Thermowell fracture 1

[Milutin]

Hi all,

On crude unit we have for pass fired heater with thermocouples on each pass exit.
We experienced thermowell fracture two time in last three years. When thermowell is broken crude oil start leaking. It is very unpleasant situation because of high crude oil temperature and possibility of fire.
Possible cause of fracture is pass vibrations on heater outlet.

I find in literature if there is possibility of thermocouple fracture it is necessary to install valve on thermovell end and in case of fracture it is possible to close valve, cut wires and stop leaking.

Does anybody know manufacturer or solution in this case?

Regards,

Milutin

 

[rzrbk]

A failure analysis of the thermowell should be done to determine if failures are caused by vibration or corrosion attack.

If vibration, a study needs to be done to determine a correction. (eg. thicker material, different length, provide thermowell with guides which support from the nozzle id, etc.) Thermowell supplier should be able to do this given your installation details.

If corrosion, a material change will be needed.

I would recommend strongly against putting a valve on the thermowell.

 

[Christine74]

I think installing a plug valve on the end of the thermowell is a good idea, as it would allow you to stop the leak should this happen again.

-Christine

 

[JLSeagull]

No valve.

Consider temperature element assemblies can be made with compression fittings drilled in a manner to detect that the element is immersed within the fluid.

Use shorter wells, better metals, perhaps erosion protection such as hard facing the well. Calculate the resonance and bending stress to assure that the selected wells are suitable for the fluid velocities, etc. Calculate based upon ASME PTC 19.3 Chapter 1.

 

[0707]

I suppose that it is impracticable to install a plug valve in thermocouples applications. If I am wrong let me know. We have already had some thermocouples leaks on heater passes of our CDU heater. Leaks have been caused by erosion due stem wear by coke particles , by broken stems due to vibration e by naphtenic acid corrosion. I agree with Milutin that such failures in thermocouples are very dangerous and can cause a fire. We have changed our thermocouple stems to 317L and all turnarounds thermocouple stems are inspected and replaced if required by inspection.

luis marques

 

[Christine74]

For a typical plug valve detail, see Figure 4.14e here:

http://books.google.com/books?id=6p...ts=DgY64a-Zn7&sig=p1rVUI5E5L5wiPH1vjy1sg0rHwI

-Christine

 

[0707]

oK Christine74

That can be applicable only in thermocouples for local reading. Thermocouples with computer instrument cabling for long distance TDC readings cannot have plug valves.

Luis marques

 

[Milutin]

Thanks all for valuable answers.
As I wrote we have four pass furnance an problem only on two passes. Reason is probably vibrations caused by pass configuration. Heater is cabin type but tubes go in spiral in firebox.
Two troublesome passes have two 90deg bends on short distance before furnance exit.Other two passes are straight on exit and we didn't have any problems with them. Please see picture below.



We also experienced abrasion caused probably by two phase flow and high velocities on furnance exit.

Regards,

Milutin