Process Flow Analysis Resolves Filter Changeout Issues at Steel Plant

A company helped a major steel product manufacturer to rectify downtime issues caused by frequent filter changeouts at its steel rolling mill. Unplanned downtime can result in reduced margins and loss of business. Therefore, finding the root cause of the problem was a priority; preferably without having to take the mill offline during the investigation.

By Chris Nixon, Global Product Director, Cleanova

Using a unique on-site Gas Filtration Test skid, the company was able to conduct a detailed analysis of the solid and liquid contaminants in the gas stream without interruption to normal operations. The tests identified the causes of poor performance and premature equipment failure in the upstream and downstream filtration systems. These were directly associated with the exhaust gas reprocessing system. The test results were then used to recommend ways to optimize the filtration system and improve operational efficiency.

Steel Production

The customer makes steel wires, rolls, and sections of different shapes, sizes, and specifications. Production involves the use of process gas to reduce iron ores into iron pellets for introduction into an arc furnace. An important byproduct of the reduction process is the exhaust process gas from the direct reduction plant reactor (DRP) reactor. The gas passes through different rehabilitation units to remove heat, solid, and liquid contaminants before being introduced to a gas compressor, which then recycles the gas to the reactor at the correct pressure.

As a final part of this gas conditioning process, the DRP has been equipped with filters intended to retain solid contaminants down to 50μm. However, the customer was experiencing higher changeout frequencies than expected on its existing filtration system. An increase in filter demand may indicate that higher levels of contamination are present, which in turn can lead to prolonged operational disruption. If ignored, higher contamination levels may lead to equipment failure and more prolonged outages. A concentration of solid and liquid contaminants in the recycled gas stream can severely damage the downstream compressor and lead to off-spec gas and rapid deposition on the compressor’s internals, resulting in frequent emergency shutdowns.

Recognizing the risks, the customer called in the company to monitor the size and quantity of solid and liquid contaminants in its process gas recycling system so an optimal filtration solution could be proposed and evaluated.

Contamination Analysis

The company mobilized the Gas Filtration Test Skid at the customer’s site to perform the analysis. The Test Skid is a recent addition and enables customers to gain a better understanding of their liquid process and filtration system requirements without interrupting production.

The Test Skid samples gas flows under real operating conditions. This allows the actual solid and liquid contaminants in gas flows to be detected and estimated with far greater precision. Delivering results based on real-time data gives customers the confidence to make more informed process and operating decisions. It also makes clear the business case for either implementing a new filtration solution or upgrading their existing system.

The tests at the steel rolling mill were conducted over five days. The specific aim of this testing program was to determine the quantity of solid contaminants, verify the quality of the contaminant as particle size distribution, and identify the volume of liquid contaminants in the gas stream.

The samples were examined at the company’s laboratory and at local laboratories to determine the actual process gas conditions. The analysis revealed that the steel mill’s existing filters were providing an initial removal efficiency of 93-95% at 50μm. However, there were significant volumes of particulates in the range of 2μm-40μm present in the flow, which the current filtration system had not been designed to cope with. These solid contaminants were found to be primarily iron or rust particles, together with a percentage of carbon compounds.

The filtration team calculated that a potential total load of almost 5kg per day of solid particulate (excluding particulate mixed with separate water) in the 10-50μm range was currently passing downstream of the existing filter. Solids loading across the entire system averaged between 14-15kg per day.

Liquid contaminants in the feed were both high and variable in volume, ranging from 7.5ml/m3 to 25.57ml/m3. Among other factors, this liquid contamination may be due to variations in the volume and timing of quench water added to the process, and/or the wash-down regime of the separator vessel. In conjunction with the finer solid particles, these high residual liquid concentrations were likely to form a slurry on surfaces downstream. This slurry can adversely affect the filter element structure, process pipework, valves, and the internal compressor surface and blades. Such high levels of liquid contamination are also likely to result in significant quantities of condensed liquid forming in the process gas filter outlet, which collects at the lowest system point.

Tests highlighted that the flow permeability of the existing 50μm filter was reduced by 20-25% due to the liquid saturation of the media. The wetting of the filter media occurs very quickly, so the differential pressure starting point is higher than that of a dry gas application.

Outcomes

The detailed findings of the gas analysis were delivered in a preliminary technical report to the customer. Based on the results, the company recommended a reconfiguration of the existing filter arrangement to 98% at 100μm. This will reduce the element differential pressure significantly and improve the operating lifetime of the gas compressor. They also recommended the addition of a finer filtration system downstream to achieve the customer’s optimum requirement of 5-10μm, while still improving the operational lifetime.

Functional removal and handling of the liquid concentrations were also proposed, consisting of a combination of modifications to the existing pipework, incorporation of a drain trap, and the addition of a dedicated liquid separator. Any additional filtration equipment will need to be configured to effectively separate and isolate the liquid content to prevent potential re-entrainment and possible flow downstream to the strainer/compressor installation.

Based on these initial recommendations, the customer will be able to determine the acceptable downstream cleanliness requirement for their gas needs, in terms of allowable particulate sizes/volumes, as well as allowable liquid levels so that a final filtration solution can be developed.

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Shopia Ketheeswararajah is a feature editor contributing to Pump Engineer, Stainless steel World Americas, Hose and Coupling World, and other related print & online media.