Metals Processing

Process Gas Compressor Flow Control – For Hot Briquetted Iron

Metals Processing, Mining and Metals /

Success Story: Process Gas Compressor Flow Control – For Hot Briquetted Iron In 2020, the largest North American producer of flat-rolled steel completed a new direct reduction plant. The plant produces Hot Briquetted Iron (HBI), a versatile iron source needed to produce steel in electric arc furnaces, basic oxygen furnaces, and blast furnaces. The plant has a closed loop gas circuit that dedusts, compresses, and reheats the gas. Natural gas is also added to the loop. To achieve the highest quality HBI product, optimum process control of product temperature and furnaceatmosphere is required.  In May 2021, the direct reduction plant’s HBI pellets were transferred to their sister company’s blast furnace. In the blast furnace, the HBI blends with coke and iron ore additions. Iron ore is rich in iron oxides, which are reduced by carbon monoxide to form elemental iron and carbon dioxide. Not only are carbon dioxide emissions reduced using HBI on blast furnaces, but also the blast furnaces run more efficiently. Four months into the HBI pellet production, the plant’s operations group had grown increasingly frustrated with their pneumatic actuators. The actuators’ were doing an inadequate job of positioning the compressor Inlet Guide Vanes (IGVs).  Accurate positioning of each compressor on the IGVs is necessary to precisely control the gas throughout, with two compressors in series. Doing this effectively ensures optimal gas flow through the compressor. The compressor IGVs used a pneumatic rotary actuator. During start-up, the pneumatic actuators had one-second delays before moving to position. The slow and inaccurate positioning led to terrible vibration, causing the compressor to shut down. In some cases, up to six manual attempts were tried before the process could restart.  In May 2022, both compressor IGV’s were upgraded with a REXA Electraulic™ Linear Actuator. REXA supported the upgrade by providing a site survey, engineering drawings, and onsite start up commissioning. Plant personnel immediately noticed improved repeatable accuracy to 0.1% of span, regardless of process conditions. The REXA actuators have the capability to open from 0-20%, in under two seconds, without overshoot. For this reason, the plant can now safely start up the compressors without vibration. The process gas stability has also improved, leading to better control of the HBI product chemistry. By utilizing REXA actuators, the steady state process gas flow oscillation was reduced from +/-2,000 m3/hour to +/- 50 m3/hour. Other benefits of improved process control are the reduction in natural gas consumption and less modulating of the 24 downstream control valves. In turn, the control valves’ maintenance intervals were extended. Producing and using HBI contributes to the steel company’s greenhouse gas reduction initiative. REXA actuators contributed to improving the HBI plant operations with improving process gas stability.  Literature Click download to further explore the ROI this plant had in this Success Story!  Download

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Nickel Mine Oxygen Compressor Inlet Guide Vanes

Metals Processing /

Success Story: Nickel Mine Oxygen Compressor Inlet Guide Vanes A large nickel mine in Ontario, Canada produces nickel metal, which is used in the manufacturing of smartphones and rechargeable batteries that power electric cars. Sulfur dioxide (SO2) is a major air pollutant emitted in the roasting, smelting, and converting of sulfide ores. Fortunately, the SO2 is recovered and utilized to make sulfuric acid (H2SO4), an important coproduct that contributes to the mine’s operational revenue. This high-grade sulfuric acid is used to make recyclable fertilizers and paper products.  Oxygen gas is an important precursor compound required in the expanded process to make sulfuric acid. The gas is generated on site and supplied to the nickel smelter where it is used in flash furnaces that produce iron oxides, sulfur dioxide (SO2), and nickel matte. The sulfur dioxide produced by flash smelting is used to make sulfuric acid,removing the major environmental effect of smelting. Pressurized oxygen must be available to the nickel smelter, or the acid plant will need to halt operations – potentially costing up to $60,000USD per day in revenue. This very scenario worried the nickel mine’s acid plant operations management team since a legacy oxygen plant was beingdecommissioned. This meant there was no longer a backup supply.  The plant has two multistage centrifugal compressors, each powered by a large electric motor. Accurate positioning of the compressor inlet guide vanes (IGVs) is necessary to precisely control the gas throughout the compressor. Doing this effectively ensures the most efficient gas flow through the compressor. The plant operations group grew increasingly frustrated with the pneumatic actuators’ poor performance in positioning the IGVs. The mechanical position switches would drift—requiring recalibration every 6–8 weeks. Due to the compressibility of air, the compressor turndown increased to safely operate without surging, at the cost of compressor efficiency. In 2016, both compressor IGVs were upgraded with six REXA Electraulic™ Linear Actuators. Plant personnel immediately noticed improved accuracy—repeatable to 0.1% of span, regardless of process conditions.  The plant also noticed significant energy savings, since the Electraulic™ Actuators accurately controlled at a higher maximum capacity flow within turndown range. Each compressor had a 0.5 MW power reduction due to the improved air flow and higher compressor efficiency. The annual savings in electricity are calculated to be $1,092,000 USD per year. Literature Read the full Success Story by downloading here!  Download

