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Common Mistakes not Commonly Understood - Part 2

Mon, 20 Nov 2017 16:44:00 -0500

Here we continue on in our exploration of what we should know but don’t know and how it hurts us. (11) Ignoring actuator and positioner sensitivity. Piston actuators are attractive due to smaller size and lower cost but have a sensitivity that can be 10 times worse than diaphragm actuators. Many positioners look fine for conventional tests but increase response time to almost a 100 times larger for step changes in signal less than 0.2%. The result is extremely confusing erratic spikey oscillations that only get worse as you decrease the PID gain. My ISA 2017 Process Control and Safety Symposium presentation ISA-PCS-2017-Presentation-Solutions-to-Stop-Most-Oscillations.pdf show the bizarre oscillations for poor positioner sensitivity. Slides 21 through 23 show the situation, the confusion and a simple tuning fix. While the fix helps stop the oscillations, the best solution is a better valve positioner to provide precise control (critical for pH systems with a strong acid or strong base due to amplification of oscillations from sensitivity limit). (12) Ignoring drift in thermocouples. The drift in thermocouples (TCs) can be several degrees per year. Thus, even if there is tight control, the temperature loop setpoint is wrong resulting in the wrong operating point. Since temperature loops often determine product composition and quality, the effect on process performance is considerable with the culprit largely unrecognized leading to some creative opinions. Some operators may home in on a setpoint to get them closer to the best operating point, but the next shift operator may put the setpoint back at what is defined in the operating procedures. Replacement of the thermocouple sensor means the setpoint becomes wrong.  The solution is a Resistance Temperature Detector (RTD) that inherently has 2 orders of magnitude less drift and better sensitivity for temperatures less than 400 degrees C. The slightly slower response of an RTD sensor is negligible compared to the thermowell thermal lags. The only reason not to use an RTD is a huge amount of vibration or a high temperature. Please don’t say you use TCs because they are cheaper. You would be surprised at the installed cost and lifecycle cost of a TC versus an RTD. See the ISA Mentor Program Webinar “Temperature Measurement and Control” for a startling table on slide 4 comparing TCs and RTDs and a disclosure of real versus perceived reasons to use TCs on slide 7. (13) Not realizing the effect of flow ratio on process gain. The process gain of essentially all composition, pH and temperature loops is the slope of the process variable plotted versus the ratio of the manipulated flow to the main feed flow.  This means the process gain is inversely proportional to feed flow besides being proportional to slope of plot. In order to convert the slope that is the change in process variable (PV) divided by the change in ratio to the required units of the change in PV per change in manipulated flow, you have to divide by feed flow. The plot of temperature or composition versus ratio is not commonly seen or even realized as necessary. The same sort of relationship holds true where the manipulated variable is an additive or reactant flow for composition or a cooling or heating stream flow for temperature. For temperature control, the slope of the curve is often also steeper at low flow creating a double whammy as to the increase in process gain at low flow. Also, for jackets, coils and heat exchangers, the coolant flow may be lower creating more dead time for a sensor on the outlet. Fortunately for pH, we have pH titration curves where pH is plotted versus a ratio of reagent volume added to sample volume although often the sample volume added is just used on the X axis. In this case, you need to find out the sample volume so you can put the proper abscissa on the laboratory curve.  In the process application the titration curve abscissa that is the ratio of reagent volume to sample volume is simply the ratio of volumetric reagent flow to volumetric feed flow if the reagent con[...]



Industry signals and indicators - November 2017

Mon, 20 Nov 2017 12:32:00 -0500

• Opto 22 announced Oct. 24 a new version of its groov edge appliance that directly embeds key IIoT technologies, including MQTT and OPC-UA drivers. Added to the groov View software for web and mobile visualization, and its open-source Node-RED  development environment, the new release offers engineers, technicians and developers...



2017 State of Technology: HMIs and industrial PCs

Mon, 20 Nov 2017 12:13:00 -0500

This State of Technology report brings together all of the latest and best articles on HMIs and industrial PCs,



Chemical/Specialty Chemical Industry Forum: Project planning means profit

Fri, 17 Nov 2017 16:52:00 -0500

Seeking process improvement opportunities during a project’s definition and design stages—and performing opportunity assessments—can pay off handsomely later for little or no cost



Engineering, Procurement and Construction Industry Forum: Save on projects with unified power and automation system

Fri, 17 Nov 2017 16:51:00 -0500

By putting packaged systems together on unmodified Ethernet, the Integrated Power & Automation offering from Rockwell Automation can register 90% savings on integration costs



Pulp & Paper Industry Forum: Desperately seeking nano-sensors

Fri, 17 Nov 2017 16:51:00 -0500

Real-time, in situ analysis of tiny particles needed to advance nanocellulose manufacturing



Automation Fair shows #HoustonStrong support

Fri, 17 Nov 2017 16:50:00 -0500

Rockwell Automation rallies volunteers in support of Hurricane Harvey relief efforts



Instant analytics aid in production optimization

Fri, 17 Nov 2017 16:50:00 -0500

Rockwell Automation advances its scalable FactoryTalk Analytics offering, from Devices to Machines and beyond



Talent shortage drives diversity initiatives

Fri, 17 Nov 2017 16:49:00 -0500

Rockwell Automation creates a culture of inclusion, earning the 2017 Catalyst Award



Live from Rockwell Automation Fair 2017

Fri, 17 Nov 2017 16:36:00 -0500

The editors of Control, Control Design, and Smart Industry are on site at Automation Fair to bring you the breaking news and best sessions from this world class event.



Water/Wastewater Industry Forum: El Paso upgrades to PlantPAx DCS

Thu, 16 Nov 2017 16:49:00 -0500

Facilities shouldn’t wait until their systems are as old as El Paso’s before reaping the benefits of the modern DCS



Run analytics where they make most sense

Thu, 16 Nov 2017 16:48:00 -0500

Data often doesn’t have to go to the cloud for context and usefulness



STEM booth sparks future engineers

Thu, 16 Nov 2017 16:47:00 -0500

“Engineering Our Future" booth highlights invest by Rockwell Automation in next-gen technical professionals



Power & Energy Management Industry Forum: Modernization without interruption

Thu, 16 Nov 2017 16:47:00 -0500

Upgrading five different manufacturers' control systems to Rockwell Automation hardware improved a power generation facility’s process and production support



Machine & Equipment Builders Industry Forum: Built for speed and agility

Thu, 16 Nov 2017 16:42:00 -0500

Consumer trends continue to direct requirements for packaging machines



Oilfield solution streamlines asset integration

Thu, 16 Nov 2017 16:41:00 -0500

Rockwell Automation ConnectedProduction solution reduces time to production, lower costs and helps oil & gas producers thrive



Motors, drives deliver force behind the scenes

Wed, 15 Nov 2017 16:55:00 -0500

Hardworking rotating devices are getting help from variable-speed devices, efficiency rules, and onboard and networked intelligence to aid process applications worldwide.



Oil & Gas Forum: ARC lifts well performance

Wed, 15 Nov 2017 16:41:00 -0500

Canadian oil & gas company ARC Resources uses ConectedProduction Well Manager to optimize artificial lift



Wellsite controls take heat, deliver safety

Wed, 15 Nov 2017 16:40:00 -0500

Oil and gas specialist Frames integrates PlantPAx DCS and AADvance for safe control of wellhead equipment in the Sahara desert



Get to the core of safety and productivity

Wed, 15 Nov 2017 16:40:00 -0500

Proper safety system methods, tools, design and analysis address the risk while providing the value in productivity