TNT The NDT Technician A Quarterly Publication for the NDT Practitioner Focus Transducer Ultrasonic Thickness Flowing water supply Bubbler Measurements at attachment High Temperatures Backing material by Ronald T. Nisbet Legend Water hy is it necessary to obtain thickness Early pulse echo instruments were often Figure 1. Early pulse-echo instrument using measurements on equipment used in tandem with resonance equipment and flowing water as couplant and coolant. W operating at temperatures from eventually replaced the resonance 300 ºF to 1200 ºF (150 ºC to 650 ºC)? Simply put, measurement technique. Pulse echo decisions about corrosion and erosion of piping instruments could be used with flowing water and pressure vessels must be made based on that acted as couplant and also kept the Transducer immediate measurements that accurately transducer cool. However, metallurgists were reflect the high temperature environment. concerned that these instruments might quench What industries require this type of the material or cause thin wall piping to corrosion information at high temperature? Oil rupture. Ensuing technology substituted a liquid Liquid filled chamber refineries and chemical plants are the principal filled chamber in place of flowing water. industries requiring measurements in such Diagrams for a flowing water system and an Test piece corrosive and heated environments and these early high temperature prototype transducer industries have adopted onstream inspection with liquid filled chamber are shown in Figs. 1 Legend Rubber diaphragm. point systems to achieve them. Measurements and 2. As technology continued to evolve, the Couplant made in onstream inspection point systems are liquid filled chamber was replaced with a solid recorded at the same point each time at cylinder of interchangeable lengths. This Figure 2. Early high temperature transducer predetermined intervals in order to accurately transducer type is typically used as a single with liquid filled chamber. assess the rates of corrosion and erosion and to element transducer and is most effective with project needed replacements. analog ultrasonic instruments. Continued on page 2. How can ultrasonic measurements be •••••••••••••••••••••••••••••• obtained at elevated temperatures? The answer, needless to say, is very carefully. ONTENTS Protective equipment is required for the C technician to prevent burns and for the Volume 3, Number 4 October 2004 ultrasonic testing equipment to prevent damage to the transducer and cables. Focus: Ultrasonic Thickness Measurements at High Temperatures. 1 History Tech Toon . 2 The earliest thickness measurements in the petrochemical industry (1958-62) were obtained FYI: Practical Contact Ultrasonics — Angle Beam Calibration using resonance measurement techniques in Using a Basic Calibration Block . 4 which resonance principles were used to determine velocity or thickness. The equipment Feature: Job Safety: Slips, Trips and Falls . 8 was of two types; the first based upon interpreting audible beeps and the second Practitioner Profile: Anthony J. Gatti, Sr. 10 based on interpreting visible indications on a cathode ray tube. The instruments were Inbox: Number Belts for Pipe Welds . 11 accurate but required surface preparation and the transducers used could not withstand A Publication of the American Society for Nondestructive Testing temperatures in excess of 150 ºF (65 ºC). Focus continued from page 1. Modern Methods Most ultrasonic thickness measurements made today use modern equipment that features both an A–scan display and digital thickness FROM THE EDITOR measurement (Fig. 3). Additionally, transducers have been developed for use at temperatures ••••••••• ranging from ambient up to 800 ºF (427 ºC). A section view of a popular transducer that can be used up to about 700 ºF (370 ºC) is shown in hen asked what advice he would offer to Fig. 4. When temperature exceeds 800 ºF (427 ºC), someone considering NDT as a career, Tony the single transducer with a heat resistant W Gatti answered, “If you’re gonna do it, do stand–off (coupling block used between the face whatever it takes.” In other words, to do it well, you of the contact transducer and the front surface of have to commit. Certainly, Anthony’s commitment is the test object) can often be used with a flaw apparent. He’s been on the decks of the many ships detector. and submarines that he’s inspected bearings for and he’s been to the very bottom of Hoover Dam and stood in front of the huge new turbines that incorporate his inspection product. Interesting and important work — he’s justly proud. Not all of us work on projects as interesting as Hoover Dam but because •••••••••••• the NDT community strives to ensure that the product is what it’s said to be, NDT is important work. This month, Ron Nisbet’s Focus article discusses thickness measurements made in high temperature environments and Jim Houf has prepared the fifth article in the popular FYI Practical Contact Ultrasonics series. The topic is ASME type angle beam calibration. There’s also a feature on falls in the workplace with an interesting