when inspected one of our boiler we found some tubes bulged. How can we tell if this tube can still be used? What are the approperiate test to measure bulge?
I would suggest that you get a boiler inspector and your AI in to to look at your situation. Your whole boiler needs to be inspected. The inspector could be from the boiler manufacturer or a group that specilizes in boiler inspection.
The reason for the bulge needs to be determined.
If in the inspectors judgement the bulge is not determintal to the integrity of tube it can be hammered back in. If its detrimental it will have to be repaired or the tube replaced or removed.
They are a lot of things that can cause a bulge in a boiler tube and all of them are detrimental to a boilers utility/life.
Could you define bulged? Is it in one spot on the tube or does it cover a length of a tube? What area of the boiler is it in: furnace, SH, RH, Radiant SH, economizer? What pressure is the boiler? What is the nominal OD of the tube and what is the OD of the bulge? [What fuel are you using? gas, oil, coal?)
In my opinion, these are probably somewhat irrelavent, but thought I'd ask anyway -- the tube has undergone plastic deformation from overheating, either long term or short term that didn't propogate to failure (yet)... it's only a matter of time before they fail..
I worked in the utilities when we repaired these when we found them -- in today's world (run by marketers and bean counters who only care about today's $$), you're probably faced with how long can you live with these -- in this case, the key may be how extensive is the bulging and how far out of spec is the OD of the tubing... you should use NDT to find the tube wall thickness and mic the OD, compare to the original specs and then I'd suggest soliciting advice from a metallurgist and make a risk assessment...
regardless of what happens, these tubes are damaged and will need to be replaced -- delaying that will expose you to tube rutures and a forced outage on the boiler / unit...
The questions Pablo02 asked are totally relevant. To further analyse the bulges we usually: measure the outside diameter of the tubes at the bulge and at undamaged areas, perform ultrasonic thickness measurements at the bulges and undamaged areas and perform micro hardness tests at the bulges and the undamaged areas. If you have the time and the resources, in-situ metallography is an excellent tool for determining the remaining characteristics of the tube metal.
As Pablo02 mentioned, the bulges are caused by overheating. But if the thickness readings and hardness tests are still acceptable, and, depending on the location of the bulges, other preventative actions are taken (such as the installation of shields, etc. depending on the location of the bulged tubes) the tubes can be returned to service with the plan of changing them out during the next planned boiler outage.
This is usually a plan which keeps the bean counters happy.
Most of the time the safer and best plan for longer term boiler reliability is, if you find a bulged tube, replace it with a new pup piece but investigate the cause of the bulging to prevent it from happening again in the future.
engtalal -- finding the wall thickness at the bulge (localized bulging -- more prevalent in the high heat area of the furnace) will be critical; if there is internal corrosion of the tubes, the wall will be significantly thinner than expected and the life of the tube will be reduced accordingly..
Boiler tube bulging is a common occurance. The most critical factor in determining your course of action is what type of surface the bulge is occuring on. There is a significant difference in the course of action between steam touched surfaces and waterwall tubing.
I have inspected boilers that have had recorded cases of bulging on waterwall tubing and are still running without failures for 20 years. In some cases the tube bulging can look like a small knuckle in the tube surface or ripples if they are close together. Overheating or internal tube depositsis not always the root cause of the bulge in some cases the bulge can be caused by distortion from the initial start up and if you monitor it over time you will find that they stay the same from year to year. I would advocate taking a tube sample for analysis to determine the root cause. In no case should you hammer the bulge back into plane with the rest of the tube surface, this will cause greater damage than leaving it alone.
Bulges on SH or RH surfaces should always be treated with a greater degree of suspicion and should be replaced at the next available opportunity. Don't forget to have the removed section sent out for analysis to determine the cause of the bulge. This may point out deficiencies in your water chemistry that you can address and prevent additional failures.
Weather in fire tube boilers or water tube boilers bulging is usually the result of hot spots that develop in those areas of the tubes. For example in fire tube boilers carbon soot adhears to certain areas which disrupts the heat transfer at that point and the area gets too hot and develops bulges(poor maintenance) in most cases. In water tube boilers you will develop calcium deposits which again will disrupt heat transfer and develop hot spots with the same results, again I say a result of poor maintenance,(water quality) These systems require regular scheduled maintenance and if the bean counters can't understand then you guessed it, a forced outage
dfurey- I think what unclesyd is refering to was common practice in the "old days" He sounds like an ex-boilermaker to me(such as myself). This process was an acceptable practice listed in the NBIC for boiler"saggs and bags". If properly done,heat,jacking,welding,pwht,etc. it should be ok. The other listings help explain this by careful evaluation.
Any process for any boiler repair must be carefuly considered.
deanc,
A little anecdotal but explains my post a little more.
You are pretty close to being right. We had 8 power boilers 6 @ 110,000 lbs/hr, the other 2 at 150,000 and 175,000 #/hr All at 650 psi steam. 8 fired Therminol vaporizers from 10 to 20 million BTUs. It fell under our group to inspect, recommend, supervise repairs.
We had a lot of tubes to practice on. On the 4 original steam boilers it was not uncommon to have several generator tubes with slight bulges form a design problem, steam blanketing. We would reform the tube. Also we reformed slightly bulged tubes due to flame impingment, making sure we corrected the burner before leaving the firebox. We worked on the water wall quite often.
All tube work was discretionary, you obviously wouldn’t try to fix a tube that was about to rupture or has excessive scale. We never touched the superheater tubes except to repair one or two over a 40 year span. We were quite fortunate in that we fired with natural gas 99% of the time. They are now on oil/gas-50/50 now and having tube trouble.
The Therminol vaporizers never had any problems with tubes due to operations. They ran 32 years before we lost one to 2 ruptured tubes and a big fire because an idiot told management that they could run with a 1/4" hole in a 2" tube as long as the scale was conductive.
