Backup condensate

Water Hammer and Condensate Backup Steam-Side Reboiler Control... 

Sub-cooled condensate Figure 8.3 Effect of condensate backup. 

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. 

Figure 8.5 Condensate backup controls steam flow.

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. 

Operators who have problems with loss of reboiler capacity often attribute these problems to condensate backup. This is usually true. To drop the level of water out of channel head, either the steam trap or the... 

The vapor is drawn into a steam jet (discussed in Chap. 16). The steam condensate flows into the boot or hot well. The water in the boot is slightly subcooled. This is accomplished by a pair of baffles that create a small zone of condensate backup. The subcooled condensate, cooled to perhaps 10°F below its boiling or bubbling point, is easier to pump. As the pressure in the hot well is subatmospheric, the hot-well pump typically develops a AP of at least 30 to 50 psi. 

Air-cooled surface condensers. Figure 8.11 shows a surface condenser elevated above the steam turbine. This creates an additional problem, in that moisture from the turbine exhaust steam will accumulate in the bottom of the turbine case. A special drain line from the turbine s case is needed to prevent condensate backup from damaging the spinning wheels. 

Figure 8.11 A leaking flange restricts condensate backup.

Does this mean that it is a lot easier to condense steam than to cool water Yes. This also explains, then, why condensate backup reduces the rate of heat transfer and condensation. 

Figure 13.3 Condensate backup in a shell-tube heat exchanger.

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. 

Effect of condensate backup. When the condensate level in an exchanger increases, the area of the condenser devoted to subcooling the condensate increases. But the area of the exchanger available for condensing decreases. That is bad ... 

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. 

Figure 13.6 Hot-vapor bypass pressure control. Note condensate backup.

The increase in the vapor outlet temperature from a condenser, as compared to a decrease in the temperature of the condensate from the same condenser, is a sure sign of condensate backup. The condensate is covering some of the tubes in the surface condenser. This subcools the condensate and does no harm. 

However, the number of tubes exposed to the condensing steam is also reduced. This forces the steam to condense at a higher temperature (as discussed in Chaps. 8 and 13). In effect, the condensate backup has reduced the surface area of the condenser, available to condense the steam. The higher the condensation temperature of the steam, the higher the condensation pressure of the steam. Just like the deaerator I described in Chap. 15. 

Surface condenser problems. These include undersized surface condenser area, water-side fouling, lack of water flow, condensate backup, and excessive cooling-water inlet temperature. To determine whether a poor vacuum in a surface condenser is due to such heat-transfer problems, plot the same point on the chart shown in Fig. 18.4. If this point is on or slightly below the curve, it is poor heat transfer in the surface condenser itself that is hurting the vacuum. 

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. 

To determine if this problem exists on a reboiler, establish a definite condensate seal by restricting the condensate effluent line. If the tower bottom s temperature increases, the reboiler has a blown condensate seal. This wastes steam and reboiler capacity. The correction is the same for condensate backup installation of a steam condensate seal drum. 

Corrosion inhibitor Condensate backup in channel head... 

Steam reboilers are subject to a wide variety of serious capacity problems. The most common is steam condensate backup into the channel head. This is checked by manually draining the steam condensate to a nearby sewer and observing if the reboiler duty increases or the steam inlet control valve closes. 

In one instance the effect of steam condensate backup was clearly observed during a unit turnaround. As shown in Figure 12-10, the rust layer on the inside of the channel head of a reboiler showed clearly where the condensate level normally ran. This was a positive indication that 20% of the heat-transfer surface area of the reboiler was waterlogged and, therefore, useless. Steam condensate backup can be due to one of the following ... 

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. 

To minimize condensate backup. If condensate fills up tubes, then those tubes are unavailable to condense steam. Thus, the rate of heat transfer will drop in proportion to the tubes filled with water. 

Figure 13.3 The effect of vapor lock is to promote condensate backup and...

To suppress the evolution of steam in the drain line, the condensate must be subcooled from 320°F to 270°F as it leaves the reboiler. This can only be accomplished by condensate backup. And if the condensate is not subcooled to 270°F, then the condensate will begin to vaporize. Which will then ... 

The problem with condensate backup and subcooling is the very low heat transfer coefficient (U) for subcooling vs. condensation. Typical heat transfer coefficients (Btu/hr/fb/°F) are ... 

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