General considerations to be evaluated for Piping Design should be the Design Conditions such as temperatures, pressures, and various forces applicable to the design of piping systems.
- Design Pressure : The design pressure of a piping system shall not be less than the pressure at the most severe condition of coincident pressure and temperature resulting in the greatest required component thickness or rating.
- Design Temperature : The design temperature is the material temperature representing the most severe condition of coincident pressure and temperature. For uninsulated metallic pipe with fluid below 38°C (100°F), the metal temperature is taken as the fluid temperature.
- With fluid at or above 38°C (100°F) and without external insulation, the metal temperature is taken as a percentage of the fluid temperature unless a lower temperature is determined by test or calculation. For pipe, threaded and welding-end valves, fittings, and other components with a wall thickness comparable with that of the pipe, the percentage is 95 percent; for flanges and flanged valves and fittings, 90 percent; for lap-joint flanges, 85 percent; and for bolting, 80 percent.
- With external insulation, the metal temperature is taken as the fluid temperature unless service data, tests, or calculations justify lower values. For internally insulated pipe, the design metal temperature shall be calculated or obtained from tests.
The following criteria must be met for a good piping design
Design Criteria for Metallic Pipe
- The piping system shall have no pressure-containing components of cast iron or other nonductile metal.
- Nominal pressure stresses shall not exceed the yield strength at temperature (see Table 10-49 and data in ASME Code, Sec. VIII, Division 2).
- Combined longitudinal stresses (SL) shall not exceed the limits established in the code (see pressure design of piping components for SL limitations).
- The number of cycles (or variations) shall not exceed 7000 during the life of the piping system.
- Occasional variations above design conditions shall remain within one of the following limits for pressure design:
- When the variation lasts no more than 10 hours at any one time and no more than 100 hours per year, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than 33 percent.
- When the variation lasts no more than 50 hours at any one time and not more than 500 hours per year, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than 20 percent
- Dynamic Effects Design must provide for impact (hydraulic shock, etc.), wind (exposed piping), earthquake (see ANSI A58.1), discharge reactions, and vibrations (of piping arrangement and support).
Weight considerations include
- live loads (contents, ice, and snow)
- dead loads (pipe, valves, insulation, etc.)
- test loads (test fluid).
- Thermal-expansion and -contraction loads occur when a piping system is prevented from free thermal expansion or contraction as a result of anchors and restraints or undergoes large, rapid temperature changes or unequal temperature distribution because of an injection of cold liquid striking the wall of a pipe carrying hot gas.
: The code uses three different approaches to design, as follows:
- It provides for the use of dimensionally standardized components at their published pressure-temperature ratings.
- It provides design formulas and maximum stresses.
- It prohibits the use of materials, components, or assembly methods in certain conditions.
: External-pressure stress evaluation of piping is the same as for pressure vessels. For piping, the design pressure and temperature are taken as the maximum intended operating pressure and temperature combination which results in the maximum thickness. For straight metal pipe under internal pressure the formula for minimum required wall thickness tm is applicable for OD/t ratios greater than 6.
|tm = ||-------------|| + C |
|2(SE + PY)|
where (in consistent units)
P = design pressure
Do = outside diameter of pipe
C = sum of allowances for corrosion, erosion, and any thread or groove depth. For threaded components the depth is h of ANSI B2.1, and for grooved components the depth is the depth removed (plus 1/64 in when no tolerance is specified).
SE = allowable stress
S = basic allowable stress for materials, excluding casting, joint, or structural-grade quality factors
E = quality factor. The quality factor E is one or the product of more than one of the following quality factors: casting quality factor Ec, joint quality factor Ej, and structural-grade quality factor Es of 0.92.
Y = coefficient for ductile ferrous materials
= minimum required thickness, in, to which manufacturing tolerance must be added when specifying pipe thickness on purchase orders.
Pipe with t equal to or greater than D/6 or P/SE greater than 0.385 requires special consideration.
Thermal Expansion and Flexibility
: Metallic Piping ANSI B31.3 requires that piping systems have sufficient flexibility to prevent thermal expansion or contraction or the movement of piping supports or terminals from causing (1) failure of piping supports from overstress or fatigue; (2) leakage at joints; or (3) detrimental stresses or distortions in piping or in connected equipment (pumps, turbines, or valves, for example), resulting from excessive thrusts or movements in the piping.
To assure that a system meets these requirements, the computed displacement –stress range SE shall not exceed the allowable stress range SA, the reaction forces Rm shall not be detrimental to supports or connected equipment, and movement of the piping shall be within any prescribed limits. Displacement Strains result from piping being displaced from its unrestrained position:
- Thermal displacements. A piping system will undergo dimensional changes with any change in temperature. If it is constrained from free movement by terminals, guides, and anchors, it will be displaced from its unrestrained position.
- Reaction displacements. If the restraints are not considered rigid and there is a predictable movement of the restraint under load, this may be treated as a compensating displacement.
- Externally imposed displacements. Externally caused movement of restraints will impose displacements on the piping in addition to those related to thermal effects. Such movements may result from causes such as wind sway or temperature changes in connected equipment.
Total Displacement Strains Thermal displacements, reaction displacements, and externally imposed displacements all have equivalent effects on the piping system and must be considered together in determining total displacement strains in a piping system. Expansion strains may be taken up in three ways: by bending, by torsion, or by axial compression. In the first two cases maximum stress occurs at the extreme fibers of the cross section at the critical location. In the third case the entire cross-sectional area over the entire length is for practical purposes equally stressed.
Bending or torsional flexibility may be provided by bends, loops, or offsets; by corrugated pipe or expansion joints of the bellows type; or by other devices permitting rotational movement. These devices must be anchored or otherwise suitably connected to resist end forces from fluid pressure, frictional resistance to pipe movement, and other causes.
Axial flexibility may be provided by expansion joints of the slip joint or bellows types, suitably anchored and guided to resist end forces from fluid pressure, frictional resistance to movement, and other causes.
Displacement Stresses may be considered proportional to the total displacement strain only if the strains are well distributed and not excessive at any point. The methods outlined in the code are applicable only to such a system. Poor distribution of strains (unbalanced systems) may result from highly stressed small-size pipe runs in series with large and relatively stiff pipe runs.