Maximum Allowable Working Pressure

File: 3783, Last Updated: June 7, 2012, By: LRB

Design Pressure vs. Maximum Allowable Working Pressure (MAWP) Alberta (ABSA) and Saskatchewan (TSASK)

Typically when doing calculations for pressure vessels you start with a set of design parameters to determine your vessel characteristics. The design pressure, temperature, corrosion allowance and several other factors contribute to the calculations, affecting the wall thickness of the shell, heads and nozzles.

For example, you may have a design pressure of 150 psi with a temperature of 400° F, and no corrosion allowance. This is on a 20″ carbon steel vessel with the shell made from SA-106 B pipe. You select 20″ SCH 10 (0.25″ wall) because it exceeds the thickness required by the calculations (0.154″ wall), and it’s a readily available material. The MAWP of the shell is 320 psi.

Most of the time you are able to list the Design Pressure as 150 psi and the MAWP as 320 psi. However, Alberta has a clause in AB-516 Rev. 3, 2011-08-22 that does not permit this. Section 15(2)(b) Calculations states the following:

“The pressure used in the calculations must be equal to or greater than the MAWP to be marked on the vessel nameplate and manufacturer’s data report. A lower pressure which must be met for process conditions must not be used in the calculations.”

In our example, we have two choices:

• Have the drawing only state 150 psi. Design Pressure equals 150 psi. If MAWP must be listed on the drawing or the nameplate states MAWP, then the MAWP also equals 150 psi.
• Run the calculations at 320 psi. The design for the shell will still pass. Now our Design Pressure equals 320 psi and the MAWP also equals 320 psi.

Either method will be accepted in both Saskatchewan and Alberta.

So you can either rate your vessel’s MAWP to be the same as the design pressure as in the first method, or you can run calculations at the higher pressure of the MAWP and then list the MAWP as both the Design Pressure and MAWP. The choice is yours, but you must abide by one of these methods in order to obtain a CRN in either Alberta or Saskatchewan.

Note: The MAWP from the calculation could be 320.8 or some other number as long as it is higher than the MAWP reported on the nameplate and drawing. Do not use decimal precision numbers for the MAWP on the nameplate or drawing. For example if your design program returns a MAWP of 320.8 psi, you could re-run the design at 320.0 psi and declare that 320.0 is the MAWP even though your design software will report a slightly higher number. The design pressure and MAWP on your Alberta or Saskatchewan destination vessel will be the same number.

According to ASME Section VIII Division 1, the definitions for Design Pressure and MAWP are as follows (VIII-1 App 3):

Design Pressure – “The pressure used in the design of a vessel component together with the coincident design metal temperature, for the purpose of determining the minimum permissible thickness of physical characteristics of the different zones of the vessel. When applicable, static head shall be added to the design pressure to determine the thickness of any specific zone of the vessel.”

MAWP – “The maximum gage pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature for that pressure. This pressure is the least of the values for the internal or external pressure to be determined by the rules of this Division for any of the pressure boundary parts, including the static head thereon, using nominal thicknesses exclusive of allowances for corrosion and considering the effects of any combination of loadings listed in UG-22 that are likely to occur at the designated coincident temperature. It is the basis for the pressure setting of the pressure relieving devices protecting the vessel. The design pressure may be used in all cases in which calculations are not made to determine the value of the maximum allowable working pressure.”

In the document AB-516, ABSA defines maximum allowable working pressure (MAWP) as:

“the pressure authorized on the design registration or a lesser pressure as indicated on the manufacturer’s data report. MAWP is the highest pressure at which the equipment may be operated at its design temperature. MAWP is measured at the top of the vessel in its operating position.”

Another similar term commonly used is maximum allowable pressure (MAP). This refers to the maximum pressure in the new and cold condition. It does not take temperature effects or corrosion into consideration and therefore cannot be confused with MAWP or Design Pressure. It is best to avoid referring to this when submitting designs to Alberta or Saskatchewan.

As an example of the differences between Design Pressure, MAWP and MAP, we have run a set of calculations in Compress using the design conditions stated earlier. From the Pressure Summary output you can see that the Design Pressure is the same for every component (150 psi), however the MAWP and MAP are considerably higher.

The lowest MAWP for these components is 182.46 psi (rated at 182 on the drawing and nameplate), therefore the MAWP of this design is 182 psi. In this case the MAP and MAWP are the same for most of the components. This is mostly due to the fact that there is no corrosion allowance. Only the nozzles N1 and N2 have different values for MAWP and MAP. MAWP is in the hot and corroded condition while MAP is in the cold and new condition.

In most normal circumstances, such as our example above, the Design Pressure is lower than the MAWP. Typically a specification sets the Design Pressure based on the function of the vessel. The calculations are then run using this pressure, and the MAWP is derived from the calculations by taking the maximum pressure from the lowest rated component in the vessel. The vessel can be used up to the MAWP. This is the reason the ASME nameplate in Section VIII-1 depicts the MAWP instead of the Design Pressure.

As mentioned previously, in Alberta and Saskatchewan you cannot use this practice. You must have the Design Pressure set equal to the MAWP.