Ultrasonic thickness testing (UTT) measures the remaining wall of pipe, vessels, tanks, hull plate and structural members from one side. Taken over time, the readings show how fast material is being lost and how long a component has before it reaches its minimum thickness.
- Finds
- Corrosion and erosion wall loss, remaining wall against minimum design thickness
- Typical use
- Piping and pressure vessels, storage tanks, structural members, hull plate
What a UT thickness survey finds
- General corrosion and erosion
- Local thinning at elbows, tees, injection points and other hot spots
- Remaining wall compared with the minimum required thickness
- Corrosion rate and remaining life, when earlier readings exist
How ultrasonic thickness testing works
A transducer sends a pulse of sound into the wall and times the echo from the far side. With the instrument calibrated for the sound velocity of the material on a reference block, it converts that time into thickness. On painted or coated surfaces, echo-to-echo techniques can measure the metal alone when the instrument and procedure support them.
What the report should contain
A reading is only useful if it can be found again. A good thickness survey report identifies each measurement location or grid on a drawing or isometric, records the readings together with the instrument and calibration used, and compares them with nominal and minimum thickness. With earlier readings it gives a corrosion rate and remaining life, which is what the API inspection codes use to set the next inspection date. When wall loss needs a closer look, ultrasonic testing and phased array corrosion mapping take over from spot readings.
Where it’s used
- Refineries, chemical and petrochemical plants, terminals and tank farms
- Process piping, pressure vessels and aboveground storage tanks
- Boilers and other power generation equipment
- Ship hull plate and maritime structure
- Structural members, bridges and wind power structure
Codes and standards
- ASTM E797/E797M, the practice for measuring thickness by manual ultrasonic pulse-echo contact
- ASME Boiler and Pressure Vessel Code, Section V, which adopts E797 as SE-797
- API 510, API 570 and API 653, which use thickness data to calculate corrosion rates, remaining life and inspection intervals for vessels, piping and tanks (see API inspection)
- Owner inspection programs and customer specifications
Who does the work
Every Midgard technician who performs ultrasonic thickness testing holds current certification in that method, issued under a written practice. For most industrial work the written practice follows ASNT’s SNT-TC-1A; for aerospace and defense work it follows NAS410. A Responsible Level III oversees the program: approving procedures and technique sheets, examining and certifying technicians, and answering the technical questions that come up on the job.
If your contract calls for a different scheme, such as ANSI/ASNT CP-189 or NAVSEA requirements, tell us when you ask for a quote.
Training in ultrasonic thickness testing
Midgard also trains in ultrasonic thickness testing, along with all the other major methods. Instruction is given at Level I, II and III by certified Level III instructors, to SNT-TC-1A, CP-189, NAS410, NAVSEA or whichever code you certify to, with general, specific and practical examinations at your facility or ours. It suits new hires working toward first certification as well as technicians due for re-examination. See training and certification, or ask about a class.
Common questions
How does ultrasonic thickness testing work?
A transducer sends a sound pulse into the wall and times the echo from the back surface. The instrument, calibrated for the material’s sound velocity, converts that time into thickness, so the wall can be measured from one side without cutting or draining anything.
Can you measure thickness through paint?
Often, yes. Instruments with an echo-to-echo mode measure the metal and ignore the coating. Where the coating is thick or poorly bonded, a small spot may need to be cleaned to bare metal. The procedure states which method is used.
What is the minimum thickness ultrasonic testing can measure?
It depends on the instrument, the transducer and the material. Standard contact gauges typically handle steel down to about 1 mm (0.040 in); thinner material needs high-frequency or delay-line transducers. Tell us the nominal wall and we will choose the setup.
How often should thickness readings be taken?
For pressure vessels, piping and tanks in refining and petrochemical service, the API codes set the interval from the corrosion rate and remaining life, within maximums in each code. For other equipment, the owner’s inspection program or the jurisdiction decides. Consistent locations matter as much as frequency, because readings taken at different spots cannot be compared.
Send us drawings or isometrics if you have them, the nominal and minimum thickness, earlier readings, and the code you inspect to. Call 1-844-MIDGARD, email contact@midgardscientific.com, or use the contact page.