Effect of Diking

To demonstrate the effectiveness of diking as a postrelease mitigation measure, an accidental release of carbon disulfide from a vertical storage tank will be evaluated. In the example, isopleths to the ERPG-2 concentration of 50 ppm (AIHA, 1992) will be considered. 

At Qatar, the liquid came out with such force that it spilled over the dike wall. Conventional dike walls also have the disadvantage that a large area of liquid is exposed to the atmosphere if a leak occurs. For these reasons it is now usual to surround cryogenic storage tanks with a concrete wall, about 1 m from the tank and the full height of the tank. If the tank is not made from crack-resistant material, then the concrete wall should be designed to withstand the effects of a sudden release of liquid. 

LOPA is a semi-quantitative tool for analyzing and assessing risk. This method includes simplified methods to characterize the consequences and estimate the frequencies. Various layers of protection are added to a process, for example, to lower the frequency of the undesired consequences. The protection layers may include inherently safer concepts the basic process control system safety instrumented functions passive devices, such as dikes or blast walls active devices, such as relief valves and human intervention. This concept of layers of protection is illustrated in Figure 11-16. The combined effects of the protection layers and the consequences are then compared against some risk tolerance criteria. 

Since the purpose of diking is to contain a spilled fluid effectively while reducing the vapor generation rate, a secondary containment system that will be impervious to the fluid must be provided. It should also have favorable thermal properties to retard the boiloff rate. 

Another technique for removing spillage from a diked area is the use of an all-purpose sorbent. The effectiveness of using sorption as a removal technique is influenced by factors such as the physical and chemical attraction between the spilled material and the sorbent, the surface geometry and area of the sorbent, the contact time between the materials, and the density ratio of the spilled fluid and the sorbent (Bauer et al., 1975). Some effective sorbents include polymethacrylate foam resins, propylene fibers, molecular sieves, expanded clays, polyolefins, polymethylmethacrylates, and polystyrene sulfonates (Bauer et al., 1975). 

Vapor fences are solid walls located downstream from an expected release point of a flammable or toxic gas. These fences are most effective in achieving an initial dilution when they are located as close to the source as possible but downwind of it, or on the side of a facility where a sensitive population is located. Heights of vapor fences can vary from 3 to 12 m. At times they are added to the tops of dikes to provide a dilution effect (see Section 5.2.1). 

Assessing the effects of enclosures or vapor boxes is similar to assessing the effects of vapor fences. Consider the case of a typical dike sized to hold 110 percent of the liquid spilled. For small releases, the dike walls would act as containment or storage for the vapor generated shortly after release occurred. The vapor that would be generated by evaporation or boiling from the dike floor would displace the air in the dike for a given period of time and then overflow the dike walls. The vapor holdup duration is easily estimated ... 

Equation (5.2) indicates that the time vapor is advected downwind will increase as zt (height of vapor containment box) increases, as this increases the time to fill the vapor box. This additional time allows for the effects of decreasing rates of conductive heat transfer from the dike floor for cryogenic materials, or decreasing convective mass transfer for materials with boiling points that are higher than ambient temperatures, to take effect. 

In the mitigated case, in which the spilled carbon disulfide is retained by the dike installed around the tank, there is a large reduction in the vaporization rates of carbon disulfide for both sets of meteorological conditions. The primary reason for the differences is that the available surface area for vaporization is significantly reduced. Second, the vaporization rate remains essentially constant for the duration of the incident because the surface area for vaporization is constant. The small variations that do show in these curves are due to small effects of heat transfer into the diked carbon disulfide, as discussed in Chapter 5. 

Several borings were taken from the top of the dike to verify the reported soil formations. pH numbers were determined for each sample recovered. These show very low values below 10 ft, verifying that seepage of the retained acids was occurring. SIROC was selected for this project, and the soil samples were used to check the effects of the low pH on the gel time. 

The thermal effect of later volcanic intrusions was further evaluated by detailed fluid inclusion study on quartz separated from silicified rings which arose by the thermal action of basalt dike. Five different groups of fluids were distinguished based on salinity and homogenization temperature. 

Coastal Engineering. The movement of water in oceans and lakes has erosive effects on their shorelines. The preservation of wetland for flood mitigation or marine ecology requires the knowledge of such effects. Use of artificial barriers such as breakwaters or dikes at a shore or a harbor can result in reducing the water wave level within protected areas, eliminate or reduce the effects of shoreline erosion, and redirect natural sediment so that new land can be created over time. 

Containment materials — In order to prevent the spread of large amounts of Uquid chemicals, a supply of diking materials needs to be maintained. Ready access to a supply of bales of straw is a great asset. Straw is cheap, easily handled, and easy to clean up afterward. In the event of a spill reaching a stream, floating booms and skimmers are useful in containing and cleaning up the spill. Booms are not effective for materials more dense than water and not water soluble. 

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