UT Testing Equipment
|The Tools Of Our Trade|
Every ultrasonic investigation follows a finely detailed written procedure which outlines our actions taken in the field to acquire the raw wall thickness, as well as our subsequent data analysis and generation of the final report. This ensures that every investigation is not only based upon accurate wall thickness data, but also ensures that no steps have been overlooked during our field investigation. The final result, as we always strive, is the most useful and informative fitness fore service report for the client. A report answering all client questions and much more.
While different piping systems may require slight alterations in our field testing procedure, such as when the pipe surface is rusted, the fundamental process of sending an ultra high frequency sound wave through the pipe to produce a measurement of wall thickness is identical. Differences in field testing procedure exist, however, depending upon the type of piping system under evaluation, the piping material, pipe temperature, internal corrosion profile, and external surface conditions.
Common issues are:
- Rusted chill water or dual temperature pipe will require external rust removal
- Steam and other hot pipe requires higher temperature couplants to move the sound into the metal
- Painted pipe requires “echo-to echo” technology in order to negate the paint thickness without removing it
- Galvanized steel pipe requires a greater level of spot testing in order to identify more random pitting
- Dry and pre-action fire sprinkler pipe requires a specific test grid in order to fully define internal rust conditions
- Yellow brass pipe requires an added visual inspection in order to identify dezincification
- Cast iron and ductile iron pipe require more frequent calibration at flanges and end samples
While the above differences exist in terms of taking the wall thickness measurement, the balance of every investigation remains the same.
- The pipe is spot tested to provide an initial condition assessment
- 12 wall thickness measurements are taken for record at each pipe section under evaluation
- All thickness data and waveforms are stored in memory for download and future verification
- All pertinent data such as pipe location, date installed, size, material, pressure, temperature, etc. is recorded in a laptop database
- Secondary information such as pipe orientation, test grid, nearby leaks or anomalies, construction, etc. are also recorded
- A photograph is taken in order to define the pipe section tested and general area inspected
- Photographs are taken of all anomalies and issues of interest
- Notes are taken related to any event or observation considered useful in the final report
Of greatest importance is the UT instrument itself. Having evaluated many other UT inspection reports which have been proven entirely inaccurate and worthless, one common denominator has been the use of low cost instruments having none of the advanced features necessary to produce an accurate wall thickness measurement every time. Equipment capable of providing only 2 decimal places of accuracy should be dismissed immediately, as should those without a waveform display or the capacity to save such information for future reference.
Operator error and inexperience also play a significant role, notwithstanding the testing qualifications and certifications presented. The need to toss out all wall thickness data provided by a Level 3 UT tech arguing that calibrating to a steel step block is acceptable in testing copper pipe because both metals are “basically the same” is just one such example.
While many excellent instruments are available from different manufacturers, we favor the Olympus 38DL Plus for our ultrasonic investigations. Ideally suited for almost every ultrasonic thickness application, this handheld thickness gauge is fully compatible with a full line of dual and single element transducers. The versatile 38DL PLUS can be used in applications ranging from wall thinning measurements of internally corroded pipes with dual element probes to very precise thickness measurements of thin materials with single element transducers. More information on Olympus equipment is provided here.
This very rugged and reliable instrument provides all of the special features needed to produce an accurate thickness reading while at the same time saving the thickness measurement and entire waveform for future review and confirmation. Its large on-board memory capacity enables us to store all waveforms and to maintain backups of each project.
Some key features include:
- Dual and single element transducer compatibility
- Wide thickness range: 0.08 mm (0.003 in.) to 635 mm (25 in.)
- THRU-COAT® and Echo-to-Echo measurements on painted and coated surfaces
- Standard resolution of 0.01 mm or 0.001 in. for all transducers
- Default/Custom dual and single element transducer setups
UT Transducer Probes
The transducer performs another critical role, which is to convert one form of energy into another. In ultrasonic NDT, transducers convert a pulse of electrical energy from the test instrument into mechanical energy in the form of sound waves that travel through the test piece. In effect, the transducer acts as an ultrasonic speaker and microphone, generating and receiving pulses of sound waves at frequencies much higher than the range of human hearing.
The correct selection of transducer is critical to acquiring an accurate wall thickness measurement. Given that most of our investigations are at internally corroded piping systems, “Dual Element” transducers are typically employed. As the name suggests, such probes incorporate separate transmitting and receiving elements mounted on a delay line at a small angle to focus sound energy a selected distance beneath the surface of a test piece.
Although wall thickness measurement with dual element transducers is sometimes not as accurate as with other types of transducers, they usually provide significantly better performance in corrosion survey applications due to their higher sensitivity to pitting and improved near surface resolution. As for all transducers, surface preparation is critically important. Most errors we find in reviewing competitive reports relate to lacking or improper surface preparation.
