The reference material states that Design Pressure = 1.5 × Working Pressure, but this does not define nominal pressure. Nominal pressure relates to flange ratings and is not directly derived from design pressure.
Here's a breakdown to clarify the terms and their relationship (or lack thereof):
Understanding the Terms
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Nominal Pressure (PN): This is a numerical designation used for reference purposes. It relates to the pressure-temperature rating of a flange and doesn't have units. The nominal pressure defines the pressure-temperature capability of a flange. Standard values for vessel flanges, as indicated by the reference text, include 0.25, 0.60, 1.00, 1.60, 2.50, 4.00, and 6.40 MPa. This rating system allows engineers to select components suitable for different operating conditions.
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Design Pressure: This is the pressure used in the design of a pressure vessel or piping system. It is the maximum pressure the equipment is expected to experience during normal operation, and is often calculated using a formula, like the one provided (1.5 x working pressure). It provides a safety margin.
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Working Pressure: The pressure at which a vessel or system operates under normal conditions.
The Relationship (or Lack Thereof)
While design pressure helps determine the required pressure rating for a system's components, there's no direct mathematical link between design pressure and nominal pressure. You use the design pressure, along with other factors like temperature, to select an appropriate nominal pressure (flange rating) for the connections in the system. You need a flange with a pressure-temperature rating equal to or greater than what the design pressure dictates.
Example:
Let's say a pressure vessel has a working pressure of 1 MPa.
- The design pressure would be calculated as: Design Pressure = 1.5 * 1 MPa = 1.5 MPa.
- When selecting flanges for the vessel, you would need to choose a flange with a nominal pressure rating (PN) equal to or greater than 1.6 MPa (the next highest standard value from the reference list, since 1.5MPa isn't a standard flange rating). You would also consider the temperature to ensure the flange can withstand both the pressure and temperature.
In short, design pressure is a calculated value used to select components with appropriate nominal pressure ratings.
