Figure 4 illustrates an override configuration for preventing the exchanger from blowing steam. It requires a measurement device for the condensate level within the shell of the exchanger but all other components are implemented in software.
As long as the condensate level exceeds its set point, the liquid outlet temperature controller determines the control valve position. However, if condensate level drops to its set point, then the condensate level controller takes over. This is implemented by using a low select to choose between the outputs of the liquid outlet temperature controller and the condensate level controller.
Figure 5. This provides another option to forestall release of steam.
Another way to prevent the exchanger from blowing steam is to install a condensate pot downstream (Figure 5). The steam pressure in the shell of the exchanger and in the condensate pot is the steam supply pressure. Therefore, the full steam supply pressure is available for condensate return. A controller maintains the desired liquid level in the condensate pot. The condensate line from the exchanger enters below the liquid level in the condensate pot, so the exchanger can’t blow steam.
The condensate flows by gravity from the exchanger to the condensate pot. So, the condensate level in the exchanger must be above that in the condensate pot. The hydrostatic head provided by this difference is the driving force for condensate to flow from the exchanger to the condensate pot.
The maximum driving force is when the exchanger is completely full of condensate; the condensate flow under these conditions would be zero. The minimum driving force is when the exchanger is completely empty of condensate; the maximum condensate flow occurs under these conditions.
Figure 6 presents the process operating lines for both a linear and an equal-percentage valve. The pressure drop across the control valve isn’t constant, which favors using the equal-percentage valve. The operating lines in Figure 6 confirm this. There’s only a modest departure from linearity from a control valve opening of 0% to 78%.
Above a valve opening of 78% for the equal-percentage valve (44% for the linear valve), the control valve has no effect on the liquid outlet temperature because no condensate remains within the exchanger. So, opening the valve further doesn’t impact the heat transfer rate or, consequently, the liquid outlet temperature. This exposes the liquid outlet temperature controller to windup.
The physical locations of the exchanger and the condensate pot affect the degree of windup that’s possible. The maximum possible level in the condensate pot corresponds to the bottom of the exchanger. If the condensate pot level is above the bottom of the exchanger, the exchanger can’t be completely drained of condensate. If the condensate pot level is exactly at the bottom of the exchanger, there would be no hydrostatic head for condensate flow when the exchanger is completely drained of condensate (and the condensate flow is at its maximum). As the condensate pot level is dropped further below the bottom of the exchanger, the hydrostatic head for fluid flow increases — but this also raises the possibility for windup in the liquid outlet temperature controller.
Windup can’t occur if the control valve is either perfectly sized or undersized. However, as previously noted, most valves are oversized at least to some degree. In this case, an additional consequence of oversizing the valve is the possibility of windup in the liquid outlet temperature controller.
Figure 6. Equal-percentage valve better suits the varying pressure drop across the valve.
This is another case where the windup prevention mechanisms as customarily configured in digital systems are ineffective. Figure 6 indicates that the upper output limit could be set at 78% (for an equal-percentage valve) but this value is neither precise nor constant. Instead, the windup prevention should be invoked at the instant the condensate is completely drained from the exchanger. Detecting this requires additional instrumentation (such as a level switch) to provide the necessary information so that the digital system can properly initiate its windup protection mechanisms.
In large production facilities where multiple exchangers can be physically located within a reasonable proximity, condensate pots are commonly installed to address the condensate return issues. (Installing a condensate pot for an individual exchanger is difficult to justify.) There are competing designs for condensate pot arrangements but in most the pressure in the condensate pot is the steam supply pressure and the condensate flows from the exchangers to the condensate pot by gravity.
Cecil L. Smith is president of Cecil L. Smith, Inc., Baton Rouge, La. E-mail him at firstname.lastname@example.org.