Saturated vs superheated steam —
why a gasket that survives one
may fail in the other
Documentation scope: Approval, compliance and certification references apply to the documented material grade, supplier certificate or listing, and stated test conditions unless a product-specific certificate says otherwise. A cut gasket size is not automatically certified as a separate product just because it is made from a documented material. For regulated applications, confirm the current certificate or listing scope, exact grade, medium, temperature, pressure and market requirement before specifying.
Scope note: This article is a selection and diagnosis reference for steam service. Do not tighten, loosen or disturb hot or pressurised steam joints from this article; follow plant procedure, isolation rules and competent-person assessment.
The physical difference — what each type of steam actually is
Saturated steam is steam at the temperature and pressure at which water transitions from liquid to vapour. At any given pressure, there is one specific saturation temperature — for example, at 10 bar (gauge), the saturation temperature is approximately 184°C.
Saturated steam may contain a small proportion of liquid water droplets — wet saturated steam — or be entirely vapour — dry saturated steam. The presence of moisture makes it a wet, chemically active environment at the gasket face.
Many heating systems, process heating and lower-pressure steam distribution use saturated steam.
Superheated steam has been heated beyond the saturation temperature at the same pressure. It contains no liquid water and behaves more like a gas than a vapour. At 10 bar (gauge), superheated steam might be at 300°C, 350°C or higher — well above the 184°C saturation point at that pressure.
The degree of superheat — how far the temperature exceeds the saturation point — is an additional variable on top of pressure. A system described as "10 bar steam" could be saturated at 184°C or superheated to 350°C. These are very different sealing conditions.
Power generation, high-efficiency process plant, and turbine inlet lines commonly use superheated steam.
Why the same pressure rating is not the same temperature
A common error in steam gasket selection is specifying a grade based on system pressure alone, without checking the actual steam temperature. In saturated steam, pressure and temperature are linked — there is one saturation temperature for each pressure. But in superheated steam, the temperature can be significantly higher than the saturation point at the same pressure.
| Gauge pressure | Saturated steam temp | Illustrative superheated temperature range | Implication |
|---|---|---|---|
| 6 bar | 165°C | 200–350°C+ | A grade rated to 180°C may be adequate for saturated, insufficient for superheated at same pressure |
| 10 bar | 184°C | 250–400°C+ | The degree of superheat determines actual temperature — should be confirmed from the system design |
| 16 bar absolute | about 201°C | 300°C+ | Standard fibre grades with NBR binder have continuous ratings that may not cover superheated service at this pressure |
| 40 bar | 250°C | 400–500°C+ | High-pressure superheated steam is outside the range of compressed fibre grades — graphite or metallic grades apply |
Saturation temperatures approximate at gauge pressure. Superheated temperatures illustrative — actual temperature depends on system design and degree of superheat. Confirm actual operating temperature from system documentation before gasket selection.
The degree of superheat is the critical missing variable. A system specification that states "16 bar steam" tells you the pressure. It does not tell you whether the steam is saturated at 201°C or superheated to 350°C or higher. These require different gasket grades. Confirming the actual steam temperature at the connection point — not just the system pressure — is the first step in correct gasket selection for steam service.
Continuous temperature — the parameter that matters most
Every gasket material has two temperature ratings: a peak temperature and a continuous operating temperature. These are different numbers and they describe different things.
The peak temperature is the maximum the material can withstand for a limited time — typically during process upsets, start-up or short transient conditions. The continuous temperature is the maximum at which the material can sustain reliable sealing performance over an extended service period.
In steam service, continuous temperature is the more relevant parameter. A steam line operates continuously at its design conditions. A gasket in that line is at operating temperature for as long as the system is running — hours, days, weeks between maintenance shutdowns.
Peak temperature is not the operating temperature. A gasket grade with a 350°C peak and a 250°C continuous rating is not normally suitable for continuous service at 300°C superheated steam — even though 300°C is below the peak. The continuous rating is the ceiling for sustained service. Marketing literature that emphasises peak temperature without stating continuous temperature is presenting an incomplete picture.
Why superheated steam is harder on gasket materials
Where compressed fibre grades end and graphite grades begin
The continuous steam temperature rating varies by gasket grade and material. In the Kinetics Line range, continuous steam ratings from Kinetics Line technical data are listed below. These ratings indicate material suitability boundaries — not automatic application approval for any specific steam joint. Actual suitability depends on the documented operating conditions, system design and relevant standards.
- GREENSEAL PRO 180 — continuous steam 120°C. Appropriate for low-pressure saturated steam within this limit.
- BLUESEAL ULTRA 350 — continuous steam 200°C. Covers a wider saturated steam range and lower-temperature superheated service where actual temperature is confirmed within this limit.
- FLEXSEAL PRO 350 — continuous steam 200°C. The source lists the same 200°C continuous steam maximum as BLUESEAL, with different compressibility and recovery data. Compare exact gasket stress, leakage, relaxation and face criteria before selecting either grade.
