Internal obstructions

Chapman and Wheeler (1926, 1927) conducted early flame-propagation experiments in tubes. They observed continuous flame acceleration and substantial increases in acceleration in tubes with internal obstructions (Table 4.4). These early findings were subsequently confirmed by many others, including Dorge et al. (1979,... 

A tube 10 m long and 2.5 m inside diameter was used for experiments with methane (Moen et al. 1982) and propane (Hjertager et al. 1984). These often-cited experiments showed that very intense gas explosions were possible in this tube, which had an aspect ratio of only 4 but which contained internal obstructions. Pressures of up to 4.0 bar for methane and 13.9 bar for propane were reported. Obstruction parameters, for example, blockage ratio and pitch, were varied. As with cylindrical geometry, explosions became more severe with increasing obstacle density. 

If the tube is provided with internal obstructions, the open area cross-section should be greater than about three characteristic-cell sizes. Then detonation... 

Axial dispersion Low Z/D and low Reynolds number flow conditions Vessel with baffles or internals obstructing flows High ZID and high Reynolds flow in open pipes... 

Radial dispersion (/ e)flow conditions b. Vessel with baffles or internal obstructing flows Large-diameter reactors with low flow rates. flow in open pipes [Usually ignored in... 

Internal obstructions. Studies with exothermic reactions have shown that internal obstructions such as thermowells, supports and other internal hardware that may obstruct the flow can cause channelling as discussed in Section 1.6. 

The elastic properties of the walls of the pipe and of its internal obstructions can alter during sound production... 

Nitrous gases originating from the combustion units in nitric acid plants carry small amounts of unreacted ammonia, NH3. The ammonia may react with the nitrous gas to form microscopic particles of ammonium nitrate that adhere to solid surfaces. Within a short time, there is a growing layer of ammonium nitrate salt covering the internal surface of the nitrous gas compressor (Figure 4-27). This layer can obstruct the flow passages because it tends to increase the power consumption, provoke excessive vibrations, and even present a safety hazard since ammonium nitrate explosions can occur. 

Vessel is final scrubber in a flare, relief, or vent system, is designed to withstand maximum built-up back-pressure, and has no internal or external obstructions, such as mist extractors, back-pressure valves, or flame arrestors. 

Having RMs available is only one aspect, obtaining them is another. There are real difficulties in distribution of RMs through different Customs and Quarantine systems. These obstructions in movement of RMs are not only in the developing world, but also in aU global areas. Materials that cross international borders must be accompanied by a (or maybe 20 ) customs document. One of the key pieces of information on that document is correct allocation of the tariff number to the product. The customs tariff number directly impacts on the resolution of such matters as, most importantly, the rate of import tax and the speed through customs. Unfortunately at present there is no specific tariff number for RMs. For most producers, tariff niunber 382200000 seems to fit the biU. As discussed in Chapter 7, this issue will need to be resolved at International levels before easy movement and distribution of RMs can occur. 

Several devices are also available to promote airway clearance. Flutter valve devices employ oscillating positive expiratory pressure (OPEP) to cause vibratory air flow obstruction and an internal percussive effect to mobilize secretions. Intrapulmonary percussive ventilation (IPV) provides continuous oscillating pressures during inhalation and exhalation. Finally the most commonly used technique is high-frequency chest compression (HFCC) with an inflatable vest that provides external oscillation. Vest therapy is often preferred by patients because they can independently perform the therapy even from an early age.5,14... 

For pipe fittings, valves, and other flow obstructions the traditional method has been to use an equivalent pipe length Lequiv in Equation 4-30. The problem with this method is that the specified length is coupled to the friction factor. An improved approach is to use the 2-K method,s-6 which uses the actual flow path length in Equation 4-30 — equivalent lengths are not used — and provides a more detailed approach for pipe fittings, inlets, and outlets. The 2-K method defines the excess head loss in terms of two constants, the Reynolds number and the pipe internal diameter ... 

An internal explosion is possible if air is drawn into the system through leaking packing glands, fittings or valves under conditions of negative pressure produced by a suction line obstruction. These explosions frequently lead to external fires. 

Internal (or covered) floating-roof tanks Vent fire Overfill ground fire Obstructed rim seal fire Obstructed full liquid surface fire Many fires in this type of tank occur as a result of overfilling. Tank will be extremely difficult to extinguish if entire liquid surface becomes involved. Fires in tanks with pan type covers can be expected to develop into obstructed full liquid surface fires. 

Finally, pins should be cleaned and inspected to ensure they are free of any obstructions of fhe bps prior to use. Manufacturers have specific recommendations on best methods of cleaning their pins. Some vendors such as TeleChem International (www.arrayit.com) offer cleaning solutions and equipment (sonic baths, holders, etc.) for their pins and print heads. In our laboratory, we routinely inspect pins under a microscope before and after cleaning and keep photographic images as records. 

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