Loop seal

Continuous water makeup and overflow on the seal loop, to ensure that the seal is always made during normal operation, and reestablished after a blow. 

The 300 mm water seal is maintained by the location of the overflows which discharge to the oily water sewer through the primary seal loop. The water... 

A secondary seal loop is provided for water withdrawal during major blows when turbulence at the downstream overflow connection to the primary seal loop interferes with normal drainage. Extending the base of the flare stack 3 diameters below the sloped inlet line provides vapor disengaging for the secondary seal leg. The bottom of the stack and inlet line up to 1.5 m above the seal water level are gunite lined for corrosion protection. 

A seal dam is provided in each drum to prevent the burner back pressure from varying the seal level. This is shown in Figure 6-8. The seal loops from the drum to the sewer should provide a seal depth of at least 10 feet. Each drum is provided with a seal water rate of approximately 20 gpm. 

Thermal-convection loops. A sealed loop filled with a liquid metal is heated in certain sections and cooled in others to cause circulation due to changes in density of the liquid metal... 

Reverse flow of vapor from the column into the reflux line would interfere with liquid downflow, condenser action, and may cause hammering if the reflux is subcooled. A seal loop (Fig. 5.1a to c) is almost always used to avoid reverse flow. The low point in the seal loop needs a drain (normally closed). 

It is best to locate the bottom of the seal loop at a lower elevation than the outlet rim of the reflux pipe (Fig. 5.1a, h, d). This prevents the seal loop liquid from siphoning back into the column. Arrangements c and e may suffer from siphoning unless an open high-point vent is installed to break that siphon (188). However, an open vent can be troublesome as described below (item 3). 

A high point in the reflux line downstream of the seal loop should be avoided (if there is no seal loop, a high point should also be avoided). Arrangement 5.1a is best, with lines downstream of the seal loop sloping toward the seal loop to avoid a high point. If the reflux nozzle is at the column head, arrangement 5.1d is best, with the vertical inlet pipe sized for vapor upflow simultaneous with liquid downflow. 

Gravity cycles often destabilize a column. Depending on the geometry and violence of fluctuations, they can also cause loss of liquid from the seal loop, vapor backflow into the reflux line, and hammering. 

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. 

Partial drawoffs (commonly utilizing downcomer trapouts) are used when a side draw does not share a pumparound drawoff. Figure 19.116 shows the preferred controls (234). Operator action is required to ensiure that the correct distillate quantity is drawn and to prevent drying of trays below the drawoff. The dryout problem can often be mitigated by drawing the side product from the bottom seal pan (i.e., just above a chimney tray). In both arrangements (Fig. 19.11a and b), note the seal loop in the line from the main fractionator to the stripper. This loop prevents vapor backflow at low liquid rates (Sec. 5.1). 

The header-box drain must have a seal loop and clean-oUt plug for inspection just above the drain hub (see Exhibit 7-37). Removal of this plug permits in-iqiection for small leaks in the tube headers in the header box. 

---