- Published on Monday, 06 August 2012 09:09
- Written by Stephen M. Wing and Bradley K. Smith
Since valves are the number one source of leakage, those faced with reducing fugitive emissions need to look at the most challenging type: control valves.
Process plant managers have a host of good reasons to minimize fugitive emissions from their facilities and reduce the amount of process fluid released into the atmosphere and surrounding environment. Those reasons include keeping employees and neighbors safe, lessening the facility’s environmental impact, complying with increasingly stringent air-quality regulations, optimizing the plant’s energy consumption and maximizing plant operating efficiency.
Industrial valves are a leading source of leakage from typical process plants. In fact, studies of refineries have shown that valves and relief valves account for about 75% of fugitive emissions.1
Because of this reality, monitoring, controlling and reducing valve leakage can make a significant impact on overall plant fugitive emissions. Of particular concern are control valves, which pose a greater challenge than other industrial valves because of their typical operating mode and potential leak paths. There are, however, techniques, technologies and strategies that plant owners and managers can implement to better manage fugitive emissions from the control valves in their facilities. This article highlights some of these options for typical control valve features, characteristics and options.
CONTROL VALVE LEAK PATHS
Standard control valve designs include a number of potential external leak paths. For example:
Globe-style Control Valves
Figure 1 shows a cross-section of a typical globe valve or linear operating valve, with the key potential leakage locations highlighted. The process fluid is contained within the valve body, which is a pressure vessel designed in compliance with standard pressure vessel codes, such as American Society of Mechanical Engineers (ASME), Deutsches Institut für Normung e.V. (DIN; in English, the German Institute for Standardization) or Japanese Industrial Standards (JIS).
There are several static joints or locations interfacing with the valve body where external leakage is possible, including the pipeline-flange connections and the valve-body-to-bonnet joint. Leakage at these joints is uncommon because of the static nature of these joints and the fact they are typically sealed with gaskets and then bolted together. Leakage is still possible, however, so these joints should be monitored.
The primary valve leak path is the stem-seal interface, which is typically sealed using packing installed within the valve bonnet. This is a dynamic interface, as the stem moves up and down through the packing box area during operation.
Control valves are typically applied in continuously throttling services to maintain a specific set point or operating range for different process variables, such as pressure, temperature and flow rate. As a result, the valve packing wears over time, allowing more leakage across this interface. In addition, thermal expansion and contraction caused by the process fluid and ambient temperature changes can further increase the leakage rate. Thus, the packing must be periodically tightened or fitted with some kind of mechanical compensation, such as “live loading” the packing with springs to maintain the seal integrity and control leakage to the atmosphere.
Another key contributor to packing wear in globe-style control valves—and, thus, another source of potential fugitive emissions—is the presence of foreign particles or debris in the surrounding atmosphere. Since the globe valve strokes in a linear motion, the valve stem moves up through the packing area as the valve opens and then moves back down into the packing area as the valve closes. As the valve cycles from open to close, a portion of the stem is exposed to the environment, creating an opportunity for particles to attach to the stem surface and potentially impact the packing wear rate and sealing capabilities. These particles also can increase the operating friction, which reduces the overall responsiveness and controllability of the valve.
Rotary-style Control Valves
Figure 2 shows a cross-section of a typical rotary-style or rotating-motion-design control valve. As with globe-style valves, the pipe-flange connections and the stem seal area are potential paths for process fluid to leak into the environment. However, many rotary-style control valves have an integrated body and bonnet, eliminating that location as a potential leak path.
A key advantage of rotary-style control valves in managing fugitive emissions is the rotating motion of the valve stem as the valve is opened and closed. The stem stays within the stem seal or packing area, minimizing the possibility of introducing foreign particles or debris into the sealing interfaces. As a result, these valves are typically more effective in reducing the possibility of fugitive emissions leakage, and normally deliver greater reliability and operating efficiency from this perspective.