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Tell Tale Holes (TTH)
A Compliment to Existing Piping & Fitting Inspection Practices
Older Practice Continues Yielding Risk Reduction
By Mike Badeen, Phillips Petroleum Borger Refinery and NGL Center, Borger, Texas
Bublished eralier on Inspectioneering Journal email:

New inspection technology, when added to the proven practice of using tell tale holes (TTHs), proves effective in reducing significant releases and or catastrophic events that are related to internal corrosion / erosion of process piping. In fact, one facility's experience indicates that this practice, when used in conjunction with current and newly advanced technology such as automated ultrasonic (AUT) and profile radiograph (PRT), is more effective than using only new technology.

Building upon success is critical to the continued improvement process". Companies need to carefully evaluate changes to be sure and not discontinue "proven" practices when adopting newer technology until the new methods are proven as equally effective. Similar to insurance, "you would not want to drop your policy until you were able to replace it with one of adequate value".

Tell Tale Drilling (TTD) has been discontinued by many facilities since the early 1960s as new NDE methods evolved. Phillips 66 integrated the evolving technology as it was developed and continued to accent their risk reductions by continuing to apply TTHs. A recent study conducted by Marsh and McLennan (property protection and risk consultants) reported on the distribution of large property losses. They were ranked according to the following categories and percentages:

Equipment Type & Percentage of Large Losses
Piping 30%
Tanks 15%
Reactors 13%
Drums 7%
Pumps & Compressors 6%
Heat Exchangers 4%
Towers 4%
Heaters/Boilers 3%
Others 18%

Based upon this study, newspaper reports and shared industry experiences, piping integrity/reliability has been an area where industry needs improvement. Industry conferences and meetings bear out the fact that most operators are in the process of improving their piping inspection programs.

TTD is the application of small pilot holes drilled in the pipe /component wall using specified and controlled patterns and depths to obtain the optimum desired protection against ruptures resulting from internal corrosion, erosion and erosion/ corrosion (see figures included). They are most economically installed during fabrication.

Special drill assemblies and depth gauges are used to assure the proper depth is attained. The TTH pattern and density may be one of three and is determined by the risk potential, which is a combination of factors including probability of leak and consequence parameters. A carefully chosen selection process should be used to eliminate the unnecessary expense of TTH drilling systems that are not likely to fail from internal corrosion / erosion mechanisms. Phillips refineries use a decision flow diagram and facility experience for determining when it is appropriate to apply TTH.

The density and depth of Phillips' THHs has been studied and determin- ed to produce no risk of catastrophic failure due to cracking or weakening of the wall/membrane, i.e. no weakening occurs with the installation of TTHs.

The pilot holes, typically 1/8" in diameter, are drilled from the OD to the outermost part of the corrosion allowance periphery. The design is such that when the corrosion allowance is consumed a "weep" occurs at that location.

Up until the general acceptance of ultrasonic (UT) wall thickness measurements in the 1960's the use of TTHs was common practice to determine when some amount of pipe wall loss had occurred. This practice, at most companies, was replaced with UT thickness testing. Phillips refined and continued the use of TTHs to reduce risk in addition to employing recognized and generally accepted piping inspection practices, e.g. UT, AUT, profile RT, etc.

In conclusion, we believe there is a tremendous cost/benefit ratio when comparing the cost to install TTHs versus risk exposure (not forgetting we install TTHs in addition to a modern, industry accepted inspection program). Comparisons of 13 published large industry losses from 1979 to 1996 strongly indicate that a TTH program would have drastically mitigated, if not eliminated, these incidents. Companies should re-evaluate this "older technology" before discontinuing or reconsider the implementation of this proven practice to supplement existing industry inspection programs.

As a result of continuing to use the TTH practice, Phillips has over 30 years of statistics proving the effectiveness of our specific approach, which has been endorsed by risk consultants.

Typical Results of a TTH Leak

  • Small Drips
  • Wet spot, drip or spray
  • Smoldering or dripping from insulation
  • Sufficient time for clamping or pinning
  • No Large leaks
  • Provides time for decision making
As an example, risk reduction can be dem- onstrated by an evaluation of a 1/8" leak compared to a 3" rupture. Release modeling indicates that the footprint of a liquid H 2 S release through a 3" hole size was reduced by an order of 2140 times when comparing the result to that of a 1/8" TTH. Benefits can be determined when evaluating the material released and the assets / environment exposed.

TTHs provide protection against:

  • Pipe ruptures resulting from general or localized corrosion or erosion
  • Nipple and plug failures resulting from internal corrosion
  • Misapplication of materials (PMI Issues)
  • Unexpected corrosion or erosion conditions
  • Newly designed processes (no documented corrosion / erosion rates)

TTHs are not a panacea for the detection of all potential material failures and do not replace the need for NDT corrosion monitoring techniques. TTHs should not be expected to detect damage mechanisms other than internal corrosion or erosion. NDT monitoring and trending are still required for effective maintenance and equipment life cycle planning.

If you have any questions about this article please contact the author via the Inspectioneering Journal at:

Inspectioning Journal
- Addr: 5315 FM 1960 West, Suite D 237, Houston, TX 77069, USA
- Phone: (281) 397-7075
- Fax: (281) 397-9996
- Email.

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