Gaskets are essential components for sealing piping systems and process equipment.
However, many people think of gaskets as something you simply “sandwich and tighten.”
In reality, two important parameters define gasket performance:
the gasket factor (m) and the minimum seating stress (y).
These values vary significantly depending on the gasket type and strongly influence bolt load, sealing reliability, and leakage risk.
This article compares typical values of m and y for common gasket types and explains practical selection points from a plant engineering perspective.
By the time you become a fully independent mechanical or instrumentation engineer, this is knowledge you should be able to visualize intuitively.
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Approximate Values by Gasket Type
Let’s first review representative values of gasket factor and minimum seating stress.
The data below is referenced from VALQUA’s gasket catalog.
Although these values depend on thickness, we fix the thickness at 3 mm, which is a standard and commonly used gasket thickness in practice.
Gasket Factor (m)
The gasket factor m is defined as:
The load applied to the gasket to determine the required bolt load during operation.
| Gasket Type | m |
|---|---|
| Compressed Sheet | 2.0 |
| PTFE | 2.5 |
| PTFE Jacketed (CS insert) | 3.5 |
| Spiral Wound | 3.0 |
| Metal Jacketed (SS) | 3.5 |
| Solid Metal (SS) | 6.5 |
Except for solid metal gaskets, most values fall within a narrow range of 2.0–3.5.
In practice, gasket selection is rarely made based directly on the m value alone.
In batch plants especially, the following assumptions are common:
- Internal pressure ≈ flange pressure rating
- JIS 10K flange is used
- Gasket stress = flange pressure × gasket factor
- Bolt size is already defined by flange standards
Because of this, engineers seldom select bolts independently based on gasket factors.
However, in high-pressure systems, the gasket factor should be explicitly considered when designing the flange–bolt–gasket system.
The reason m is often remembered is simple:
it is an easy numerical indicator to describe gasket characteristics.
Minimum Seating Stress (y)
The minimum seating stress y is defined as:
The load applied to the gasket to achieve sealing during initial tightening.
| Gasket Type | y (N/mm²) |
|---|---|
| Compressed Sheet | 11.0 |
| PTFE | 19.6 |
| PTFE Jacketed (CS insert) | 14.7 |
| Spiral Wound | 68.9 |
| Metal Jacketed (SS) | 44.8 |
| Solid Metal (SS) | 179.3 |
Here, the differences are significant.
The y value is closely related to gasket hardness and seating behavior.
In batch plants, discussions often focus on compressed sheet, PTFE, and PTFE-based gaskets, where differences are moderate.
When using spiral wound or metal jacketed gaskets, it is essential to design the entire system—flange, bolts, and gasket—together.
Comparison of Low-Pressure Gaskets
For low-pressure applications, the following topics often arise.
“This gasket feels hard”
Hardness is often discussed based on hand feel or tightening sensation.
This becomes important for equipment such as glass-lined vessels, where excessive bolt load must be avoided.
PTFE gaskets tend to feel harder, which is reflected in their higher y value (19.6 N/mm²).
Interestingly, PTFE jacketed gaskets were developed partly to improve conformability and seating behavior.
“PTFE gaskets tend to leak”
This was a common belief in the past.
It is related to the gasket factor m.
PTFE jacketed gaskets have a higher m value (3.5), meaning higher bolt load is required for the same operating pressure.
If tightened with the same “feel” as softer gaskets, leakage is more likely to occur.
Gasket paste is often used as a countermeasure, but in processes where paste is undesirable, careful handling is essential.
Effect of Gasket Thickness
In this article, we fixed gasket thickness at 3 mm.
As a general reference:
Thicker gaskets have lower m values and lower y values.
In simpler terms:
Thicker gaskets are easier to tighten.
Lower m and y values mean lower required bolt load.
Lower tightening force reduces the risk of uneven bolt loading, especially in large-diameter flanges.
For glass-lined equipment or large vessels, this consideration can be particularly useful.
Conclusion
The gasket factor (m) is a guideline for required bolt load during operation,
while the minimum seating stress (y) reflects tightening difficulty and gasket hardness.
Understanding the large variation in y values among gasket types helps prevent leakage and over-tightening problems.
This knowledge is especially valuable when selecting low-pressure gaskets in batch chemical plants.
If you have questions or challenges related to gasket selection, plant design, maintenance, or operation, feel free to leave a comment below.
All comments are carefully reviewed and answered.
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