How to choose a ball valve to curb fugitive emissions

In developed countries worldwide, more and more attention is being focused on fugitive emissions — equipment leaks, as opposed to point-source emissions from reactor vents or boiler exhaust stacks. Governments worldwide are focusing on fugitive emissions and legislating to force utilities and plant operators to curtail them.

In July 2008, the Australian government instituted the National Greenhouse and Energy Reporting System, which monitors the emissions that cause climate change. Under the system, some corporations are required to share data with the public regarding their greenhouse gas emissions. The Australian government is using the data collected from this system to develop an emissions trading program, the Carbon Pollution Reduction Scheme, which will begin in 2010.

Not all equipment leaks are considered fugitive emissions. Equipment leaks may be either 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 does not vent to atmosphere, an internal leak would not result in a fugitive emission. In 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 are fugitive emissions.

External leaks from fittings, valves and other fluid system components can add up over the course of a year to major financial losses. For example, for a plant with 50,000 fittings, the average annual economic loss due to leakage from fittings alone is estimated at more than US$25,000. Such examples make the case for a total cost of ownership approach to system design, product selection and maintenance.

To control fugitive emissions from ball valves, the critical point is to select the right ball valve for the application. While this article cannot address all ball valve types, we will focus on two design features that are especially important in controlling fugitive emissions and overall cost of ownership: body seal design and stem seal design.

Body seal design

Two common types of body seals are the screw type and the 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, an important benefit.

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 therefore it can be an especially efficient seal, enabling effective sealing at pressures as high 10,000 or 20,000 psig (689 or 1378 bar). In addition, the nature of the design enables the manufacturer to offer an especially 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 (Figure 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.

Figure 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.

The manufacturer’s standard sealing material is not always the answer. System designers should take care to research sealing materials in conjunction with their system operating conditions, considering the full range of options, including metal gaskets, many different types of elastomer O-rings and Grafoil packing, which may offer a more robust valve design. The bolts in the flange-type body seal should be of high-grade material, such as strain-hardened 316 stainless steel, to ensure sufficient sealing load is maintained.

Beyond sealing materials, an advantage of the flange-type design is the ease of maintenance. Once the bolts are removed, the valve’s body swings out for easy repair, eliminating the need to remove the entire valve from the system.

Stem seal design

In a ball valve, there must be some means of ensuring that the system media, whether liquid or gas, 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. Based on the application, a deliberate choice between design types should be made.

One-piece stem packing

The most basic and primitive 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.

Unfortunately, PTFE and other similar packing materials are subject to cold flow, which is the tendency for certain materials to change shape over time; cold flow can be exacerbated by pressure and temperature. In some cases, the material may extrude into areas where it was not intended to go, undermining its effectiveness and leading to leakage of system media.

To compensate for cold flow, the packing bolt may need to be tightened more frequently to increase the compression load on the stem seal, especially as application pressures and temperatures change and as the valve is repeatedly cycled.

To reduce the risk of fugitive emissions, the one-piece packing design should be reserved for applications where fluctuations in temperature and pressure will be minimal, where cycling will be limited and where inspection and monitoring will be frequent and regular.

Two-piece chevron stem packing A two-piece chevron stem packing design is an improvement on the one-piece design and therefore allows for wider temperature and pressure ranges, as well as regular and easy actuation without excessive wear. A chevron packing consists of two matched gaskets, one fitting inside the other. The cross-section of the gaskets is triangular in shape. Fitted together, the two gaskets form a rectangular cross-section (Figure 2). As force is applied from the stem’s packing nut, the two gaskets are pushed against each other along the diagonal point where they meet, which sends the force horizontally and evenly against the stem and body housing. With minimal pressure from the packing nut, a substantial seal is created between the stem and the body housing.

Figure 2: Cross-sections of two different types of packing. On the left is the standard one-piece packing. On the right are the two ferrules that make up a chevron stem packing.

For the chevron seal to work correctly, the two PTFE gaskets — the packing — must be held in place to reduce cold flow during thermal cycling. The packing in the chevron design, therefore, must be adequately contained and supported by packing support rings and glands, which evenly distribute pressure to the packing.

To reduce the interval of inspection and adjustment, the chevron design may also 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. These springs provide a constant bias force against the seal and the body to ensure that the appropriate amount of sealing force is provided. At high temperature, the springs compress and allow space for the packing to expand. At low temperature, they expand and maintain the correct amount of pressure on the packing. This live loading system enables the chevron design to maintain a constant seal using this steady biasing spring force. The result is easy actuation and minimal wear to the packing. Without the springs, the packing would have to expand and contract in a relatively fixed space. As the packing expanded at high temperature, load on the stem would increase and cold-flow could occur. The result would be increased wear on the packing and difficult actuation.

O-ring seal

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, such as a cylinder, where, for example, pressure may drop from 2300 psig (158.5 bar) when full to 100 psig (6.9 bar) as it nears empty.

The O-ring is usually made from a highly elastic material, such as fluorocarbon FKM. The O-ring is energised by pressure in the media stream. As pressure in the stream increases, the O-ring further deforms and increases pressure on the stem. Conversely, as pressure in the gas stream decreases, the O-ring relaxes, filling the space between the stem and the body.

In terms of temperature, pressure and chemical attack, the design is limited by the specifications of the elastomer. The user must take the initiative to understand the system media and the potential for chemical interaction with the elastomer.

Stem misalignment 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. If for any reason the stem becomes tilted or forced to one side, there may be uneven wear to the stem seal, resulting in leakage. 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 or askew, 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. A free-floating ball (Figure 3) can be pushed downstream — too far downstream. In the absence of an advanced seat design — such as a spring energised seat, with an O-ring and spring on each side — the ball may not return to the centre position. As a result, the stem will tilt to one side and, with time, uneven stem wear will occur.

Figure 3: A cross-section of a floating ball valve in the shut position, with downstream pressure pushing the ball to seal on the right-hand side. Arrows point to the seat seals.

An alternative to the free-floating ball design is the trunnion design (Figure 4). This design employs a ball, but the ball is not a discrete sphere. Rather, its geometry includes two cylinders — which are called the trunnions — affixed to the ball at the top and bottom. The unit fits into a space in the valve body and cannot move along the flow axis. As the ball rotates to the open and closed positions, it glides on the trunnions, which can be fitted with bushings or bearings. Even with a ‘hammer effect’ — where a non-compressible medium, like water, produces a pressure spike — the trunnion design will keep the ball centred and the stem properly aligned.

Figure 4: A trunnion ball valve.

Conclusion

Most ball valve designs have their appropriate applications. However, different designs have different strengths and relative merits, and these have a direct impact on fugitive emissions. When choosing a ball valve, a system designer should give due consideration to material compatibility, pressures, temperatures, desired frequency of inspection and adjustment, and frequency of actuation. Further, 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, and direct and indirect costs associated with frequent maintenance, the costs associated with valve 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.

Swagelok Corporation
www.swagelok.com