32 safety in the plant
Choosing a ball valve to control fugitive emissions
To control fugitive emissions from ball valves, the critical point is to select the right ball valve for the application. Two design features that are especially important in controlling fugitive emissions: body seal design and stem seal design. Michael Adkins and Pete Ehlers report.
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. To the extent that leaks pose harm to the environment, they are fugitive emissions.
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Two common types of body seals are screw type and flange type. While the screw type is a stronger seal, enabling higher system pressure, the flange type allows fast and easy maintenance with the valve in line. The screw type consists of one or two threaded ‘end screws’ that screw onto the body of the valve after the ball and seat packing have been loaded inside.
The sealing area of a screw-type fitting is relatively small and it can be an especially efficient seal, enabling effective sealing at pressures as high as 10 000 or 20 000psig (689 or 1378bar). In addition, the design enables a wide range of end connection choices.
In valves employing the
flange-type body seal, the valve body consists of three discrete sections that are joined together with flanges, seals, and bolts (Fig. 1).
Because the sealing area across these components is larger, this design usually results in a lower pressure rating.
Since 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.
ore and more attention worldwide is being focused on fugitive emissions. Fugitive emissions are defined variously and may refer to a wide
Another advantage of the flange-type design is the ease of maintenance.
In a ball valve, there must be some means of ensuring that the system media does not leak from the stem and body interface. This is the role of the stem seal. With sufficient cycling frequency, all stem seals are subject to wear, and wear can lead to leakage. However, some seals are more effective than others in certain applications.
The most basic and primitive technology is a one-piece gasket (Fig. 2) 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.
Unfortunately, PTFE and other similar packing materials are subject to cold flow, which can be exacerbated by pressure and temperature. In some cases, the material may extrude into unintended areas, leading to leakage of system media.
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Fig. 1. Valves employing the 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.
To reduce the risk of fugitive emissions, the one-piece packing design should be
reserved for applications with minimal fluctuations in temperature and pressure, limited cycling, and where inspection and monitoring will be frequent and regular.
A two-piece chevron stem packing design (Fig. 2) allows for wider temperature and pressure ranges, and regular and easy actuation without excessive wear.
A chevron packing consists of two matched gaskets, one of which fits inside the other. With minimal pressure from the packing nut, a substantial seal is then created
between the stem and the body housing. For the chevron seal to work correctly, the two PTFE gaskets must be held in place to reduce cold flow during thermal cycling. The packing must be adequately contained and supported by packing support rings and glands, which evenly distribute pressure. To reduce the interval of inspection and adjustment, the chevron design also may include Belleville washers, which are springs that create a ‘live load’ on the packing. Live loading enables even pressure on the packing, as
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