Some Flanges Leak
Due To Bolt Tension Variations
Over 50 web pages on torque, tension,
& bolted joints.
designers know that their flange seal may leak, if their flange bolt tensions
vary too much between each bolt. Yet, most installations are done with a torque wrench,
not a tension wrench. SureBolt is a new
technology that measures actual
flange bolt tension more reliably. It is
not just flange bolt torque meter. Steel flange, pipe flange, flange adapter, flange
mount, flange gasket, gasketed & non-gasketed mechanical seals, ..., rely on tension, not torque.
After all the engineering classes on flange design, why
do some flanges leak? Here is a list of
information that may be useful for a flange engineer or flange designer. Verify your flange design.
1. Flange seals sometimes leak. What is
the most common cause of flange gasket leaks (as stated by bolting experts and flange joint
designers)? Lack of proper - evenly distributed - pre-load
tension. What are some of the factors that cause pre-load
tension errors, see this link?
These all affect a flange design, especially the assumptions
of the gasket tension being applied if only a torque wrench is used. Why does
the tension in a flange bolt vary so much, bolt to bolt? Tightening one
bolt loosens some others.
No matter how many passes, the tension can vary over
50% bolt to bolt. How can you tighten each flange bolt to closer
tolerances? How can you use SureBolt on
Update says: " SureBolt outperformed the one-point bolt gauges
on every bolt and every test in reliability and accuracy."
2. Flange bolt tension, not
bolt torque. Tension not torque. You want to put a known amount of tension
into your nut and bolt combination, not just a known amount of torque. Your
flange design often relies on consistent pre-load tension. See
3. Measure tension errors. How can a
flange designer measure torque wrench errors?
Do these bolt tension errors matter?
Flange engineers know that bolt and nut friction consume over 80% of the
But, how does the actual tension vary
bolt to bolt? What if you use the same type of bolt?
different lubricants, multiple passes, different tightening sequences, ...
Notice the variation in bolt tensions on each of three passes with a
Bickford for this graph. He mislabeled the first pass (it should
show 100 Pound-Feet instead of 200 Pound-Feet). This is corrected in his
new edition. Mr. Bickford is probably the world's expert on bolted joints.
X-axis = bolt position. 16
Y-axis = bolt stretch (tension).
Large tension variations are typical with torque control. Notice
the > 250% variations.
5. Flange bolt's stress, strain & yield.
A stress strain curve can be plotted using SureBolt. Stress strain and
yield. SureBolt can help a structural engineer verify
his bolted joint design. Did his bolt reach yield?
6. Bolt gage. Your flange's bolt stress
analysis can be verified using a bolt gage. What is an ultrasonic bolt gage?
See an animated demonstration of an ultrasonic
echo inside of a bolt. Mechanical engineering labs rarely have enough
resources to let each engineer learn ultrasonics. Are bolt gages
reliable? Are they easy to use? New patented
technology has finally made ultrasonic bolt gages reliable and easy to use.
7. SureBolt. How does SureBolt differ
from any other ultrasonic bolt gauge? What is
the difference between a one-point bolt gage and SureBolt's patented whole echo
method? Are their prices similar? (Yes)
8. Flange Engineers. Some engineers use
a strain gauge to measure bolt tension because they know a torque
wrench is not directly measuring tension. Yet their flange
design software may assume a limited amount of tension error (+- 25%) when
calculating bolt tension from bolt torque.
9. Tension errors. What are some of the
tension errors associated with using a torque
wrench? Does your flange design software let you enter the tension error ranges?
10. Flange engineering. This site is a
flange engineering resource for torque versus tension in bolted joints. SureBolt
is a nondestructive testing - NDT - method of
measuring bolt tension. Non destructive testing includes the use of ultrasonics.
SureBolt's Whole Echo
patented technology greatly increases reliability by using the whole echo
instead of just one point.
All other bolt gages use just "One
Point" (one zero crossing), that leaves you susceptible to peak
jumping (20% or more error). Surebolt is a much more reliable bolt gage due to
this new technology. See comparison table.
Bolt gages have been around for over 20
years. Their "Peak Jumping"
problems and difficult learning curves, have kept them from being widely used. When your bolted joint
fails or flange leaks, then you find out your one-point bolt gage may have
Example, with a lot of training and laboratory testing, NASA has used bolt
gages for years. NASA physically takes Polaroid pictures of the echoes
before and after tensioning. Then they use their trained human
experience to determine if the echo has distorted
"too-much". Even after all
this, they have to throw away approximately one third of the ET
(External Tank umbilical bolts) measurements because they are not reliable. When NASA used the
SureBolt prototype, no measurements had to be thrown away.
SureBolt gives you a 13.3" TFT, portable PC
(Panasonic Toughbook Model 72). The ultrasonic pulser and digitizer are built into the CD ROM
and PCMCIA slots. Click here to see
the actual SureBolt screens. See the animated DEMO page
to see how an ultrasonic bolt gage can measure tension. SureBolt has been used
on special high strength titanium bolts, to standard "off the shelf"
Time". See animation below of how to ultrasonically
measure the change in bolt tension using echo time.
This animation requires FLASH
(version 4 or higher).
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No more guessing as to
which "one point" is the right point.
The FIRST whole
echo method (patented DSP Technique).
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