MOP-MAOP-MAWP-DESIGN PRESSURE 1

[miyanui]

thread798-229469

BigInch, thank you for the awesome contribution in the above referenced thread, I think there is a lot of confusion even amongst experienced engineers with this subject.

 

[rotw]

From rotating equipment standpoint at least, for me it is exactly what Zdas posted in the referenced thread:

quoted
MAWP and MAOP are the same thing. Design pressure is a less formal term that sometimes means "design point" or the nominal operating pressure that the system will operate at most of the time (<MAWP) and sometimes it means MAWP depending on the country/company.
unquoted

Somehow it is a no brainer problem and I'd stick to the above explanation.

 

[bimr]

Actually, that definition is incorrect.

The ASME BPV Code requires that equipment under its Code stamp be protected from over pressurization; there is
no such requirement for piping systems. When a pressure relief device is specified for an equipment item the
MAWP of the equipment is the set point for the relief device.

After the tank is fabricated the actual material thickness, weldments and other as-built physical characteristics of the tank are compiled and used in a series of complex calculations to determine the actual MAWP of the tank. Once determined, the MAWP will replace the initial design pressure for the tank. It is also used to determine the relief device set point pressure.

With regard to a Piping System, the design pressure, with respect to the equipment MAWP, is determined based on service conditions, not the limits of the system components. Once the design conditions are determined based on service conditions, the piping system pressure Classification can be specified. While the pressure Classification of the piping system will exceed that of the design pressure it does not constitute a reassignment of the design conditions as in equipment design. The design conditions are specific to the service not to the piping material.

http://www.wmhuittco.com/images/Relative_Design_Test_Pressures_and_PSV_Set_Point.pdf

 

[bimr]

Another source with the same definition.

The design pressure is that pressure that the specifying engineer decides is the value of the pressure at which the vessel will normally operate (or which it must withstand under operating conditions). This value must include any normal excess pressure that can occur during the vessel’s operation. This is a discretionary value that depends on the background and experience of the design engineer. Sometimes the design value can be 10% over the pressure calculated (as in a simulation) or as much as 25% more. Good engineering judgment is employed in arriving at this design figure.

Once the Specification Sheet is received by the vessel fabricator, mechanical fabrication design takes place in which alloys, fabrication techniques, available materials, and other factors are taken into consideration to generate a fabrication drawing. Although the design pressure given is employed to generate the required vessel physical characteristics, some practical factors – such as available materials, fabrication efficiency factors, and alloys employed – will result in a vessel that not only meets the required design pressure, but often EXCEEDS it. This is a fortunate and conservative procedure because it ensures that the vessel will meet pressure safety expectations. The Maximum Allowable Working Pressure (MAWP) is a result of back-calculating the ultimate resulting fabricated vessel and is the prime factor in setting the pressure at which the corresponding vessel Safety Relief Devices will be activated. The MAWP the most important pressure value attached to a vessel and one that should be clearly understood and stamped on the vessel for all to clearly read. The MAWP will change with time (as will the related design value) due to wear, corrosion, and vessel fatigue. This is why it is so important to religiously keep and maintain current and accurate data sheets and calculations on all pressure vessels as they are inspected and repaired through the years of service.

http://www.stb07.com/process-safety-management/process-safe-limits.html

•Maximum Allowable Working Pressure (MAWP) - maximum pressure permissible at the top of a vessel in its normal operating position at the operating temperature specified for the pressure. It is the least of the values found for maximum allowable working pressure for any of the essential parts of the vessel by the principles established in ASME Section VIII. The MAWP is shown on the vessel nameplate. The MAWP may be taken same as the design pressure, but for the most part the MAWP is based on the fabricated thickness minus the corrosion allowance. MAWP applies to pressure vessels only.

•Maximum Operating Pressure (MOP) - The highest pressure expected during operation. This is usually 10-20% below the MAWP.

http://www.wermac.org/others/ndt_pressure_testing.html

 

[rotw]

bimr,

For rotating equipment a casing is not a pressure vessel.

 

[bimr]

Within the ASME BPV Code Section VIII, Division I, the terms Maximum Allowable Working Pressure (MAWP)
and Vessel shall be interpreted to mean Design Pressure and Piping System respectively when applied to piping
systems. This is in relation to pressure relieving devices.

 

[Ripz]

To put it simply, the pressure you're designing the vessel to. Duh.

The thickness which is calculated will then be used to manufacture the vessel. However, the thickness you specify isn't always obtained (since plates are mass manufactured at specific thicknesses), so you'll usually get a slightly thicker vessel.

Thus, calculating using ASME's Formula, the Maximum Pressure this vessel is now Allowed to Work at is higher than your Design Pressure.

And consider your vessel in service and corroded thus having a lower wall thickness, the calculated Maximum Pressure this vessel is Allowed to Work at will be lower than Design Pressure.