Heat Exchanger tube Failure mechanism
Heat Exchanger tube Failure mechanism
(OP)
I am very interested in a failure mechanism that is associated with low and intermittent tube flow that causes tube failure and tube movement. This has been observed in verticaly oriented, u-tube heat exchangers and requires a hot shell (secondary) side, cold tube (primary) side, liquid medium, a thick tubesheet and double rolled tubes - each roll is 1.25 x tubesheet thickness and located adjacent to the secondary and primary tube sheet faces.
The tube flow is comprised of a constant tempering flow in parallel with a control valve that responds to shell vapour pressure. The cooling flow requirement is slightly more than tempering, therefore, the valve modulates cyclically, alternating between a minute of low tempering flow to 10 seconds of flows 20x higher.
My theory is that, as a result of this flow transient, the following mechanism occurs:
- during periods of low tempering flow, the radial thermal gradient through inlet tubes and tube sheet is small due to the insulating laminar heat transfer coefficient
- when flow quickly increases, the tube gradient increases sufficiently to relax the upper rolled joint and permit axial tube contraction reaction forces to develop between the lower rolled joint and the seal weld.
-by the time tube contraction reaches the lower rolled joint, the upper tube sheet has cooled and locks in some residual tensile tube strain.
- tensile stress in the tube section between upper and lower rolled joints is released when the lower rolled joint relaxes.
- after this 10 second cooling cycle, there follows 60 seconds of tube sheet heating
- with 500,000 cycles occurring each year, cracks are initiated at stress risers in the root of the fillet/seal weld and propagate at 45 degrees from tube OD to ID along the heat affected zone adjacent to the fillet weld.
- when the tube has cracked circumferentially through wall, it is free to inch-worm down the tube sheet in response to the 'thermal peristaltic' action.
Does this sound plausible, and would eliminating the double roll by rolling the section between, prevent tube cracking and movement (if the flow transient could not be changed)?
The tube flow is comprised of a constant tempering flow in parallel with a control valve that responds to shell vapour pressure. The cooling flow requirement is slightly more than tempering, therefore, the valve modulates cyclically, alternating between a minute of low tempering flow to 10 seconds of flows 20x higher.
My theory is that, as a result of this flow transient, the following mechanism occurs:
- during periods of low tempering flow, the radial thermal gradient through inlet tubes and tube sheet is small due to the insulating laminar heat transfer coefficient
- when flow quickly increases, the tube gradient increases sufficiently to relax the upper rolled joint and permit axial tube contraction reaction forces to develop between the lower rolled joint and the seal weld.
-by the time tube contraction reaches the lower rolled joint, the upper tube sheet has cooled and locks in some residual tensile tube strain.
- tensile stress in the tube section between upper and lower rolled joints is released when the lower rolled joint relaxes.
- after this 10 second cooling cycle, there follows 60 seconds of tube sheet heating
- with 500,000 cycles occurring each year, cracks are initiated at stress risers in the root of the fillet/seal weld and propagate at 45 degrees from tube OD to ID along the heat affected zone adjacent to the fillet weld.
- when the tube has cracked circumferentially through wall, it is free to inch-worm down the tube sheet in response to the 'thermal peristaltic' action.
Does this sound plausible, and would eliminating the double roll by rolling the section between, prevent tube cracking and movement (if the flow transient could not be changed)?





RE: Heat Exchanger tube Failure mechanism
you should look at old technology: locomotives,
they had the same problem, most adopted a certain tube rolling technique called... you have to look at Sect. I, they still mention it there. I got very interested and I called the retrofit shop at the Museum in Sacramento CA.
RE: Heat Exchanger tube Failure mechanism
RE: Heat Exchanger tube Failure mechanism
you can try calling the shop and talk to a senior boiler maker.
they will treat you well.
RE: Heat Exchanger tube Failure mechanism
Vertical , heated u-tube configuration will have flow instability unless inlet orifice ( or capillary tube) frictional pressure drop is added to each tube.
research older ( 1950's) topic of heated parallel channel instability or ledineg instability. Or add inlet capillary tubes or an efective inlet orifice.
RE: Heat Exchanger tube Failure mechanism
RE: Heat Exchanger tube Failure mechanism
RE: Heat Exchanger tube Failure mechanism
RE: Heat Exchanger tube Failure mechanism
500,000 cycles a year sounds like a lot of cycles. If the equipment is older it probably wasn't designed for that kind of cycling. What kind of ramp rates does your exchanger see? If you keep heating and cooling cycles in the 100-200 F/hr your cyclic fatigue limits will greatly increase. Also, the highest stresses due to pressure loadings are in the outer most edge of the outer most tube (generally).
