Nominal pressure and design pressure are two distinct but related concepts used in the design and operation of pressure vessels, piping systems, and other equipment. They relate to the pressure handling capabilities of the equipment.
Nominal Pressure (PN)
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Definition: Nominal pressure (PN) is a numerical designation used for reference purposes. It's an alphanumeric code that indicates the approximate pressure-bearing capability of a component, at room temperature. It is a convenient round number for classification and identification. Think of it like a pressure class.
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Usage: Primarily used for flanges, valves, and fittings.
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Units: Typically expressed in bar or MPa (megapascals), but it is a dimensionless designation. This means the actual allowable pressure corresponding to a specific PN value depends on the material and temperature.
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Discrete Values: Nominal pressures are standardized and come in discrete values. Common values (in MPa) include: 0.25, 0.6, 1.0, 1.6, 2.5, 4.0, 6.4, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100.
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Example: A flange designated as PN16 is suitable for a certain pressure at room temperature, but its pressure rating decreases as the temperature increases. You must consult the relevant standards (e.g., ASME, EN) for the specific material to determine the allowable pressure at a given temperature for the PN16 designation.
Design Pressure (P)
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Definition: Design pressure is the pressure used in the engineering design of a vessel or system to ensure safe operation under the most severe expected conditions. It is the pressure that the equipment is specifically designed to withstand.
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Usage: Used for calculations of vessel wall thickness, nozzle reinforcement, and other structural design aspects.
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Units: Usually expressed in pressure units, such as psi (pounds per square inch), bar, kPa (kilopascals), or MPa.
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Determining Design Pressure: The design pressure is typically determined by considering:
- Maximum expected operating pressure (MEOP).
- Static head pressure.
- Pressure surges (water hammer).
- Control system limitations.
- Safety factors.
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Relationship to Operating Pressure: Design pressure must be greater than or equal to the maximum expected operating pressure to provide a margin of safety.
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Relationship to Nominal Pressure: As per the linked document, it is expected that
Nominal Pressure >= Design Pressure.
Key Differences Summarized
| Feature | Nominal Pressure (PN) | Design Pressure (P) |
|---|---|---|
| Definition | Numerical designation for reference; approximate pressure class | Pressure used for engineering design to ensure safe operation |
| Units | Dimensionless (designation); actual pressure depends on material & temp. | Pressure units (psi, bar, kPa, MPa) |
| Usage | Classification of components (flanges, valves, etc.) | Structural design calculations |
| Values | Standardized, discrete values | Calculated based on operating conditions and safety factors |
| Temperature Dependence | Yes | No - specific to the worst case |
Example Scenario
Imagine designing a steam piping system. The maximum expected operating pressure is 10 bar. To account for pressure surges and control system limitations, a design pressure of 15 bar might be selected. Components would then be selected with a nominal pressure rating (PN) greater than or equal to 15 bar. A PN20 or PN25 component would likely be chosen.
In short, nominal pressure is a convenient classification, while design pressure is a specific calculated value used for engineering.
