This section discusses criteria to determine how many supports are needed in a pipe section to ensure that it does not get overstressed or sag too much. This is based on the weight of the pipe and components, type of fluid service, design pressure and temperature, pipe material, and diameter and wall thickness. Once the maximum span is determined, the engineer can determine the number of supports that are needed and where they are needed. This will allow him to calculate the load on each support, and therefore permit doing the detailed design of each support. For example, if the load is high, the support must be designed to spread it out over a larger area on the pipe to reduce localized pipe stresses. Depending on the complexity of the piping layout, additional weight loads to be applied, the nature of the fluid service and operations, the applied loads at support points and the required support spacing may be determined by hand calculations, available tables, or by a detailed analysis using a piping flexibility computer program.
The discussion here will be confined to supports in straight horizontal sections of single-diameter pipe without other weight loads imposed. More complex systems must be evaluated by using other equations to account for differences in pipe geometry or loading, or a piping flexibility analysis computer program. Discussion of other hand-calculation techniques is beyond the scope of this course. The general requirements for a computer analysis are discussed later in this module. With the general availability and ease of use of computer programs, use of hand calculations and table solutions are generally confined to relatively simple systems (i.e., piperack runs or offsite piping systems), or initial screening studies.
Determining the maximum spacing between two supports consists of:
• Establishing stress and deflection criteria.
• Identifying and using the applicable Saudi Aramco or industry table to determine the maximum permitted span.
• If the situation is beyond the limitations of the tables, calculating the maximum permitted span given stress and deflection criteria, using either hand calculations or a computer program, as appropriate.
Piping Weight Stress and Deflection Criteria
Support spacing for horizontal pipes in open areas is governed by the strength of the pipe.
Support spacing for pipes in process plants is determined more by the spacing of conveniently located structural steel. Spacing of the supports in a pipe rack is usually based on supporting the weakest pipe, although larger spans are acceptable if sagging and pockets in smaller lines is not objectionable. Small lines can be supported off larger lines, bundled with other small lines, or increased in size to be self-supporting.
Allowable spans for horizontal lines are influenced by limits on longitudinal stress or deflection to avoid interference with the nearby pipe or structure, or to avoid excessive sagging that could be detrimental to fluid flow. The span may also be chosen to change the pipe's natural frequency to avoid a resonant-vibration condition.
Stress Criteria
Stress criteria for a particular situation is a function of material, pressure, and temperature.
The value for the allowable longitudinal stress is obtained by using the applicable ASME/ANSI B31 Code equation and table. The sum of the longitudinal stresses due to weight and pressure must be limited to the pipe material allowable stress.
MEX 101.03 discussed calculation of the required pipewall thickness based on design pressure considerations, and this is based on limiting the pipe circumferential stress to the allowable stress. The longitudinal stress in a pipe due to internal pressure is half the circumferential stress. Thus, if the pipewall thickness is exactly the value that is required for internal pressure, then half the allowable stress is still available as a limit for longitudinal weight stress.
Deflection Criteria
Deflection under weight effects is generally of secondary importance in piping just as it is in structures. In most process units, however, the deflection should be kept within reasonable limits to minimize pocketing of liquids at low points. Appearance may also be a factor. The maximum deflection is typically limited to the smaller of 25 mm (1 in.) or half the normal pipe diameter, unless a smaller deflection is required due to pocketing concerns.
Determining the Maximum Allowable Span
The maximum span between two supports is based on the allowable stress and deflection criteria. This is determined through two calculations:
• A calculation based on stress limits:
L ≤ 0 . 8 Z f s
W
• A calculation based on deflection limits:
L = EI∆ 13. 5 W
4
where: L = Length of span, ft.
fs = 1/2 x allowable stress at design temperature per applicable ASME/ANSI B31 Code. Note that this assumes that the pipewall thickness exactly matches that required for internal pressure. The longitudinal pressure stress thus also equals half the ASME/ANSI B31 Code allowable stress. This is a simplified assumption, but is conservative for most situations.
