Condensate Pots

Some of the main considerations in their design and operation are 

A pressure-equalizing line must be provided. This is a small line (often 1 in diameter) connecting the top of the reboiler with the top of the condensate pot. Without this line, it will be impossible to maintain a steady pressure and level in the condensate pot. 

Arrangements 17.1c, d, and f require a large surge volume in the condensate pot in order to prevent reboiler level variations from flooding or draining the pot. In arrangement 17.le (with or with- 

Bertram (32) has discussed sizing condensate pots. The paper presents a sizing chart and a few examples, but no firm guidelines. Based on the author s interpretation of the information presented by Bertram, the following guidelines can be inferred  

In steam reboilers, a small atmospheric vent should be provided on top of the condensate drum and always left cracked open (234) (except when steam chest pressure dips below atmospheric). This will 

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Kondens-rohr, n. condenser tube, -topf, m. condensing pot steam trap, -wasser, n. condensation water, condensed water, condensing water. 

A condensing pot at the top of the reference leg is incorporated to condense the steam and maintain the reference leg filled. As previously stated, the effect of the steam vapor pressure is cancelled at the AP transmitter due to the fact that this pressure is equally applied to both the low and high pressure sides of the transmitter. The differential pressure to the transmitter is due only to hydrostatic head pressure, as stated in Equation 3-3. 

Relief line headers, individual relief lines, blowdown lines and flare lines should be self-draining from all relief valve outlets to the knock-out drum, flare stack or a point at the plant limit. A pocketed relief line system is more expensive, because usually an extra condensate pot is required with instruments, valves and pumps. To eliminate... 

Figure 7-64. With exchanger elevated just above condensate pot, flooding of tubes can be avoided. 

Figure 7 -64 shows an example. This arrangement is really a high capacity steam trap. In the layout, the top of the condensate pot should be at least in line with the bottom of the exchanger to avoid flooding the tubes with condensate and adversely affecting the exchanger heat transfer duty. 

When a condensate pot is shared by a number of reboilers, each reboiler should have a separate condensate line leading to the pot... 

Unless it is intended to maintain a liquid level in the condenser, all liquid outlet piping should leave the condenser at the bottom and slope toward the reflux drum (or condensate pot) with no high points. This practice is recommended in most cases, and is essential for gravity draining. The condenser in Fig. 15.15c experienced a liquid-removal problem (134) because its outlet was not at the bottom of the condenser. A similar problem has been reported by others (381). 

Figure 17.1 Reboiler steam control, (o) Inlet control, no condensate pot (6) outlet control, no condensate pot (c) outlet control via condensate pot level (d) outlet control, with condensate pot level override.

Rgure 17.1 Continued) Reboiler steam control, (e) inlet control, with condensate pot, no level in reboiler if) inlet control, with condensate pot, level kept in reboiler g) inlet control with a pressure regulator ih) inlet control with a seal loop. 

The condensate outlet scheme is often troublesome. If the condensate valve cannot handle the amount of condensate that the reboiler can generate, a m lximum vapor flow rate may be reached with condensate still covering a portion of the tubes. The remedy depends on the cause of the problem. If there is ample pressure difference between the reboiler and the condensate system downstream of the valve, it may be sufficient to resize the valve or condensate line. If the pressure difference is small, a condensate pot with a pump (Fig. 17.1c) may be needed to overcome the problem. 

To overcome this problem, a submerged condensate pot is often installed instead of the steam trap (Fig. 17.le) as described earlier (item 5 above). An alternative remedy is replacing the steam trap by a level condensate pot (Fig. 17.1/). By varying the level control set point, the surface in the reboiler can be adjusted so that the reboiler operates at a pressure high enough to ensure condensate removal at all times without a pump. Note that the bottom of this drum is located below the bottom of the condensing side of the reboiler (189) otherwise, "dry reboiler operation at high rates will not be possible, and reboiler capacity will be reduced. 

It is important to properly design and operate the condensate pot. In one case history (351a), a column preheater equipped with a steam inlet control scheme and with a condensate pot (no pump) experienced condensate removal problems upon turndown. It was not stated whether the Fig. 17.1e or / arrangement was used. Arrangement 17.1c needs the pump for avoiding this type of problem. Arrangement 17.1/ needs a sufQdently tell condensate pot (Sec. 17.1.3) and adequate operation of the level controller at tumed-down rates in order to avoid this problem. The author suspects that in this case (351a), one of these needs was not fulfilled. 

Successful application of the condensate pot technique above has been reported (155, 280). Sometimes, selfipriming condensate pumps without condensate pots (155) or pumping traps (20) are installed in the Fig. 17.1c arrangement. A cheaper alternative to both, but one which suffers the disadvantages described in items 1 to 3, 5, 6, 12, and 13 is using the condensate outlet scheme. 

Loop seal. In some low-pressure steam reboilers the condensate pot is replaced by a loop seal (Fig. 17.Ih). In this arrangement, increasing the flow to the reboiler raises the pressure in the reboiler shell, which in turn lowers the liquid level in the reboiler and exposes more tube area. The dynamics of this system are similar to that of the condensate outlet scheme. The height of liquid in the loop is typically 5 to 10 ft. The system can be troublesome when the reboiler heat load or the steam mains pressure tend to fluctuate, and it is usually best avoided. 

Bertram, C. G., "Sizing and Specifying Level-Controlled Condensate Pots, Hydrocarbon Proc. 60(8), 151, 1981. 

Large condensate loads are frequently present in evaporator systems. Large quantities of condensate are best handled in an instrumented condensate pot or tank in which liquid level is controlled. Liquid-level control will reduce steam consumption by 2-17%, depending on the steam pressure and trap selected for... 

The only way to change the vapor flow is therefore to change the temperature difference. Since the contribution of the sensible heat can be neglected, heat transfer is determined by the difference between the temperature of the condensing steam and the evaporation temperature of the liquid in the vessel. The evaporation can therefore be affected by increasing the pressure on the steam side or lowering the pressure on the evaporating hquid side. It should be noted that a condensation pot is essential, since it separates the vapor and condensed steam. Without it, no pressure can be built up. 

The adjustable variables can be separated into two categories, namely the flow that is used for control of the throughput and flows that are used for control loops. In case of the evaporator, shown in Fig. 3.3, three flows can be adjusted the steam flow, the feed flow and the vapor flow. The vapor flow ould not be used, since a valve in the vapor line causes an undesirable pressure drop. A valve is only useful if the vapor should be delivered at a desired pressure. The condensate flow does not have to be adjusted either, the condensate pot contains a level controller. 

A special version of a flooded reboiler designed for low-boiling materials (requiring low-temperature steam) is shown in Figure 4.4. Steam to the reboiler is throttled in a conventional fashion—usually flow controlled or flow-ratio controlled—but there is neither a trap nor a condensate pot. Instead condensate is removed through a loop seal whose top is vented to atmosphere. The height... 

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