Tubesheet vent

Tubes are fastened to tubesheets by welding, mechanical rolling, or both. However, these joints are susceptible to thermal and pressure stresses and may develop leaks. In those instances where mixing of the shell and tube fluids would result in corrosive or other hazardous conditions, special designs such as double tubesheets are used, with the space between the tubesheets vented. A double tubesheet can be used only in the following rear head designs fixed tubesheets (L, M, or N) and the outside packed head (P). Bimetal tubes are used when corrosive conditions of shell and tube fluids require the use of different metals. 

It is good practice to provide tubesheet vents (134, 254, 358 Fig. 15.106) opposite the top vapor inlet nozzle in a vapor-heated vertical reboiler (condensing on the shell side). This is essential in steam reboilers, where a small steam purge should be continuously vented to atmosphere. Shell corrosion due to CO2 accumulation is known to have occurred in steam reboilers that had no such vent (234, 254). 

Sometimes a combination of backwashing and air injection is used with good results. During backwashing, silt deposits may remain in the space between the back face of the tubesheet and the nozzle. Air can be injected through the lower tubesheet vent or drain connections to remove these deposits. 

Proper venting of the condenser to eliminate the dead space below the top tubesheet is extremely important. This is to ensure that there is no vapor space and that the tubes will be wet all the time, thus avoiding wetting and drying cycles.10 About one-third of SCC failures were caused by the existence of a vapor phase.12,13 The proper method of venting vertical water-cooled condensers, is given in the MTI Manual.4... 

Vertical steam-heated exchangers must also be vented to remove carbon dioxide and other gases which can accumulate under the top tubesheet. Corrosion of the shell, tubes, and tubesheet, especially in the area just opposite the steam inlet nozzle, may result if adequate venting is not provided. Sometimes a continuous steam purge or intermittent venting is recommended. 

Venting inerts from the floating head end of a horizontal reboiler can be difficult. A novel method which successfully accomplished this (232) was installing a 1-in internal pipe that extended a top tube from the channelhead tubesheet to a vent in the channel head (Fig. 15.10c). The top tube was thus converted into a "vent tube. The internal pipe was coupled to the vent from inside the channel head, to permit removal. This technique cured a CO2 corrosion problem attributed to poor venting at the floating head end. 

Downcomers must be sized to be self-venting and to minimize liquid holdup on the top tubesheet. In order to avoid entrainment of vapor in the downcomer, the superficial liquid velocity should not exceed 0.4 ft/sec, based on the expected liquid circulation rate. Several downcomers are usually preferred in order to reduce the flow path liquid must take in order to reach the downcomer. 

The tubesheet passages can be pressurized, purged, vented, or monitored to detect leaks. Separate vents and drains are required in the gap in each pass of multipass units. The integral design construction requires fairly expensive tooling and careful attention to tool speeds and coolants. The method is usually limited to relatively free-machining materials. 

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