Types of Systems

With so many processes employing steam, no single system can be expected to handle every variable. Six fairly basic systems that can be used individually or in combination to close a steam/condensate loop and save all of the available flash steam and British thermal units lost to the atmosphere are shown in figures 1 through 6.

Figure 1 illustrates an approach to saving heat by which condensate is pumped directly back to a boiler. This approach is not responsive to gas removal, which is extremely important for maximum heat transfer. The pump operates continuously in a semi-steam binding condition, which usually results in considerable maintenance.

FIGURE 1. Direct pumping system augmenting boiler feed system.

Figure 2 illustrates a variation of the direct-pumping concept, with pump discharge modulated in exact proportion to condensate flow and operating with a flooded suction at all times. Excellent gas removal maximizes production, while the absence of traps enhances condensate flow and speeds startup. Selection of a pump with extremely low net positive suction head is critical.

FIGURE 2. Direct pumping system with modulated discharge and gas-dispeller line.

Figures 3 through 6 illustrate pressurized boiler feedwater systems, which consist of a receiver operating at a controlled pressure usually at or slightly higher than the plant heat balance. These systems save energy by not only conserving all high-pressure condensate, but reducing pump-horsepower requirements with their additional suction pressure. Annual savings per horsepower are at or near $500. Because makeup is minimal in a properly designed closed-loop condensate system utilizing a pressurized boiler feedwater system, a smaller deaerator, one sized for actual requirements, usually is incorporated. Often, existing feedwater systems can be used.

Figure 3 illustrates a drainage module used for a typical process. The module incorporates a pressure-insensitive system that senses inflow and automatically adjusts outflow to match. Because the module does not have a fixed orifice, such as a trap, high pressure drops are not necessary for good drainage. The module can drain to a pressurized feedwater system with very little differential. Depending on the processes involved and the plant's configuration, several modules may be used in conjunction with the system.

FIGURE 3. Pressurized boiler feedwater system with process operating at boiler pressure.

The system in Figure 4 is similar to the one in Figure 3, except the modules are equipped with pumps (for processes with variable pressures) or temperature controls. The pumps continuously evacuate process condensate, regardless of pressure.

FIGURE 4. Pressurized boiler feedwater system with pumping modules for temperature-controlled systems operating at variable pressures.

In the system illustrated in Figure 5, several processes operate at a common pressure. Process equipment is drained first to a common receiver, then in a single line to a remotely located pressurized feedwater system. Good pressure control enables this single line to operate overhead and drain to a receiver located above grade. Excellent non-condensable-gas removal is characteristic of this type of system.

FIGURE 5. Pressurized boiler feedwater system with unit receiver for processes operating at common pressures.

Figure 6 illustrates a typical process that uses steam at several pressures. The cascading of flash steam from the high-pressure process to the low-pressure process enables the use of a less-expensive type of pump at the condensate system. Flash steam from the high-pressure process often provides a significant portion of the steam for the low-pressure process.

FIGURE 6. Pressurized boiler feedwater system using cascading flash steam for low-pressure process.

The systems in figures 1 through 6 are merely the tip of the iceberg. Variations and combinations are innumerable and impossible to apply without a good plant survey.