Cement plants are large scale operations that require large scale machines to produce the materials required to build a wide variety of structures. The non-destructive testing requirements of a cement plant are very unique and required a broad range of NDT testing methods to assess reliably the critical plant infrastructure. Important cement plant machines and components that require non-destructive testing include cement ball mills, pinion gears, kilns and bull gears. NDT methods relevant to cement plants include visual testing (VT), magnetic particle testing (MT), eddy-current testing (ET), ultrasonic thickness testing (UTT), phased array ultrasonic testing (PAUT), and liquid penetrant testing (PT). This article presents some examples of cement plant mill, gear, and kiln non-destructive testing applications.
Description of Cement Plant Machinery
Figure 1: Cement plan ball mill non-destructive testing is performed with a variety of different methods for cracks.
Cement plant machinery and processes are supported by unique and critical infrastructure including ball mills, a variety of gears, kilns, and conveyor components. Ball mills are large very large cylinders made from structural steel that include multiple full penetration girth and seam welds. Over a finishing mill’s lifetime, it is subjected to millions of cycles of fatigue and complex stress loading. Thermal stress cycling is also relevant give the high temperatures at which the mill operates. While the shall of the finishing mill is very thick, fatigue crack do initiate and propagate along the axial and girth weld toes, roots, and subsurface. Cement plant finishing mill NDT uses magnetic particle testing, ultrasonic testing, eddy current testing, and in some cases alternating current field measurement (ACFM) testing.
Figure 2: Cement plan ball mill pinion gear non-destructive testing is performed with a variety of different methods for cracks.
Pinion and bull gears used to drive the mills and other machinery operate at an intense level based on demand. Gear surfaces become embrittled and eventually chip resulting in material loss or form cracks that case gear tooth failure which may require downtime and replacement. Cement mill gear NDT uses magnetic particle testing, eddy current testing, alternating current field measurement (ACFM) testing and in some cases liquid penetrant testing depending on the size of the gear.
Figure 3: Cement plan kiln non-destructive testing is performed with a variety of different methods for cracks and wall thinning.
Cement plan kilns are very large cylinders, sloped slightly, that rotate relatively slowly compared to other machinery. Kilns, like the finishing mills, are also made from a series of axial and circumferential welds. Over time the kiln wall may thin and periodic ultrasonic thickness testing (UTT) is required. Additionally, Cement mill kiln NDT uses magnetic particle testing, ultrasonic testing, eddy current testing, and in some cases alternating current field measurement (ACFM) testing.
Non-destructive Testing of Cement Plant Ball Mills
Due to the extreme temperatures that the mall mills operate at, the mill are usually tested non-destructively during scheduled plant maintenance shutdowns. While cement mill fatigue cracks initiate on both the inside and outside surfaces, NDT is most commonly performed from the outside since it is very difficult to prepare the mill for interior entry. A quick visual inspection can locate very large mill cracks but more sensitive methods like magnetic particle testing (MT) are advisable for smaller tighter cracks. Magnetic particle testing, however, is not always practical since the mill outside diameter is almost always caked with fine dust from the process. Surface cleaning to prepare for proper NDT is time consuming due to the requirement for fall protection, limited access, and repeated rotation of the ball mill. If only a thin layer of cement powder is present on the mill OD, eddy current testing (ET) or alternating current field measurement (ACFM) is a possible alternative to magnetic particle testing. These eddy current techniques search for surface breaking or near surface breaking fatigue crack on the mill. The eddy currents are inserted into the mill using a surface coil and induction. Selection of the right eddy current probe and frequency enables the magnetic field generated by the primary ET probe to penetrate into the mill and generate eddy-currents. In turn, the eddy currents in the test part generate a secondary magnetic field that opposes and is detected by the primary coil. Material defects in the ball cause disturbances in the test part eddy current and magnetic field which are then exhibited by changes in the primary coil measured impedance.
Figure 4: Phased array ultrasonic testing of cement plant ball mill weld. Measurement cursors identify a through wall vertical crack.
Non-destructive Testing of Cement Plant Pinion and Bull Gears
Cement mill pinion and bull gears are high torque and fatigue applications. Over time bending fatigue and/or contact fatigue will initiate surface cracks that may propagate deeper and lead to gear failure. Bending fatigue generally leads to failure in the gear tooth root. Contact fatigue leads to surface pittings can grow to larger craters of significant depth and diameter. The pittings and craters are stress concentrations from which fatigue cracks may grow. The standard visual non-destructive testing methods enhanced with magnetic particle and liquid penetrant testing are adequate for locating most bending and contact fatigue cracks. However, the gear teeth surfaces must be cleaned at a very high quality level which is very time consuming. For this reason, eddy-current (ET) and alternating current field measurement (ACFM) techniques applied in many cases.
Figure 5: Cement plant pinion and bull gear non-destructive testing searched for bending and contact fatigue cracks using magnetic particle, liquid penetrant, ultrasonic testing, and eddy-current techniques.
Non-destructive Testing of Cement Plant Kilns
The steel cylinder containing the refractory liner may thin and crack overtime due to erosion, corrosion, mechanical and thermal stresses. Additionally, there are many welded connections to the outside kiln shell that are common locations for cracking. Cracks commonly occur at retainer blocks to shell welds. The areas are effectively inspected with wet visible magnetic particle testing. In this NDT method, a light coating of white contrast paint is applied to the shell and weld surfaces. A wet black bath then sprayed onto the desired areas during magnetization.
Figure 6: Cement plant kiln shell NDT inspection wet visual magnetic particle testing (WVMT).
Summary – NDT of Cement Plant Mills, Gears, and Kilns
NDT of cement plan machinery requires optimization of one or more non-destructive testing techniques to detect and quantify defect in early stages. Candidate NDT methods include visual testing (VT), magnetic particle testing (MT), eddy-current testing (ET), ultrasonic thickness testing (UTT), phased array ultrasonic testing (PAUT), and liquid penetrant testing (PT). TKS staff are skilled in applying these NDT methods across a spectrum of plant machinery. Additionally, TKS NDT staff are skilled with the requirements for the confined space, lockout – tagout, and fall protection requirements associate with the NDT inspection of the equipment.