Fired heater tube fretting. 2

[AJerry]

I have two vertical fired heaters, the heater’s tubes are supported at the bottom on the U tube and at the top the tube is held to the furnace wall with a Bull horn bracket. (The bull horn bracket holds two tubes and is held to the wall with a pin that is connected to a wall clip between the tubes. The tubes are to slide vertically through this bull horn bracket.
The wall clip is square at the ends and this is gouging in to the tubes at the 4 o’clock or 8 o’clock side of the tubes, the bull horn bracket is fretting the front side of the tubes (12 o’clock). The bull horn brackets and clips seem to be tight at 4, 8, or 12 o’clock positions.
I have asked manufacture if they have a plan for stopping this but all they say is I have a vibration problem and they can replace the tubes. We do not seem to have any vibration but we could see some water hammer from time to time.
Has anyone seen this and do they know how to prevent the tube wear other than stopping the water hammer.

 

[rzrbk]

It sounds like the tubes grow up from the bottom on these heaters. If so then it seems like once hot, they would not have any weight holding them against one side of the bracket, making them very susceptible to rattling around even in very low vibration.

I am more used to seeing tubes in vertical heaters supported at the top and just pinned at the bottom to keep them from swinging. In these heaters the entire weight of the tube rests on the hook at the top and the tubes grow down.

If the bottom u-bends are just pinned, check to make sure they have plenty of room to travel. I have had the sleeves of these pins fill with trash, bottoming out the ping and causing the tubes to grop up and off of the top hooks before.

 

[Jonkatz]

GL,
I used to design these types of heaters, are they in steam/water service or petroleum product heating?
Is this "fretting" as you call it occurring on all the tubes or just a few of them?
On bottom suppoted heaters, this is a normal situation - the "guides" have to be pretty tight to prevent excessive vibration, I've never heard of a tube rupturing due directly to the guide causing wear on the tube.

I would suggest monitoring it, and during the next shutdown, get someone in there to inspect the tubes, try and measure the errosion depth for starters...see if its significant, then decide if:
1. the tube is in danger of rupturing and should be replaced, and
2. is the guide design inadeqaute, usually the guides I have used are smooth where they contact the tube to allow sliding when the tube expands, and not so tight to prevent the expansion, just tight enough to reduce vibration.
3. If the guide design seems to be the culprit, have a heater service company either adjust the existing guides, or replace/modify the guide with one that will not Gouge the tube.
4. the last possibility is there may be some uneven heating problem which is warping the tube and putting too much pressure onto the guide, causing the excessive contact.

HOPE THIS HELPS YOU.
J. katz

 

[AJerry]

Just to fill in some of the blanks, the heaters has a water/glycol (heat medium) mixture in it; the temperature of the heat medium on exit is 290 F. All of the tubes show wear from contact with the guides.
The heaters have just been inspected and the tubes for now are safe but the deepest gouge is 0.04”.
Thanks for your input.