Another consideration:
Depending on the temperatures you are running with the heat exchanger, you can
be doing annealing and softening, either on the bolts, washers or gasket
material. I have had the experience of using a copper sealing ring between
steel flange and body - you tighten the bolts and everything is sealed, and
works well as it is heated up and operates. But the copper ring anneals and
flows slightly. Then, when you shut down and cool, the bolts are loose and the
gasket leaks. So, tighten the bolts and OK again, until the next shutdown, and
it repeats. After a period of time, the copper gasket had essentially extruded.
Solution, soft iron gasket with same coefficient of expansion as steel.
Recommendation, look at the whole closure system, bolt torque and temper (grade), gasket material, and washers. Personally, I like the castellated locking nuts.
George McKinney
On May 17, 2005, at 9:18 AM, alejandroperezposada wrote:
> I'm having some problems with the heads of a Heat Exchanger related
> with the bolts. The Bolts are loosening in operation.
Typically bolts loosen because they aren't tightened properly. When the
bolt is subject to cyclic loading the joint slips and the nut loosens.
There are lots of reasons why bolts won't maintain a preload, from
bolts that are too short or flanges that squash out over time or poor
understanding of the actual loading. Bellevilles and those worse than
useless split washers won't maintain a preload. Depending on the
circumstances, you might have some luck with thread locking compounds
or special locking threads. Nylock nuts sometimes work. As a last
resort you can stake the threads, but you can't reuse the bolt and it's
a bear getting it loose.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
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[Non-text portions of this message have been removed] Received on Tue May 17 11:04:00 2005
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