Consider what befell a large site that originally had conventional condensate handling. As plant operations pushed capacity, lots of services had difficulty in keeping the condensate at high enough pressure to return it to the boiler house. So, many steam exchangers dumped condensate to the concrete pads in the units. In winter the condensate froze, turning large areas of the site into skating rinks. Something had to be done — so the plant installed a fourth, low-pressure condensate system.
Figure 1. Too simple a solution -- Low pressure of
Adding a new condensate system doesn’t necessarily create problems. How you add the system may. The plant buried a vessel for condensate collection on the new system (Figure 1). To minimize pressure, the vessel had an atmospheric vent. A submerged pump sent condensate to the boiler house. A large amount of excess capacity was provided to deal with future expansions.
Condensate flashes when depressurized. The amount of flashing as condensate enters the new drum depends upon the upstream pressure of the condensate. To eliminate equipment, most of the plant’s steam heaters used partially flooded bundles to control exchanger duty (“Don’t Let Heater Control Get you Steamed,” www.ChemicalProcessing.com/articles/2004/81.html). One consequence of using condensate level in a partially flooded bundle for duty control is that the exchanger pressure is always at a maximum on the steam side.
Because increasing exchanger duties created further problems with the other condensate systems, more and more condensate sources were routed to the new atmospheric-pressure system (the one with the extra capacity). The combination of partially flooded exchangers and the low pressure of the new system maximized the pressure drop between the normal consumer and the condensate recovery drum. Significant amounts of condensate vaporized.
The condensate vaporization created two problems. First, much of the steam was vented. The same winter conditions that led the dumped condensate to freeze also led the vented steam to freeze out as icicles on surrounding equipment. Over weeks a large area crusted with ice developed. This caused significant safety and operating problems. (Plus the vented steam is a lost opportunity for energy savings.) Second, the volume expansion from the condensate flashing started to create pressure drop and erosion problems in the low pressure condensate lines.
Placing strategically located intermediate flash drums and replacing the buried drum with a properly designed condensate recovery system solved the plant problems. Steam from flashing condensate was recovered close to groups of major steam consumers. This dramatically reduced condensate flashing in the existing plant lines. The recovered steam went to the next lower pressure level. The energy savings alone paid for much of the work required.
Don’t let simple solutions grow over time into unmanageable problems. Keep in mind that eventually systems reach their limits. At that point, make a step change to improve plant operations instead of letting poor situations fester.