Gillian Chew

Success Story: Collection Main Predictive Maintenance Coke oven main pressure control is a critical control application for the safe operation of coke making. A plant’s actuator not only must position the cross-over valve to achieve a pressure of 12+/-1 mmWC, but it must avoid pressure spikes from volatile gases soon after coal charging, as shown in Figure 1. If the actuator is slow to respond,the collection main and coke oven gas pressure will increase and leak toxic gas at the oven doors. If the actuator overshoots, the coke oven pressure can decrease to a vacuum, entering oxygen in air which ignites in the over. Both pressure scenarios present major safety concerns.  During normal operation, the cross-over valve becomes coated in tar originating from the coke over gases. Tar buildup increases the required torque to turn the valve. Left unchecked, this condition can potentially damage the valve, causing unnecessary downtime to repair. Regular cleaning ensures proper valve operation. Therefore, plant operators and technicians prefer to optimize the cleaning schedule.  The preventative maintenance schedule for cleaning the cross-over at a major integrated mill in Canada was based on fixed time intervals without any data related to current condition. The mill recently retrofitted the existing valve with a REXA Rotary Electraulic™ actuator, including the diagnostic capability to monitor operating pressure. The previous actuator’s oil reservoir was open to the environment, requiring frequent scheduled oil changes. The actuator also used a mechanical linkage to actuate the valve stem. These linkage assemblies introduce hysteresis, which can worsen over time and ultimately lead to poor control. REXA consulted plant personnel and developed a solution which eliminated the need for mechanical linkage assemblies by mounting directly to the valve stem. With accuracy to 0.1% of desired position, less than 70ms deadtime, and zero overshoot, the collection main pressure control began operating better than ever before. Theself-contained, positive pressure, non-vented Electraulic system never communicates with atmosphere, improving reliability and eliminating routine fluid maintenance. Our Bluetooth graphical user interface tool displays the operating pressure and records it over time. User-configurable warning and alarm limits let our customer know when a pressure threshold is reached — signaling a need to initiate the valve preventative maintenance cleaning procedure. This ensures the maintenance group canschedule the cross-over valve cleaning at an interval based on the valve’s health instead of an arbitrary time. Cleaning the valve only when necessary enabled a reduction in cleaning frequency, increasing uptime and personnel safety. Unscheduled shutdowns due to tar build-up on the cross-over valve were eliminated.  Literature Download the full Success Story here!  Download

Success Story: Hot Blast Temperature Control Steel production is vital to many industries around the world. Some industries include general construction, machinery manufacturing, automotive, marine, and transportation. The world needs large volumes of high quality steel to sustain the high demand. This is especially true during times of economic growth. The blast furnace is a key component of many integrated steel mills. A chemical reduction process occurs within the blast furnace. Carbon dioxide converts iron oxides into “pig iron,” an elemental iron with carbon and sulfur containing impurities. Pig iron undergoes additional processing to make a variety of commercial steel products. In Indonesia, a tier one steel producer uses REXA actuators in multiple applications, for the blast furnace. One application is positioning the cold blast mixing valve. This valve’s purpose is to blend cold blast air with hot blast main air, so the feed air temperature is constant. Constant air volume and temperature enables the reactions in the blast furnace to occur in acontrolled manner. The outcome is a reliable blast furnace production of pig iron. Two or three stoves are used to preheat the air to around 1200 °C (2190 °F). The stoves are cylindrical steel structures lined with insulation and filled with checker brick. This is where the heat is stored and then transferred to the cold blast air. Air blowers feed 200 °C (392 °F) air to the ovens. The ovens cycle between preheating the air and being in reheat mode. Air entering a preheated oven exits at higher temperatures that dissipate over time until the next heated oven is utilized. The actuator needs to position the mixing valve, without delay, so the mixed air has a constant temperature.  For seven years, the iron making section used pneumatic actuators to operate their mixing valves. These actuators were part of the original plant equipment scope. While pneumatic actuators are inexpensive and can control 1-2% of span, they have limitations in accuracy and reliability. The pneumatic actuators are associated with many issues including piston sticking, poor accuracy, and slow response time. The operations group upgraded to REXA actuators and are pleased with the results. REXA Electraulic™ Actuation delivers the position accuracy and reliability necessary to keep the blast furnace operating and producing high quality pig iron.  The REXA actuator and control panel, shown in Figure 3, are installed on a cold blast mixing valve, and is mounted on a36-inch butterfly valve that is used to add cold blast air to the hot blast main mixer. The actuator is capable of operating with 70ms deadtime and <0.15 positioning accuracy. This allows for reliable mixing valve positioning and ensures consistent blast air temperature. The Figure 4 graph shows the difference in gas flow with use of a pneumatic actuator versus a REXA actuator. The REXA has an advantage of fast and accurate valve movement without overshooting the control signal setpoint.The REXA actuator’s performance undeniably impressed the plant operation team to the point they decided to upgrade their blast furnace top gas damper actuators to REXA, and also plan to retrofit other actuators to improve overall operation! Literature Download the full Success Story to learn more!  Download

Success Story: Steam Turbine Actuator Retrofit Hydroprocessing in the O&G refining industry refers to two separate, yet similar, processes: hydrocracking and hydrotreating. The hydrocracking process converts a variety of petroleum fractions, including oil recovered from solvent extraction of petroleum residues, into more valuable, lower-boiling fuel products. Hydrotreating refers to the process of upgrading the quality of petroleum distillate fractions, such as catalytic and thermally cracked distillates, by decomposing the contaminants with a negligible effect on the boiling range of the feed. Hydroprocessing units include applications such as charge pumps and recycle compressors, driven by steam turbines. Steam turbines convert thermal energy to rotational energy-ultimately controlling the input speed or power of the driven device. Maintaining reliable and repeatable control of the turbine is essential for refineries to achieve efficiency goals and eliminate costly downtime for maintenance. This is possible by identifying and implementing the right actuation technology. REXA recently conducted a steam turbine actuator retrofit on a Nuovo Pignone steam turbine driving a hydrogen recycle compressor at a refinery in Mexico. The OEM actuator was the original Nuovo Pignone system, which consisted of a pilot valve and power piston. This actuator required an external oil supply from the turbine oil skid(pump). One of the main issues with this actuator type is its susceptibility to oil contamination and varnishing, which causes internal stiction. The actuator exhibited poor control and “hunted” for position, ultimately leading to large and unwanted RPM swings, as well as costly nuisance trips.  Refinery personnel met with REXA to develop the right solution. Following review of all the issues, REXA proposed the “Rotating Equipment Total Integrated Solution” package. This all-inclusive upgrade solution includes onsite evaluation, engineered actuator mounting hardware, and installation drawings, TAR supervision and support, both mechanical and electrical integration/calibration, as well as on-site REXA product training. By utilizing the self-contained design of Electraulic™ Actuation, the need for the external oil supply to the OEM actuator was eliminated. All segments of the REXA hydraulic circuit are positive pressure. The oil inside the actuator is never exposed to atmospheric conditions and excessive heat, ensuring no contamination or varnishing. Replacing the old flyball governor and installing our Electraulic actuators allowed tighter speed (RPM) control. This also enabled fully automatic startup of the machine from the control room, meaning they were able to start the unit and put it onlineconsistently without variants in operator methods. With the implementation of the digital speed controller and our actuator to control the process loop, our customer now has the capability to fully modify the setpoint from the control room from 0-100% speed, a much-needed upgrade from the analog control loops of the older OEM equipment. Literature Click here to download the full Success Story!  Download