Steam Reheater Mystery Marks

[Bambie]

In my plant there is a 30’ high, 14’ diameter steam reheater with 500 psig HP steam on the tubeside and 70 psig LP steam on the shell side. Each of five vertically hung rectangular tube bundles has 4”dia inlet and outlet pipes with a four foot thermal loop located in the bottom of the vessel before exiting. The mystery marks are found only on the inside surfaces of these loops. Inspectors describe them as saw cuts, surface gouges, arc strikes and grinding marks.
I would like to know if anyone has seen this type of damage before and whether it could possibly be attributed to impact from internal debris like u-bolts, nuts & bolts swirling around the bowl. Please view the pictures attached.

 

[dougarthur42]

WOW, interesting never seen them before. Looks like steam gouging to me but I don't know. Possibly debris but you would have found some thing when you opened it up. Sorry I couldn't help. Doug

 

[eyec]

i am assuming that this is a relatively new construction.
i agree with the inspectors except for the 3rd picture of the bend. i would say that is a thermal crack.
regardless, if you did not find evidence of FOD (foreign object damage), like loose material in the bottom of the vessel, i would have these sections cutout and replaced with new material.
whoever, installed and welded these loops should have brought these marks to the attention of the job supervisor.

but you know what they say about "assume"!

 

[Bambie]

Dougarthur42 and eyec, This vessel has been inservice for 25 years and u-bolts and nuts have been found in the bowl. The tube bundles have developed leaks and were replaced at least once during that time, which requires the thermal loop to be cut, usually just below the tube bundle. Unless the welders were using this thermal loop nook to balance their tools on, I don't see how the damage could be maintenance related. The material is A53B, and these inside surfaces would be in compression during operation so I don't see how a crack could form...and the tips are not crack like.

 

[metengr]

I would suspect the ID surface defects were from original tube fabrication and some of the other locations are internal debris striking the ID surface of the tube.

 

[Bambie]

I have an alternate, in-service explanation for the thermal loop surface damage for comment:

At 100% load the Reheaters superheat 70 psia saturated HP turbine steam approximately 135 degF which increases its specific volume 1.3 times.
At 25% load they superheat 23 psia saturated HP turbine steam approximately 260 degF, a specific volume increase of 1.5 times.

There is not much baffling inside the Reheater shell to channel the steam through the heater coils, therefore, as the expanding and heating turbine steam moves through the coils, the increased backpressure causes a flow split over and under the tube bundle obstruction resulting in significant vertical flows inside the shell.

Heater coils have been replaced in the past because of leaks. When heater coils leak, Boiler steam at 585 psia (100% load) and 728 psia (25% load) blows into the Reheater, creating a buoyant mass in the upper shell, causing a significantly larger obstruction to flow through coils, resulting in very significant velocities. These flows could carry a hail storm of u-bolts and nuts through the thermal loops cutting and gouging the pipes in the process.

This might explain many of the marks, as metengr points out.