table that breaks out the principal types of work-related falls and the relative incidence of each type. The second leading cause of accidental death on the job, how falls happen and what we can do to eliminate their potential is good information we should always keep in mind. Hollis Humphries ••••••••••••• TNT Editor PO Box 28518, Columbus, Ohio 43228 Figure 3. Modern UT equipment features both (800) 222-2768 X206; fax (614) 274-6899 A–scan display and digital thickness reading. <[email protected]> Backing material Transmitting crystal Receiving crystal Tech Toon element element Temperature resistant stand–off material •••••••••••• Wear plate Figure 4. Diagram and section views of a “Seems like we do this drop test earlier every year, Harve.” transducer suitable for temperature up to •••••••••••• 700 ºF (370 ºC). 2 · 10/2004 · The NDT Technician Understanding the Effects of High Compensating for Error in High • It is difficult to determine when the Temperature Environments Temperature Measurements calibration block has reached the temperature of the test piece. To conduct effective high temperature The two main sources of error that occur in •It is often difficult to find an exposed portion measurements, it is important to understand high temperature measurements are due to: of the test piece operating at the desired how increases in temperature affect material •reduction in velocity in the heated measured temperature. properties, the ultrasonic signal and couplant. material and Corrected Measurements. As an alternative to Effects of Heat in Signal and Material •change in zero calibration that occurs when heating the calibration block, method two Properties. Increase in temperature affects the stand–off material becomes heated. compensates for changes in velocity by both velocity of the sound wave and When the velocity of sound is decreased, the correcting measurements after they are taken. attenuation. In high temperature environments, pulse takes longer to return to the transducer This is done by using a device to measure the velocity of sound decreases. As and the measurement appears higher than the temperature and a correction table (Table 1) or temperatures increase within a material, the actual material thickness. The error is greater in by applying a formula that makes a correction molecules of the material begin to bounce thicker material and increases as temperature of one percent per 100 ºF or 55 ºC of rapidly in a random motion and as a result, the increases (about one percent additional temperature change. ultrasound signal or wave of vibration transmits thickness for every 100 ºF or 55 ºC). As the less and less effectively. In carbon steel the stand–off material between transducer and Ta = Tm x [1.007 – (0.0001 × t)] decrease in velocity is about one percent for workpiece heats up, the velocity of sound in the Where: every 100 ºF or 55 ºC. For the same reason, stand–off itself decreases. This changes the zero Ta = corrected thickness of the part attenuation, or loss of energy, increases in high offset of the transducer and results in an Tm = measured thickness of the part temperature materials and it becomes more increase in the apparent thickness reading as t = surface temperature of the part in ºF difficult to obtain a strong recognizable signal. the transducer heats up. The amount of change Properties of Couplant. Couplant properties depends on the transducer and stand–off Correcting measurements after they are taken factor significantly among the difficulties material and is the same for the material being using a correction table or formula is more cost incurred in high temperature environments. It is examined regardless of thickness. As a result, effective than heating the calibration block and couplant that excludes the air gap between transducers must be cooled between readings. equally as accurate. Correction or compensation transducer face and workpiece and permits The problematic changes in sound velocity that methods have been incorporated into the sound to be transmitted into the material under occur in heated materials can be addressed in technology of many new ultrasonic instruments. test. At high temperatures, typical couplant either of two ways. The calibration block can be Accessing Inspection Points. Another practical evaporates. Special couplants have been placed on the heated test piece and allowed to complication to high temperature ultrasonic formulated for use at various ranges of reach the temperature of the test piece before thickness measurement is that most piping and temperature. Liquid viscous couplants are most calibration is made on the heated block or vessels operating at high temperatures are effective at ambient temperatures. At very high measurements can be corrected after they have insulated. Access for ultrasonic measurement is temperatures, couplants that are paste at room been taken to compensate for the effect of through holes cut in the insulation. When temperatures become liquid when applied to a temperature on sound velocity. thickness of the insulation and/or diameter of hot surface.