Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation

 

 

 

Machine Design Case Study No. 81: Open ended Steam Sparger Creates Excessive Vibration

When designing steam spargers for heating liquids in tanks, avoid opened ended pipes. Yes, it's quick, and yes, it's cheap ... but it can be deadly.

The u-bolt shown to the left had the awesome responsibility of trying to hold a 2" diameter open ended 125 psi steam pipe at bay. The pipe was vertically located down the sidewall of a tank that required heating. The pipe was attached to the baffle supports by way of u-bolts. Open ended steam pipes, or "spargers" as they are referred to, are the poor man's heat exchanger. They don't cost much to install, but they sure create a lot of damage to the tank. If anyone saw the movie "U-571" in that movie the submarine is chased by a destroyer that throws depth-charges overboard. Besides being exciting, the movie was technically accurate. The men in the sub were literally thrown about as each depth-charge exploded under water. The shock waves created by those depth-charges were tremendous. That's how they work.

View of tank

 

An open ended steam sparger is no different than a depth-charge. The 3700+ steam immediately vaporizes the liquid next to it. The rapid expansion of the liquid to steam gets a tank rockin' and rollin' pretty quick. This is the second tank that has failed like this in as many years. The tank that failed is shown to the left. The steam sparger went all the way to the bottom of the tank, where the red circle is located.

Maintenance had been welding tank sidewall leaks in this vicinity for months. The leaks were getting worse and the welds would not hold. The verdict by some was that the tank was too old and fatigued, the fact that the welds wouldn't hold was "testament" to that "fact." This tank is solid stainless steel. Wall Street commodity dealers in chromium and nickel drool at stuff like this. They would just love for us to throw this tank out and get a new one.

When all else fails, open the darn tank up and LOOK inside the thing. Lo and behold, there's the steam sparger attached to baffle supports, two of which are located EXACTLY where the weld repairs are located. Gee, what a coincidence.

Is the stainless steel tank sidewall fatigued? You bet!! Can it be repaired? No problem.

Tank Sidewall Weld Repair - Top Half

 

For illustrative purposes, the baffle supports are shown by a dotted red line in the pictures to the left and lower left. The weld repair technique consisted of using a full penetration weld, backgrinding the root pass, and finishing off with a couple of passes. Notice the location of the tank sidewall tears with respect to the angle iron supports. The angle iron supports created very sharp corners with which to concentrate and transfer the loads to the tank sidewall. The repair also consisted of installing circular pads underneath the angle iron supports. The circular shaped heat affected zones of those interior welds can be seen in both pictures. The pads were installed so as to spread out the vibration created by the steam sparger.

 

 

Tank Sidewall Weld Repair - Bottom Half

 

Oh yes, the bottom of the steam sparger was plugged, and small holes drilled throughout the length of the pipe, so as to minimize the shock waves and implosions created when the steam mixed with the liquid.

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