Gillian Chew

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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Pump Station Pressure Control Systems Explained

Midstream /

Pump Station Pressure Control Pressure control of incoming and discharge flows at the pump stations directly correlates to overall pipeline safety and security. A critical part of the control systems is pressure control in the pipeline. Pressure control serves two essential purposes. The first is to control the discharge pressure of the pump station to achieve the proper flow rate and to attenuate pressure fluctuations in the transmission to downstream pump stations. The pressure control systems, which include valves and actuators, must ensure the pipeline is operating below the system design pressure. The second purpose of the pressure control system is to ensure that the incoming pressure to the pump station is kept above the NPSH (Net Positive Suction Head) required by the pumps in order to prevent cavitation within the pumps and the possibility of damage that could result from the cavitation. Pressure control is interactive from one pump station to another, which means that pressure fluctuations at one station will affect all of the other pump stations along the pipeline. The control system must be able to provide fast response to pressure disturbances during starts, stops, and flow rate changes. It must also provide fast response to set point adjustments at one station in order to achieve the desired flow rate and pressure at another Literature Download the Pump Station Pressure Control Application Spotlight! Download

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Terminal Inlet Pressure Control

Midstream /

Terminal Inlet Pressure Control Background Pressure control is critical when transferring crude oil and petroleum products from the main line to the terminal. Therefore, reducing the variation in delivery pressure will result in uniform flow rates that provide protection from over-pressure conditions in terminal systems. Installing a terminal inlet pressure control valve in the delivery line reduces pressure variation. The inlet pressure control valve is also known as a “holding pressure” or “delivery pressure” control valve. Achieving Poor valve performance could potentially cause unwanted pressure excursions – leading to unscheduled pipeline shutdowns and unit downtime. Problem High pressure differentials are concerning due to the potential cavitation in the flow through the terminal inlet pressure control valve. Cavitation will occur when the flow stream pressure falls below the fluid’s vapor pressure, therefore forming bubbles which will implode once the fluid pressure recovers. These bubble implosions cause severe damage to the valve and, consequently, to the downstream pipeline itself. Solution REXA Electraulic™ Actuators are the solution for fast response to signal command and precise modulation of the terminal inlet pressure control valve. This ensures proper flow control and stable pressure for safe operation. Literature Download the Terminal Inlet Pressure Control Application Spotlight! Download

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Pump Recycle Control

Midstream /

Pump Recycle Control Pump Recycle Control, also known as pump re-circulation flow control, is one of the most common applications found within the Liquid Pipeline industry. Pump Recycle Control keeps pumps operating at a point on their curve. This preventative measure prevents over-pressure of downstream piping and components. The loop directly controls pressure and (by the nature of the system hydraulics) flow rate.  When selecting control valves for this application, it’s important to understand cavitation and consider designs that will limit it. Globe valve designs are generally preferred for pump recycle control due to their higher cavitation coefficient. Response times should be moderately fast.  Explore the significance of Pump Recycle Control, a crucial application in the Liquid Pipeline industry. Learn how it maintains optimal pump operation, prevents over-pressure, and the importance of selecting appropriate control valves to mitigate cavitation effectively. Literature Download to read the full Pump Recycle Control Spotlight! Download

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Distribution Water Pumping

Pump Stations /

Distribution Water Pumping Water Pumping Background Reliable access to drinking water is an overlooked necessity to the modern world. When using pump stations, it is critical to ensure full capacity is always readily available. Standard design practices were created to promote reliable operation, but the technology available has not always been up to the task. Valve actuation on the discharge side of a pump requires careful consideration. Reliability is critical to ensure full water pumping capacity, especially during high demand periods. Equally important is for any discharge valve actuator to provide a guaranteed fail-safe system to protect the pumps in the event of an emergency (when applicable). Why do other actuation technologies come up short in this application? Water often gets pumped across long distances and elevated terrain. Any failure or emergency condition that halts active pumping can result in a flow reversal where gravity forces the flow back down the line towards the pump. This causes the pump to spin backwards. To prevent this, there is typically a check valve on the discharge side of the pump installation. In some cases, a check valve may be combined with an actuator to serve as both the in-line check and control valve package. Commonly used actuator technologies do not represent the most reliable options available and typically require frequent maintenance. A need exists for a more reliable alternative that requires lower cost of ownership. Pneumatic-based systems employ a central compressor system that generates air pressure fed to multiple pneumatic cylinders driving valves. Air is a compressible medium, so it is not a rigid form of actuation. As such, pneumatic actuators are susceptible to “sticking” and “sliding” (also known as deadtime and overshoot), resulting in poor response and control. Hydraulics also present a new set of concerns regarding the oil medium itself. In any hydraulic system, roughly 80% of all failures are attributed to the breakdown of oil. Maintaining oil within these systems is of critical importance. Hydraulic actuators require periodic oil changes, as well as a method of filtering that also requires periodic attention. This task can be quite burdensome in larger pumping stations where massive volumes of oil may be required in these systems. The REXA Solution REXA offers the most reliable actuator solution for a pump discharge valve service. Individual actuators are supplied for each pump discharge valve, eliminating the single point failure hazard posed by centralized systems. REXA delivers smooth and controlled pump discharge valve operation during both normal and emergency conditions. Any speed requirements can be met to maintain a pump curve while also eliminating water hammer / surges that can damage piping infrastructure. REXA is the most energy efficient actuator available, reducing your power costs. With dramatically reduced oil volumes compared to traditional hydraulic systems, REXA eliminates any environmental contamination or insurance related concerns. Literature Download to read the full Distribution Water Pumping Application Spotlight! Download

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Hot Blast Temperature Control

Uncategorized /

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 a controlled manner. The outcome is a reliable blast furnace production of pig iron. A simplified process diagram is illustrated in Figure 1. 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 complex sequencing of the three stoves, switching modes, is illustrated in Figure 2. The actuator needs to position the mixing valve, without delay, so the mixed air has a constant temperature.

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