The 1-1/4" roller expansion is a function of the roller expansding mandrel, any longer an the mandrels tend to break. Thicker tubesheets are step expanded several times to about 1/8" from the shell side of the tubesheet. If the tubesheet is thick (6"+), it may be better to use explosive expansion (or hydro-expansion) for a uniform and gradual transition from expanded area to un-expnaded tube. Either of these expansion methods could be performed again on-site to make sure all the joints are tight, and repair any tubejoint welds that may not be holding.
RE: Heat Exchanger tube Failure mechanism
The 100 degF water in the tubes condenses 300 psig saturated steam on the shell side. Temperature gradients in the tube walls are due to heat conducted from the condensing steam, through the tubesheet into the tubes at the two rolled joints and convected into the cooling water. Thermal fatigue from changes in these temperature gradients are only due to cooling water flow fluctuations in the tubes - tempering to full flow. Shell thermal cycles are very few, limited to vessel warm-up and cool down.
Circumferential tube cracking has been observed on both inner and outer tube locations. Tube bend radii are all greater than 4 x tube OD (5/8"dia). Tube material is Inconel 800. In approximately two years of operation, one tube cracked through-wall and inch-wormed 2" down the tubesheet. More than half of the other tubes were cracked circumferentially and had moved various amounts up to 1/4".
Continuously rolling the 6" thick tubesheet with explosive or hydro-expansion is being proposed, as you recommend. The effectiveness of this repair to stop tube cracking and inch-worming would be masked by re-instating tube seal welds. The plausibility of this failure mechanism is the question...
RE: Heat Exchanger tube Failure mechanism
rmw
RE: Heat Exchanger tube Failure mechanism
The upper tube sheet face is cladded with 1/4" thick Inconel 800 (to which the tubes are seal welded with 1/4" fillets), while the lower face is not cladded (and not seal welded).
A few more construction details that might bear on the thermal fatigue mechanism:
- this is a four pass, ten foot long tube bundle with tube cracking and movement only occurring in the first quadrant
- each pass is 39 tubes and the first pass is encapsulated on all sides by a two foot long, pie shaped off-gas cover
- off-gases are not vented at any time during two year periods of continuous operation
RE: Heat Exchanger tube Failure mechanism
if the gasses need not be there, can you store the gasses in an added vessel?
does the problem exist at the section with accumulated gasses?
is the first pass the coolest of the four or th ehotest?
Seems to me that the movement, expand and contract of the tubes create the cracking, all will agree to that.
finding the cure:
incoloy tends to corrode and gas in the system could affect the tube at the most weak part.
design: we mke small shell steam generators and we never pay attention to expansion and contraction of the tubes because they are for lower pressure 150 psi sat.steam.
If these were for higher pressure and temp, we wold have the same problem you are having, some tubes would crack at the hotest ends due to expansion and contraction; we would have to address the problem and find a solution to avoid loosing tubes.
Since the solution can not be done by working the tube and yes, you can corrugate (bellows) each tube but then how would you insert them.
but why not corrugate(bellow) the shell, that way the expansion and contraction of the tubes will push the shell and not the tubes.
now the test would be how many cycles the shell will widstand.
I do not know that asnwer because I do not have the equipment, software and expertise. I think you do!
RE: Heat Exchanger tube Failure mechanism
are you certain the fluid pressure on the tube side (cold water) is above 300 psig at all times? If not , the low flow / low friction cold fluid may be boiling and easily cause flow stagnation and flow instability.
RE: Heat Exchanger tube Failure mechanism
Genb - gases are produced whenever water condenses - there is no deaerator in the process. Tube cracking and movement occurs only adjacent to seal welds in the cold (inlet) quadrant. There is no possibility of thermal interaction between shell and tube bundle - the u-tubes are free to expand vertically down (axially) from the tube sheet, which is clamped between shell flanges.
RE: Heat Exchanger tube Failure mechanism
If you are not venting non condensables, then you may also have pockets in the Hx where the tubing is blanketed by the gasses and not being heated as are other parts of the tubing.
rmw
RE: Heat Exchanger tube Failure mechanism
A cooler lower tubesheet face means a tighter lower rolled joint which means higher tensile stresses generated in the tubes during the full flow thermal transients...
RE: Heat Exchanger tube Failure mechanism
I have been searching high and low for any evidence of this 'inch worm', 'slinky motion', 'catch and release' or 'thermal peristalsis' occurring between autofrettaged tubes and their tube sheets. My Schmidt plots indicate that the through-wall thermal gradients are steep enough and the tube contraction loads are high enough to cause slip, but how do I prove that cyclic flow transients can coordinate this interaction between rolled joints?
Perhaps there is empirical evidence in the field of reciprocating compressors?