- GRAPHITESEAL ULTRA 350 — continuous steam 250°C. The highest continuous steam rating in the Kinetics Line compressed sheet range. Appropriate for applications where temperatures approach or exceed the limit of standard fibre grades, and where the graphite content is not a constraint (note: not food contact approved).
Above the continuous steam ceiling of standard compressed fibre grades, graphite-based, mica-based or metallic sealing classes are typically the next candidates for evaluation. These grades operate at continuous temperatures that compressed fibre grades cannot sustain, and they do not rely on rubber binder for sealing — which removes the oxidation failure mode relevant to rubber-bound fibre grades in high-temperature dry steam. The suitable grade within these classes depends on the specific operating conditions, flange standard and design requirements.
Compressed fibre grades — where they fit
- Low to medium pressure saturated steam
- Continuous steam temperature within grade rating
- Systems where moisture is present at the gasket
- Where NBR binder oxidation is not a primary concern at the confirmed operating temperature
- Confirm continuous steam rating — not peak — for the actual temperature
Graphite-based grades — where they fit
- Higher-temperature saturated and superheated steam
- Where continuous temperature exceeds the ceiling of fibre grades
- Dry steam conditions where rubber binder oxidation is a concern
- Applications where high residual stress retention at temperature is required
- Ordinary cut sizes are not automatically covered for food contact; the underlying material may have conditional EC 1935/2004 documentation only when specifically ordered with the required traceability. Verify the supplied batch and scope
What to confirm before selecting a gasket for steam service
- Is the steam saturated or superheated? Check the system P&ID, process data sheet or equipment specification. Do not assume from pressure alone.
- What is the actual steam temperature at the connection? For superheated systems, the temperature at any given point depends on the degree of superheat and any temperature drop along the line. Confirm from the system design documentation.
- What is the continuous operating temperature? Not the peak. Not the start-up condition. The sustained temperature during normal operation.
- Does the gasket grade's continuous steam rating cover the actual temperature? Check the material technical datasheet for the specific grade, not the product family headline figure.
- Is the system subject to thermal cycling? Repeated transitions between operating and ambient temperature accelerate creep relaxation. Material grades with better residual stress retention at temperature are preferable in heavily cycled systems.
The continuous steam temperature rating is not the same as the continuous temperature rating. Some materials have a lower continuous rating in steam than in dry heat — because steam at elevated temperature and pressure is a more aggressive environment than dry heat alone. Check the steam-specific continuous rating in the datasheet, not just the general continuous temperature figure.
Saturated and superheated steam are different sealing problems, not the same problem at different pressures.
Superheated steam is hotter and drier than saturated steam at the same pressure. The actual temperature should be confirmed from the system design — pressure alone does not determine it. For gasket selection, continuous temperature rating is more relevant than peak temperature. Above the continuous steam ceiling of compressed fibre grades, graphite-based, mica-based or metallic sealing classes are typically the next candidates for evaluation. Confirm the steam type, actual temperature and continuous rating before specifying — not after.
Practical FAQ
What is the difference between saturated and superheated steam?
Saturated steam is steam at the temperature and pressure at which water transitions from liquid to vapour — the boiling point at that pressure. It may contain a small proportion of liquid water droplets (wet saturated steam) or be entirely vapour (dry saturated steam). Superheated steam is steam that has been heated beyond the saturation point at the same pressure. It is entirely dry, has a higher temperature than saturated steam at the same pressure, and behaves more like a gas than a vapour. The distinction matters for gasket selection because superheated steam is hotter and drier than saturated steam at the same system pressure, and these conditions affect gasket material behaviour differently.
Why does a gasket that works in saturated steam sometimes fail in superheated steam?
Several factors contribute. First, superheated steam is hotter than saturated steam at the same pressure — the actual temperature at the gasket may be significantly higher than the system pressure would suggest in a saturated steam application. Second, the dryness of superheated steam means the gasket binder material is exposed to dry heat rather than wet steam — conditions that can accelerate oxidation and thermal degradation of some rubber binder compounds. Third, the continuous temperature in superheated steam service may exceed the continuous temperature rating of a gasket grade even if the peak temperature is within the material's stated limit. Peak temperature and continuous temperature are different parameters, and continuous temperature is the more relevant one for sustained steam service.
What gasket materials are appropriate for superheated steam?
For superheated steam where the documented continuous operating temperature exceeds the continuous steam capability of compressed fibre grades, graphite-based, mica-based or metallic sealing classes are generally the next candidates for evaluation. Compressed fibre grades with NBR binder are usually outside their practical range in high-temperature superheated steam service, especially where temperatures exceed 300°C. Flexible graphite and graphite composite grades typically offer higher continuous temperature capability than rubber-bound fibre grades at elevated temperatures, and may be preferable where rubber binder oxidation becomes a concern. The specific temperature and pressure conditions, along with the flange standard and gasket dimensions, determine the suitable grade — consult the gasket material datasheet for the actual operating conditions.