The Problem

A major US conglomerate ships petroleum coke throughout the Pacific Rim. Delays in the loading operation created by flow stoppages can disrupt schedules, leading to higher shipping costs. The facility encountered severe plugging problems with the chute system that feeds into their main storage silos. These stoppages occurred in several chutes at conveyor transfer points. The water used in cleanups ended up in the silos, which increased the moisture content of the stored coke and thus created additional clean-up problems downstream.

A tripper car chute in the ship loading system also experienced similar problems. Blockages at this location, where flow rates at times reach 2,000 tons per hour, sometimes halted ship-loading operations for an entire shift.

Jenike & Johanson's Solution

During the investigation into this failure by Jenike & Johanson engineers, calculations showed that the silo was under-designed and did not identify or account for a phenomenon called “thermal ratcheting”. The walls of outdoor metal silos expand during the day and contract at night as the temperature drops. If there is no discharge taking place and the material inside the silo is free flowing, it will settle as the silo expands. However, the material cannot be pushed back up when the silo walls contract, so it resists the contraction, which causes increased tensile stresses in the wall. The effect is repeated each day that the material sits at rest. The investigation also illustrated that the silo supplier did not implement the correct bolts.

Our engineers calculated pressures exerted on the silo walls by the stored bulk solid. We analyzed two different conditions: loads applied after the initial filling of the silo, prior to any material discharge, and loads after material had started to discharge (flow loads).

The Result

The petroleum coke handling system has been in operation for over three years with minimal downtime for maintenance. According to the plant operations manager, "Maintenance costs have been reduced and handling improved significantly by installing the new system.

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