Understanding Maximum Allowable Pressure: MAWP and How to Calculate It

Maximum Allowable Working Pressure (MAWP) is defined in ASME Section VIII, Division 1, Appendix 3, and in ASME Section VIII, Division 2, paragraph 4.1.5.2. MAWP is the maximum internal pressure permitted for a pressure-retaining component at a coincident metal temperature, based on the available thickness and the applicable allowable stress.

At least one MAWP must be established for a pressure vessel, although multiple MAWPs may exist. When multiple MAWPs are listed, all must be provided with a coincident temperature. In practice, a single temperature, typically the design temperature, is commonly used as the governing temperature for the listed MAWP, as it controls the applicable allowable stress.

MAWP is established for a specific component, temperature, and loading condition. The governing MAWP for a vessel is the lowest applicable MAWP among all pressure-retaining components and applicable load cases.

Relationship to Design Pressure and Test Pressure

Design pressure is a value selected by the user (or a proxy) and used by the designer in the mechanical design and code validation. MAWP is a calculated value derived from code equations, material properties, geometry, and loading. A designer may elect to set the MAWP equal to the design pressure, thereby avoiding explicit calculation of a higher allowable value. In all cases, the MAWP must be at least equal to the design pressure.

The MAWP marked on the vessel cannot be greater than the value used to determine the test pressure. For example, a typical test pressure is determined as 1.3*(Allowable Stress Ratio)*MAWP. If the value used in this equation is 100 psig, then the MAWP marked on the vessel may not exceed 100 psig, even if a MAWP calculation resulted in 105 psig.

Temperature and Corrosion Basis

MAWP is defined at a coincident metal temperature. Allowable stress values are temperature-dependent, and each MAWP must be associated with the temperature at which it applies. When multiple MAWPs are listed, each must be provided with its corresponding temperature.

For new construction, MAWP is typically governed by the corroded condition. Corrosion allowances represent the anticipated corrosion on the vessel for the intended life of the vessel. To determine the MAWP in the corroded condition, the new dimensions are altered by the corrosion allowance, and the calculation is performed. When determining a MAWP for a vessel in service (e.g. a re-rate), the same methodology is often employed as for a new vessel, except the measured dimensions are now considered as the new geometry. A new corrosion allowance is employed, the geometry is altered for the calculation, etc.

Component- and Chamber-Specific MAWP

MAWP is calculated on a component-by-component basis. Each pressure-retaining component, including shells, heads, nozzles, and flanges, must be evaluated independently. The minimum MAWP among these components governs for the specified condition.

MAWP may also differ between chambers within the same vessel. A shell-and-tube heat exchanger, for example, may have different MAWPs on the shell side and tube side. In all cases, coincident coincident temperatures and all applicable load cases must be considered when establishing MAWP.

Flanges and Pressure–Temperature Ratings

Pressure–temperature ratings published in ASME B16.5 and ASME B16.47 represent combinations of temperature and pressure for various material groups. The pressure listed at a given temperature is a Maximum Allowable Pressure (MAP).

MAWP is determined as the MAP minus static head. External loads acting in combination with pressure may reduce the MAP and therefore reduce the MAWP. External loads may not increase the MAWP above the MAP established by the pressure–temperature rating.

Typical Workflow for Establishing MAWP

A typical workflow for establishing MAWP for a vessel component at a given temperature and loading condition is as follows:

1. Establish material properties at the required temperature.

2. Establish the design pressure, static head, and external loadings.

3. Corrode the dimensions.

4. Determine the required thickness using the applicable code equations, as well as other required design methods.

5. Pick a nominal thickness that meets or exceeds the sum of the required thickness, corrosion allowances, forming allowances and tolerance requirements.

6. Determine the thickness available for loading as the nominal thickness minus applicable corrosion allowances, forming allowances, and tolerance requirements. This value must be at least equal to the required thickness.

7. Solve for the Maximum Allowable Pressure (MAP) of the component.

8. Solve for the Maximum Allowable Working Pressure (MAWP) of the component. 

Workflow Example, slight variation on PTB-4 2021 E 4.3.1: for a cylinder under internal pressure for S8D1

• SA-516 Grade 70, normalized, at 300 degrees Fahrenheit, with an allowable stress of 20,000 pounds per square inch for a Section VIII, Division 1 vessel.

• Internal design pressure of 350 psig, static head of 6 psig, and no external loadings. The pressure used in the design equations is the sum of these values, P = 356 psig.

• New inside diameter of 90.0000 inches with an inside corrosion allowance of 0.125 inches. The diameter used in the design equations is the corroded inside diameter, D = 90.2500 inches. The equation requires R, where R = 0.5D (corroded).

• Required thickness is determined as the greater of the circumferential stress equation in paragraph UG-27(c)(1) or the longitudinal stress equation in paragraph UG-27(c)(2). With all joint efficiencies equal to 1.0, the circumferential stress equation governs, resulting in a required thickness of 0.8119 inches.

• Nominal thickness is selected to meet or exceed the sum of the required thickness and applicable allowances. The minimum required nominal thickness is 0.9369 inches, and a nominal thickness of 1.0000 inch is selected.

• Thickness available for pressure loading is determined as the nominal thickness minus the corrosion allowance, resulting in 0.8750 inches.

• Using the circumferential stress equation in paragraph UG-27(c)(1), rearranged to solve for pressure with the available thickness, allowable stress of 20,000 pounds per square inch, inside radius of 45.125 inches, and joint efficiency of 1.0, the Maximum Allowable Pressure is 383.35 psig. 

• Maximum Allowable Working Pressure determined as the Maximum Allowable Pressure minus static head, resulting in an MAWP of 377.35 psig.
  
  Note: This example assumes new construction. For a rerating evaluation, the actual measured thickness of the shell may be used as the nominal starting thickness when determining the thickness available for loading.

Summary

MAWP is a calculated pressure limit defined by code and established for a specific component, temperature, and loading condition. It may vary between components and chambers within a vessel and is commonly governed by the corroded condition. MAWP is determined from a component-level evaluation that considers coincident pressure, static head, and external loads. When established in accordance with Section VIII requirements, MAWP provides the pressure limit used for design, testing, and operation.

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