Plant Services - August 2008 - (Page 43) Choose a ball valve with leak-tight seals to curb fugitive emissions ore attention worldwide is being focused on fugitive emissions, which are equipment leaks, as opposed to point-source emissions from reactor vents or boiler exhaust stacks. e trend is toward stringent limitations and more scrutiny, and fugitive emissions will be in the vanguard as regulators attempt to impose the next set of emissions standards. Fugitive emissions are defi ned variously and might refer to a range of emissions not confi ned to a stack, duct or vent, including emissions from bulk handling or processing of raw materials, windblown dust and other industrial processes. Not every leak is considered a fugitive emission. Leaks might either be internal or external. In the case of a ball valve, an internal leak could refer to a leak across the seat, from the upstream to the downstream side. So long as the valve doesn’t vent to atmosphere, an internal leak wouldn’t result in a fugitive emission. By contrast, an external leak refers to a leak from inside the valve into the environment, for example, by way of the stem seal or body seal. To the extent that leaks pose harm to the environment, they’re fugitive emissions. Let’s focus on discrete component leaks, in particular, the external leaks from ball valves, a widely used valve type that enables high flow and effective shutoff s in many industries, including the chemical, petrochemical, oil and gas exploration, power and alternative fuels industries. To control fugitive emissions from ball valves, the critical point is to select the right valve for the application. Begin with accurate information about the application. en, choose the valve technology that most closely accommodates your operating variables. is article can’t address every ball valve type, so it focuses on two design features that are especially important in controlling fugitive emissions and overall cost of ownership: body seal and stem seal designs. Two common types of body seals are screw type and flange type. e screw type provides a stronger seal that tolerates higher system pressure, but the flange type allows for fast and easy maintenance with the valve in line. e screw type consists of one or two threaded end screws affi xed to the valve body after the ball and seat packing have been loaded inside. e sealing area of a screw-type fitting is relatively small and can be an especially efficient seal, enabling effective sealing at pressures as high as 10,000 psig or 20,000 psig (689 bar or 1,378 bar). Valves using the fl ange-type body seal have three discrete sections that are joined together with fl anges, seals and bolts (Figure 1). e sealing area across these components is larger, so this design usually results in a lower pressure rating. Because the fl anges are sealed with gaskets, there are fewer geometric constraints on the sealing material and, therefore, a wider choice of sealing materials is available. A .PLANTSERVICES. 43 M By Michael Adkins and Pete Ehlers Body seal design http://www.PLANTSERVICES.com
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