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Steel Making Reheat Furnace Pressure

Metals Processing /

Success Story: Steel Making Reheat Furnace Pressure Steel companies around the world continuously introduce and enhance processes that emphasize sustainable development. Key sustainable targets include minimizing the environmental footprint and increasing energy efficiency. These companies proactively develop actions to decrease fossil fuel consumption, thus reducinggreenhouse gas emissions. Through all these initiatives, companies proves it is possible to be socially responsible while reducing operational cost to produce quality steel. Modern continuous rolling mills produce large quantities of thin sheet metal but consume significant amounts of energy—particularly with reheat furnaces. These furnaces are gas fuel energy intensive, second only to blast furnaces.  The reheat furnace is located at the hot strip mill and is the process step before hot rolling. In this step, steel billets, plates, or blocks are usually heated from room temperature to ~1,200 C°. At this temperature, the billet can be hot rolled and achieve the desired metallurgical, mechanical and dimensional properties of hot rolled products. Optimal operations require the minimization of fuel consumption, while maintaining a controlled steel billet thermal soak.  From economic, production, and environmental standpoints, the operation of reheating furnaces is of great importance to the steel making process. Economically, the consumption of fuel needed for reheating can represent up to 15% of the operational cost of a rolling process. With respect to productivity, a reheat furnace capacity often dictates the production rate for the rollers, which means that reheating is usually the bottleneck in achieving the maximum production volume. The furnace operation must be reliable, as any downtime will cost the average sized mini mill nearly $45,000 per hour in lost product revenue.  Maintaining a controlled ramp up and holding soak temperature is not a straightforward task. Heat transfer to the steel billets is influenced by internal temperature setpoint of the furnace, flow rate of fuels and flow rate of air. Maintaining furnace pressure is very important. If the furnace pressure is too low, air will ingress in the furnace with oxygen, potentially causing scale formation, impacting quality and productivity. If the furnace operates too high, then the fuel consumption will increase operational cost.  Literature Download the full Success Story here!  Download

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Furnace Fuel Gas Pressure Control

Metals Processing /

Furnace Fuel Gas Pressure Control Producing steel is an energy intensive process. Utilizing byproduct gases as a fuel source is vital to the plant’s operational efficiency. These byproduct gases, which are recovered from blast furnaces and coke ovens, are commonly used as a mixed gas fuel source throughout the plant. Reheat furnaces can use mixed gas as the primary fuel source. Mixed gas feed lines can have variable incoming pressure and constantly changing gas demand due to heating cycles and exit door losses. This creates challenges in regulating air/fuel mixture and temperature within the reheat furnace. Reheat furnace operation directly impacts steel properties. Austenite grain structure, which is sensitive to time and temperature, directly effects the strength and mechanical properties of steel.  Minimal variation of incoming mixed gas pressure feeding the reheat furnace is critical to producing high quality steel. Accurate and responsive mixed gas pressure control enables efficient combustion for a controllable temperature profile within the furnace. Efficient combustion will reduce SOx emissions and gas consumption. This leads to stable slab heat up and soak, resulting in a more efficient furnace producing a higher quality finished product with fewer slab defects.  Literature Download the Furnace Fuel Gas Pressure Control Application Spotlight! Download

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Coke Oven Cross Over Valve Pressure Control

Metals Processing /

Coke Oven Cross Over Valve Pressure Control Coke ovens process coal into coke that is used as a fuel source in blast furnaces. Coke ovens run continuously and do not use air as a combustion source. The co-product gas exiting the coke oven is tightly pressure controlled to +/- 1 millibar. Excess pressure may cause the gas to be diverted from the co-product plant to atmosphere. Insufficient pressure may cause air to enter the oven, ruining the coke.  REXA  Linear  and  Rotary  Actuator’s 100% duty cycle capability is used to precisely control the coke oven collection line gas pressure.  Benefits: Precise Modulating Control  Reliability  90 degree normal, 355 degree maintenance rotation  Literature Download the Coke Oven Collection Control Application Spotlight! Webinar Download

