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Difference Between MAWP and Design Pressure

MAWP vs Design Pressure

When equipment is designed, MAWP, or Maximum Allowable Working Pressure, and design pressure are very much taken into account.

In designing equipment, it is seen that it sustains the stress imposed on it from the external and the internal pressures. This pressure is termed as design pressure. The design pressure of any vessel or equipment is determined from its maximum operating pressure which is generally increased by some margin for balancing the probable pressure surge.

MAWP is the maximum pressure at which the vessel or equipment is allowed to function at a specific temperature. The mechanical design, which is based on Material of Construction (MOC) and thickness of the vessel is taken into account in MAWP.

One of the main differences between Maximum Allowable Working Pressure and design pressure is that the latter one is always equal or less than the MAWP of the vessel or equipment. The PSV set pressure is based on design pressure, and it can be equal to the MAWP but not higher.

Design pressure is the condition of coincident temperature and pressure that is expected in a normal condition. Maximum Allowable Working Pressure is the maximum pressure that is permissible at the top of the equipment or vessel in normal operating conditions.

The MAWP will be changing with time because of corrosion and vessel fatigue. Design pressure depends on the kind of system (fuel, water, steam, etc.) that is used in the vessels. Both the design pressure and MAWP have many applications in steam drums, boilers, piping, and different types of pressure vessels.

Summary:

1.In designing equipment, it is seen that it sustains the stress imposed on it from the external and the internal pressures, and this pressure is termed as design pressure.
2.MAWP is the maximum pressure at which the vessel or equipment is allowed to function at a specific temperature.
3.Design pressure is the condition of coincident temperature and pressure that is expected in a normal condition.
4.Maximum Allowable Working Pressure is the maximum pressure that is permissible at the top of the equipment or vessel in normal operating conditions.
5.One of the main differences between Maximum Allowable Working Pressure and design pressure is that the latter one is always equal or less than MAWP of the vessel or equipment.
6.The MAWP will be changing with time because of corrosion and vessel fatigue. Design pressure depends on the kind of system (fuel, water, steam, etc.) that is used in the vessels.


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3 Comments

  1. Design Pressure(DP) – It is function of process condition like pressure and temperature , chain reaction etc. i.e key process parameters.

    Maximum allowable working pressure (MAWP) – It is the maximum process pressure that can be allowed in a pressure depending on the current metallurgical condition of the pressure vessel. Due to corrosion and other environmental effects it is considered to decrease with time.

    Maximum Allowable Operating Pressure (MAOP) : Maximum allowable operating pressure or MAOP refers to the wall strength of a pressurized cylinder such as a pipeline or storage tank and how much pressure the walls may safely hold in normal operation

  2. How is it explained then, that in process datasheet design pressure is always higher than the operating pressure?

    In my opinion the design pressure is based on what process engineering determines to be the possibility of pressure surge in the complete system. Which will always mean that DP > MAWP.

    As long as Operating pressure remains below MAWP, the equipment would be compliant.

    However, in several cases manufacturers do point out the MAWP, but are not showing what the equipment is actually set out for, being DP.

    DP, in most cases, is however of the essence, as this determines hydrostatic test pressure for the equipment.

    The story above is inconsistant, as the first paragraph mentions that design pressure is MAWP with a safety increase and the rest of the explanation is stating that DP is mostly equal to or less than MAWP!

    Fact is, that asking for a guarantee on DP is mostly dependant of what type of equipment is required. For instance, if a hose has a MAWP of 14 barg and manufacturer of the equipment states that they use a safety factor of 3, is the DP than 42 bar(g)? To date there is no-one that can give a definitive answer to above question.

    Anyone?

  3. No, the DP is not 42 bar(g). That is the likely rupture pressure.

    If the normal operating pressure of a water hose is 70 psig.

    then the designer chooses a slightly higher pressure as the design pressure. Say in this case, somewhere between 30 to 50 psig higher. Say 100 to 150 psig.

    So lets set our design pressure at 100 psig.

    Then you run the calculations for the temperature and pressure to determine the thickness of the hose and set the Maximum Allowable Working Pressure.

    As a general practice, I like to ask that the MAWP be set for the highest attainable pressure over the the design pressure.

    —-

    Say I have a small pressure vessel that I intend to operate between o to 5 psig at ambient temperature.

    So I set a design pressure of 30 psig at 120 F and ask the designers to build me a vessel.

    It is a small vessel, so when they spec it out, the 30 psig design rating is easily met. Their calculations allow the vessel to be rated for an MAWP of 150 psig at 300 F.

    So they could set the vessel MAWP at 30 psig and 120F, what I asked for.

    Or they could set the vessel MAWP at 150 psig at 300 F.

    Since it is a pain in the but to rerate a pressure vessel once it is built, I would prefer the vessel came in with a MAWP of 150 psig.

    Nothing is more annoying than to be working with what is essentially a 8″ diameter schedule 40 pipe with a 30 psig MAWP when it is clearly capable of having a much higher pressure rating.

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