What is the difference between external pressure vessels and internal pressure vessels?

External-pressure vessels include those that operate under negative pressure, as well as those in which the pressure inside the jacket is higher than the pressure inside the inner vessel; all other cases are internal-pressure vessels.

The difference between external-pressure vessels and internal-pressure vessels lies in the direction of the pressure

The failure modes are somewhat different; external pressure usually leads to instability and collapse due to insufficient stiffness

The design calculation methods for the two are different...

The pressure in a pressurized vessel increases from the inside outward, and the mode of failure is explosion； The pressure in a externally pressurized vessel acts from the outside inward, and the mode of failure is collapse.

For externally pressurized vessels, stiffness is the main consideration, while for internally pressurized vessels, strength is important

The main difference: one applies force from the outside, the other from the inside； Secondly, the modes of failure are different: one is stiffness, and the other is strength

Internal pressure: mainly strength failure. External pressures, mainly strength failure and stiffness failure. Many thin-walled containers under external pressure often suffer from local or overall collapse due to insufficient stiffness, before reaching the material’s strength limit, which prevents the equipment from functioning properly. Therefore, for thin-walled structures, the primary mode of failure is buckling failure.

1. Different failure modes: For internal pressure, fractures resulting from insufficient strength or toughness are a concern; for external pressure, instability and leakage caused by insufficient stiffness are the main issues. Thick-walled vessels under external pressure also need to take into account the problem of insufficient strength； 2. In terms of material selection: for internal pressure, strength and toughness are primarily considered, while for external pressure, the elastic modulus is mainly taken into account ； 3. In terms of design calculations: (1) For cylindrical and spherical shells, the internal pressure is calculated using the mean diameter formula, while the external pressure is calculated using the formula for circumferential instability under external pressure ； No difference between flat covers and flanges ； (2) If the internal pressure strength is insufficient, the wall thickness must be increased; there is no other solution ； An external pressure reinforcement ring can be added to apply external pressure ； (3) Allowable stress: For internal pressure, it is calculated by dividing the yield strength, tensile strength, and endurance limit of the material within its elastic range by a safety factor; whereas the allowable stress for external pressure is determined by reversing calculation based on the allowable strain, using the material’s tensile curve. 4. In terms of manufacturing: External pressure vessels require a higher degree of circularity in their cylinder bodies