Chemical Processing - October 2007 - (Page 47)

PO;; process gain that also is inversely proportional to perature flow. In either case, once a secondary flow loop is added so there’s a cascade loop of pH to reagent flow or temperature to coolant flow, we’re back to the nonlinearity considerations for the flow loop. Major reasons to add these secondary flow loops are to better reject pressure disturbances and provide a more accurate implementation of flow feedforward control [1]. A quick-opening trim characteristic provides initially a very high process gain followed by a very low process gain. This nonlinearity is accentuated in the installed characteristic and is generally undesirable because it magnifies resolution problems near the seat and causes an excessive loss of sensitivity even at mid-range throttle positions. Pinch valves and isolation valves designed for on/off service tend to have this characteristic. Turning on a dime or at least a quarter Dead time and response time quantify dynamic response. The dead time, Td, is the time to a first change in closure member position after a change in signal. The response time, T86, is the time required for the position to reach 86.5% of its final value and includes the dead time. These parameters are defined in the ISA standard and report for the test and measurement of the response of the complete control valve assembly [3, 4]. If you’re looking just at the actuator, you can estimate the pre-stroke dead time and stroking time from an individual fill or exhaust parameter for the actuator type and volume divided by fill or exhaust flow coefficient for the positioner, I/P or booster. The response time for large changes is estimated as 86% of the desired change in valve position (%) divided by the slewing rate (%/sec.). For changes between 1% and 10% the actuator response time becomes relatively fixed except for large valves and dampers. It always was rather obvious that large valves were slow because it takes time to fill or exhaust enough air in a large actuator volume to make a change in actuator pressure large enough to overcome the torque or friction load to move the closure element. However, until recently, it wasn’t known that the response time of even small valves was dramatically slower, to the point of almost no response, because of the design of pneumatic positioners. Figure 2 shows how the response time increases to 10 to 100 seconds from 1 to 2 seconds as the size of the step change in signal decreases below 1%. Note that in this plot we are primarily seeing the effect of the positioner because the step changes aren’t big enough to see strokingtime limitations. Spool-type positioners have an extremely slow response, to the point of no response for changes less than 0.2%. The response time for even good positioners can rise by an order magnitude for small steps. HART and fieldbus digital positioners generally have eliminated this positioner resolution problem. When you also consider that pneumatic positioners tend to lose their www.chemicalprocessing.com http://www.a-tcontrols.com http://www.a-tcontrols.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - October 2007

Chemical Processing - October 2007 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Biofeedstocks See Real Growth Become a Cyber-Security Pacesetter Go Beyond Condition Monitoring Disposable Equipment Earns Lasting Role Improve Control Loop Performance Ethanol Plant Boosts Output and Saves Energy Process Puzzler Plant InSites Equipment & Services Ad Lits Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - October 2007

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