thePetroStreet

We have a primary reformer furnace of Foster Wheeler (Terrace Type). It is assumed that the thermal expansion system of counterweight (hanging concrete weights designed at 105% the weight of catalyst tubes) is not working properly due to one reason or the other. This perception is based on the issues we have been facing on fatigue/creep cracking @ outlet pigtails.

does anyone has some literature or procedure on "effective monitoring & adjustment of these counter weights"?

Regards

What does Foster Wheeler recommends?

Correspondence with FW is underway. But our in-house RCA recommends "effective monitoring & adjustment of CW in hot/cold positions". Topic is initiated to seek experience of fellow engineers at their sites on the matter.

Look it's a very serious topic and need to be understood clearly before giving any comment.
I would like to provide you some directions to think about this but before I do that please mention the following:
1. Is there any counterweight on the lower side of the catalyst tubes also? Where that has been connected with? Outlet flange of catalyst tube?
2. Are the catalyst tubes connected with each other in the each entire row via some lug arrangement?
3. Guides at the bottom of catalyst tubes have been provided or not for allowing horizontal movement and restricting vertical at the same time?
Any suitable drawing would help me before guiding you in the proper direction.

Dear Mechcolor,

Appreciate your interest. Please find the related information below:

1. No ! There is no counterweights on lower side of catalyst tubes. Outlet pigtails are connected with Cat. tubes at right angle using weldolet near the bottom outlet flanges of Cat. tubes. Counterweights are only available at upper side of Cat. tubes (connected with inlet flanges using hangers).

2. No ! Cat. tubes are not connected with each other with any lug. However, a group of some 4 tubes are pulled up at upper side by a Single concrete block, equivalent to 105% counterweight of all 4 tubes (single contilever beam connects 4 hangers of Cat. tubes).

3. Yes ! Positioning guides are provided at bottom flanges of the Cat. tubes. These individual guides are moved within a horizontal H-beam, controlling limits of vertical movements as per thermal expansion & contraction.

A layout sketch is attached for more understanding.

Regards

Things are bit more clear but still I need to see the mechanical layout not this graphical outline sketch.
Let me now tell you few very basic things about such design.
i) Outlet pigtails is purposely kept in tension by using the moment of counterweight in order to avoid any sagging
ii) Catalyst tubes are kept restricted along the vertical direction and thus stresses travel along side the tube length from the bottom fixed point to the top counterweight

Now talking specifically about the failure case:
i) If the counterweight system is not being monitored properly, there is a likelihood of having stresses at the outlet pigtail weld joint with catalyst tube or with outlet header
ii) Stress in the weld joint can be either of tensile or compressive nature depending upon the malfunctioning of the counter weight system
iii) In order to check for any excessive tensile or compressive forces on the outlet pigtails, it is necessary to see the guides working properly. You must make a check for the same. Stuck guides can induce certain unbalanced forces exceeding the endurance limit of the outlet pigtail.
iv) Another check necessary to be made is the movement of counterweight in cold and hot condition, if any. This would also provide you an indication of tube bending. Improper counterweight system can cause tube bending and failure.
v) As not individual tube is being supported by counterweight, it is quite possible that each tube in a group of 04 is not having the exactly same horizontal movement along the axis of outlet pigtails.

These counter weight are self supportive for the catalyst tubes and it takes care of tube movement also. Further,there is no mechanism involved and it is just a support.The designer specified the concrete based on thermal stresses during start-up and shutdown. Pigtail weld crack is very common in reformer furnace due to so many reasons and one such a issue is improper insulation which will induce lot of stresses on the pig tail tubes causing the weld to crack. Please check the pigtail and header piping insulation.

Failure in the outlet pigtails for terrace wall type reformers can occur due to following factors:
1. Thermal shock during start-up and shutdown (thermal fatigue phenomena)
2. Sagging (there must be tension force always present in the outlet pigtails to avoid sagging and eventual failure)
3. Improper insulation
4. Improper supporting system to avoid stresses
5. Arrangement of tubes (in-line or staggered) - Unbalanced radiation can cause bowing of catalyst tubes and induce stresses in pigtails

Terrace wall reformers with counterweight systems are sensitive ones if one talks about stresses pattern throughout the catalyst tube and outlet pigtails. In this case, a balance force beneath the outlet flange of catalyst tube is absent which ensures a positive tension force in the outlet pigtails reducing the chances of sagging, creep and then a failure. Counterweight system if not properly working can be a cause of tube bending and failure.

@ Mechcolor:
Thank you for your detailed answer & guidelines. We have not been aware of proper monitoring & adjustment of counterweight system to overcome the thermal stresses caused. Now a consultant from Haldor Topsoe is contacted for rechecking the expansion system and seeking advise on proper monitoring & adjustment.

@ Piiengineers:
Appreciate your comments. You are stressing on Improper Insulation of pigtails / header, but the designer (Fooster Wheeler in our case) has not suggested insulation wrapping on outlet pigtails / header and so these are kept bare. What is the basis of your advise? Please share your practical experience.

@ Ben:
Thank you too. Your RCA on the problem is very close to the conclusion of investigation team at our site. We are now working on three key factors (No. 3, 4 & 5 in your list) to improve the reliability and service of furnace components.

Regards

I recently found a relevant document on subject matter. Sharing with all for knowledge and understanding.