Overflow line

Fig. 3. Solvent-processing equipment using partial condenser. Level a on the water overflow line to the receiver should be about 3 cm below level b on the solvent-return line. Dimension b—c must be great enough to overcome pressure drop in the vapor piping, condenser, solvent piping, and rotameter. In a 4 m (1000-gaI) ketde, dimension b—c would be at least 1.25 m. The volume of the piping described by the dimension c—d—e should contain twice the volume of dimension b—c, thus providing an adequate Hquid seal against normal ketde operating pressures.

Use open vent or overflow line discharged to a safe location... 

Provide overflow line from filter to drain... 

For filter boxes, provide overflow line to safe location... 

This is the charge buildup just before reaching the overflow line. 

Compute the accumulated charge and energy for a 100,000-gal vessel being filled with a fluid at a rate of 200 gpm and having a streaming current of 2 X 10-6 amp. Make the calculation for a fluid having a conductivity of 10-18 mho/cm and a dielectric constant of 2.0, Repeat the calculation for (a) a half full vessel, (b) a full vessel, and (c) a full vessel with an overflow line. 

Obviously, a multidisciplined safety review committee may have detected the problems of the ill-advised use of a combination vent/overflow line, but this type of collapse is viewed as somewhat of an oddity. Several variations of this type of collapse have been reported on low-pressure and atmospheric tanks. Another incident occurred when a company requested the operating team to fill a tank as high as possible for storage needs, so they ignored the high-level alarm. After the tank level reached the overflow, the liquid started pouring out of the overflow faster than it was being pumped in and the tank collapsed. [4]... 

Every low-pressure, closed-top tank with an overflow line should be equipped with some type of device to prevent collapse from an overfilling operation. A large open nozzle, a high-level alarm and/or shutdown system, vacuum breakers or separate vent lines are some appropriate safeguards. 

Within chemical and petro-chemical plants, conservation vents and goose-neck vents protect many low-pressure tanks. This was especially true in the period 1960—1980, when industry tolerated minor volatile organic emissions. Some of these tanks did not have an overflow line. 

This 12-ft. (3.7 m) diameter and 24-ft. (7.3 m) high tank was equipped with a fill line from the process, a 4-inch (10 cm) overflow line, a 3-inch (7.5 cm) vent line and a 6-inch (15 cm) vacuum relief device. Overpressure protection was intended to be supplied by the vent system piping or the overflow line. The designers did not include a high-maintenance pressure-relief device, because the 3-inch (7.5 cm) valve-free vent line was the overpressure device relieving into the scrubber. Neither the vent line nor the overflow line were equipped with block valves. The 4-inch (10 cm) overflow line was routed to a chemical collection/treatment sewer. 

The fiberglass acid tank was out-of-service because it had just been washed. If the level in the acid tank was ever overfilled, the 4-inch (10 cm) overflow line would relieve into the adjacent chemical collection sewer. The sewer was scheduled for maintenance. As a precaution to protect the individuals assigned to repair the chemical collection sewer, an operations supervisor authorized the maintenance crew to blind the 4-inch (10 cm) overflow line. The operations supervisor provided instructions to the operators to limit the liquid level within the acid tank to a well-defined maximum. 

Figure 5-12 A large acid tank fails due to blocked vent and a blocked overflow line.

With a newly installed precautionary blind in the overflow line, the hidden blind in the vent line, and no additional overpressure devices, the tank was destined to fail. As liquid was pumped into the vessel the inerts had no place to go and the tank was pressurized to destruction. To the best of everyone s knowledge and the evidence, this hard-to-detect blind may have been inserted over a year prior to the incident for a previous internal inspection of the acid tank. Obviously the mechanic(s) who removed the other blinds a year or so before and on this occasion did not notice this one. 

However, during the steam-out process, some steam and air formed a mist which contained traces of caustic soda that escaped from the overflow line. If anyone were in the immediate vicinity during the steam-out, they might experience a bee sting sensation. A resourceful employee placed an ordinary 5-gallon plastic bucket on that overflow line and filled it with water. In this way, any steam and air would bubble through the water, capturing any possible caustic mist. 

