Controlling control-valve emissions

By keeping leakage low, plants can reduce frequency of expensive monitoring programs

There's a new criterion for selecting and maintaining valves, pumps and flanges: leakage. Because of the 1990 Clean Air Act, many plants must eliminate even the slightest equipment leaks. This requirement can lead to major changes in equipment selection and maintenance procedures as plants take steps to detect leaks and minimize repair costs.

Much of the equipment designed to meet low-leakage requirements also has a long service life and requires little or no maintenance. As a result, many users find this equipment to be cost-effective even if they are not required to meet EPA leakage requirements.

Fugitive emission sources
Not all air pollution comes from defined points, such as stacks and exhaust pipes. Approximately 33 percent of air pollution in the United States comes from fugitive emissions, such as leaking equipment and evaporation from sewers and waste treatment facilities. The 1990 Clean Air Act and its supporting EPA regulations attack these fugitive emissions with a vengeance.

Although fugitive emission regulations apply to pumps, flange gaskets, and other equipment, they have the greatest impact on valves. That is because a typical plant has 15 to 20 times more valves than pumps, and the potential for leakage from a static flange gasket is much less than for either valves or pumps.

Control valves present a special challenge in leak reduction. Valve stem movement contributes to leakage through the stem packing and shortens the life of any sealing system. Sliding stem valves (that is, globe style) pose more sealing challenges than do rotary motion valves. And, many stem-sealing systems, in an attempt to prevent leakage, overly constrain stem movement, reducing controllability.

Faced with these potential stem-sealing dilemmas, plant engineers often look to packing improvements to meet the EPA standards. Most valve packing sets now in use will meet the low leakage rates required by the EPA, but in many applications they will do so only for a short time. Because of friction damage and extrusion loss, these packing sets require frequent maintenance and replacement to keep leakage low. Valves that stroke frequently control valves, for example, and valves subjected to high or varying temperatures, are especially susceptible to high leakage and low packing life.

Many stem-sealing systems, in an attempt to prevent leakage, overly constrain stem movement, reducing controllability.

However, new packing systems are available that meet EPA leakage requirements in adverse applications, and do so without attention over the maintenance cycle of the valve itself. Because of this long life and low maintenance, many plants will use these new packing systems even if they are not required to monitor fugitive emissions.

How the new packing works
To provide low leakage and long life, these new packing systems are designed according to five principles:

• confine the packing with anti-extrusion rings;
• use a minimum amount of packing to reduce thermal expansion;
• use a live load to maintain
• packing stress;
• keep the stem aligned with a
• bushing system, and
• use smooth valve stems.
• use smooth valve stems.

Polytetrafluoroethylene (PTFE), although an excellent all-purpose packing material, is especially subject to extrusion loss because of its tendency to cold flow. This extrusion is intensified by the high thermal expansion coefficient of PTFE - 10 times that of steel. If the valve temperature increases, the PTFE expands more than the packing box space, further increasing
extrusion loss.

As packing is lost by extrusion, the stress on the packing declines, and the bolts on the packing follower must be tightened to maintain a seal. With periodic tightening of the packing, soon all the packing is lost and the valve must be re-packed.

This extrusion problem can be virtually eliminated by using anti-extrusion rings and minimizing the amount of packing used. Anti-extrusion rings must deform less than the packing so that they themselves do not extrude, and fit the stem snugly enough to contain the packing, yet without constraining stem movement or scoring the stem.

For some packing systems, two sets of anti-extrusion rings are used. A slightly pliable ring is installed adjacent to the more-pliable packing, and a non-deformable ring is installed on the outside of the packing set.

Packing quantity must be minimized to reduce the effect of thermal expansion. This is especially important with PTFE packing. Tests show that packing seals the stem over a very small area, so using less packing does not affect the strength of the seal.

Packing must be kept under a constant and correct amount of stress for it to maintain its seal. This stress is provided with an external spring or "live load."

A stack of Belleville washers (springs) mounted on the valve stem usually provides this live load. The load required to maintain the proper stress on the packing is a function of the type of packing, the valve design, and the application. However, it is usually 1,500 psi or greater --which is much more than the loading provided by the small springs internal to the packing.

Other design principles include using a smooth stem finish so as to minimize leak paths and prevent erosion of packing. Also, a bushing system is required to ensure that the stem stays aligned. A wobbling or off-center stem reduces packing life.

These design criteria apply equally well to rotary or sliding stem valves, whether they use graphite or PTFE packing. However, the optimum amount of packing, the design of the packing retention system, and amount of live load required varies depending upon the valve design, type of packing material, and the temperature and pressure of the application. In all cases, however, proper application of these principles allows the packing to meet EPA leak limitations.

Not all air pollution comes from defined points, such as stacks and exhaust pipes.

A graphite-based packing system provides emission control to EPA requirements and fire-safe operation to 450 degrees F. It also reduces the compression needed to achieve a seal.

This packing system employs two sets of packing rings and uses live-loading to maintain a consistent packing ring stress. The top set of packing rings features a PTFE and carbon composite that provides superior sealing to below the EPA minimum of 500 ppm. This composite allows smooth valve operation over an extended period without needing maintenance. The second tier packing rings are constructed of a proprietary graphite composite that provides fire-safe protection at temperatures to 450 degrees F as well as a sealing capability of less than 500 ppm. This added capability proves advantageous in hydrocarbon processes where fire safety is a concern.

Overall, the graphite-PTFE duplex combination offers a significantly lower operating force requirement--compared to all-graphite ring sets--excellent sealing capability, and fire-safe protection. Currently, it is available for sliding-stem style control valves with stem diameters ranging from 3/8 to 1-1/4 inches.

Graphite and rotary valves
Use of graphite packing systems within rotary valves offers special challenges. The design principles of graphite systems, while identical to those for sliding stem valves, must be implemented differently to accommodate the rotary motion of the stem against the packing.

The key problem is adhesion of  the graphite to the valve shaft, a situation that is similar to galling. This occurs because of the high force between the two materials, with two serious consequences.

First, small graphite particles adhere to the shaft and create a rough surface that erodes the packing. Second, the adhesion rips off large sections of the packing, making it mechanically unstable.

The result can be catastrophic failure of the packing system because much of the graphite packing literally blows out as a fine dust. Even when catastrophic failure does not occur, adhesion causes high and erratic leak rates. Note that adhesion does not occur in sliding stem packing systems since the sliding action relieves the adverse effects of adhesion.

Lubricating the shaft/graphite interface eliminates some adhesion. As with sliding stem duplex graphite/PTFE packing, positioning trace amounts of PTFE in the graphite lubricates the shaft interface. Besides providing lubrication, the PTFE improves leak rates by filling tiny imperfections in the shaft/graphite interface.

As with the duplex packing for sliding stem valves, the rotary valve system uses anti-extrusion rings that double as stem bushings and Belleville washers that provide a live load. This system performs as shown by testing for 25,000 cycles at 600 degrees F with methane gas on three valve shaft sizes. Throughout the tests, stem leakage remained less than 50 ppm.

Final guidelines
Key to a packing system's emission-control performance is how well it implements the four basic principles of containing (or retaining) the packing materials, reducing thermal expansion, maintaining packing stress, and ensuring a smooth stem surface.

When both emission control and fire-safety are essential, a duplex graphite/PTFE system, which again follows these principles, can be the answer. In most cases, packing systems designed originally to meet the stringent EPA also yield an extended service life, reduce overall maintenance costs, and provide long-term answers to valve sealing problems.