Excessive Pipe Stress

In the event that the nozzle loads stiU result in excessive shell stresses after the iterative work process is conducted, the loadings may be reduced by recalculating the piping loads utilizing the vessel spring rate . Often times the loads determined using the vessel spring rate will be much... 

From Table 3-8 it becomes obvious that although a 15-ft. span between pipe supports would be satisfactory from a pipe-stress standpoint, excessive sag or deflection of the pipe between the supports would take place over the service life with this span length. To avoid exceeding the specified maximum deflection of 0.25 in.. Table 3-8 tells us that the span between supports should not exceed 6-8 ft. 

The code provides no guidance for analysis but requires that external and internal attachments be designed to avoid flattening of the pipe, excessive locahzed bending stresses, or harmful therm gradients, with further emphasis on minimizing stress concentrations in cyclic service. 

Double Layer Pipe expansion is a significant factor at tenmera-tures above 600°F (3I6°C). Above this temperature, insiilation should be apphed in a double layer with all joints staggered to prevent excessive heat loss and high surface temperature at joints opened by pipe expansion. This procedure also minimizes thermal stresses in the insulation. 

Make static and dynamic analyses of pipe systems to avoid excessive stresses or excessive vibrations. 

Elastic Behavior. Stresses may be considered proportional to the total displacement strains in a piping system in which the strains are well-distributed and not excessive at any point (a balanced system). Layout of systems should aim for such a condition, which is assumed in flexibility analysis methods provided in this Code. 

Overstrained Behavior. Stresses cannot be considered proportional to displacement strains throughout a piping system in which an excessive amount of strain may occur in localized portions of the system (an unbalanced system). Operation of an unbalanced system in the creep range may aggravate the deleterious effects due to creep strain accumulation in the most susceptible regions of the system. Unbalance may result from one or more of the following ... 

External and internal attachments to piping shall be designed so that they will not cause undue flattening of the pipe, excessive localized bending stresses, or harmful thermal gradients in the pipe wall. It is important that attachments be designed to minimize stress concentration, particularly in cyclic services. 

Limitations on Design Pressure, P, in Para. PL-3.7.1 (a). The design pressure obtained by the formula in para. PL-3.7.1(a) shall be reduced to conform to the following P shall not exceed 85% of the mill test pressure for all pipes in the pipeline, provided, however, that pipe, mill tested to a pressure less than 85% of the pressure required to produce a hoop stress equal to the specified minimum yield, may be retested with a mill type hydrostatic test or tested in place after installation. In the event the pipe is retested to a pressure in excess of the mill test pressure, then P shall not exceed 85% of the retest pressure rather than the initial mill test pressure. It is mandatory to use a liquid as the test medium in all tests in place after installation where the test pressure exceeds the mill test pressure. This paragraph is not to be construed to allow an operating pressure or design pressure in excess of that provided for by para. PL-3.7.1(a). 

Checking of alignment of all piping fitup to equipment and removal of excessive stresses. 

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