Chemical Processing - August 2007 - (Page 34) >> Distinctive noise Normal Increased noise Reduced noise Standard deviation increases or decreases with changing noise level Mean is unchanged shown on the local display or communicated across the control network. By applying algorithms based on field data collected from real applications over the years by Siemens field service engineers, the “Sonic Intelligence” signal processor is said to automatically ignore obstructions to give accurate and reliable level readings. Bridges also points to recent diagnostic developments in flowmeters. “With our magnetic flowmeters,” he says, “we’re looking at changes in conductivity that could show up problems like coating on the electrodes. In the old days solids bouncing off the electrodes could create a lot of noise, but now we’re using that to determine what the likely solids content is in the fluid.” The company’s Optiflux magmeters now feature three levels of diagnostics — of the meter itself (microprocessor, memory, software, etc), of the application (problems that NAMUR recommends a meter should be able to detect, such as gas bubbles, electrode corrosion and fouling, changes in conductivity, and so on), and “out-of-spec” diagnostics, in which the accuracy of the meter is checked against a test signal. The latest versions of Rosemount E-series magmeters also feature diagnostics that alert users when instrument variability is caused by process noise and not actual flow variation. This allows for adjustment to a higher coil drive frequency to stabilize the output without additional damping. Coriolis challenges It’s probably fair to say that in the early days of Coriolis meters their reputation suffered because of a perceived inability to respond to rapidly changing process conditions such as entrained gases or the onset of two-phase flow in the pipe. While acknowledging that, Bridges notes that manufacturers in recent years have addressed these issues, putting Coriolis flow metering in greater demand than ever before. Figure 2. The effect of changing noise levels on the mean and standard deviation values for a process signal can help identify problems. Source: Rosemount. high and low side clogging by using an algorithm to monitor the changes in process noise over time. “So we can not only detect that plugging has occurred, we can also predict over time when plugging will start to develop,” says Henk van der Bent, marketing manager for process control instrumentation, Yokogawa Europe, based in Amersfoort, the Netherlands. Enhanced electronics Emerson’s SPM advanced diagnostics has been available on Foundation Fieldbus devices (including temperature transmitters) for a few years but was extended to HART devices last December, a move Schmeling credits particularly to advances in low-power electronics. This is enabling instrument manufacturers to pack more functionality into their devices. Bridges, for example, says “Krohne has been doing this for some years with our radar level devices, where different signals can indicate unusual events in the vessels, such as broken baffles or damaged mixer impellers.” Siemens Automation and Drives, Spring House, Pa., and Nuremberg, Germany, offers similar capabilities in its recently introduced Sitrans LR250 radar level transmitter. A new graphical local interface displays echo profiles and diagnostic information, allowing the user to determine dynamics in the tank at a glance. Self-diagnostic features are 34 • August 2007 >> Plugging detection Figure 3. Foundation Fieldbus differential-pressure transmitter features built-in plugged-impulse-line detection diagnostics based on “self-adaptive” algorithm. Source: ABB. www.chemicalprocessing.com http://www.chemicalprocessing.com
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