Chemical Processing - March 2008 - (Page 37)

Liquid outle 180 140 100 L 100 0 14 0 20 14 20 51 40 60 Steam valve position, % 51 80 80 100 PO;;The minimum heat transfer rate corresponds to an lines. opening of 29% for the equal-percentage valve and 6% for the linear valve. Attempting to operate below this minimum leads to a cycling condition. The following scenario occurs for the equal-percentage valve: a. The controller positions the valve to less than 29%. b. The shell pressure drops to atmospheric and the shell begins to fill with condensate. This reduces the effective heat transfer area and the heat transfer rate. c. The liquid outlet temperature drops below target, causing the controller to increase its output to the control valve and admit more steam to the shell. d. When the shell pressure exceeds atmospheric pressure, the condensate is forced from the shell, exposing the entire heat transfer area. e. The heat transfer rate increases, which drives the liquid outlet temperature above its target. f. The controller decreases its output to the control valve, eventually giving a valve position less than 29%. This causes the cycle to repeat. How do we avoid operating the process below the minimum heat transfer rate? For normal operating conditions, we could impose a minimum of 200°F for the liquid-outlet-temperature set point. Unfortunately, a variety of variables, including throughput, affect the value for this limit. Perhaps the surest way to avoid such conditions is to monitor the shell pressure and provide a shell pressure override (Figure 5). For digital implementations, the only additional hardware is the transmitter for the shell pressure. The remainder is implemented in software. For exchangers that discharge the condensate to a drain, the set point for the shell pressure controller would be slightly above atmospheric. For exchangers that discharge into a condensate return system, the set point for the shell pressure controller must be slightly above the pressure required to force the condensate back to the boiler house. >> Minimum heat transfer rate 300 Liquid outlet temperature, °F 260 220 200 180 140 100 0 6 29 6 20 29 40 60 Steam valve position, % 80 100 Linear valve Equal-percentage valve 40 60 Steam valve position, % 100 300 Liquid outlet temperature, °F 260 220 Linear valve Equal-percentage valve 200 180 140 100 20 40 60 100 Figure 4.0Attempting to operate below the minimum80 transfer rate opening leads heat Steam valve position, % to cycling. >> Protection against cycling > Steam > PC TC TT PT TC TT Liquid out PC Steam PT Liquid out T Liquid in T Condensate Liquid in Implications Figure 6 shows the process operating lines and limits on Figure 5. Monitoring shell pressure and providing a pressure override can combat cycling. operation. Let’s examine the aspects previously cited: Limits of process operation. When expressed in • or a linear valve, the minimum and maximum F terms of the liquid outlet temperature the operating limits correspond to valve positions of about 5% limits are 200°F and 274°F, regardless of the inherent and 15%, respectively. These are small valve openvalve characteristics. But when expressed in terms of ings coupled with a narrow operating range. As is the control valve position the operating limits definitely normally the case for an oversized valve, a linear depend upon the type of valve: valve isn’t a good choice. • or an equal-percentage valve, the minimum and F Departure from linearity. Over the region of 30% to maximum limits correspond to valve positions of about 30% and 50% (again rounding to the nearest 50% for the equal-percentage valve, the departure from 5%), respectively. So, we’re using the valve only over linearity is modest and shouldn’t necessitate any form of 20% of its full range. This is the customary conse- scheduled tuning or the like. Regions where the operating line is flat. For the equalquence of oversizing an equal-percentage valve. Forpercentage valve, such a region exists for valve openings tunately, we’re often able to get by with this. www.chemicalprocessing.com March 2008 • 37 Condensate 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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