More 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. The 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.
View more content on PlantServices.com
Fugitive emissions are defined variously and might refer to a wide range of emissions not confined 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 shutoffs 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. Then, choose the valve technology that most closely accommodates your operating variables. This 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.
Body seal design
Two common types of body seals are screw type and flange type. The 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.
The screw type consists of one or two threaded end screws affixed to the valve body after the ball and seat packing have been loaded inside. The 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 flange-type body seal have three discrete sections that are joined together with flanges, seals and bolts (Figure 1). The sealing area across these components is larger, so this design usually results in a lower pressure rating. Because the flanges are sealed with gaskets, there are fewer geometric constraints on the sealing material and, therefore, a wider choice of sealing materials is available.
Valves having a flange-type body seal consist of three discrete parts that are joined together with flanges, seals and long bolts. Such valves come apart for easy repair in situ.
Beyond sealing materials, an advantage of the flange-type design is ease of maintenance. Once the bolts are removed, the valve’s body swings out for easy repair without removing the entire valve from the system.
A ball valve requires some means of ensuring that the system medium, whether liquid or gas, doesn’t leak from the stem and body interface. This is the role of the stem seal. With sufficient cycling frequency, stem seals are subjected to wear, and wear can lead to leakage. However, some seals are more effective than others in certain applications.
One-piece stem packing
The most basic technology is a one-piece gasket that encircles the stem. As the packing bolt is tightened down on the stem, the gasket, usually made of polytetrafluoroethylene (PTFE), is crushed, filling the space between the stem and the body housing.