Emerson Automation Solutions’ technologist Wade Helfer and simulation engineer Sarah Witte recently published an article in the December 2021 edition of Processing. The article is titled “Simplifying control valve packing selection” and it describes how increasingly stringent environmental regulations have complicated valve packing selection. A summary of the article follows.
Historically, valve packing design has been a relatively simple part of the control valve selection process. A control valve was chosen to provide the necessary flow performance at specific process temperatures and pressures, and the packing was usually specified by the vendor to match the process data.
Packing selection has become a much more involved process as the EPA lowered emission limits to 500 ppm, then 100 ppm, and even 50 ppm in some cases. Now, the packing design process can be very challenging and critical to valve selection.
What is packing?
Before discussing the packing selection process, it is best to describe how packing functions. Control valves typically have a reciprocating rod or twisting stem that move the valve plug to control flow. Regardless of the design, the valve stem must exit the body and be capable of relatively friction-free movement, yet still contain the process media and avoid leaks. Valve packing makes that possible.
The sealing portion of a valve packing usually consists of a series of Teflon (PTFE) or graphite rings that encircle the valve shaft (Figure 1). The rings are compressed from above, squeezing them against the shaft, to seal and contain the process.
Figure 1: This picture shows a typical rising stem control valve. The packing consists of the packing box (#3), the packing rings just above it (#2), and the packing follower, packing flange and bolts mounted above.
While a tight seal against process leaks is necessary, it is just as important that the compressed rings allow the valve stem to move freely, since a stuck or bound stem impacts a valve’s ability to control flow of the process media. In the past, emission requirements were less stringent, so free movement was considered more important. This changed with the new Clean Air Act Amendments, as the authors explain:
Packing designs were simple, and so was packing ring selection, as it was primarily based on process temperature since only very high process pressures impacted packing. All that changed when fugitive emission reduction became a priority for the EPA.
Valve packing emissions for hazardous and targeted chemicals have been steadily dropping with each round of regulation, and this has driven a whole host of alternative packing designs to meet the standards.
Environmental packing designs
One way to meet low emission requirements is “live loading” the packing (Figure 2). Spring-loaded Belleville springs are compressed during installation to maintain a constant force on the packing, ensuring it seals even as the rings wear.
Figure 2: Modern packing design uses compressed Belleville or other special springs to maintain constant pressure on the packing rings. This ensures the fugitive emissions are limited to 100 PPM or less, even as the rings wear.
Live loaded packing designs greatly reduce fugitive emissions, but the high compression increases stem friction and degrades control valve performance. PTFE seals well and lubricates the shaft, but it is only rated to 450 at limited pressure. Graphite rings can handle temperatures to 1000 and pressures to 4000 PSI, but they do not seal nearly as well as PTFE. Graphite also tends to restrict stem movement at lower temperatures.
Some valve vendors address these issues by utilizing advanced materials and improved packing configurations (Figure 3). Layers of carbon or glass reinforced PTFE and graphite, as well as alternative materials such as KALREZ, are used to extend pressure and temperature limits, while still meeting emission requirements.
Figure 3: Advanced environmental packing arrangements use carbon reinforced PTFE, graphite, or combinations of reinforced PTFE and graphite rings, to handle higher process temperatures and pressures while keeping emissions low.
Unfortunately, there are some environmentally sensitive applications where the process conditions exceed the temperature and pressure ratings of these advanced packing designs, but there is yet another design option.
High temperature modeling
In most valve configurations, the packing is located on the valve stem and somewhat separated from the valve body. This often means the packing will not encounter the full process temperature as heat radiates away from the valve bonnet. The resulting temperature differential provides an opportunity to utilize the superior sealing and lubrication of PTFE-based packing above its temperature rating. The authors describe how this works:
Finite element analysis can be employed to model the heat signature of a control valve body and bonnet during process conditions. This thermal model can be used to predict the maximum temperature encountered by different sections of the packing.
The simulation models (Figure 4) are based on an advanced understanding of the valve’s heat transfer properties as well as extensive lab testing, and this effort enables new packing design options.
Figure 4: Advanced modeling and simulation programs allow vendors to predict the maximum temperature of the packing components, allowing superior performing packing materials to be used.
Once temperature models are created and validated, the data is used to create custom packing designs that include a combination of graphite packing in the high temperature zones and reinforced PTFE packing in the lower temperature areas. This revised design handles high temperature and pressures, meets the emission requirements, and has improved lubrication and longer service life.
Packing selection summary
The authors suggest end users follow these steps when selecting control valve packing:
Wade Helfer is a technologist at Emerson and is responsible for developing and evaluating new control valve technologies with an emphasis on mechanical systems. He has 23 years of industry experience in the design and evaluation of control and isolation valves for a variety of industries. He completed his BS degree and graduate coursework in mechanical engineering from Iowa State University.
Sarah Witte is a simulation engineer at Emerson. She has four years of industry experience at Emerson in the evaluation of control and isolation valves utilizing Finite Element Analysis with a focus on thermal analysis. She completed her B.S. degree in Mechanical Engineering from Iowa State University.