However, the overflow line also served as a vent line. In short, during one of the times the tank was in the process of being pumped out, it partially collapsed. The water in the bucket overflow line sealed out the air and the resulting partial vacuum was so severe that the tank was completely destroyed. 

Figure 6-16 A caustic tank was fitted with a bucket on the overflow line. Courtesy of...

The fiberglass acid tank was out-of-service because it had just been washed. If the level in the acid tank was ever overfilled, the 4-inch (10 cm) overflow line would relieve into the adjacent chemical collection sewer. The sewer was scheduled for maintenance. 

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. 

Suppose that you have two tanks in series, as diagrammed in Fig. P6.12. The volume of liquid in each tank remains constant because of the design of the overflow lines. Assume that each tank is filled with a solution containing 10 lb of A, and that the tanks contain 100 gal of aqueous solution each. If fi esh water enters at the rate of 10 gal/hr, what is the concentration of A in each tank at the end of 3 hr Assume complete mixing in each tank and ignore any change of volume with concentration. 

A stream containing a radioactive fission product with a decay constant of 0.01 hr (i.e., dn/dt = 0.01 n), is run into a holding tank at the rate of 100 gal/hr for 24 hr. Then the stream is shut off for 24 hr. If the initial concentration of the fission product was 10 mg/liter and the tank volume is constant at 10,000 gal of solution (owing to an overflow line), what is the concentration of fission product ... 

The fluidized-bed section of the calciner contained the air distributor plate, the bed drainage outlet, the feed nozzle, the product overflow line, and the bed charging line. An expanded filter section containing porous stainless steel filters and the off-gas vent line was located directly above the fluidized-bed section. 

There was a slow buildup of a layer of scale on the walls of the first reactor. Insufficient agitation may have contributed to the deposition of this scale which was primarily made up of calcium sulfite/sulfate solids. The formation of this scale was also evident in the overflow pipe connecting the first and second reactors. This pipe had to be replaced in early March as the 6-inch line had been reduced to a 3-1/2 inch line due to scale buildup. This overflow line was rather long—6 feet—and had an angle of incline of only 3° because of limitations in the existing plant layout. A greater angle of incline may have reduced this scale buildup. 

CONTINUOUS GRAVITY DECANTER. A gravity decanter of the type shown in Fig. 2.6 is used for the continuous separation of two immiscible liquids of differing densities. The feed mixture enters at one end of the separator the two liquids flow slowly through the vessel, separate into two layers, and discharge through overflow lines at the other end of the separator. 

Provided the overflow lines are so large that frictional resistance to the flow of the liquids is negligible, and provided they discharge at the same pressure as that in the gas space above the liquid in the vessel, the performance of the decanter can be analyzed by the principles of fluid statics. 

For example, in the decanter shown in Fig. 2.6 let the density of the heavy liquid be and that of the light liquid be p. The depth of the layer of heavy liquid is and that of the light liquid is Zj,. The total depth of liquid in the vessel is fixed by the position of the overflow line for the light liquid. Heavy liquid discharges through an overflow leg connected to the bottom of the vessel and rising to a height Z j above the vessel floor. The overflow lines and the top of the vessel are all vented to the atmosphere. 

Equation (2.14) shows that the position of the liquid-liquid interface in the separator depends on the ratio of the densities of the two liquids and on the elevations of the overflow lines. It is independent of the rates of flow of the liquids. Equation (2.14) shows that as approaches pjj, the position of the interface becomes very sensitive to changes in Z 2> Hi height of the heavy-liquid leg. With liquids that differ widely in density this height is not critical, but with liquids of nearly the same density it must be set with care. Often the top of the leg is made movable so that in service it can be adjusted to give the best separation. 

In Fig. 3.19a F, = A, since the equilibrium solutions in both underflow and overflow have the same composition. Also F/ = 0 in the overflow when there is complete drainage and the carrier is not soluble in the solvent. In the y-x diagram, the underflow line AB is parallel to the overflow line FD, and the extrapolated tie lines (e.g., FE) pass through the origin (100% inerts). 

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