Explosions, conduit

A.m blent Environment. The environment around the flow conduit must be considered in meter selection. Such factors as the ambient temperature and humidity, the pipe shock and vibration levels, the avadabiHty of electric power, and the corrosive and explosive characteristics of the environment may all influence flow meter selection. Special factors such as possible accidental flooding, the need for hosedown or steam cleaning, and the possibiHty of lightning or power transients may also need to be evaluated. 

Explosion-proof enclosures are characterized by strong metal enclosures with special close-fitting access covers and breathers that contain an ignition to the inside of the enclosure. Field wiring in the hazardous environment is enclosed in a metal conduit of the mineral-insulated-cable type. All conduit and cable connections or cable terminations are threaded and explosion-proof. Conduit seals are put into the conduit or cable system at locations defined by the National Electric Code (Article 501) to prevent gas and vapor leakage and to prevent flames from passing from one part of the conduit system to the other. 

Flexible cord approved for extra-hard service, flexible metal conduit, and liquidtight flexible conduit for limited flexibility. A suitable grounding conductor must be provided inside the flexible cord s outer jacket. Flexible conduit must be bonded with an external jumper or an approved internal system jumper external bonding jumpers are disallowed for flexible conduit exceeding six feet. Typical liquidtight and flexible cord connectors and an explosion-proof flexible connection are shown in Figure 17-23. 

Conduit systems have the advantage of offering greater mechanical protection to enclosed conductors, but they can easily lose this advantage through corrosion if not properly maintained. A conduit system which corrodes on the inside can provide false security although tlie outside appears completely sound, the system may not contain an internal explosion. Extremely rapid corrosion will occur in salt-air environments for most conduit systems of ferrous materials. In offshore environments. 

A box or fitting must be installed at each conductor sphce connection point, receptacle, switch, junction point, or pull point for the connection of conduit system. In Division 1 areas only explosion-proof boxes or fittings are allowed. General purpose gasketed cover type fittings are allowed in Division 2 areas. 

Figure 17-24. Standard explosion-proof junction boxes ond conduit fittings. [Courtesy of Crouse-Hinds Electrical Construction Materials, a division of Cooper industries, Inc. ...

Confine internal explosions to explosion-proof enclosures and conduit systems. 

Seals are required at entries by conduit or cable to explosion-proof enclosures containing arcing or high-temperature devices in Division 1 and Division 2 locations. It is not required to seal IM in. or smaller conduits into explosion-proof enclosures in Division 1 areas housing switches, circuit breakers, fuses, relays, etc., if their current-interrupting contacts are hermetically sealed or under oil (having a 2-in. minimum immersion for power contacts and 1-in. for control contacts). 

Seals are required where 2 in. or larger conduits enter explosion-proof enclosures containing taps, splices, or terminals in Division 1 areas (but not Division 2 areas). 

Except for conduit or cable entries into explosion-proof enclosures containing arcing or high-temperature devices (as described in Item I above), cables that will leak gas through the core at a rate of less than 0.007 ft /hr at 6 in. of water pressure need not be sealed if they are provided with a continuous gas/vapor-tight sheath. Cables with such a sheath that will transmit gas at or above this rate must be sealed if connected to process equipment that may cause a pressure of 6 in. of water at the cable end. 

These devices are tested only for internal explosions and not for external explosions pressurizing the devices from the outside. As an example, a factory-sealed push-button start/stop station connected to an explosion-proof motor starter cannot suffice as a seal for the motor starter conduit entry. A separate seal must be installed at the point of conduit entry. 

To prevent the accumulation of moisture in conduits and enclosures, drains should be installed at all low points. In classified areas, breathers and drains must be explosion-proof. Figure 17-28 shows typical explosion-proof breathers and drains. 

