Ultrasonic minimum thickness

The minimum wall thickness found with a manual ultrasonic wall thickness meter was 4.0 mm ( 2.3 mm wall thickness loss). This is in good accordance to the estimated wall thickness loss by radiography, which shows for small spots even a higher loss up to 2.8 mm. The expected accuracy for these measurements is about 0.2 mm. 

In these tests it is to be determined if iron inclusions with a mass greater than a minimum value are present and, if yes, at what location within the disk. The large thickness of the disk is a special challenge. While eddy current and ultrasonic test generally fail we operate as follows. 

Off-bottom suspension defines a condition in which individual particles never come to rest for longer than a second at the bottom of the vessel. Zwietering was the first to work with this visual suspension criterion. He determined minimum shaft speeds Wjs (js = just suspended) at which this condition first occurs. The off-bottom suspension point can be determined by methods such as the ultrasonic Doppler anemometer, which do not, however, take into account the uniformity of distribution in the vessel. This means that particularly with partides with high settling velocities, even when the criteria on-bottom motion and off-bottom suspension are fulfilled, there are often clear zones near to the liquid surface containing no particles. In these cases, a further assessment process is employed, the so-called layer thickness criterion, at which the layer of particles in suspension has reached 90% of the height of the liquid. 

Some cylinders have thicker walls due to different manufacturer s designs. Values shown are for absolute minimums allowed by specifications. Higher values may be used if information showing thicker walls than those listed is obtained from the manufacturer of the cylinder. Use of ultrasonic testing is suggested for determining actual wall thickness. 

The boilers were periodically inspected visually and by non-destructive tests (such as dye penetration, magnetic particle or ultrasonics), and any cracks found were ground out. Eventually, however, the minimum wall thickness was reached. At that point, weld repairs were done to build the wall thickness backup again. The repairs were done by Lloyd Werft in Bremerhaven, Germany, between 1987 and 1990. The weld repair procedure was written by Deutsche Babcock and reviewed and approved by Bureau Veritas. 

Molding of CSM requires some special care in the type of metal due to the acidic nature of CSM even with the presence of an acid acceptor. A hard surface (minimum 45 Rockwell C), abrasion and corrosion resistant and non-porous steel needs to be employed. A hard chrome plate at least 0.1 mm thick is recommended. Stainless steel is a more expensive but more effective alternative. Lower mold temperatures reduce mold fouling and corrosion if economics will allow. Molds need to be cleaned at least weekly with an alkali solution with ultrasonic vibration. 

---