Chemical Processing - March 2008 - (Page 36)

T Liquid in Condensate >> Steady state behavior 80 Steam supply pressure 75 Liquid outlet temperature, °F 60 Shell pressure, psig 40 20 0 -20 0 6 20 29 40 60 Steam valve position, % 80 Linear valve Equal-percentage valve 300 260 220 200 180 140 100 0 20 40 60 Steam valve position, % 80 Linear valve Equal-percentage valve a 100 b 100 Figure 2. Because valve characteristics are incorporated, there are separate lines for linear and equal-percentage valves. of 90% of 138°F or 124°F. This corresponds to a liquid outlet temperature of 274°F. Figure 3 indicates the valve openings for a liquid outlet temperature of 274°F. For an equal-percentage valve, this is a valve opening of 50% (actually 51%, but let’s round valve openings to the nearest 5%). For a linear valve, this is a valve opening of 15%! This is typical for oversized valves with linear characteristics. If you install a linear valve, it must be properly sized for the process operating conditions. One of the issues previously raised pertained to flat regions of the process operating line. The slope of the process operating line is the process gain or process sensitivity (again, “process” includes the control valve). A decrease in the process sensitivity has the same effect on loop performance as a decrease in the controller gain, that is, the loop responds more slowly. For an equal-percentage valve, the controller can be effectively tuned and will give consistent performance for valve positions up to 50%. But above 50% the process gain is very small and approaches zero as the valve opening is increased. Even process operators will observe this, saying “Once the valve is half open, it has no effect on anything.” If the controller gain is increased to compensate for the reduced process gain, the higher gain will give problems at lower liquid outlet temperatures. This is one case where some people will propose characterization functions, scheduled tuning or other approaches commonly referred to as adaptive control (but, in reality, these are nonlinear controls). A simple approach (scheduled tuning) would be to use one value of the controller gain for valve openings (controller outputs) less than 50% and a higher value for controller outputs above 50%. Unfortunately, the actual cutoff point depends upon operating conditions, especially throughput. Minimum heat transfer rate Let’s assume that the condensate from the steam trap flows into a drain. Figure 2a indicates that the shell pressure is below atmospheric when the steam valve position is less than 29% for the equal-percentage valve or 6% for the linear valve. Under these conditions, the condensate won’t flow out of the shell. If the condensate flows into a condensate return system, the minimum shell pressure is approximately atmospheric pressure. This imposes a minimum heat transfer rate on the exchanger. Atmospheric pressure in the shell corresponds to a shell temperature of 212°F. Because the liquid enters at 150°F, heat will be transferred to the liquid. This heat transfer rate is the minimum that can be continuously sustained by the exchanger in Figure 1. The minimum heat transfer rate gives a liquid outlet temperature of approximately 200°F, regardless of the valve characteristics. Figure 4 illustrates the effect on the process operating www.chemicalprocessing.com >> Maximum heat transfer rate 300 Liquid outlet temperature, °F 274 260 220 180 140 100 0 14 20 51 40 60 Steam valve position, % 80 100 Linear valve Equal-percentage valve Figure 3. Linear valve opening is far less than that of an equal-percentage valve. 36 • March 2008 300 http://www.chemicalprocessing.com

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Chemical Processing - March 2008 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Distillation is Bubbling Feel Secure About Vulnerability Assessments The Door Opens For Membranes Achieve Effective Heat Exchanger Control Epoxy Maker Finds the Right Glue for Its Business Process Puzzler Plant InSites Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - March 2008

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