External Pressure

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ASME Code – Name Plate Stamping Requirements If the markings required are stamped directly on the vessel, the minimum letter size is 5⁄16 in. If they appear on a nameplate, the minimum letter size is 5 ⁄32 in. Pressure Vessel Plate Material - You have to be careful when choosing Pressure Vessel Plate Materials; there are lots of requirements and specific prohibitions in the ASME code. Some of these requirements are ASME and Non ASME plate material, plate.

File: PVE-3473, Last Updated: July 5,2012, By: Laurence Brundrett

Design and Fabrication Specification for Vessels March 2012 ASME Code Section VIII, Divisions 1 and 2 Process Industry Practices Page 2 of 44 1. General Requirements 1.1 Purpose This Practice provides design criteria and construction requirements for pressure vessels constructed in accordance with ASME Boiler and Pressure Vessel.

External pressure (vacuum) calculations start off more complex than internal pressure calculations, and once jackets or other sources of pressure are added the difficulty increases. The external pressure rating depends upon more variables and the failure mechanism is more difficult to understand. This article is only an introduction, but it also covers many of the areas of external pressure that often need to be explained. Common mistakes made with external pressure calculations are listed. Sample vessel calculations are included throughout this article based on a vessel 48″ diameter x 96″ long with a Flanged and Dished (F&D) head at one end and a Semi Elliptical (SE) head at the other. As the design conditions change, the required shell thickness is updated. Internal pressures are calculated at 30 psi for comparison. Download the sample calculations at the end of this article.

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1 – The Basics – Failure Mechanisms

The mechanism of external pressure failure is different from internal pressure failure. Different methods are required to design vessels to safely handle this different failure mechanism. Internal pressure failure can be understood as a vessel failing after stresses in part or a large portion exceeds the materials strength. In contrast, during external pressure failure the vessel can no longer support its shape and suddenly, irreversibly takes on a new lower volume shape. The following three pictures show vessels of reduced volume after external pressure failure. The fourth picture shows an internal pressure failure for contrast.

A storage tank tested crushed after hydrotest when a plastic sheet blocked the top vent during draining. The volume of vessel decreases.

Vacuum failure of a barometric condenser (vacuum vessel) after the internal support rings failed. The volume of the vessel decreases.

A storage tank tested to destruction with an internal vacuum (external pressure). The volume of the vessel decreases.

Unexpected INTERNAL pressure failure during hydrotest (caused by pressure stress and helped by unacceptable material properties) . Here volume increases until failure occurs. compare this with the external failure examples.

External pressure can be created three ways:

From a vacuum inside a vessel and atmospheric pressure outside
From a pressure outside a vessel greater than atmospheric (typically from some types of jacket or a surrounding vessel)
From a combination of the first two – vacuum inside + pressure greater than atmospheric outside.

Two simple and unexpected sources of vacuum – unloading a vessel or a tank that is not adequately vented – or – cooling down a vessel filled with steam that condenses to water. It is good practice to design any vessel exposed to steam service for full vacuum, the cool down rate can be very fast, overloading vacuum protection equipment.

External pressure generated from an internal vacuum

External pressure failure can be understood as a loss of stability. The vessel no longer has the ability to hold its shape and suddenly collapses to a shape with less internal volume. Good videos can be found on YouTube – see the links at the bottom of this article. ASME type pressure vessels use code rules to calculate the safe external pressure load. The stability of a straight shell under external pressure depends on four variables:

diameter (the larger the diameter, the less stable)
length (the longer, the less stable)
thickness (the thinner, the less stable)
material properties – a lower yield point is less stable from plastic collapse and a lower modulus of elasticity is less stable from elastic collapse. Elastic and plastic collapse will not be discussed further here.

Large vessels typically have lower external than internal pressure ratings. These four variables are used in Code equations which can specify the thickness of a safe straight shell. Similar methods are used to calculate heads. Nozzles are calculated using the familiar area replacement rules. In addition we can use burst (proof) testing or finite element analysis to calculate components not covered by the code rules. The code rules are on average very conservative, but greatly simplify what would otherwise be very difficult calculations – this has been a good trade off. The finite element analysis and burst testing are done to a usually less conservative 3x factor of safety (See ASME VIII-1 UG-101(p)) where exact testing or analysis information replaces generalized code rules.

2 – Designing for External Pressure

The easiest way to design for external pressure is to make the shell thick enough to make the vessel stable with an acceptable factor of safety (pass code calculations). The length of the vessel used in the calculations includes some of the head at each end. The calculations are found in ASME VIII-1 UG-28. The shell calculations are for a cylinder with supported ends (the heads at each end). Calculations are also given for the heads which are treated as spheres.

The effective length of a vessel includes some of the length of the heads. The head is calculated as a sphere.

(See the companion calculation set part 2 starting on page 4.) A typical vessel 48″ diameter with a straight shell 96″ long needs a shell 0.056″ thick for an internal pressure of 30 psi, but needs to be 0.225″ thick for a 15 psig external pressure (full vacuum) per VIII-1 UG-28. The F&D head on the left end needs to be 0.082″ thick for the interior pressure but 0.142″ for the vacuum. A SE head needs to be 0.046″ thick for the interior pressure but 0.127″ for the vacuum. The F&D and SE heads are both calculated as if they are part of a sphere, but the two heads are given different equivalent radiuses resulting in different required thicknesses.

4x factor of safety in external pressure buckling analysis

4x factor of safety – the design exceeds the code required 3x factor of safety

FEA results show that the shell will collapse at 4.04x the 15 psi applied pressure. This is greater than the 3x safety factor expected in the code and shows the code results to be acceptable.

3 – Vacuum Rings

Instead of making the shell thick enough to handle the external pressure, an economical vessel can often be designed by reinforcing the shell. When the reinforcement is as strong as required by UG-29, the effective length of the shell is reduced and thinner shells can be used. In this case, L1 and L2 are calculated separately. Each zone independently passes the code calculations. The UG-29 reinforcing calculation ensures that the reinforcing is strong enough that whatever happens on one side of it has no impact on the other.

A vessel divided into zones using a stiffening ring

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(See companion calculations starting on page 9.) The sample vessel shell reduces in required thickness from 0.225 to 0.168″. Not bad for the addition of a 0.25 x 2.5″ bar rolled the hard way. More rings could be added to lower the required thickness further. The required head thickness could also be reduced by adding some type of reinforcement. VIII-1 allows for this, but provides no guidance on the design.

8.1x factor of safety with a thinner she

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