Chemical Processing - July 2008 - (Page 27)

Liquid 140 Equal-percentage valve Liquid in 100 0 20 40 54 60 84 80 100 Condensate Condensate valve position, % Override control A simple step TC eam Liquid outlet temperature, °F Inserting a steam trap into the condensate line upTT TC stream of the control valve (Figure 3) will prevent TT Liquid out release of steam into the condensate return piping. Liquid out Steam As long as there’s some condensate within the exchanger, the trap has no effect on condensate flow. However, should the condensate completely drain < LC LT from the exchanger, the trap prevents steam from flowing into the condensate return system. Unfortunately, the configuration in Figure 3 exposes the liquid outlet temperature controller to T Liquid in windup. The test for the possibilityin windup is very Liquid of simple: Are there situations where changes in the Condensate controller output (the condensate valve position) have Condensate no effect on the controlled variable (the liquid outlet temperature)? Here, the answer is “yes” — whenever Figure 4. Such a configuration also can prevent the the trap is preventing the steam from flowing into the exchanger from releasing steam. 300 condensate return system. 288 put attains its upper Linear valve limit (normally set at a value As customarily configured, the windup preTT TC 260 above 100%). Based on the operating lines in Figvention mechanisms provided by digital control systems are ineffective for this situation. TheseLiquid outure 2, the appropriate upper output limit is 54% Steam 220 for a linear valve and 84% for an equal-percentage mechanisms are invoked when the controller out180 140 Equal-percentage valve ©2008 Siemens Energy & Automation, Inc. LC LT Condensate Liquid in 100 0 20 40 44 60 78 80 100 Condensate valve position, % Condensate SIMATIC PCS 7 takes you beyond the limits. SIMATIC PCS 7 How far can your system go? Traditional DCS limits your vision – and potential – but SIMATIC® PCS 7 process control system breaks through and gives you a whole new perspective on system performance. Only PCS 7 enables full integration of all the automation systems in your plant: process, batch, discrete and safety, and all the field devices: instrumentation, analytics, motors, drives, and safety into a single platform, with common tools for engineering, visualization, and facilitywide asset and maintenance management. Take flight with over 5,000 satisfied PCS 7 users and see the new possibilities. 1.800.964-4114 • ref. code PCS7V7 • www.sea.siemens.com/pcs7 Answers for industry. 27 chemicalprocessing.com July 2008 http://www.sea.siemens.com/pcs7 http://www.sea.siemens.com/pcs7

Table of Contents Feed for the Digital Edition of Chemical Processing - July 2008

Chemical Processing - July 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Who’s a Big Hit? Succeed with Condensate Control Take Some Basic Steps with pH Measurements Steam Projects Provide Fast Payback Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds/Ad-Lits Ad Index End Point

Chemical Processing - July 2008

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