The schedule in a flange refers to a number that denotes the wall thickness of the flange. It's a standardized way to specify the pressure-containing ability of a flanged connection.
Understanding Flange Schedules
Flange schedules, similar to pipe schedules, indicate the relative wall thickness of the flange. A higher schedule number means a thicker wall and therefore greater pressure resistance. The schedule number does not directly translate to a specific dimension in inches or millimeters. It is a dimensionless number that is related to the pressure rating and material strength.
Why Use Schedules?
- Standardization: Schedules provide a standardized method for specifying flange thickness, ensuring compatibility between different manufacturers and applications.
- Pressure Rating: The schedule is related to the pressure rating of the flange. Selecting the correct schedule ensures the flange can safely handle the intended operating pressure.
- Material Strength: The appropriate schedule also depends on the material of the flange. Stronger materials can withstand higher pressures with thinner walls, while weaker materials require thicker walls for the same pressure rating.
How Schedules are Determined
Flange schedules are typically determined using formulas that take into account:
- Design Pressure: The maximum pressure the system is expected to experience.
- Allowable Stress: The maximum stress the flange material can withstand at the operating temperature.
- Diameter: The nominal diameter of the flange.
Common Flange Schedules
While many schedules exist, some of the more common ones include:
- Schedule 5S, 10S, 10, 20, 30, STD, 40, 60, XS, 80, 100, 120, 140, 160, XXS
Generally, "STD" is often the same as Schedule 40 for smaller diameter flanges and "XS" is often the same as Schedule 80 for smaller diameter flanges. "XXS" is extra extra strong. The exact correlation depends on the specific size and standard.
Importance of Proper Selection
Choosing the correct flange schedule is crucial for safety and reliability. An undersized flange can fail under pressure, leading to leaks, equipment damage, and potential injury. An oversized flange is unnecessarily expensive and adds weight to the system. Consulting with a qualified engineer or referring to relevant standards (like ASME or API) is essential when selecting a flange schedule.