∆ = Maximum deflection, in.
W = Weight of pipe, including commodity lining, and insulation if any, lb/ft.
E = Hot modulus of elasticity of pipe at design temperature, psi.
I = Moment of inertia of the pipe, in.4 Z = Section modulus of the pipe, in.3
The values for the weight of the pipe, W, and the section modulus, Z, are obtained from the Pipe Properties Table discussed in MEX 101.03. The weight of the pipe must include consideration of the pipe material, contained fluid, external insulation, and internal lining, in lb/ft. Determining the weight of insulation and lining is beyond the scope of this course. The equations used are based on a mean between a uniformly loaded beam simply supported at both ends and one with both ends fixed.
The maximum allowable span is the lower value that results from the two calculations.
Maximum Span Tables
Saudi Aramco Standard Drawing AC-036697 also provides maximum allowable spans for unrestrained pipelines based on pipe sizes from 350 mm to 1,500 mm (14 in. to 60 in.) of specified thicknesses, maximum allowable internal pressure, and specified wear pad or saddle details to distribute the load or saddles. This is included in Work Aid 1, and may be used as a convenience for pipelines that are within its limitations.
Sample Problem 2
Refer again to Figure 6 of Sample Problem 1. It is now necessary to determine if the 10.7 m (35 ft.) support span between Locations 3 and 4 is excessive, and estimate the number of supports required in the 45.7 m (150 ft.) North/South run. For this work, assume the following:
• The specific gravity of the liquid in the system is equal to that of water.
• The allowable stress for the pipe material based on ASME/ANSI B31.3 requirements is 130.3 MPa (18,900 psi.).
• The Modulus of elasticity at 260°C (500°F) is 188.2 x 103 MPa (27.3 x 106 psi.).
• There is 75 mm (3 in.) of calcium silicate insulation on the pipe. Its weight may be assumed to be 18.9 kg/m (12.7 lb/ft.).
Solution
This problem will be solved using Work Aid 1.
From the Pipe Properties Table in MEX 101.03, obtain the following information:
Weight of pipe = 49.6 lb/ft.
Weight of fluid = 49.0 lb/ft.
I = 279 in4.
Z = 43.8 in3.
Then W = 49.6 + 49.0 + 12.7
= 111.3 lb/ft.
fs = 0.5 x 18,900 = 9,450 psi.
Stress limit calculation:
L ≤ 0.8Z f s
W = 0.8 x 43.8 x 9450 111.3
L ≤ 54.5 ft Deflection limit calculation:
L ≤ EI ∆
13.5W
4
= 27.3 x 10 6 x 279 x 1 13.5 x 111.3
4
L ≤ 47.4 ft
Thus, the maximum allowable span is 14.5 m (47.4 ft.). Therefore, in the 45.7 m (150 ft.) run:
150 = 3.16 spans, rounding up to 4.
47.4 Therefore, five supports are needed.
Loads on Supports
The loads imposed on supports must be considered in the detailed support design to ensure that they are not overstressed, and that they do not overstress the pipe, locally. The loads on the supports will, in turn, be transmitted to other structural members and foundations which also must be designed considering the applied loads. The design of these elements must ensure that the support will perform its intended function in the piping system. For example, if the structure under a support is not rigid enough, it will deflect excessively under the applied load which will let the piping system deflect as well.
The details that are used to attach the support to the pipe must consider the local stresses in the pipe wall resulting from the applied load. In the extreme, high-weight loads at support points could cause the pipe wall to locally deform. Therefore, the support attachment detail must spread the load enough along and around the pipe wall to keep the local stresses in the pipe wall within reasonable limits.
These detailed support design considerations may, in some cases, require the support span to be reduced even if the overall pipe stress and deflection criteria are met. This would occur if the support load is so high that the detailed design becomes impractical, or is more expensive than adding an additional support location to reduce the load.
