C. Codes Governing Piping Design and Stress Analysis⬇️⬇️
(i) ASME B31.3, ASME B31.4 and ASME B31.8
(ii) Other codes including applicable local codes
(iii) Role and scope of codes
(iv) Information available from codes
(v) Typical organization of code material
(i) ASME B31.3, ASME B31.4 and ASME B31.8
(ii) Other codes including applicable local codes
(iii) Role and scope of codes
(iv) Information available from codes
(v) Typical organization of code material
BACKGROUND⬇️
Occasionally, we work on projects that require pipe to be laid out across the prevailing terrain.
The pipe may simply be placed on the ground surface, or it may be suspended or “cradled” in support structures. Above ground installations may be desired due to the economic considerations of a temporary piping system, the presence of rock and the cost for blasting a trench, land rights, cultural resources, or an easement that prevents burial of the pipe.
Polyethylene (PE) pipe provides good joint integrity, toughness, flexibility, and low weight to make its use practical for many “above ground” applications. This technical note presents design criteria and prevailing engineering methods that are recommended by The Plastic Pipe Institute for above-ground installation of polyethylene (PE) pipe. This technical note will discuss the effects of temperature extremes, chemical exposure, ultraviolet radiation and potential mechanical impact or loading. Engineering design considerations for both on grade and suspended, or cradled, polyethylene pipe installations are also discussed.
Occasionally, we work on projects that require pipe to be laid out across the prevailing terrain.
The pipe may simply be placed on the ground surface, or it may be suspended or “cradled” in support structures. Above ground installations may be desired due to the economic considerations of a temporary piping system, the presence of rock and the cost for blasting a trench, land rights, cultural resources, or an easement that prevents burial of the pipe.
Polyethylene (PE) pipe provides good joint integrity, toughness, flexibility, and low weight to make its use practical for many “above ground” applications. This technical note presents design criteria and prevailing engineering methods that are recommended by The Plastic Pipe Institute for above-ground installation of polyethylene (PE) pipe. This technical note will discuss the effects of temperature extremes, chemical exposure, ultraviolet radiation and potential mechanical impact or loading. Engineering design considerations for both on grade and suspended, or cradled, polyethylene pipe installations are also discussed.
DESIGN METHOD:
ALLOWABLE DESIGN PRESSURE⬇️
The exposure of above ground pipe to sunlight can result in extremely high outside surface temperatures. In the majority of cases, the water flowing in the pipe is substantially cooler than the exterior of the exposed above ground pipe and water flowing through the pipe tends to moderate the surface temperature of the exposed pipe. This can result in a pipe wall temperature that is only slightly above the temperature of the water flowing through the pipe. However, in pipeline systems with occasional flow, the temperature increase can be much higher. The site specific design needs to determine the allowable pressure rating of the PE pipe based upon the expected maximum service temperature.
ALLOWABLE DESIGN PRESSURE⬇️
The exposure of above ground pipe to sunlight can result in extremely high outside surface temperatures. In the majority of cases, the water flowing in the pipe is substantially cooler than the exterior of the exposed above ground pipe and water flowing through the pipe tends to moderate the surface temperature of the exposed pipe. This can result in a pipe wall temperature that is only slightly above the temperature of the water flowing through the pipe. However, in pipeline systems with occasional flow, the temperature increase can be much higher. The site specific design needs to determine the allowable pressure rating of the PE pipe based upon the expected maximum service temperature.