It is important that the fire and explosion hazards of an area be carefully examined, because the expense of consistent installation of all the motors, controls, switches, instruments, and wiring can be considerable. Tables 14-8A and 14-8B summarize the National Eire Code for hazardous locations. It is equally important to be consistent and not install explosion-proof motors with nonexplosion proof wiring, because a failure in the conduit can still cause considerable damage. 

The explosion-proof design includes the use of conduit with special sealed connections around all junction boxes. 

Specifically note that Type 7 (explosion-proof) enclosures and their associated conduit systems are neither gas or liquid tight. Consequently, corrosive gases such as hydrogen sulfide and water from rain or internal condensation can accumulate with the enclosure. Premature failure of electrical devices and interconnections often results when preventive measures such as drains, air purges, and dual rated enclosures are not used to remove or exclude these corrosive elements. Type 7 enclosures are intended for indoor use. 

Many vent microbes are symbionts. Others form dense filamentous mats, such as the sulfide oxidizers Beggiatoa (oxic) and Thioploca (hypoxic). Similar mats probably develop subsurface within hydrothermal conduits. These subsurface mats are ejected into the water column during first stage of vent formation. The resulting explosive discharge of this biomass, which has the appearance of a white floe, has given rise to the term snowblower vent. The ejected microbes are thought to eventually settle back down onto the seafloor where they increase in number to form surficial mats that support the successional colonization of vent animals. 

In Germany a machine of the Niepmann type has been adapted for cartridging dynamite [7]. A schematic view of its operation is shown in Fig. 179. The explosive supplied through funnel (/) is pushed out through worm (2) into conduits (4) and (5), from which it is ejected by pistons (5). The empty paper cases are fed by conveyer ( ) into slots on the rotating table (7). When the cases reach position (8), a charge of plastic material, extruded from the orifice (6), is slipped into them. Further rotation severs the rods of dynamite while the cases, already filled, are passed to... 

Given terrorists focus on the news media as a conduit of their psychological warfare, the crisis response community competes with the perpetrators of violence for media and public attention. In this respect, a terrorist event is very different from other disasters—whether an explosion in a chemical or nuclear plant, hurricanes, floods, or other natural catastrophes. 

Jaupart C. (1998) Gas loss through conduit walls during eruption. In The Physics of Explosive Volcanic Eruptions. Geological Society of London Special Publication 145 (eds. J. S. Gilbert and R. S. J. Sparks). Geological Society of London, pp. 73—90. 

Melnik O. and Sparks R. S. J. (2002a) Modelling of conduit flow dynamics during explosive activity at Soufriere Hills Volcano, Montserrat. In The Eruption of Soufriere Hills Volcano, Montserrat, from 1995 to 1999. Geological Society of London Memoir 21 (eds. T. H. Druitt and P. Kokelaar). Geological Society of London, pp. 307-317. 

Using intrinsically safe cables/connectors allows for live connections and disconnection in hazardous areas without danger of explosion or the need for a gas test and a safe work permit. Additional savings will be realized by eliminating the need for conduit, condulets and conduit/cable seals and glands. 

Development work in the pilot-plant production of the vinyl ether intermediate was performed on a Mettler Toledo ReactlR MP m. This instrument was the hrst generation of explosion-proof manufacturing technology available from Mettler Toledo. Setup requirements for the incorporation of the ReactlR monitoring system into the vinyl ether process in the pilot plant were minor. Installation of a fiber optics cable and protective conduit from the pilot-plant operations area to the control room, allowing communication between the acquisition computer (contained within the ReactlR MP ) and the monitoring station, was the most extensive mechanical modification. ... 

Design for Hazardous Areas. This type of area classification tends to simplify the electrical design by directive means, but poses alternate choice problems, dictated by economics. The cost of an explosion-proof installation is as much as twice that using standard equipment. As an example of more costly electrical equipment, rigid metal conduit with threaded explosion-proof boxes and fittings are required for all Class I, Division 1, hazardous locations either rigid metal conduit or electrical metal tubing is approved for both Class I and II, Division 2, locations. 

---