Field Damage to Metal Bellows
Subprime – time news
I can’t help watching the financial news these days. It’s fascinating. OK, not good fascinating but train-wreck fascinating. The media always drags out some financial expert and asks him/her the same tiresome question, “So, would you say it’s time to panic?” To which nobody in the history of financial melt-downs has ever said, “Why, yes Dan, this is an excellent time to panic”.
On a smaller scale, metal bellows damage (we’re talking non-leaking) may create panic among the plant’s reliability group. Here are some guidelines to consider before going to red alert:
Dents, and really big dents
The most common causes of mechanical damage to bellows are arc strikes or wrenches and hammers dropped from the deck above. Bellows material is thin and dents easily. Protective covers are meant to prevent such damage, but for many flanged designs they must be removed to install bolts – thus the occasional wrench slip dent. Everybody always blames the previous shift for such damage.
(original article with photo’s can be viewed on the website noted below)
(Photo showing shallow dent)
Unsightly, yes. Time to panic? No.
Shallow dents do not cause a significant enough reduction in the bellows cycle life to require replacement. Don’t accept them on a new part from a supplier, but live with them if they occur in the field.
Sharp dents will reduce the cycle life. The sharper the dent the sooner a fatigue crack will form as the bellows flexes. This could quickly lead to a failure.
Scratches and arc-strikes
As long as a scratch or arc strike does not remove more than 10% of the bellows material thickness (maybe even a little more, but you didn’t hear it from me), you can just sand and buff it out. If you can catch your fingernail in a scratch, it should be sanded out. And sure, the QC guys will feel good by performing an LP exam after the repair but trust me, there won’t be any cracks.
(photo showing severe dent)
An excellent time to panic
The bottom Line
We have a saying in the industry – Dent shallow, let it fallow; Dent deep, don’t keep. Just kidding, I just made that up. Our industry doesn’t even allow sayings.
Just remember that if you ever drop your wrench onto a bellows down below – make sure your name isn’t on it. If it is… it’s time to panic.