Remote - Special SCADA issue 2012 - (Page 16)

FEATURE Hardwire vs. Wireless Failsafe Control Systems In today’s industrial automation world, the debate continues. Is wire more reliable then wireless? The answer is no. OleumTech In any industrial control environment, it is imperative to return process conditions to its normal or safe conditions to prevent fires, petro-chemical spills, waste spills and other preventable disasters when failure within a control loop is detected. Implementation of a “failsafe control system” is required to perform this function. Prior to wireless technologies, aluminum and copper wires have been the only means to control and monitor remote instrument devices. The advantage of a hardwired system is that it is “reliable.” The notion is: I know it’s connected so I trust it. The disadvantages are more apparent as the cost of wires, trenching, installation and technical aspects of operability start to mount. In addition, there are physical drawbacks: environmental factors such as lightning hazards (fire), galvanic corrosion, electrolysis, and other debilitating wear and tear on the wiring over time. Modern day wireless technology is proven to offer an equally reliable fail safe solution when replacing long distance hardwired applications. Wireless solutions offer additional advantages of the added benefits of lower cost and faster startup and commissioning of new projects. Wireless solutions eliminate the inevitable costs mentioned of hardwiring. And, when an OleumTech WIO Wireless System is utilized for replacing the hardwire method, it reduces additional cost by completely removing the need for long distance analog (4 to 20 mA) I/O modules and analog to digital converters from the control instrumentation loop. All inputs and outputs (I/Os) are relayed via Serial communication (Modbus) to and from a control device, which further reduces the complexity of wiring and installation time. The block diagrams below depicts a typical hardwire and wireless control loop for valve control with feedback. the control device’s output current rating is inadequate to drive the greater current required by the solenoid valve. Figure 1 depicts a control loop that is in a safe operating condition. The example below shows the valve to be closed because the SCADA system that monitors and controls the Instrument loop has initiated the valve to close, or the logic in the control device is reading a no flow rate from the Flow Meter. In either case, the output of the control device is de-energized thereby removing the ground source to the interposing relay coil. The valve solenoid is wired to the open contact of the interposing relay. In this condition the valve solenoid coil is de-energized and no instrument air is supplied to the valve actuator diaphragm keeping it closed. Figure 1. Hardwired - Open Loop Diagram (Valve Closed) Figure 2 depicts an operating failsafe instrument loop. In this condition the SCADA computer system has initiated the control device to open the valve. The control device has energized the output and supplied ground to the interposing relay coil. Once the interposing relay coil is energized the contact will switch from open to close. By closing the contact, ground is delivered to the high current solenoid valve. Once the solenoid valve is energized, instrument air pressurizes the valve actuator diaphragm to open, allowing water to flow. The water flow meter is monitored by the control device for feedback. This condition is operating as a failsafe instrument closed control loop because if any part of the loop fails the valve will close. Hardwired Instrument Chain Wireless Instrument Chain Using OleumTech WIO System Figure 2. Hardwired - Closed Loop Diagram (Valve Opened) Typically a Distributive Control System (DCS), Programmable Logic Controller (PLC), or other Remote Terminal Unit (RTU) is used to control a valve (control device). Figure 1 depicts a failsafe control loop that uses an air to open acting actuator to control the valve. This de-energized condition is commonly referred to as an open loop. In general, the use of an interposing relay protects the output I/O circuit of the control device that is controlling the solenoid valve. The output of the control device drives the interposing relay coil, which in turn switches the valve open or close. The control device output is isolated from the solenoid valve coil by the interposing relay. Utilizing this concept is necessary because the voltage ratings between the control device I/O and solenoid are often dissimilar, or because 16 Identifying that the control valve may be a critical part of the described control loop. It would be inaccurate to say that the control valve is the most important part of the loop when using a failsafe concept, because all the components are interrelated and dependent upon each other. Think of the above example control loop as an instrumentation chain. Like any other chain, the whole chain is only as strong as its weakest link as displayed below.

Table of Contents for the Digital Edition of Remote - Special SCADA issue 2012

Remote - Special SCADA issue 2012
Re-Inventing SCADA for the Next Generation Enterprise
Securing Automation within Critical Infrastructures for SCADA Security
As the Brain, So the Body: The PLC Market and Automation
Reducing Labor Costs and Increasing Investor Confidence With Real-Time Monitoring of Photovoltaic Energy Network
Advantages of the DNP3 Communications Protocol in Water & Wastewater Telemetry Systems
Hardwire vs. Wireless Failsafe Control Systems
Advantech Introduces IEC 61850-3/IEEE 1613 Compliant Substation Automation Platform
CalAmp Launches Fusion-LTE 4G Broadband Router Platform for Smart Grid Applications
El Paso Water Utilities Implements Trimble Fieldport Mobile Software to Streamline Customer Field Service

Remote - Special SCADA issue 2012