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Coke Plant Quenching Flow Control

Metals Processing /

Coke Plant Quenching Flow Control The coking process takes place over a span of 12-36 hours in a coke oven. Once pushed out of the vessel, the hot coke is often quenched with water to cool it before storage.  After the coke has been cooled, it is transferred to the blast furnace for use in iron making. REXA  Linear  and  Rotary  Actuators are used to effectively deliver quenching water to cool the coke by controlling the header pressure. Benefits: Precise modulating control  Reliability 

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BF Steam Turbine RPM Control

Metals Processing /

BF Steam Turbine RPM Control What is a Blast Furnace? The mining industry uses blast furnaces for smelting to produce industrial metals, such as pig iron. It is top-charged with iron ore, coke and limestone while huge quantities of air blast enter the bottom of the furnace from the blower. Oxygen within this air first reacts with the coke to produce carbon monoxide, which then reacts with the iron ore in a reduction reaction. Molten slag and iron leave the bottom of the furnace, while carbon monoxide, carbon dioxide and nitrogen leave the top. This is a continuous process, which if interrupted by unscheduled downtime, leads to increased costs.  Steam Turbine Control Solution Steam turbine speed control is a critical application requiring superior reliability. REXA Electraulic™ Actuators enable a smooth and reliable supply of cold blast “wind” flow to your blast furnaces.  The consequences of an actuator fail are devastating to plant iron making operations.  Loss of “wind” causes the molten burden in the furnace to drop, therefore resulting in damage to the furnace and an interruption of production.  Our self-contained Electraulic™ actuators require zero oil maintenance, making them virtually maintenance-free.  Benefits: Reliable Operation in Tough and Demanding Environments  Accurate (0.1%) and Repeatable Positioning  Smart Communications for Reviewing Actuator Health/History  The BF Turbo Blower has many applications that REXA can provide an immediate solution for including:  Inlet Guide Vane Control  Anti-choke Control  Blower Anti-Surge Control 

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Blast Furnace Gas Pressure Control

Metals Processing /

Blast Furnace Gas Pressure Control Background Steel making is an energy-intensive process. At a tier one integrated steel mill in Jamshedpur, India, the mill energy consumption required for steel production meets with power generation produced at the power houses on site. As their primary fuel sources, this steel mill uses by-product gases from blast furnaces and coke ovens.  What is Blast Furnace Gas? Blast Furnace Gas (BFG) is the key fuel source used at the combustion boiler to generate steam. This steam is used in various plant processes, as well as the steam generator, to produce electrical power. There are three power houses in the plant, each with five to seven boilers. BFG feed has two issues: a variable incoming flow rate and a low, fluctuating calorific energy constant.  Problem The key in producing high quality steam is to have minimal process variation of incoming BFG entering the boiler. Furthermore, greater process control enables efficient combustion and boiler product steam pressure – leading to stable-pressured steam.  Unfortunately, even with the use of smart positioners, pneumatic actuators lack precision due to the compressibility of air. REXA Electraulic™ Actuators, however, are able to work through this and provide more precise and accurate performance than pneumatic and traditional hydraulic technologies.  Solution Check out the full Blast Furnace Gas Pressure Control Application Spotlight below! Be sure to also view our webinar showcasing how our actuation technology can enhance this application’s efficiency.  Literature Download our Steel Mill Blast Furnace Gas Pressure Control Application Spotlight! Download

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Gas Pressure Control – Baghouse

Metals Processing /

Gas Pressure Control – Baghouse A baghouse is an air pollution control device commonly used in steel mills. Baghouses remove particulates out of hot gases from various steelmaking processes.  The use of air dampers controls the gas pressure to optimize particulate recovery and process availability. Rexa actuators are used to precisely maintain vacuum pressure; even a slight positive pressure inside the bags can result in reduced cleaning efficiency.  Benefits: Precise modulating control  Reliability 

