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ASME B31.3 Limitations on Threaded Joints

Most threaded joints in ASME B31.3 piping systems are made with taper threads in accordance with ASME B 1.20.1. Figure 5.1 shows a taper thread. These threaded joints are generally acceptable for Normal Fluid Service and Category D Fluid Service. Paragraph 314.1(a) states that threaded joints should be avoided in any service where crevice corrosion, severe erosion, or cyclic load may occur. Cyclic loading is not defined and this limitation is rather unclear, because all systems are subject to some cycles and threaded joints are commonly used in process piping.

Threaded joints that are to be seal-welded are prohibited from using thread sealing compounds. However, when seal welding is not an intent of the design, but is intended simply to seal leaks that are detected on hydro-test, threaded joints that use thread sealing compound may be seal-welded, provided the sealing compound is removed from the exposed thread. The joint does not have to be undone and remade without the compound.

Threaded joints are prohibited from use in severe cyclic service, except for two conditions. One is the use of threaded components of a specialty nature, which are not subject to external moment loading, such as thermometer wells. However, by the definition of severe cyclic conditions, if there is no moment, there is no Code thermal expansion stress and such a joint cannot be in severe cyclic conditions. The other condition where threaded joints are permitted in severe cyclic conditions is provided in para. 314.2.2, which permits straight thread joints in which the tightness of the joint is provided by a seating surface other than the threads, provided the joint is safeguarded.

ASME B31.3 provides minimum thicknesses for male and female portions of threaded joints. For the female portions, it requires that they have equivalent strength to listed components (in Table 326.1, e.g., ASME B16.11). For die male portion, the minimum thickness is provided in Table 314.2.1. #Little_PEng.

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