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Choosing the Right Valve Stem Sealing Method

What is Valve Stem Sealing?

Control valves and on-off valves typically fall into one of two categories: linear or rotary. Linear valves have a stem that extends from the valve body, rising and falling to drive the valve open and closed. Rotary valves have a stem connected to a plug, disc, or ball. When the stem rotates, it opens and closes the valve. In both designs, the stem moves within the valve body with relatively low friction while also contacting the process medium.

Measuring Valve Stem Seal Performance

Three major industrial standards measure valve stem leakage: TA Luft, FCI 91-1, and ISO 15848. However, each standard differs significantly in testing performance and methodology.

TA Luft is the earliest standard, providing leakage rate criteria without detailed specifications for test parameter classification.

FCI 91-1 aligns more closely with EPA’s LDAR program and uses EPA Method 21 to “sniff” valve packing and determine leakage rates. This standard provides valve testing methods and various classification ratings based on stem leakage rates after specified numbers of mechanical and thermal cycles.

ISO 15848 covers a broader scope, classifying several leakage rates for control and on-off valves based on mechanical cycles, thermal cycles, and stem size. It allows testing with helium or methane and specifies precise methods for measuring leakage.

Important: When comparing valve stem seal performance, it is crucial to first determine which standard was used for valve testing. Achieving very low leakage rates is relatively easy with fewer mechanical cycles. Maintaining very low leakage rates becomes much more difficult when valves undergo thousands of mechanical cycles while experiencing thermal cycling.

Packing Seal Valve Stems

Most valves use PTFE or graphite sealing rings to provide stem sealing. A combination of packing gland, packing gland flange, and bolts compresses the sealing rings, pushing the packing rings down against the stem. This design allows stem movement while preventing process medium from escaping.

To achieve and maintain low stem leakage, packing should use “live loading” to maintain constant pressure on the sealing rings. Fisher’s ENVIRO-SEAL™ packing uses compressed disc springs to maintain constant pressure on the packing rings, ensuring seal integrity even after stem movement wear. However, increased pressure tends to restrict valve movement, so sealing materials and stem finish must be carefully selected.

Bellows Seal Valve Stems

Another option is the bellows seal valve. Bellows sealing utilizes a welded or mechanically formed metal barrier around the stem that compresses and extends like an accordion. This bellows-style metal seal achieves zero leakage.

Bellows seals use two main types:

Welded plate bellows — Constructed from a stack of washer-like thin plates welded together, providing many folds in a given length. Typically one-third the length of formed bellows.
Formed bellows — Made from a flat metal plate formed and welded into a mechanically fixed tube. Generally offers longer service life.

Packing Seal vs. Bellows Seal

Comparison	Packing Seal	Bellows Seal
Cost	Lower	Higher (total cost of ownership much higher)
Leakage Rate	Low leakage, cannot achieve zero	Zero leakage
Material Options	Various packing materials and designs	Metal construction, layers and stroke determine life
Maintenance	Can be adjusted and replaced without valve disassembly	Repair requires complete valve disassembly
Applications	Most general applications	Toxic services (where zero leakage is critical)
Failure Prediction	Minor leaks can be resolved by tightening	Failures are more predictable

How to Choose the Right Valve Stem Sealing Method

Each stem sealing method has its advantages and disadvantages. The ideal approach is to select the appropriate stem seal based on actual application requirements.

Choose Packing Seal When:

Budget constraints exist
General service, non-toxic media
Frequent maintenance adjustments needed
Low leakage rates meet requirements

Choose Bellows Seal When:

Toxic or hazardous media service
Zero leakage is mandatory
Strict environmental emission controls
Product loss costs are extremely high

Conclusion

Proper selection of valve stem sealing is a critical component of the valve specification process. When chosen correctly, the design provides reliable long-term performance, significantly reducing environmental emissions, product loss, and maintenance costs.

The range of design options is extensive. End users are advised to consult with valve suppliers to determine the ideal seal design, construction materials, and other application-specific details.