Understanding operating, metal, design, and minimum design metal temperatures in pressure vessels

In the design and operation of pressure vessels, several key temperature parameters are critical for ensuring safety and functionality. These include operating temperature, metal temperature, design temperature, and the minimum design metal temperature (MDMT). Below is an explanation of each of these terms, their significance, and how they are applied in practice.

Operating Temperature

1.Definition: Operating temperature, sometimes referred to as service temperature, is the temperature of the material inside the vessel during normal operation.

2.Usage: In technical documents within the pressure vessel industry, “operating temperature” refers specifically to the temperature of the substance contained in the vessel.

3.Variation: Different parts of a vessel may experience varying operating temperatures, and it is sometimes necessary to specify the operating temperature for each section.

Piping system test temperature measurement

Metal Temperature

1.Definition: Metal temperature refers to the temperature of the vessel’s components, which may differ across different elements or sections of the same component.

2.Application: For example, in a shell-and-tube heat exchanger, the metal temperature typically refers to the wall temperature when calculating the temperature for the tube sheet.

Design Temperature

1.Definition: Design temperature is the calculated average metal temperature of a vessel’s components under normal operating conditions.

2.Usage: This temperature, along with the design pressure, is used to establish the design load conditions of the vessel.

3.Variation: If different parts of the vessel experience different metal temperatures under normal operation, each part may have its own design temperature.

Minimum Design Metal Temperature (MDMT)

1.Definition: The MDMT is the lowest anticipated temperature that a vessel’s components might experience under various potential operating conditions.

2.Considerations: When calculating MDMT, not only the normal operating temperature but also other factors must be considered, such as the lowest ambient temperature, operational anomalies, automatic cooling systems, and environmental conditions.

3.Component-Specific: Different parts of a vessel may have different MDMT values.

4.Material Selection: MDMT is a key factor in selecting appropriate materials for a vessel. The materials must meet the required performance standards at the lowest anticipated temperature and ensure the impact toughness of both the material and welded joints.

Test Temperature

1.Definition: Test temperature refers to the metal temperature of the vessel during pressure or leakage testing.

2.Application: In practical applications, the test temperature is usually set to the temperature of the testing medium.

3.Safety Considerations: To prevent brittle fracture during pressure or leakage tests, the metal temperature of the vessel’s shell must be maintained above a certain threshold, as specified by regulations such as the TSG21-2016 “Safety Technical Supervision Regulations for Fixed Pressure Vessels.”

Each of these temperature parameters plays a critical role in the design, operation, and testing of pressure vessels. Understanding and properly accounting for them ensures the structural integrity and safe operation of the vessel throughout its service life.