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Compressor Anti-Surge

Downstream, Metals Processing /

Compressor Anti-Surge Control Background Axial and centrifugal compressors are expensive, critical pieces of equipment that are found at the heart of many industrial processes and across a multitude of industry segments. Implementing the correct, high performance compressor control strategy has direct impact upon the control of the process and the profitability of the plant. It is important to have control that is both responsive and stable to improve yield and ensure compressor availability is maximized. One of the main applications to ensure maximizing availability and throughput is Compressor Anti-Surge Control. Process dynamics that are common during start-up and shutdown sequences, as well as when a compressor is operated at a reduced throughput, can cause compressor surge events. The surge phenomenon is created when a flow reversal through the compressor occurs due to it’s inability to overcome the pressure stored in the discharge volume. An Anti-Surge Valve (ASV) is utilized to protect the compressor by safely recycling gas from the discharge side to the suction side in order to keep the compressor operating point safely away from the surge limit. The Anti-Surge Valve requires an actuator capable of immediate response to signal change (minimal deadtime), rapid stroking speed, and precise modulating control. In a surge scenario, the ASV is designed to rapidly open in order to protect the compressor, typically under 2 seconds, and occasionally as fast as 500msec. This action moves the operating point away from the surge limit line and allows the compressor to resume normal operation. Accurate and precise control, with minimal deadtime for steps of all sizes, is critical to keep the compressor out of surge in the first place, but equally as important in stabilizing this extremely fast process. Valve positioning hysteresis and overshoot can create and accelerate upset conditions within the process. Problem Anti-Surge Control Valve suppliers offer actuation packages as part of the valve assembly. These valve assembly packages are commonly offered with the supplier’s brand of pneumatic positioner (smart or traditional). Great efforts have been made to engineer the specifications of the application to meet the best performance that can be achieved with pneumatic actuation. What is constant across all pneumatic actuation is the effect of Boyle’s Law. Boyle’s Law explains the inverse relationship of pressure and volume with respect to a fixed amount of gas. This relationship states that P₁V₁ = P₂V₂. This principle explains the effect of jump and overshoot (hysteresis) inherent to all pneumatic actuation. Every time a pneumatically operated valve is required to move it needs to overcome the force of static friction (P₁)(area) to initiate movement. Once it overcomes static friction the dynamic friction force is greatly reduced (P₂)(area). Plugging into a Boyle’s Law formula, the drastic reduction in pressure requires a drastic increase in volume to equalize the equation. That increase in volume is directly translated to overshoot in a pneumatic system. The goal of a smart positioner, regardless of the manufacturer, is to limit the amount of jump/overshoot as pneumatic response can be predicted. The trade off with “controlling” this effect is a greatly slowed response, which is artificially adding deadtime into the loop. On small step changes (less than 0.5%) the impact can be many seconds, in some cases greater than 10 seconds. In this application where the impact on compressor efficiency and throughput can be so easily affected by process disturbances, pneumatic actuation on compressor control can have a refinery FCC cost of downtime over $1,500,000 per day in gasoline production alone. Solution In modern compressor control theory, a Surge Control Line (SCL) is developed to protect the compressor, so a required safety margin is necessary to prevent approaching the Surge Limit Line (SLL). See the graph below. The SLL is the minimal flow point before the compressor becomes highly unstable. Crossing above the SLL causes the compressor to enter a surge condition. The SCL is typically established at around a 10% flow margin of safety to the right of the SLL but is dependent upon both the process dynamics of the system surrounding the compressor, and system’s response at different loads (ASV speed of response). The performance of critical valves, like the ASV, is a significant determining factor on loop performance. Some processes require a large margin of safety between the SCL and SLL to prevent the compressor from going into frequent surge cycles. As the required margin of safety increases, the envelope of the compressor map decreases in size resulting in lower operational efficiency of the compressor. A less efficient compressor is not desirable but is better than a compressor that is frequently in surge. There are two critical requirements of an ASV that the Anti-Surge Control application needs to meet to ensure the compressor control is optimized. The first requirement is to open the ASV in a stable and precise manner. The compressor controller uses suction pressure, discharge pressure, flow, and temperature inputs while utilizing Proportional Integral Derivative (PID) controller close loop response to cope with typical process changes. The ASV receives command from the controller and begins to open before the unit reaches a surge condition. Minimal deadtime, precise control, and minimal overshoot from the ASV are critical to loop performance, particularly on small operating point changes. The second requirement of the ASV is to be able to open rapidly when a rapid surge approach does occur due to a major process upset. To meet the critical requirements of Compressor Control, REXA actuators, based on the company’s proprietary Electraulic™ Technology, are the perfect choice. The self-contained actuators combine the simplicity of electric operation, the power of hydraulics, the reliability of solid state electronics, and the flexibility of user-configured control. Electraulic™ Technology is comprised of an actuator assembly and electrical control enclosure. The principle behind its technology is a unique hydraulic circuitry called the Flow Match Valve (FMV) system. The actuator incorporates a bi-directional gear pump coupled to either a stepper or a servo motor that provides a highly-efficient method of pumping hydraulic fluid from one side of a double-acting

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