Maximum Allowable Working Pressure defined

Maximum Allowable Working Pressure -As defined in the construction codes for pressure vessels, the maximum allowable working pressure depends on the type of material, its thickness, and the service conditions set as the basis for design. The vessel or equipment may not be operated above this pressure, or its equivalent stress at any other metal, temperature. However, ANSI B31.3, Petroleum Refinery Piping Code, permits variations above the maximum allowable working pressure, as follows ... 

All pressure easing eomponents (ineluding seal glands and seal chambers) as defined in 1.4.40 shall be hydrostatically tested with liquid at a minimum of 1.5 times the maximum allowable working pressure, with the special provisions specified below ... 

The first, 1914 edition of the ASME Code did basic things like defining the term maximum allowable working pressure, and defining the minimum allowable distance between rows of rivets. The 1914 Code stipulated that the maximum allowable pressure on cast iron boiler headers should not exceed 160 psig (about 10 atmospheres or 10 bar). Minimum capacities for boiler pressure relief valves were defined. 

In field applications, it is assumed that the equipment has already been fabricated and been shop-tested for the maximum allowable pressure (MAP). The MAP is defined as the maximum allowable pressure of the vessel in the new and cold condition. It is more often determined in the shop before delivery. After the vessel is delivered, any test performed after operation begins is the maximum allowable working pressure (MAWP). The MAWP can also be used for new construction. The maximum allowable working pressure is defined as the maximum gauge pressure permissible at the top of the completed vessel in its operating condition for a designated temperature. Thus, in the field, you are likely to hear the term MAWP much more than MAP. 

The entropy concept allows us to define two new extensive thermodynamic variables the Helmholtz Free Energy, F, which is the maximum amount of work a system can do at constant temperature (isothermal changes) and the Gibbs Free Energy, G, which is the maximum amount of work a system can do at constant pressure (isobaric changes) and is a minimum for closed systems at equilibrium with a fixed temperature and pressure. 

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