Dip pipes

Note Section A-5-6.3 of the 1996 edition of NFPA 30 (reiterated in NFPA 385) contains erroneous information about switch loading. 

When a tank is emptied of Class I liquid, there is left a mixture of vapor and air, which can be, and often is, within the flammable range. When such a tank is refilled with a Class I liquid, any charge that reaches the tank shell will be bled off by the required bond wire. Also, there will be no flammable mixture at the surface of the rising oil level because the Class I liquid produces at its surface a mixture too rich to be ignitable. 

NFPA 30 also fails to recommend flow rate restrictions except a slow start until the downspout is submerged. Section 5-4 of this book provides for restricted flow rates throughout filling this should be applied wherever charge accumulation is possible due to low liquid conductivity and where flammable mixtures involving gas, mist or froth may be formed. 

Prevention of a flammable atmosphere may be accomplished using any of the alternatives presented in NFPA 69. In cases where fuel concentration cannot be limited, the most common technique (inerting) is to add a suitable inert gas such as nitrogen, so that the residual oxygen concentration is insufficient to support a flame. A safety factor is then applied. For most flammable gases and vapors this typically involves reducing the oxygen concentration to less than 5-8 vol% (see Chapter 2-7 of NFPA 69). 

FIQURE 5-1.4.4- Schematic effect of droplet diameter on MIE of typical petroleum products. 

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When the solid is one of the reactants, such as in ore roasting, the flow must be continuous and precise in order to maintain constant conditions in the reactor. Feeding of free-flowing granular solids into a fluidized bed is not difficult. Standard commercially available sohds-weighiug and -conveying equipment can be used to control the rate and dehver the solids to the feeder. Screw conveyors, dip pipes, seal legs, and injectors are used to introduce the solids into the reactor... 

An important practical question is, what is the representative pipe diameter in loading circuits comprising different sizes of pipe This has a large effect on the values calculated for velocity and velocity-diameter product. As an example, static ignition of ester mist in a rail car (5-1.3.1) involved 1450 gpm through a 6-in. pipe (v = 5 m/s and vd = 0.76 mVs) followed by a short 4-in. dip pipe assembly (y = 11 m/s and vd = 1.15 mVs). Were nonconductive liquid flow rate restrictions applied to the semiconductive ester (time constant —0.01 s) involved in this fire, the flow rate based on the 4-in. pipe would be unacceptably large based either on a 7 m/s maximum velocity or a 0.80 mVs maximum vd product. However, based on the 6-in. pipe upstream the flow velocity is less than 7 m/s and also meets API s vd < 0.80 mVs criterion. 

In reality the charging current increases in the smaller diameter dip pipe. Also, small diameter spiral-wound hose can in some cases greatly increase charging current. This effect is presumed due to increased turbulence at the inner wall [8]. Eor overhead filling of road and rail tankers the maximum vd products recommended in this book are respectively 0.38 mVs and 0.50 mVs, which are smaller than the maximum recommended values given in... 

API 2003 and BS 5958. This took into account the possibility that credit might be taken for larger upstream pipe as described in BS 5958 and that operational errors such as partial dip pipe insertion might occur. 

Note 2. A fixed dip pipe avoids possible hazards of partial dip pipe insertion (5-1.3.1). 

The use of fixed plastic tanks is especially problematic for nonconductive flammable liquids because the efficiency of grounding aids (such as a conductive dip pipe or a submerged grounded metal plate) decreases as liquid... 

Tanks with conical bottoms discharge cakes by gravity and those with dished bottoms have a spade that rakes and conveys the cake towards the outlet. Hence, the conical types require more headroom as compared to the dished type having the same filtration area. Conical tanks also have often an additional scavenging plate at the lower part of the cone to filter the residual slurry heel that remains below the main plates. The slurry heel that remains at the very bottom of the tank is removed through a special dip pipe to avoid discharging a wet cake. To facilitate better cake... 

Step 3. Heel Removal - Onee the filtration eyele is eompleted it is neeessary to remove the slurry heel that surrounds the plates. This is done by blowing air into the tank whieh displaees the slurry down to the lowest plate and further to the seavenger plate if one exists. The remaining slurry at the very bottom is reeireulated through a dip pipe baek to the feed tank until the entire slurry has been evaeuated. 

Step 3. Heel Removal - Once the filtration cycle is completed it is necessary to remove the slurry heel that surrounds the leaves otherwise the cake will be wet while being discharged. For this purpose a special dip pipe at the very bottom of the tank evacuates the remaining slurry heel which is recirculated back to the feed tank. 

The unit shown in Figure 4-49 has been used in many process applications with a variety of modifications [18,19,20]. It is effective in liquid entrainment separation, but is not recommended for solid particles due to the arrangement of the bottom and outlet. The flat bottom plate serves as a protection to the developing liquid surface below. This prevents re-entrainment. In place of the plate a vortex breaker type using vertical cross plates of 4-inch to 12-inch depth also is used, (Also see Reference [58].) The inlet gas connection is placed above the outlet dip pipe by maintaining dimension of only a few inches at point 4. In this type unit some liquid will creep up the walls as the inlet velocity increases. 

Flash steam. This will result from the blowdown heat recovery referred to in Section 7.13. Flash steam is introduced into the tank through a dip pipe terminating in a distribution manifold near the bottom of the tank. 

The third method would be continuous blowdown through a regulating or micrometer valve. The take-off position for this should preferably be about 250 mm below the working water level and may either be on the side of the shell or on the crown with a dip pipe down to the correct level. If a connection is not available, it is possible to install the valve on the bottom connection prior to the main blowdown valve. 

The blowdown from the boiler(s) will be run to a flash-steam vessel mounted adjacent to the feed tank. Flash steam will be introduced into the feed tank through a dip pipe terminating in a distribution manifold. The drain from the flash vessel may then be taken to a residual blowdown heat exchanger. Any remaining heat is then transferred to the make-up water to the tank before the blowdown runs to drain. 

Anon., CISHC Chem. Safety Summ., 1984, 55(218), 34-35 Diethylamine fumes from a reactor were usually absorbed in a glass scrubber through which sulfuric acid was circulated, but an unresolved fault in the level sensor caused the acid circulation pump to operate intermittently. While the pump was not running, amine fumes condensed in the dip pipe, forming a sohd crust (of the sulfate) which allowed a quantity of condensed amine to accumulate out of contact with the acid. When the pump was restarted, the neutralisation exotherm was sufficient to shatter the scrubber and distort the mesh guard around it. 

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