DESIGN METHOD:
EXPANSION AND CONTRACTION⬇️
The expansion and contraction for an unrestrained PE pipe can be calculated by the following
equation:
Change in Length = delta(L) = α *(T2-T1)*L
Where:
delta(L) = theoretical length change, inches
Where: positive values = expansion,
negative values = contraction
α = coefficient of linear expansion
α = 1.0 to 1.1 x 10-4 in/in/°F for PE 3408 materials
α = 1.0 x 10-4 in/in/°F for PE 2406 materials
T1 = initial temperature oF
T2 = final temperature oF
L = length of pipe, inches (at temperature T1)
EXPANSION AND CONTRACTION⬇️
The expansion and contraction for an unrestrained PE pipe can be calculated by the following
equation:
Change in Length = delta(L) = α *(T2-T1)*L
Where:
delta(L) = theoretical length change, inches
Where: positive values = expansion,
negative values = contraction
α = coefficient of linear expansion
α = 1.0 to 1.1 x 10-4 in/in/°F for PE 3408 materials
α = 1.0 x 10-4 in/in/°F for PE 2406 materials
T1 = initial temperature oF
T2 = final temperature oF
L = length of pipe, inches (at temperature T1)
DESIGN METHOD:
LONGITUDINAL STRESS VS. TEMPERATURE CHANGE⬇️
Longitudinal Stress = σT = α (T2-T1) Es
And
Longitudinal Force = Ft = (σT) A
Where:
σt = Theoretical longitudinal stress, psi
Where: positive values = expansion
negative values = contraction
α = coefficient of expansion or contraction (as in Equation 1)
α = 1.0 to 1.1 x 10-4 in/in/°F for PE 3408 materials
α = 1.0 x 10-4 in/in/°F for PE 2406 materials
T1 = initial temperature oF
T2 = final temperature oF
ES = apparent short term modulus of elasticity, psi
(see Table 1) at lowest temperature
Ft = Theoretical longitudinal force, lbs
A = pipe wall cross sectional area, in2
LONGITUDINAL STRESS VS. TEMPERATURE CHANGE⬇️
Longitudinal Stress = σT = α (T2-T1) Es
And
Longitudinal Force = Ft = (σT) A
Where:
σt = Theoretical longitudinal stress, psi
Where: positive values = expansion
negative values = contraction
α = coefficient of expansion or contraction (as in Equation 1)
α = 1.0 to 1.1 x 10-4 in/in/°F for PE 3408 materials
α = 1.0 x 10-4 in/in/°F for PE 2406 materials
T1 = initial temperature oF
T2 = final temperature oF
ES = apparent short term modulus of elasticity, psi
(see Table 1) at lowest temperature
Ft = Theoretical longitudinal force, lbs
A = pipe wall cross sectional area, in2
Anchor and Support Design⬇️
Proper design of anchors and supports is as important with PE piping as it is with other piping materials. A variety of factors must be considered.
Proper design of anchors and supports is as important with PE piping as it is with other piping materials. A variety of factors must be considered.
1-Some installations of PE pipe have the pipe lying directly on the earth’s surface. In
this type of installation, the surface under the pipe must be free from boulders,
crevices or other irregularities that could create a point-loading stress situation on the
pipe.
this type of installation, the surface under the pipe must be free from boulders,
crevices or other irregularities that could create a point-loading stress situation on the
pipe.
2-On grade placement over bed rock or hard pan should be avoided unless a uniform
bed of material is prepared that will cushion the pipe. If the PE pipe rests directly on
a hard surface, this creates a point loading situation and can increase abrasion of the
outer pipe surface as it “wanders” in response to temperature variations.
bed of material is prepared that will cushion the pipe. If the PE pipe rests directly on
a hard surface, this creates a point loading situation and can increase abrasion of the
outer pipe surface as it “wanders” in response to temperature variations.
3-Intermittent pipe supports should be spaced properly using the design parameters
discussed in the preceding pages. Anywhere unusual loadings or excessive
temperatures are encountered, continuous support should be considered.
discussed in the preceding pages. Anywhere unusual loadings or excessive
temperatures are encountered, continuous support should be considered.
4-Supports that simply cradle the pipe, rather than grip or clamp the pipe, should be
from one half to one pipe diameter in length and should support at least 120 degrees
or one third of the pipe perimeter. All supports should be free from sharp edges.
from one half to one pipe diameter in length and should support at least 120 degrees
or one third of the pipe perimeter. All supports should be free from sharp edges.
5-The supports should have adequate strength to restrain the pipe from lateral or
longitudinal deflection given the anticipated service conditions. If the design allows
free movement during expansion, the sliding supports should provide a guide without
restraint in the direction of movement. If, on the other hand, the support is designed
to grip the pipe firmly, the support must either be mounted flexibly or have adequate
strength to withstand the anticipated stresses.
longitudinal deflection given the anticipated service conditions. If the design allows
free movement during expansion, the sliding supports should provide a guide without
restraint in the direction of movement. If, on the other hand, the support is designed
to grip the pipe firmly, the support must either be mounted flexibly or have adequate
strength to withstand the anticipated stresses.
6-Heavy fittings or flanges should be fully supported and restrained for a minimum
distance of one full pipe diameter on both sides of the PE pipe. This supported fitting
represents a rigid structure within the flexible pipe system and should be fully
isolated from bending stresses associated with beam sag or thermal deflection.
distance of one full pipe diameter on both sides of the PE pipe. This supported fitting
represents a rigid structure within the flexible pipe system and should be fully
isolated from bending stresses associated with beam sag or thermal deflection.
Flange analysis is done based on the Kellogg equivalent pressure method. In addition to internal and external pressure, axial force and bending moment can act on flanges as equivalent pressure. The values of axial force and bending moment are determined from analysis without pressure thrust effect.