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.
More and more attention worldwide is being focused on fugitive emissions. Fugitive emissions are defined variously and may 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. To the extent that leaks pose harm to the environment, they are fugitive emissions.
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 10000 or 20000psig (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. 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.
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 fitting inside the other. With minimal pressure from the packing nut, a substantial seal is 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 temperatures and pressures fluctuate. The result is easy actuation and minimal wear to the packing.
Another effective stem seal technology is the O-ring design. When properly designed, this technology provides flexibility for applications requiring high pressure, low pressure, or a broad pressure range.
The O-ring is usually made from a highly elastic material, such as fluorocarbon FKM. A proper stem design with an O-ring configuration requires a back-up ring or some other mechanism, usually made of PTFE, which will contain the O-ring under high pressure. In terms of temperature, pressure, and chemical attack, the design is limited by the specifications of the elastomer.
Beyond issues relating to stem seal design, there are some additional causes of leaks from the stem. These have to do with alignment of the stem. There are two basic causes of misalignment.
In the first case, misalignment may result from improper installation of the actuator. If the centre line of the actuator and the centre line of the stem are not properly aligned, the stem will become tilted, resulting in uneven wear of the stem seal.
In the second case, damage to the seat seal inside the valve may cause the stem to tilt. Ball valves can employ either a floating or trunnion ball design.
In a floating ball design, the ball is not fixed inside the housing but floats between two seats. In the shutoff position (Fig.3), the ball seals against the seat on the low-pressure side, pushed downstream by a positive pressure differential.
By contrast, the trunnion design employs a ball, but the ball is not a discrete sphere. Rather, its geometry includes two cylinders, the 'trunnions', affixed to the ball at the top and bottom (Fig.4). The unit fits into a space in the valve body and cannot move along the flow axis.
In the case of high differential pressure across the seat, a free floating ball can be pushed too far downstream. In the absence of an advanced seat design, such as a spring energised seat, the ball may not return to the center position. As a result, the stem will tilt to one side, and uneven stem wear will occur. The trunnion design prevents excessive movement of the ball downstream. The trunnions, which are fitted in place, keep the ball centered and the stem properly aligned.
This article intends to show that different designs have different strengths and relative merits, and these have a direct impact on fugitive emissions. The real cost of a valve is not the purchase price but the overall cost of ownership. With raw material feedstock prices increasing, as well as the frequency and severity of environmental non-compliance fines, direct and indirect costs associated with frequent maintenance, failure and replacement must be considered.
Michael Adkins is general industrial valve product manager and Peter Ehlers is alternative fuels market manager for Swagelok Company, Solon, Ohio, USA. www.swagelok.com