. When was the last crude tank cleaning?
5. Do you have desalter? it is suppose to remove some of the sediment (if it works properly)
6. I propose you use Kepner-Tregoe method of troubleshooting. 5-why method is also possible, but my experience using the method shows that it sometime do not lead to the right root cause of problem.

Edited by S.AHMAD, 23 October 2011 - 10:37 PM.

[Flame]

It is very improbable that in 6 months, so much significant coke deposition may have occurred inside the heater tubes. By the way, what crude are you using?

It is suggested to accurately verify the process absorbed duty across the heater, even though the current capacity might be 88% of original design, but the terminal temperature difference might be significantly higher. Maybe changes in the feedstock might have changed the thermal properties from design values. Check whether the Heater is being stretched too much...

Check whether all thermocouples are giving higher readings. Maybe some of the thermocouples are damaged.

FLIR - P series thermal cameras can be used to perform the thermography. You can check their website - www.flir.com

Another resource is attached for your reference, Hope it helps.


Regards,

Attached Files

•  Furnace_Skin temp.pdf   290.3KB   179 downloads

[pavanayi]

Skyline,
In the last six months, has there been any heater trips?