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Our DuraSquirt® DTIs Improve Flange Bolting | Applied Bolting

DuraSquirt® Direct Tension Indicators (DTIs) are bumpy washers that show how much bolt stress, or tension, has been developed in a fastener. They function independent of torque values and nut factors and indicate load directly when a nut is torqued along the threads of a stud or bolt. Once the induced tension/bolt stress has reached a predetermined minimum, the orange DuraSquirt® material is expelled and tightening effort is stopped. The tool operator moves the tool to the next fastener, in the joint, and repeats the tightening & orange material expulsion process, until all bolts within a joint have been tightened.

The whole process can be reduced to a few lines of instructions:

  1. Install DTIS on all bolts of a joint.
  2. Tighten all bolts to snug the joint.
  3. Further tighten each bolt until indication media appears at each DTI.
  4. Move tool to the next bolt within the joint and repeat until all bolts have been tightened.

DTIs have been used for structural connections since the late 1960s and Applied Bolting has been manufacturing them for the construction market since the early 1990s. While demonstrating our DTIs to engineering groups worldwide, we were often asked if the same technology (visual bolt stress/load) indication, could be applied to pipe flange bolting. While intrigued, pipe flange bolting is different than structural bolting solid steel together. Bolting solid steel is simple point loads in order to clamp steel plies together. Pipe flanges are circular steel plies, with a hollow center, separated by a springy gasket, whose bolts are cantilevered with an air gap between each bolted surface.

We learned what the piping world already knew, that tightening each bolt influences every other bolt in the ring. That is why flange bolting is conducted in multiple passes, at increasing torque values, to bring the flanges together, in separate steps, known as passes. Tightening each bolt may loosen or tighten other bolts in the ring, simultaneously. The effect is known as elastic interaction.

These elastic interactions between bolts renders point load DTI indications pointless, since any indication of any DTI, at any stage of tightening, offers no value, except to say that a particular bolt, was at a particular load, at a particular moment during the multi-pass tightening process.

In 2016, we started working on the problem and found that a solution had existed since the early 1990s. In 1992 the University of Akron Ohio, sponsored by BP America and the Pressure Vessel Research Council (PVRC), developed an improved flange bolt-up procedure that described a method to fully tighten a flange in a single bolting pass.

The basics of the method is to compensate for elastic interactions by tightening each bolt to a different load. If tightening one bolt loosens or tightens other bolts in the ring, adjust how much load is induced to compensate for its load, as well as its effect on other bolts.

For example, imagine there is a two-bolt flange and the target bolt stress for all bolts is 40ksi:

  1. Tighten bolt #1 to 40ksi
  2. Tighten bolt #2 to 40ksi.
  3. Bolt #1 drops to 30ksi final due to elastic interaction.
  4. Bolt 2 has a -25% interaction affect to bolt #1.

In equation form, bolt #2 interaction is described as:
40 = N -25% of N or 40 = N - .25xN
Solving for N:
N = 53.3333333

Repeat the tightening compensating for elastic interaction between 1& 2.

  1. Tighten bolt #1 to 53.333333333ksi
  2. Tighten bolt #2 to 40ksi.
  3. Bolt #1 drops 25% to 40ksi leaving both bolts at 40ksi.

This concept can be extended to all flange sizes and bolt counts. So long as the interactions are known, tightening can be conducted in a single tightening pass.

This was a breakthrough, but it required tightening a flange several times, while measuring load in each fastener. This can be done with instrumented bolts, measuring stretch, either ultrasonically or physically, or adding electronic load cells to each bolt.

While none of these methods are cost effective in the field, making small bolt load cells (DTIs) has been Applied Bolting’s business for 30 years, so we developed the DuraSquirt® Flange System. The DuraSquirt® Flange bolting system consists of a kit of numbered DTIs, calibrated to account for elastic interactions between each bolt. Like our structural DTIs, bolts are tightened until DuraSquirt® indication media appears. The only difference is the DTIs must be placed in numerical order along the flange, each quadrant snugged, and then tightened in numerical order.