The following loads should be considered in the design of supports:
• Weight of pipe and insulation, and internal lining (if any). The weight of other piping components such as valves and fittings, must also be accounted for.
• Weight of the line contents based on water or the operating fluid, whichever is larger. If the line is not hydrostatically tested, the weight of the line contents is sufficient. Spring hangers are normally designed for the weight of the line contents, so additional support may be needed during hydrostatic tests to avoid overstress if the line contents is a gas.
• Lateral loads due to wind. Since a support acts only in the vertical direction, wind load must be considered to the extent that it influences the structure to which the support is attached. Structural movement deflects the support and, in turn, moves the pipe.
• Lateral loads due to movement of the pipe. Pipe movement causes a frictional load to be applied to the support that acts opposite to the direction of pipe motion. The support and associated structure must be designed for this frictional load.
Support loads can be calculated using the equations of statics once all support locations are
Requirements for Pads and Saddles
Loads that act on or in the pipe create stresses in the pipe wall as previously discussed. The magnitude of these stresses determines whether or not the load needs to be distributed over a wider area. If the load needs to be distributed, then reinforcement pads, saddles or wider pipe shoes are typically used.
Saudi Aramco Standard Drawings AD-036253, AD-036252, and AD-036999 provide standard details for pipe shoes, pads and saddles. Note that these details are based on pipe diameter. More load spreading is required as the diameter increases since the pipe wall becomes more flexible and less able to absorb and transmit loads without being overstressed.
Prevention of Wind-Induced Vibration
• All external loads must be considered in support and flexibility design. Preventing wind-induced vibration is particularly important in support design because it can have a profound impact, as indicated on the next page by the requirements of SAES-L-002 and SAES-L-011. Vortex shedding induced vibration caused by wind may become a problem with piping that is more than about 10 m (30 ft.) long. This generally occurs with piping that runs up along the length of a vertical tower, or for long horizontal runs in exposed locations such as a section of aboveground pipeline.
• When wind flows past a circular pipe section, the air behind the pipe is no longer smooth.
There is a region of pressure instability where vortices are shed in a regular pattern, alternating from one side of the pipe to the other. These vortices cause an alternating force to act perpendicular to the wind direction and can make the pipe vibrate. If the frequency of vortex shedding corresponds to a mechanical natural frequency of the piping system, resonant vibration could cause pipe fatigue failure. Analyzing and solving vortex shedding vibration problems is best handled by applying certain principles that include dimensionless parameters and experimental data, which often requires using computer programs. Further discussion regarding vortex shedding is beyond the scope of this course.
SAES-L-002, Paragraph 6.2, specifies:
• Exposed piping systems shall be designed for wind loading based on 35 m/s (78 mph) fastest mile wind speed and shall take into account the effects of wind-induced vibration where applicable.
• The wind speed causes a uniform lateral load to be exerted on the pipe. This lateral load is resisted by friction that acts at support points, as long as they are not hanger-type supports which will allow the pipe to be moved by the wind. Thus, even though supports are installed to carry weight load, their presence may also provide sufficient resistance to wind loads in many cases so that additional restraint is not required.
• The location of supports influences the mechanical natural frequency of the piping system. Thus, they will affect any evaluation of wind-induced vibration since the vibration forcing frequency must be compared to the piping system mechanical natural frequency to determine if a problem exists.
SAES-L-011, Paragraph 3.3 specifies requirements for support spacing due to vibration:
When aboveground, cross-country pipelines with diameters larger than 450 mm (18 in.) are supported at regular intervals, every seventh span length shall be reduced by 20% to mitigate wind-induced resonant vibration of the pipelines. The basic support spacing shall be selected so that the natural frequency of the pipeline in the operating condition is outside the range of wind-induced frequencies, plus or minus 10%, for any wind speed above 9 m/s (20 mph) which will cause vortex shedding.
DETERMINING THE NEED FOR A PIPING THERMAL FLEXIBILITY / WEIGHT