Figure 2 presents the process operating lines for both the linear valve and the equal-percentage valve. With a linear valve, the liquid outlet temperature is linearly related to the valve position. The process operating line for the equal-percentage valve clearly reflects the inherent characteristics of such a valve.
Theoretically, the minimum heat transfer rate is zero. With the control valve closed, the exchanger completely fills with condensate, resulting in no heat transfer.
The maximum heat transfer occurs when the exchanger is completely drained of condensate. The value for the maximum heat transfer rate is the same, regardless of whether the control valve is on the steam supply or the condensate.
However, the consequences of attempts to exceed the maximum are very different. The exchanger in Figure 1 has the potential to “blow steam” into the condensate return system. As the control valve opens, the condensate level within the exchanger drops. If the valve is opened too much, the level drops entirely out of the exchanger and steam flows into the condensate return system, a somewhat unpleasant situation.
Figure 3. Adding a steam trap to the condensate line keeps the exchanger from blowing steam.
Both operating lines in Figure 2 terminate at a liquid outlet temperature of 288°F. At this point, the exchanger is completely drained of condensate and the entire tube area is exposed to condensing steam. Figure 2 suggests that this occurs at a valve opening of 54% for the linear valve and 84% for the equal-percentage valve. What about valve openings greater than these values? In practice, the liquid outlet temperature usually drops. When the exchanger blows steam, the shell pressure usually drops below the steam supply pressure, giving a lower liquid outlet temperature.
A simple step
Inserting a steam trap into the condensate line upstream of the control valve (Figure 3) will prevent release of steam into the condensate return piping. As long as there’s some condensate within the exchanger, the trap has no effect on condensate flow. However, should the condensate completely drain from the exchanger, the trap prevents steam from flowing into the condensate return system.
Unfortunately, the configuration in Figure 3 exposes the liquid outlet temperature controller to windup. The test for the possibility of windup is very simple: Are there situations where changes in the controller output (the condensate valve position) have no effect on the controlled variable (the liquid outlet temperature)? Here, the answer is “yes” — whenever the trap is preventing the steam from flowing into the condensate return system.
As customarily configured, the windup prevention mechanisms provided by digital control systems are ineffective for this situation. These mechanisms are invoked when the controller output attains its upper limit (normally set at a value above 100%). Based on the operating lines in Figure 2, the appropriate upper output limit is 54% for a linear valve and 84% for an equal-percentage valve. However, these limits depend on operating variables, especially throughput.
The windup prevention should be invoked at the instant the condensate is completely drained from the exchanger, which is when the steam trap begins to block the steam from flowing into the condensate return system. Unfortunately, with the exchanger instrumented as in Figure 3 there’s no way to detect this event (the exchanger completely drained of condensate).
Figure 4. Such a configuration also can prevent the exchanger from releasing steam.
However, there’s a way to detect this — by equipping the exchanger with instrumentation not customarily provided. For instance, a level switch or level transmitter for the condensate could indicate when the exchanger is drained of condensate and the maximum heat transfer rate is attained. Under these conditions, the liquid outlet temperature controller should be inhibited from increasing its output. Digital systems are capable of this — but only if the necessary information is available.