Each transducer requires a new calibration; with calibrations performed at regular intervals during the inspection process. Adhering to this critical step ensures that all wall thickness measurements taken are as accurate as possible. Since all thickness values and waveforms are saved by the instrument, we can always look up and view any wall thickness value and verify its accuracy.
A standard laptop serves as the coordinating basis of every investigation, and starts as a blank page on which to record all pertinent information. This includes a written description of the test location, and all physical characteristics of the pipe such as size, its construction, operating pressure and temperature, pipe material, orientation, pipe schedule, etc.
Any observed conditions or features of the pipe, such as wet insulation, nearby pipe clamps, or even an abnormal pipe temperature are noted for possible inclusion in our final report.
A standard digital camera is used to record the general location of each test location. That photo is then cross referenced to the test location in our laptop database. Tags, labels, tape, and other forms of identification to a test location are quickly lost, painted over or removed, whereas a photograph provides a lasting document to where testing was performed.
Any unusual or abnormal conditions of the pipe itself, such as wet insulation, severe external corrosion, or even broken pipe hangers and supports are provided in the report as an aid to building owners and operators. Important defects, such as a leaking pipe section or any other indicator of a pending failure are bought to the attention of building operators and engineers immediately.
At request, we can provide copies of all photographs taken in a zipped file.
While factory service ensures the proper functioning and reliability of the instrument, it is the calibration performed in the field which determines its accuracy. Most pipe metals have a well known and reliable sound velocity available in reference tables. For carbon steel or galvanized steel, the sound velocity is 0.2341 in./μs., (inches per microsecond) and for copper it is 0.183 in./μs. This provide close but still not acceptable accuracy.
For those metals, we carry stepped calibration blocks which enable us to produce the closest accuracy to near 0.001 in. Calibration is performed at the beginning of the investigation, whenever transducer probes are changed, for significant differences in temperature, and at least during every hour of use.
Ultrasonic couplants are required in UT testing in order to facilitate the transmission of sound energy between the transducer and the test piece. Their use is necessary because sound energy at the ultrasonic frequencies typically used for nondestructive testing is not effectively transmitted through air, and will escape between the surface of the transducer and subject material. Even the extremely thin air gap between the transducer and the test subject will prevent efficient sound energy transmission.
The use of a liquid or gel of some form is required with every contact of the transducer to the pipe surface. Various forms of couplants exist from simple glycerine serving most needs to high temperature gels and pastes necessary for evaluating live high temperature steam lines. Thicker gels may be required when addressing pipe having a rough exterior surface, such as cast iron or ductile iron pipe.
Pipe Measuring Tool
While the difference between 12 in. and 8 in. pipe is obvious to anyone trained in this field, errors can still exist in differentiating between 1-1/4 in. and 1-1/2 in. pipe. Likewise between 22 in. and 24 in. pipe. Working in low light conditions, above ceilings, or reaching inside restrictive access hatches where one can touch the pipe to test it but not clearly determine its diameter is one other common issue we face.
Such errors then surface later on during our office analysis when wall thickness dimensions do not coincide with pipe diameter, or if one pipe size does not match drawings and specifications. If recognized, a follow-up check of the pipe size with building engineers may provide the answer. If not, basing a corrosion rate and service life conclusion on the belief that 5 in. pipe was evaluated when in fact it was actually 4 in. then introduces substantial error into the report.
For this example, much higher wall loss from an incorrectly estimated larger diameter pipe having higher wall thickness would produce a falsely high corrosion rate and therefore an inaccurate remaining service life estimate.
In order to eliminate this common source of error we find in many competitive reports, CorrView utilizes our own pipe measuring tool. Calibrated for both steel and copper pipe in sizes 1/4 in to 24 in. for steel and 3/8 in. to 6 in. for copper, this simple tool triangulates against the pipe to always ensure a correct pipe size determination.
We provide this tool with our UT pipe inspection reports to those clients who may find it useful.
Scraper, Wire Brush, Grinder
Most investigations of fire sprinkler, natural gas, and condenser water pipe require no added pipe preparation. Pipe operating at cold temperatures and especially any outdoor pipe, however, frequently requires some level of surface preparation in order the acquire an accurate wall thickness measurement. In that respect, we follow a strict procedure of using minimal impact against the pipe in order to define its condition and potential vulnerability prior to advancing further. Only after identifying safe and acceptable wall thickness doe we progress to more deteriorated areas of the pipe using more aggressive methods.
Once reaching minimum acceptable limits, or an area of pipe showing severe corrosion indicating the potential to fail, we cease any further rust removal.
Various other tools are used including knives for removing insulation, thermal infrared gun, depth micrometer, caliper, etc.
© Copyright 2022 – William P. Duncan, CorrView International, LLC