Condensate backup causes

The reason is condensate backup. The condensate backup causes subcooling that is, the liquid is cooled below its bubble point, or saturated liquid temperature. Perhaps a rat has lodged in the condensate outlet pipe. The rat restricts condensate drainage from the shell side. To force its way past the dead rat, the propane backs up in the condenser. The cold tubes in the bottom of the shell are submerged in liquid propane. The liquid propane is cooled below its bubble-point temperature. 

We can see, then, that either condensate backup, or blowing the condensate seal, will cause a steam reboiler to lose capacity. If you think either of these two problems could cause a loss in reboiler duty, try opening the bypass around the steam trap. If the reboiler duty goes up, the problem was condensate backup. If the reboiler duty goes down, then the problem might be a blown condensate seal. If it looks like a blown condensate seal problem, close the steam trap bypass. Then, partially close the valve downstream of the steam trap. If this increases the reboiler duty, a blown condensate seal failure is proved. 

When high-pressure steam (>100 psig) is used, rather significant leaks of hot condensate and steam can be caused by a variable condensate level in the channel head. For such higher-pressure steam sources, control of steam flow with condensate backup, as shown in Figs. 8.5 or 8.6, is best avoided. 

Heat removed by condensation is easy. The heat-transfer coefficient U for condensation of pure, clean, vapors may be 400 to 1000 Btu per hour per ft2 of heat exchanger surface area, per °F of temperature-driving force. The U value for subcooling stagnant liquid may be only 10 to 30. Condensate backup is the major cause of lost heat transfer for heat exchangers, in condensing service. 

Thermostatic traps are temperature-sensitive traps there are several basic types. They respond to either a temperature difference between the steam and the condensate or directly to a temperature of either steam or condensate. All thermostatic traps are operated and controlled by the temperature in the line upstream of the trap time Is necessary for the operating elements to either absorb heat to cause the valve to close or dissipate heat to cause the valve to open. They usually discharge condensate below the steam temperature and require a collecting leg before the trap to permit some subcooling of condensate. Therefore, sufficient piping length should be provided at the trap inlet to prevent the condensate backup from interfering with the heat transfer surface. 

The opposite problem to steam condensate backup is blowout of uncondensed steam through the reboiler and out the condensate drain line. This phenomenon causes a loss in heat transfer entirely out of proportion to what might be expected. Literally half of a reboiler s duty can be lost by an apparently small amount of steam blowing out the condensate drain line. 

Condensate backup is also caused by a large number of problems associated with the barometric drain line, and especially with the seal drum, shown in Fig. 23.2 ... 

Figure 11.11 A leaking flange causes condensate backup.

Under conditions where reactor afterheat would be insufficient to provide adequate backup to the reactor plant, the boiler must be operated within its normal control range, available to provide steam to the drive turbines and deaerator if required. Regardless of the standby status of the boiler, a reactor scram shutdown will automatically cause the boiler to be brought into normal control range as rapidly as possible. An equivalent amount of steam generated by the boiler will be indirectly dissipated to the dump condensers when the reactor is operating at normal levels. 

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