Self-venting lines

When vapor disentrainment is the controlling consideration, and it appears attractive to minimize the sump size, the liquid pipe downstream of the sump can be sized to provide a self-venting flow. In this case, the liquid residence time in the pipe contributes to the residence time requirement, and the sump itself can be designed for a smaller residence time. The high points on the self-venting lines should be vented back to the column. 

A short-cut correlation (354) for sizing self-venting lines is shown in Fig. 4.5. The author has had a lot of favorable experience with this correlation. With foaming systems, pipe diameter should be increased in proportion to the foaming factor (355). Caution with applying Fig. 

When the bottom sump does not supply liquid to a reboiler (e.g., when a reboiler trapout pan is used, or when the column has a bottom feed and no reboiler), the design in Fig. 4.86 can be used with thermally unstable materials. This arrangement eliminates the need for a surge drum and self-venting lines, and immediately quenches liquid reaching the column base. 

Condensate must often flow by gravity from the condenser. Hence, nozzles must be properly sized to ensure that the condensate line will be self-venting. Liquid in a self-venting line occupies a portion of the flow area, and gas moves freely with, or counter to, the liquid. Condensate nozzles should also be sized to prevent excessive liquid levels, especially in horizontal condensers. An equation for determining proper nozzle size is ... 

An 800-gal reaetor eontaining a styrene mixture with a speeifie heat of 0.6 eal/gm °C has a 10-in. rupture disk and a vent line with equivalent length = 400. The vessel MAWP is 100 psig and the rupture disk set pressure is 20 psig. The styrene mixture had a self-heat rate of 60°C/min at 170°C as it is tempered in a DIERS venting test. Determine the allowable reaetor mixture eharge to limit the overpressure to 10% over the set pressure. 

The molten material, after holding for 4 h at 78°C in a stainless steel vessel, underwent a thermal runaway reaction and 500 kg erupted through the vent line. It was later found that addition of 0.1% of rust to the hot material led to an accelerating self-condensation Friedel-Craft reaction, catalysed by iron(III) chloride, which led to formation of poly-benzyls accompanied by evolution of hydrogen chloride. 

During maintenance work, simultaneous release of chlorine and acetylene from two plants into a common vent line leading to a flare caused an explosion in the line [10]. The violent interaction of liquid chlorine injected into ethane at 80°C/10 bar becomes very violent if ethylene is also present [11]. The relationship between critical pressure and composition for self-ignition of chlorine—propane mixtures at 300°C was studied, and the tendency is minimal for 60 40 mixtures. Combustion is explosive under some conditions [12]. Precautions to prevent explosions during chlorination of solid paraffin hydrocarbons are detailed [13]. In the continuous chlorination of polyisobutene at below 100°C in absence of air, changes in conditions (increase in chlorine flow, decrease in polymer feed) leading to over-chlorination caused an exotherm to 130°C and ignition [14],... 

B). If downstream pressure is greater, then a barometric leg may be used to maintain a seal, as shown in Figure 3.5(C). The overflow line shown must be adequately sized to self-vent otherwise, it may begin to siphon. 

FORMAT (lOX, LINE IS SELF-VENTING THEREFORE NO VIBRATION, ) WRITE (1, 200)... 

If no liquid level is maintained in the condenser, and the liquid is not substantially subcooled, vapor bubbles are likely to be entrained in the liquid condensate. The outlet line should therefore be sized for self-venting flow (Fig. 4.5). Moore (290) has suggested a slightly more conservative rule of thumb for gravity drainage of condensers (assuming no liquid accumulation)... 

This equation predicts nozzles that act as circular weirs. No whirlpool will exist the line will be self-venting and the ratio of liquid height above the nozzle to nozzle diameter will be 0.25. 

Lines will be self-venting if the following correlation is used to determine drain nozzle sizes ... 

The pump suction line is self-venting with continually upward sloped pipe from the pump suction to the hot leg. This arrangement prevents entrapment of air and minimizes system... 

Self-contained control systems combine the sensor, controller, and controlled device into one unit. In this system, a change in the controlled variable is used to actuate the controlled device. A self-contained pressure relief valve is an example of this type of control device. If the pressure in the line exceeds the maximum set-point— which may be fixed or adjustable—it is vented to relieve pressure and protect the end devices, along with ensuring the safety of the operator and occupants. 

A seal should maintain its integrity for six years, provided it is operating correctly. Most often seal life is compromised by improper start-up procedures. For example, starting up a pump without venting the seal flush lines to the inner seal on a self-flushed pump may reduce the operational seal life by six months, even though the seal faces have run dry for only a few seconds. 

For self-flushed pumps, it is necessary to vent and fill the seal flush lines completely. Otherwise, on starting the pump, there will be no seal flush flow for a second or so, which will materially damage the seal. 

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