Emerson Exchange 365

Emerson Automation Solutions Senior Engineering Manager Lisa Miller recently published an article in the May/Jun 2022 issue of InTech. It is titled “Proper Valve Stem Sealing Best Practices” and it describes how control valve stem sealing works, and shows how to select the best option for each particular application. A summary of the article follows.

There are many different methods of sealing valve stems on control and isolation valves. When chosen wisely, a valve stem seal can provide years of reliable service, reduce environmental emissions, and minimize product loss. When chosen poorly, valve stem seals can leak consistently, increasing costs, creating environmental issues, and even placing operating personnel at risk.

This article explains various methods of achieving optimal valve stem sealing, helping end users evaluate the best choice for their application.

What are valve stem seals?
Control and block valves are typically one of two types: sliding stem or rotary. A sliding stem valve has a rod protruding from the body that rises and falls to actuate the valve. A rotary valve has a shaft extending out the side that is connected to a plug, disc, or ball. As the shaft turns, the rotary valve opens and closes. In either design, the valve stem must exit the body and be capable of relatively friction-free movement, yet contain the process media. This is not an easy task, as the author explains:

Valve stem seals must accomplish two contradictory goals. First, they must seal the valve stem completely and reduce - ideally eliminate - any fugitive emissions from the process. Secondly, they must accomplish this feat while allowing the valve stem to move freely and continue sealing, even as the valve stem cycles thousands of times.

Measuring stem seal performance
The three main industrial standards addressing valve stem leakage are TA Luft, FCI 91-1, and ISO 15848. However, the required performance and test methods for each standard varies significantly.

TA Luft is the least comprehensive standard, offering leak rate standards but no test parameter details. FCI 91-1 is more closely aligned to EPA’s LDAR program and uses the EPA Method 21 to “sniff” the valve packing and determine the leak rate (Figure 1). This standard does provide details on how a valve is to be tested, offering various classification ratings based on the leak rate after a specified number of mechanical and thermal cycles.

Figure 1: This Fisher GX control valve is subjected to EPA’s Method 21’s “sniff” test to determine the fugitive emission leak rate after a prescribed number of mechanical and thermal cycles.

ISO 15848 is much more involved, listing several leakage classification rates for both control and isolation valves based on mechanical cycles, thermal cycles, and stem size. It allows testing using helium or methane, and it dictates specific ways to precisely measure leakage.

Lisa notes that it is important to determine how a valve was tested when comparing stem seal performance.

It is relatively easy to achieve very low leakage rates if the valve is mechanically cycled a small number of times. It is much more difficult to achieve and maintain very low leakage rates when the valve is mechanically cycled thousands of times while enduring thermal cycles as well.

Sealing valve stems with packing
Most valves use a series of PTFE or graphite rings that encircle the valve shaft to provide a stem seal (Figure 2 - left). The rings are compressed with a combination of a packing follower, packing flange, and bolts to push down and squeeze the packing rings against the shaft. This arrangement allows the valve stem to move, while keeping process media from escaping.

Figure 2: This picture on the left shows a rising stem control valve with standard packing. More modern packing designs, such as the Fisher ENVIRO-SEALTM shown on the right, employ compressed Belleville springs to maintain constant pressure on the packing rings.

To achieve and maintain low emissions, packing must be ‘live loaded’ to keep constant pressure on the sealing rings (Figure 2 - Right). Compressed Belleville springs maintain a constant force on the packing, ensuring it seals even as the rings wear from stem movement. Unfortunately, the increased pressure tends to restrict valve movement, so the sealing materials and valve stem finish must be carefully chosen.

Sealing valve stems with bellows
An alternative option is a valve bellows seal. A bellows seal utilizes a welded or mechanically formed metal barrier around the valve stem that can compress and stretch like an accordion (Figure 3). The metal seal achieves virtually zero leakage.

Figure 3: Bellows seal designs usually employ a welded leaf design (detail left and middle) or a mechanically formed design (right). A formed design can withstand many more cycles than a welded leaf design, but it is usually about three times longer.

Bellows seal either use a welded leaf bellow seals (Figure 3 left and middle) or a formed bellows (Figure 3 right). The welded leaf bellows employs a stack of welded washer-like plates, providing many folds over a given length. A formed bellows uses a flat sheet of metal formed and welded into a tube which is mechanically formed. The welded leaf design is usually one third smaller than a formed bellows, but a formed bellows typically lasts significantly longer.

Packing versus bellows
Each method of valve stem sealing has pros and cons, and the best choice is very application dependent. Standard or environmental packing usually costs much less, and there is a wide variety of valve packing materials and designs to suit most applications. Valve packing can also be adjusted and replaced without disassembling the valve. The disadvantage of packing is the variable nature of its performance over its lifetime and the fact that it cannot achieve zero leakage. Small leaks can be addressed by tightening the packing, but at some point, the packing must be replaced.

The biggest advantage of a bellows design is its ability to deliver zero leakage, which is critical for lethal service applications. However, the operational life of a bellows seal is based on the number and length of strokes, so at some point it will fail. While this failure is more predictable, the repair requires the valve to fully disassembled, so the total cost of ownership for a bellows seal is much higher.

Ultimately, the best design decision will vary with each application, but an optimum choice can dramatically impact a plant’s bottom line as Lisa describes:

Proper selection of valve stem sealing is a critical component of the valve specification process. When chosen wisely, the design offers reliable long-term performance, translating into significant reductions in environmental emissions, product losses, and maintenance costs.

The number of design options are extensive, so end users may find it helpful to consult with their valve vendor to determine the best sealing design, materials of construction, and other details for their specific application.

All figures courtesy of Emerson

About the Author

Lisa Miller is a senior engineering manager for Fisher sliding stem valves at Emerson Automation Solutions. She has been the primary technical consultant for Fisher packing and bellows for over 20 years, and she has 25 years of expertise with cryogenic valve design, testing, and manufacturing. Miller is the chairperson of ISA75.27.01 "Cryogenic and Low Temperature Seat Leakage Testing of Control Valves" committee, and she has been a member of ISA for 10 years. She holds a BS in mechanical engineering degree from the University of Iowa.