High liquid level

K. Reboiler Return. The top of the reboiler return nozzle should be at least one nozzle diameter plus 12 in. (18 in. minimum) below the bed support. The bottom of the reboiler return nozzle should be 12in. minimum above the high liquid level and at least one nozzle diameter (18in. minimum) above the normal liquid level. ... 

The minimum vapor clearance height above high liquid level is 20% of drum diameter. If possible this should be greater than 10 inches. 

Avoid vapor entry close to a liquid level. Reboiler vapor should enter the bottom of a fractionator a distance of at least tray spacing above high liquid level. Tray damage can result if the liquid is disturbed. 

Vertical Drums - The wetted vessel surface within 7.5 m of grade, based on normal high liquid level, is used. If the entire vessel is 7.5 m or more above grade, then only the surface of the bottom head need be included. 

Fractionators and Other Towers - An equivalent "tower dumped" level is calculated by adding the liquid holdup on the trays to the liquid at normal tower bottom (high liquid level). The surface that is wetted by this equivalent level and which is within 7.5 m of grade is used. 

Liquid Overflow Inflow exceeds outflow Liquid slug How Blocked or restricted liquid outlet Level control system failure High liquid level... 

Too high liquid level above design could cause reboiler to have less capacity... 

Remember, though, that the increased tower-bottom liquid level will not be reflected on the indicated bottom level seen in the control room, which is actually the level at the end of the kettle reboiler. This is a constant source of confusion to many operators, who have towers that flood, as a result of high liquid levels, yet their indicated liquid level remains normal. 

High liquid levels in the bottom of the stripper will also reduce stripping efficiency. A liquid level above the steam inlet will cause the stripping trays to flood. Flooding vastly decreases tray efficiency, and hence stripping efficiency. 

Compressors are also served by high-liquid-level trips in their upstream knockout drums. These high-liquid-level trips work in the same way as the low-level boiler trips discussed above, except that the mercuroid switch is activated by a rising, rather than a falling, liquid level so as to protect the compressor from a slug of liquid. 

The improper gasket may have been installed months or perhaps years before. Typically this gasket is exposed to a high vacuum and caustic soda vapors. Unrelated problems within the unit that day created a high liquid level in the vessel that exposed the gasket to atmospheric pressures and molten-liquid 98-percent caustic soda. 

The receiver tank s liquid level indicator will not be in service during the transfer. A separate, high-liquid level switch is interlocked to stop the transfer when the tank is full. The quantity transferred can be monitored from the distribution system, but it will not be specified. 

Figure 5 Nucleate boiling curves. F + (PF-5060), F- (PF-5050), LL+ (high liquid level), LL- (low liquid level), P+ (high pressure), and P- (low pressure)...

TRAPS OR LIQUID COLLECTION DRUMS. GAUGE GLASS. DRAIN OUT VALUE, HIGH LIQUID LEVEL ALARM CONTACTS TO BE FURT ISHED BY COMPRESSOR MANUFACTURER PURCHASER... 

In large-diameter packed colxunns where I-beams support the packing support plate, a larger space between the reboiler return nozzle and the packing support plate may be required. The prime consideration here is allowing sufficient open area between the bottom of the beam and the high liquid level for adequate vapor distribution. Detailed discussion is in Sec. 8.2. 

An overflow pipe (or downcomer), with an opening located below the top of the risers, is recommended for preventing liquid from overflowing the risers at high liquid levels (Fig. 4.10). The overflow pipe should be liquid-sealed at the bottom to avoid vapor rise through it. Two experiences have been described (57, 237) where failure to provide an overflow pipe caused liquid to overflow the risers and prematurely flood the column section above the chimney tray. In a third case (334), liquid overflowing the risers caused entrainment (Sec. 8.2). 

I-beam interference at the column base (Fig. 8.6a) can be prevented either by installing two feed inlets, one on either side of the I-beam, or by providing sufficient area under the I-beam for vapor cross flow. For a single feed inlet, it has been recommended (237) that the vertical distance between the bottom of the I-beam and the high liquid level be set such that the calculated pressure drop for half the feed vapor flowing under the I-beam is less than 10 percent of the anticipated pressure drop in the packed bed above. 

Low liquid levels can be as troublesome as high liquid levels. When bottom level is lost, vapor can flow out of the column bottom. In one incident (210), such vapor flow ruptured the bottom product storage tank. Low bottom levels can also cause cavitation and overhehting of bottom pumps. In some services, a low bottom level can excessively cottCentrate some chemicals, inducing an undesirable reaction. If these chemicals are unstable (e.g., peroxides, acetylenic compounds), an explosion may result. Some reported accidents (97, 275) were initiated by low liquid levels at the bottom sump. 

Figure 15.6 illustrates the sequence of events of surging. Once lights are vaporized, the mixture temperature rises, and heat transfer is reduced. This retards thermosiphon action, increases fractional vaporization, and further depletes the lights. As vaporization rate drops, or even stops, the column dumps, giving a high liquid level and excessive lights in the bottom. Vaporization is resumed, and the cycle repeats itself. 

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