Chemical Processing - July 2008 - (Page 28)

Condensate Condensate Condensate pot Liquid outlet temperature, °F TC TT 300 288 Linear valve T 260 preventing the exchanger from blowing steam. It Liquid out Steam requires a measurement device for the condensate 220 level within the shell of the exchanger but all other components are implemented in software. 180 As long as the condensate level exceeds its set point, the liquid outlet temperature controller Equal-percentage valve 140 determines the control valve position. However, if condensate level drops to its set point, then the 300 100 288 condensate level controller takes 44 This is impleover. 78 Liquid in LC LT 0 20 40 60 80 mented by using a low select to choose between the Condensate 260 outputs of the liquid Condensate valve position, % outlet temperature controller and the condensate level controller. Linear valve 220 Condensate Another way to prevent the exchanger from blowing steam is to install a condensate pot down180 Figure 5. This stream (Figure 5). The steam pressure in the shell valve. However, these limits depend on operating provides another option to forestall of the exchanger and in the condensate pot is the variables, especially throughput. Equal-percentage valve 140 release of steam. steam supply pressure. Therefore, the full steam The windup prevention should be invoked at the supply pressure is available for condensate return. instant the condensate is completely drained from 100 84 54 the A controller maintains the desired liquid level in the exchanger, which is when 60 steam trap begins 0 20 40 80 100 the condensate pot. The condensate line from the to block the steam from flowing into the condensate Condensate valve position, % exchanger enters below the liquid level in the conreturn system. Unfortunately, with the exchanger indensate pot, so the exchanger can’t blow steam. strumented as in Figure 3 there’s no way to detect this The condensate flows by gravity from the exchangevent (the exchanger completely drained of condensate). er to the condensate pot. So, the condensate level in However, there’s a way to detect this — by TT TC the exchanger must be above that in the condensate equipping the exchanger with instrumentation not Liquid out pot. The hydrostatic head provided by this difference customarily provided. For instance, a level switch or Steam is the driving force for condensate to flow from the level transmitter for the condensate could indicate exchanger to the condensate pot. when the exchanger is drained of condensate and the The maximum driving force is when the exmaximum heat transfer rate is attained. Under these < LC LT changer is completely full of condensate; the conconditions, the liquid outlet temperature controller should be inhibited from increasing its output. Digital densate flow under these conditions would be zero. The minimum driving force is when the exchanger systems certainly are capable of this — but only, of is completely empty of condensate; the maximum course, if the necessary information is available. Liquid in condensate flow occurs under these conditions. Figure 6 presents the process operating lines for More options Condensate both a linear and an equal-percentage valve. The presFigure 4 illustrates an override configuration for sure drop across the control valve isn’t constant, which Condensate pot process operating lines favors using the equal-percentage valve. The operating lines in Figure 6 confirm this. There’s only a modest 300 departure from linearity from a control valve opening 288 of 0% to 78%. Linear valve 260 Above a valve opening of 78% for the equalLiquid out Figure 6. percentage valve (44% for the linear valve), the control Equal-per220 valve has no effect on the liquid outlet temperature centage valve better suits because no condensate remains within the exchanger. the varying 180 So, opening the valve further doesn’t impact the heat pressure drop across the transfer rate or, consequently, the liquid outlet temEqual-percentage valve valve. 140 perature. This exposes the liquid outlet temperature controller to windup. 100 The physical locations of the exchanger and 44 78 Liquid in 0 20 40 60 80 100 the condensate pot affect the degree of windup Condensate valve position, % that’s possible. The maximum possible level in 100 July 2008 chemicalprocessing.com Liquid outlet temperature, °F Liquid temperature, °F 28 http://chemicalprocessing.com

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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