Baffles pass-partition

Shell-and-tube exchangers contain several types of baffles to help direct the flow of both tube-side and shelbside fluids. Pass partition baffles force the fluid to flow through several groups of parallel tubes. Each of these groups of tubes is called a pass, . since it passes the fluid from one head to another. By adding pass partition baffles on each end. the tube-side fluid can be forced to take as many passe.s through the exchanger as desired. 

This improved heat-transfer rate, promoted by low velocity, applies not only for condensing steam but also for condensing other pure-component vapors. And since condensation rates are favored by low velocity, this permits the engineer to design the steam side of reboilers and condensers in general, for low-pressure drops. For example, if we measured the pressure above the channel head pass partition baffle shown in Fig. 8.1, we would observe a pressure of 100 psig. The pressure below the channel head pass partition baffle would typically be 99 psig. 

Figure 8.6 Venting below the pass partition baffle stops carbonic acid corrosion.

One important feature of Fig. 8.6 is the condensate drum balance line. Note, that this line is connected below the channel head pass partition baffle. This ensures that the pressure in the channel head, below the pass partition baffle, and the pressure in the condensate drum, are the same. If these two pressures are not identical, then the level in the condensate drum cannot represent the level in the channel head. For this reason, never connect the condensate drum vapor space to either the steam supply line or the top vent of the reboiler s channel head. 

Steam produced from demineralized water is free of carbonates. Steam produced from lime-softened water will be contaminated with carbonates that decompose in the boiler to carbon dioxide. As the steam condenses in a reboiler, the C02 accumulates as a noncondensable gas. This gas will be trapped mainly below the channel head pass partition baffle shown in Fig. 8.6. As the concentration of C02 increases, the C02 will be forced to dissolve in the water ... 

The pass partition baffle shown in Fig. 14.5 makes this cooler a two-pass exchanger. These baffles are subject to failure, due to corrosion. More often, they break because of excessive tube-side pressure drop. The differential pressure across a two-pass pass partition baffle equals the tube-side AP. 

Once the pass partition baffle fails, the process fluid may bypass the finned tubes, and cooling efficiency is greatly reduced. This is bad. But worse yet, during a turnaround of the cooler, there is normally no way to inspect the pass partition baffle. There is no easy way to visually verify the mechanical integrity of this baffle. A few air coolers have removable inspection ports for this purpose most do not. 

Inside the exchanger s channel head, we have the pass partition baffle, which divides the channel head into two equal portions. This baffle forces the total flow only through the bottom half of the tubes. The tubes themselves are pipes of either 3/4 or 1 in OD (outside diameter). The front end of each tube is slipped into a slightly larger hole drilled into the channel head tubesheet. This tubesheet is a disk about 2 in thick, slightly larger than the inner diameter of the shell (shell ID). 

The center channel head pass partition baffle is cut out. 

Two off-center channel head pass partition baffles are welded in place, so that 25 percent of the tubes are above the upper baffle and 25 percent of the tubes are below the lower baffle. 

A new, center, pass partition baffle is welded in the floating head. The floating-head tubesheet must also be remachined. 

Accupfiulations of non-condensables in the channel head will also retard the condensation rate of steam and lead to a loss in reboiler duty. Typically, CO2 contained in the steam supply collects below the bottom channel head-pass partition baffle. If left to accumulate, the CO2 will dissolve in the steam condensate and form corrosive carbonic acid. 

Parallel passes (coking heater), 78 Partially coked resid, 40 Pass partition baffle, 27 Pass partition failure (heat transfer equipment), 433—435 Performance test planning, 486-497 strategy, 487-490 laboratory, 487 instruments, 487-488 interfacing units, 488 shift operators, 488 mechanical department, 488 flag sheet, 488-489 onstream analyzers, 488 day before test, 490-491 unit optimization, 490-491 test day, 491 data correlation, 491-496 capital projects, 495 checklist, 497 Performance testing (process unit), 482-483. See also Performance test planning. 

Excessive tube-side pressure drop due to fouling can cause the channel head pass partition baffle to fail. (Consult the TEMA data book for exchanger details. ) This pass partition baffle prevents the crude from bypassing the tube bundle. However, as the tube-side AP rises, this baffle will eventually fail and lead to a sudden loss in the preheat exchanger duty. The maximum allowable pressure difference across this baffle should be listed on the exchanger data sheet. 

Most of my experience with this subject is a loss in reboiler duty (over one day) due to a tube leak, or the more gradual loss of reboiler duty (over one month) due to CO accumulation in the channel head. Regardless, to alleviate the loss of reboiler duty, vent the channel head just belozv the bottom channel head pass partition baffle. 

Often the problem with CO accumulation inside the channel head results not so much in corrosion, but in the loss of heat transfer. This is due to the tubes below the bottom pass partition baffle filling with non-condensable CO gas. I have observed this problem even in plants using demineralized boiler feedwater. Venting from beneath the bottom pass partition baffle will restore heat transfer rates. Venting above the pass partition baffle or from the top of the channel head is futile. Unfortunately, most such channel head vents are installed on the top of the channel head, and thus are completely ineffective. 

There is always a small amoimt of CO in steam. It originates from residual carbonates in the BFW. Compared to steam, the CO is a noncondensable with a limited solubility in H O. As the steam condenses, the few ppm of CO will be trapped on the steam side of the tube bundle inside the channel head. However, none of the CO will be trapped above the pass partition baffle shown in Fig. 13.1. And all of the COj will be trapped below the pass partition baffle. Therefore, the problem is that, venting from valve C in Fig. 13.1 will vent off steam, but no COj. To purge the accumulated CO from the tube bundle, vent valve B must be used. 

If CO is not vented off from valve B, then it will begin to dissolve in the condensate to form H CO (carbonic acid). This acid causes tube failures due to acidic corrosion. Venting gas below the pass partition baffle will stop this corrosion, according to the Shell expert. I don t have any personal experience in stopping CO corrosion with such venting. But it seems to make good engineering sense to me. 

Converting back to four passes is very difficult, because the tube ODs are so tightly packed with twisted tubes, and there is very little space to accommodate additional new pass partition baffles. 

I see this frequently when tube bundles are extracted from the shell during a unit turnaround. The distorted tubes interfere with the proper fluid flow through the shell side of the exchanger and likely promote both shell-side fouling and shell-side bypassing. Also, as the tubes plug off, tube-side AP increases. If half the tubes plug, then the differential pressure across the channel head pass partition baffle will increase by a factor of four and may result in the failure of the channel head pass partition baffle. 

One of the mistakes 1 have made in the past is due to air accumulation in the channel head (see Fig. 32.1). That is. I ve confused the effect of trapped air with fouling. Especially on start-up, air may be trapped both above and below the pass partition baffles. The air can fill some of... 

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