Pressure tests MAWP

The function of a relief valve is to protect a vessel, piping system, or heat exchanger from exceeding its maximum allowable working pressure (MAWP). The MAWP is shown on the code stamp (a metal plate) attached to each process vessel, and by design is determined by the size of the impeller on the pump upstream of the vessel (see Sec. 34.4.1 in Chap. 34). Normally, the relief valve is set to open at this MAWP pressure. The relief valve setting is adjustable and is done by a machinist at a shop pressure-testing facility. 

The test pressure is generally 1.5 times the design pressure or MAWP (see below for an explanation of this term). Therefore, once a vessel has been installed, or anytime it is opened (say for inspection), it will be tested to that pressure before the process fluids are introduced into the vessel. 

An 800-gal reaetor eontaining a styrene mixture with a speeifie heat of 0.6 eal/gm °C has a 10-in. rupture disk and a vent line with equivalent length = 400. The vessel MAWP is 100 psig and the rupture disk set pressure is 20 psig. The styrene mixture had a self-heat rate of 60°C/min at 170°C as it is tempered in a DIERS venting test. Determine the allowable reaetor mixture eharge to limit the overpressure to 10% over the set pressure. 

The pneumatic test may be used in lieu of the hydrostatic test for hydrogen systems designed or supported so they cannot safely be filled with liquid, or if the vessel or system cannot be readily dried or is to be used in services in which traces of the testing liquid cannot be tolerated. The substitution requires that the parts of the system, when possible, are previously tested by hydrostatic pressure. The pneumatic test pressure should be 1.25 times the MAWP. 

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. 

The strain gage and deflection methods require the vessel to be pressurized and then released at successively higher pressures until a permanent deflection of 2% is measured. The MAWP of the vessel is established by dividing this test pressure by 2.5. 

General Vessel Formulas, 15 External Pressure Design, 19 Calculate MAP, MAWP, and Test Pressures,... 

Vessels of forged or rolled steels and aluminum alloys, metals with reasonably high ductility, are stressed at the final inspection, and in reinspection tests, at a test pressure 1.3 times the maximum allowable working pressure (MAWP), the tank shell is stressed up to 90% of the material strength, which is usually the yield strength... 

Vessels of metals with limited ductility, for example, cast iron, have to withstand a test pressure that is twice the MAWP... 

In the second test, at least two vessels are built by identical means. One vessel is pressured to either failure, leakage or bursting. This failure pressure is then used to develop an MAWP for the other vessel. The surviving vessel is then given a standard hydrostatic or pneumatic test based on the MAWP established. 

The pressure at which relief valves are set to open is the vessel design pressure. The vessel design pressure (maximum allowable working pressure, MAWP) is stamped on the manufacturer s nameplate. It is illegal to set the relief valve at a higher pressure. The vessel is probably hydrostatically tested at a pressure 50 percent greater than its... 

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. 

As discussed above, tightness test pressures should not exceed the equipment MAWP or system design pressure or the vessel s relief valve set pressure. 

No detailed inspection of the system should ever take place while it is at a pressure greater than the MAWP, and only the personnel directly involved with the test should be permitted within a 25-meter radius of the system being tested. 

Procedures for strength testing must be followed if the MAWP for a piece of equipment will be exceeded or if any relief valves are to be disabled. Never exceed the recommended strength test pressure of any equipment. 

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