ASME coded pressure vessels, MAWP

Process heat exchangers are ASMS coded pressure vessels with a MAWP (Maximum Allowable Working Pressure), just like any other vessel, for both the shell and tube sides. However, they normally are... 

Maximum Allowable Working Pressure (MAWP) the maximum pressure pounds per square inch gauge permissible at the top of a completed vessel in its operating position for a specific designated temperature corresponding to the MAWP pressure. This pressure is calculated in accordance with the ASME code (Par. UG-98) [1] for all parts or elements of the vessel using closest next larger to calculated value nominal thickness (closest standard for steel... 

It is critical that such an examination be made to be certain that the bursting or set pressure at this temperature does not exceed the MAWP of the vessel at the operating temperature per the ASME code [1] (see Figures 7-31A and 31B). 

The PED, as does the ASME VIII, accepts that the SRV is set at the MAP or PS that is the exact equivalent to MAWP in the ASME code. Also in PED, MAP or PS are the design pressure of the weakest component of the equipment used in a pressure vessel, which needs protecting from potential overpressures. This pressure vessel may be an assembly of different components - pipes, flanges, nozzles, shells, and so on - and each may have a different design pressure. It establishes the limit of the pressure vessel for very short and exceptional increases of pressure above MAP, specifically to enable the SRV to operate properly and reach its rated capacity. This exceptional increase is called accumulation. So accumulation is specific to the individual pressure vessel and does not relate to the SRV. The pressure increase of the SRV to nominal flow is called overpressure. 

The ASME code formula for the thickness of a cylindrical shell is listed in UG-27, as t = PR/(SE — 0.6P)S In this formula, t is the minimum thickness of the shell (in.), P is the maximum allowable working pressure (MAWP) (psi), R is the internal radius of the vessel (in.), S is the allowable stress in the material listed in ASME Section II, and E is the weld joint efficiency. 

The ASME code (UG 99) requires a hydrostatic test of each pressure vessel to validate the design, materials, and construction before that vessel is put into service. The minimum value for the test pressure is 1.3 times the MAWP, corrected for the temperature at which the test is conducted. This temperature correction is the ratio of the stress value at the hydrostatic test temperature (usually room temperature) to the stress value of the material used in the design formulas, usually at the maximum allowable working temperature of the pressure vessel. To conduct the test, water is pumped into the vessel to increase the internal pressure to the level of the test value and then released. 

MAWP Maximum aUowabie working pressure legally permitted for a vessel. The vessel will have an ASME code stamp. Heat exchangers are also rated with MAWP. 

The minimum thickness, MAWP, and equations for the membrane sttess are given in the ASME, Section VIII, Division 1, Boiler and Pressure Vessel Code, paragraph UG-27 [1], as follows ... 

The ASME BPV Code specifies that the minimum thickness shall he the greater value determined from equations 13.41 and 13.42. If these equations are rearranged and used to calculate the maximum allowable working pressure (MAWP) for a vessel of a given thickness, then the maximum allowable working pressure is the lower value predicted by the two equations. 

A relief valve or burst disk with a rating equivalent to or less than the rating of the vessel is not only a prudent accessory, but is also required by most safety codes. ASME recommends that the burst rating of a safety rupture should not exceed 110% of the MAWP. Rupture disks may be of a prebulged or scored type. Prebulged disks can be operated repeatedly to pressures up to 70% of their rating. Scored disks can be used repeatedly to pressures up to 90% of their rating. 

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