SLAG GRINDER – ENGINEERING & DESIGN

Crushers and Slag grinders are often applied in Materials Handling and Gasification plants to reduce the size of quenched slag from the gasifier’s tap-hole or break clinkers from slow moving bed reactors. These clinkers are typically brittle and relatively easy to crush, but may be larger and very hard and abrasive when sintered during a reactor transient or upset conditions. Also, these crushers are often of the toothed roller type, integrated in a pressure vessel subject to charge/discharge cycles, slow rotating yet transmitting very high torques and crushing forces. This makes it simultaneously a ‘machine’ and a ‘pressure vessel’.

A slag breaker dating from the 1970’s required replacement. The new design had to incorporate lessons learned in operations and maintenance, fit accurately into the existing structure and inlet-outlet flange face-to-face envelope, and re-use the existing drivetrain. The crusher must reduce slag as well as occasional refractory tiles or bricks, dislodged from the gasifier hot face, to no more than 1” in size to avoid bridging and arching in the outlet. The equipment must be designed, manufactured, tested and delivered to site under schedule constraints, in time for the next scheduled shutdown.

KnightHawk Engineering digitized hand drawn blue-prints and developed 3D CAD models (below) applied in concept development, client reviews and manufacturer reviews and design. Detailed 2D CAD drawings and part lists as well as geometry and meshing for Finite Element (FEA) designs, were developed from this model. The pressure vessel was designed as per ASME VIII Division 1 with supplementary analysis by FEA as per ASME/API 579-1 Fitness for Service (FFS) by using the methods and guidelines from ASME VIII Division 2 Part 5. The rotating parts and breaker element were integrated into the pressure vessel by shaft, breaker tooth, breaker plates, gland seals, end bearings and a bolt-on cantilever drive unit (not shown) consisting of a motor, gearbox and coupling. The breaker element is protected from overpower, including an auto reverse, circuit breaker, shear pin and hydraulic coupling.

The breaker element and teeth must be accurately sized and designed for particle size reduction, throughput and slag crush strength requirements, to ensure effective crushing yet avoid jammed rotor and lockup conditions. KHE determined the required grinding energy by the 3rd comminution theory as per Bond’s equation using a ‘Work Index’ to define motor horsepower, torque and crushing power requirements as per its experience for this type of equipment.

The pressure vessel design must include for considerations of (i) internal pressure-temperature design, (ii) fatigue loading from batched feed and pressure cycles, (iii) bending and crushing forces from drivetrain weight and work, (iv) thermal loading from batch cycles, and (v) modal analysis to ensure no coupling between any fundamental natural frequency, fN, and any of the forcing functions or excitations from the rotating parts result in resonant vibrations.

Materials selection, stainless steel and tungsten carbide weld overlay of internal surfaces subject to corrosion, impact, crushing and flow-erosion, must be carefully selected to provide a robust yet manufacturable and cost effective equipment fit for service over its design life.