Sunday, April 12, 2009

Energy Efficiency in Steam Systems

Energy Efficiency in Steam Systems

In today’s typical process plants, preventing steam loss and improving condensate return are key opportunities to make a process more energy efficient.

To be the most effective, steam generally needs to be dry (such as for process usage), or superheated (for instance, for use in turbines). These requirements dictate utility-system operating procedures for generating the highest quality steam possible, and then distributing it to the points of use with minimal deterioration. Since steam becomes condensate after its heat energy is expended, strategies must be in place to remove condensate as quickly as it is formed, in the steam-supply portion of the circuit and during steam usage alike.

Furthermore, superheated steam is typically desuperheated by injecting hot condensate into the system. As a result, excessive wetness can also occur downstream of the desuperheating station. In either case, if such condensate is not removed from the steam supply, the negative impact on the steam system can be substantial, as seen in Table 1.

Improving condensate return. At many plants, the operators admittedly realize that condensate must be removed as quickly as it is formed, but a suitable condensate drainage or transportation system is not in place. In such cases, the condensate is often sewered or sent to a field drain. Some possible outcomes of removing condensate but not handling it effectively are outlined in Table 2.

Figure 1. Preferred method to drain jacketed pipe for high-melting-point fluids, such as sulfur

Condensate is traditionally removed from steam systems by steam traps or by equipment combinations involving level pots and outlet control valves. Process situations in which high backpressure from the downstream portion of the condensate-return system tend to create a "stall." Then, a different system incorporating both a pump and trap in the design is needed to drive the condensate while also trapping the steam; this process may be referred to as pump-trapping or power-trapping.

Because there are at least three condensate-drainage alternatives, it makes more sense to think in terms of required "condensate discharge locations" rather than referring to condensate removal devices indiscriminately as "steam traps." This broader mind-set helps avoid any predisposition to install steam traps in applications that need a different type of condensate drainage solution.

Figure 2. Alternative, practical redesign method for existing installations to drain jacketed pipe for high-melting-point fluids, such as sulfur (no “stall”)

Engineered separator-drains remove condensate that is entrained in a moving steam supply (including flash or regenerated steam). The result is highest quality steam delivered for plant use. Compare that to steam traps, which remove condensate that has already fallen out of the steam. As their name suggests, steam traps remove condensate and "trap steam." Meanwhile, level pots can be used in certain instances where steam traps cannot meet the high pressure or capacity requirements.

Special situations. There can be many situations in a plant where effective condensate removal requires specialized drainage designs. For instance, Figures 1 and 2 show two options for condensate drainage from a jacketed pipe that conveys high-melting-point materials, such as liquid sulfur or high-boiling hydrocarbons.

Other examples of specialized applications include options to effectively drain steam-supplied heat exchangers. A key consideration is to first determine whether a stall condition exists or not; when it does, condensate will not drain effectively through a simple steam trap. Such a situation typically arises when modulating steam pressure creates a negative pressure differential across the condensate drain device. So-called, Type II secondary pressure drainers of the pump-trap type are used on equipment with a negative pressure differential. Because wasted condensate is a valuable resource to be saved, use Type I secondary pressure drainers of a "pump only" type to recover collected condensate and power it back to the boiler.

TABLE 1. RESULTS OF FAILING TO REMOVE CONDENSATE FROM THE STEAM SUPPLY

TABLE 2. RESULTS OF REMOVING CONDENSATE, BUT FAILING TO HANDLE IT EFFECTIVELY




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