Vented Construction

For increased power requirements, electrode constractions have been developed which bring the electronic conductors into closer contact with the active material particles first, around 1930, the sinter electrode [109], recently in sealed cells largely replaced by the nickel-foam electrode, and then, around 1980, the fiber stracture electrode [110]. In order to take full advantage of their increased performance, the electrodes have to be as close together as possible that is, a uniformly thin, highly porous separator is required with suflfidently small pores to prevent any 

Better mechanical stability can be expected from irradiated polyethylene or microporous polypropylene ( Celgard ) membranes, but these account for increased electrical resistance values. 

One version of the microporous, filled polyethylene separator ( PowerSep ) [113], which is so successful in the lead-acid battery, is also being tested in nickel-cadmium batteries. This separator is manufactured largely in the same way and also has similar properties to those described in Section 11.2.2.1. Of course, silica cannot be used as a filler, but has to be replaced by an alkali-resistant substance, for example, titanium dioxide. The resulting separator membrane excels, with very small pore sizes and low electrical resistance as well as outstanding mechanical properties. A comprehensive presentation of the different separation materials follows in Section 11.3.5. 

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The voltage profile of a sealed nickel-cadmium batteries is different from the one for a vented one, as illustrated in Fig. 28.26 The end-of-charge voltage for the sealed battery is lower. The negative plate does not reach as high a state of charge as it does in the vented construction because of the oxygen recombination reaction. 

Most major producers of mercury—zine eells and batteries use a vented construction, which renders their products leakproof and free from bulging under all normal working conditions. Sell-venting oeeurs automatically if, for example, excessive gas is produeed within the cell under sustained short-eireuit eonditions. This type of cell is manufactured in a wide variety of structures to provide batteries of varying size, voltage and capacity. A wide range of purpose-built batteries can also be obtained. For maximum reliability intereell coimection is by spot welded strips of niekel-plated steel. Outer containers are made from materials ranging from cardboard to metal. 

Volume 1 of National Fire Codes (National Fire Protection Association, Quincy, Massachusetts) contains recommendations (Code 30) for venting, drainage, and dike construction of tanks for flammable liquids. 

FIG. 11-44 Typical construction of a tube bundle with plug headers (1) tube sheet (2) plug sheet (3) top and bottom plates (4) end plate (5) tube (6) pass partition (7) stiffener (8) plug (9) nozzle (10) side frame (11) tube spacer (12) tube-support cross member (13) tube keeper (14) vent (15) drain (16) instrument connection. (API Standard 661.)... 

FIG. 25-72 Typical detail of an isolated gas vent. (From Bagchi, A., Design, Construction, and Monitoring of Sanitary Landfill, Wiley, 1.9.90,)... 

Design criteria and materials of construction (for example, vents, relief valves)... 

Pistons may be of segmented construction to permit the use of one-piece wear bands. One-piece wear bands are a requirement in API 618. Pistons have a problem in common with humans—a weight problem. Weight in a piston contributes directly to the compressor shaking forces and must be controlled. For this reason, aluminum pistons are often found in larger low pressure cylinders. Hollow pistons are used but can pose a hazard to maintenance personnel if not properly vented. If trapped, the gas will be released in an unpredictable and dangerous manner when the piston is dismantled. 

At the Lipari Landfill site in New Jersey, the gas venting system installed in 1983 consisted of two underground 4-inch (10-cm) perforated PVC pipes and five vent risers. Two parallel PVC pipes were installed 200 feet apart prior to cap emplacement during the period of wall construction and were placed about 3 feet beneath the finished grade. The vent risers were connected to the buried manifold pipes and consisted of open pipe installed perpendicular to the underground pipes and the ground surface. Filters were not attached to the riser exit points (U.S. EPA, 1987). 

No particular industrial hazards are encountered in handling liquid alum. However, a face shield and gloves should be worn around leaking equipment. The eyes or skin should be flushed and washed upon contact with liquid alum. Liquid alum becomes very sick upon evaporation and therefore spillage should be avoided. Storage tanks may be open if indoors but must be closed and vented if outdoors. Outdoor tanks should also be heated, if necessary, to keep the temperature above 450F to prevent crystallization. Storage tanks should be constructed of type 316 stainless steel, FRP, steel lined with rubber, polyvinyl chloride, or lead. Liquid alum can be stored indefinitely without deterioration. 

Outdoor air is generally less polluted than the system return air. However, problems with reentry of previously exhausted air occur as a result of improperly located exhaust and intake vents or periodic changes in wind conditions. Other outdoor contamination problems include contaminants from other industrial sources, power plants, motor vehicle exhaust, and dust, asphalt vapors, and solvents from construction or renovation. Also, heat gains and losses through the building envelope due to heat conduction through exterior walls, floor, and roof, and due to solar radiation and infiltration, can be attributed to effects from external sources. 

Another somewhat similar incident occurred when butadiene from a reactor flowed in the wrong direction up a line used for adding emulsifier. The check valve, which should have prevented the flow, was obstructed. The emulsifier tank was in a building and had an open vent. Butadiene came out and exploded. The explosion was heard 15 km away, but damage was minimized by the light construction of the building, w hich ruptured at the junction of the roof and walls [13]. 

Gas-fired water heaters use the same general method of construction, except that the elements are replaced with a burner beneath the tank. The combustion products from the burner are vented through a flue made out of the same thickness steel as the tank, that goes up through the center of the tank. To increase heat transfer from the hot flue gases to the inner wall of the flue, a baffle is inserted down the flue. This baffle is a twisted strip of sheet metal with folds and tabs on it. The folds and tabs are designed to... 

The designer must examine the specific valve selected for a service and evaluate the materials of construction in contact with the process as well as in contact or exposed to the vent or discharge system. Sometimes the corrosive... 

Mold construction FRP spray metal, cast aluminum gusket seal, air vents, self-sealing injection port FRP FRP, spray metal, cast aluminum, pinch (land) Metal, shear edge High grade steel shear edge... 

Hot gases rise by thermal lift. Hence in the open air they will disperse. Within buildings this is a serious cause of fire escalation and toxic/asphyxiation hazards if smoke and hot gases are able to spread without restriction (or venting) to upper levels. A balanced flue can serve to effectively isolate a combustion process in a gas-fired appliance, but must be sound in construction and unrestricted to avoid leaks. 

Mitigate the effects of fire or explosion, e.g. by detection provision, spacing, appropriate construction materials, shielding, venting, extinguishment, provision for evacuation of personnel. 

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