Greg Palmer
Palmer Technologies Pty Ltd, Australia
Introduction
The non-destructive assessment of refractory lined vessels in industrial plants is an important tool in monitoring structural condition and safety. At present the assessment of refractory linings in industrial vessels is generally undertaken visually or audibly by tapping the hot-face with a steel hammer. The ring tone of the hot-face is generally indicative of a crack in the refractory hot-face. A serious drawback with these techniques is that they cannot quantify the extent of damage to the refractory.
Damage to refractory linings can occur in a number of ways. Initially refractory concrete layers are reasonably well bonded together. However, in the green state slump can occur if the setting time or water content is too high, conversely too dry a mix can result in poor cohesion. This can result in delaminations in the concrete layer particularly overhead (roofs). During the first heating cycle the concrete materials will shrink due to dehydration and solid state chemical reactions. The amount of shrinkage and the temperature at which this occurs is highly temperature dependent. The shrinkage of the material will result in cracking of the concrete which will reduce the material’s stiffness.
Damage to a refractory lining then occurs over time due to corrosion, spalling and failure of refractory anchors. The types of damage that can occur are, missing sections of hot-face, a hot-face that has moved away from the insulation layer with a void present (from a few mm’s to tens of mm’s), voids immediately below the steel shell, delaminations in the insulation layer, large sections of poorly consolidated concrete (honeycomb) and severely cracked concrete layers.
A non-destructive testing technique where the refractory integrity can be assessed from outside the vessel would be an obvious advantage for industry.
The patent pending P-response® technique has been developed to address the limitations of other techniques, eg impact echo or ground penetrating radar, (GPR). The impact echo [1] technique, an acoustic method for nondestructive evaluation of concrete and masonry was invented at the U.S. National Bureau of Standards (NBS) in the mid-1980's, and developed at Cornell University, in the last 1980’s. The Impact-echo technique is a method for NDT assessment of concrete and masonry structures. The problem for refractory linings with the impact echo technique is two fold, firstly transmitting the stress wave through lightweight insulating materials and secondly, transmitting the stress wave across small interface gaps that may exist between the different concrete layers (eg hot-face, insulation and steel shell). The GPR technique has a different set of problems in that it can only be used from inside the vessel and it cannot detect refractory anchors which are in the near vertical direction (normal to the surface). Other techniques, like time of flight, Varghese et al [2] has shown great promise for particular ceramics but as with the impact echo technique small delaminations can result in back-wall signal scatter leading to incorrect interpretations.
These problems make the use of such NDT techniques for in-field testing of refractory linings difficult or impractical.
Modal testing and analysis, which is used extensively in the automobile and aerospace industry, has now been extended to structural health monitoring and damage identification of civil structures. The impact response technique is slowly gaining acceptance for assessment of concrete structures. There are various references [3, 4, 5, 6, 7] which deal with the integrity assessment of bridge structures, fly-ash silos, stacks, cooling towers.
A similar technique has been developed by Li, Samali and Crews [11, 12, 13, 14] for the cost effective integrity assessment of bridge structures through a series of R/D projects. In this technique the structure is assessed by using an array of sensors and impacting in the centre of the bridge. The technique is obviously faster than point to point testing but the same results can be achieved.
This paper shows that the patent pending P-Response® technique can be used to test refractory concrete and/or brick linings in process vessels to determine refractory lining condition from outside or inside the vessel. This is particularly important for industries such as the cement, petroleum, petro-chemical, minerals and alumina industries.
