By Michael Adkins and Pete Ehlers, Swagelok Co.
More and more attention worldwide is being focused on fugitive emissions (equipment leaks) as opposed to point-source emissions from reactor vents or boiler exhaust stacks. United States regulations are honing in on fugitive emissions in regions such as the Gulf Coast. And the European Union’s Integrated Pollution Prevention and Control Bureau (IPPC) issued a comprehensive directive to curtail 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.
With respect to emissions in general and fugitive emissions in particular, the trend is clearly toward higher standards and more scrutiny. Fugitive emissions will be on the vanguard as regulators attempt to impose the next set of emissions standards, especially as these standards concern highly reactive volatile organic compounds (HRVOC).
Leaks may be either internal or external and not all leaks are considered fugitive emissions. 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. By contrast, an external leak refers to a leak from inside the valve into the environment, by way of the stem seal or body seal, for example. To the extent such 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 $25,000. Such examples make the case for a total cost of ownership approach to system design, product selection and maintenance.
This article focuses on discrete component leaks and in particular, on external leaks from ball valves, a widely used type of valve that enables high flow and effective shutoffs.
To control fugitive emissions from ball valves, the critical point is to select the right ball valve for the application. Begin with accurate information about the application: pressure and temperature ranges, cleanliness of the medium, frequency of cycling, frequency of maintenance desired, allowable leak rate, flow requirements and potential for contamination. Then, choose the valve technology that most closely accommodates your operating parameters, giving due attention to design and performance features, as well as material compatibility. 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 (1) screw type and (2) 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. As a result, it can be an especially efficient seal, enabling effective sealing at pressures as high as 10,000 or 20,000 psig (689 or 1,378 bar).
In valves using the flange-type body seal, the valve body consists of three discrete sections that are joined together with flanges, seals and bolts. 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.
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 to ensure sufficient sealing load is maintained.
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, eliminating the need to remove the entire valve from the system. Seat and body seals are easily accessible. As regulations targeting fugitive emissions get tougher, ease of maintenance and repair will become more important.
Leaks may occur not just at sealing points but also through body materials, such as castings. When specifying valves, system designers should inquire about the integrity and inspection of body material, whether cast or machined. What specifications does the valve manufacturer hold the metal supplier to? What quality controls are in place? A Certified Materials Test Report provides many answers to the most critical questions concerning the quality of body material.
Stem Design
In a ball valve, some means must exist 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, which 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 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. The additional tightening increases the force against the stem, requiring more force for actuation. With all the occasional retightening, it is possible that the packing bolt will bottom on the valve body, at which point the packing will need to be replaced.
This basic packing technology requires frequent inspection and adjustment or leakage may occur. 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.
A two-piece chevron stem packing design is an improvement on the one-piece design and 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. 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.
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 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. These springs provide a constant bias force against the seal and the body to ensure 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. 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.
Some valve designs may allow system pressure to push up on the stem. A live-loaded mechanism accounts for this movement—as well as expansion and contraction of the packing—and enables consistent pressure on the packing.
One-piece packed valves may contain springs and purport to be live-loaded but are not as effective. The springs will enable the PTFE packing to contract and expand to some degree, but without the chevron design they cannot ensure consistent pressure on the stem. By definition, a single-piece packed valve requires heavy biasing spring force on the packing so it can bow outward and create a tight seal. With repeated actuation, wear to the packing can be considerable. The wear will require frequent replacement of the packing and may lead to leakage.
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 2,300 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. Like the two-piece chevron design, the O-ring design does not require excessive pressure from the packing nut. Rather, the O-ring is energized 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. Because it is elastic, the O-ring’s cross section deforms and reforms to make the necessary seal.
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. This back-up ring is designed to reduce the extrusion gap of the O-ring gland and keep the O-ring contained. If the O-ring is permitted to extrude beyond its specific bounds, the O-ring may be sheared during actuation. Extrusion may lead to leaks and will make actuation difficult.
The O-ring design is highly effective at high pressure. In terms of temperature, pressure and chemical attack, the design is limited by 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, additional causes of leaks from the stem have to do with stem alignment. If for any reason the stem becomes tilted or forced to one side, uneven wear may occur, resulting in leakage. There are two basic causes of misalignment.
In the first case, misalignment may result from improper actuator installation. If the center lines of the actuator and 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.
To understand this issue, we must first review basic ball valve anatomy. Ball valves can use 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, the ball seals against the seat on the low-pressure side, pushed downstream by a positive pressure differential.
By contrast, the trunnion design uses a ball, which is not a discrete sphere. Rather, its geometry includes two cylinders—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.
The trunnions are fitted in place and keep the ball centered and the stem properly aligned, preventing excessive movement of the ball downstream. Even with a “hammer effect” the trunnion design will keep the ball centered.
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.
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.
Author: Michael Adkins is general industrial valve product manager and Peter Ehlers is alternative fuels market manager for Swagelok Co.