Chemical Processing - March 2008 - (Page 35)

PO;; We experience a variety of disruptions to production (changes in raw materials, upsets in utility systems, equipment problems, etc.). This is when we are likely to attempt to operate the process beyond the limits. The consequences include cycling conditions (induced by the process, not the controller tuning), windup that isn’t addressed by the windup prevention mechanisms as normally con gured, etc. The controls need to recognize the presence of the limits and take appropriate actions instead of attempting to operate beyond the limits. Consider the exchanger with its control valve on the steam supply (Figure 1). The condensate is discharged through a steam trap into the condensate return system. The liquid is a hydrocarbon ﬂuid that enters at 150°F. The steam supply pressure is 75 psig. Under normal operating conditions, the liquid ow rate is 1,000 lb/min; however, occasionally liquid ow rates up to 4,000 lb/ min are experienced. Consequently, the control valve must be oversized for the requirements of normal operating conditions. Of course, further oversizing is the norm. For this example, the control valve is oversized by about a factor of four (relative to what’s required for normal operating conditions). A common misconception is that every control issue pertains to some aspect of process dynamics. In practice, many if not most of the problems with the controls have their root in the steady state behavior of the process. The steady state behavior of the exchanger can be understood from the graphs in Figure 2. Figure 2a presents the shell pressure as a function of the steam valve position. Figure 2b shows the liquid outlet temperature as a function of the steam valve position — this is the process operating line for the exchanger. The term “process” is somewhat of a misnomer because the valve characteristics (valve size, inherent valve characteristics, etc.) also are incorporated into the operating line. This is why there are two lines, one for a linear valve and one for an equal-percentage valve. www.chemicalprocessing.com Photo court esy o f Perry P roduc ts. Maximum heat transfer rate Two constraints can impose the upper limit for exchangers and other heat transfer processes: Media limited. The process is capable of condensing more steam than can ow through the control valve. In this case, the control valve would be effective up to 100% open. Heat transfer limited. The maximum steam ow is determined by heat transfer, that is, by UA∆T. Because control valves are commonly oversized, this is the most common situation. The maximum possible control valve opening is always 100%. This gives the maximum shell pressure, the maximum heat transfer rate and the maximum liquid outlet temperature. Increasing the steam valve opening always increases these three variables. But for large valve openings, the increase will be very small — so small, in fact, that it’s practically zero. Once the shell pressure approaches the steam supply pressure (75 psig), the exchanger approaches the heat transfer limit and is said to be “maxed out.” For heat transfer processes, the control valve normally is effective up to about 90% of the maximum heat transfer rate. For a constant liquid ow and liquid inlet temperature, the heat transfer rate is proportional to the liquid temperature rise from inlet to outlet. If the shell pressure is equal to the steam supply pressure, the shell steam temperature is 288°F. The maximum possible increase in liquid temperature is from 150°F to 288°F or 138°F. The control valve should be effective for temperature increases March 2008 • 35

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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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