Above Ground Pipeline Design
Above Ground Pipeline Design
Above Ground Pipeline Design
1.0 Introduction.
Application engineering involves designing a fiberglass piping system to ensure the successful installation. It considers the specific service conditions, thorough engineering analysis, and selection of the specific fiberglass piping system to meet the service conditions. The total process involves the application of engineering skills and product knowledge to achieve the desired overall performance. 1.0.1 Test methods and physical properties. The ultimate and allowable design stresses and physical properties for fiberglass pipe are arrived at from standardized test methodologies and some use modified techniques of standardized test methods. These properties evolve around the minimum reinforced wall thicknesses. These properties can be found in the Conley Product Data and Engineering Guide. 1.0.2 Internal pressure rating. The hydraulic design basis for internal pressure rating of fiberglass piping comes from a long-term test performed in accordance with ASTM D 2992. The hydraulic design basis is the hoop stress (or strain) that provides an estimated life of 100,000 hours for static pressure applications. 1.0.3 Thermal expansion and contraction. Fiberglass pipe has a thermal expansion rate in the axial direction about twice that of steel pipe. The total expansion or contraction for a piping system is determined by the following equation: Lc = 12 Ct L Tc (Eq. 1)
Where: Lc = Length change, inches Ct = Coefficient of thermal expansion, in/in/F L = Length of line between anchors, feet Tc = Temperature change, F (Maximum operating temperature minus installation temperature for expansion. Installation temperature minus minimum operating temperature for contraction.) Example 1: Find the change in length per linear foot of pipeline for a 2-in. nominal pipe size with a temperature change of 60F and coefficient of expansion of 9.5 x 10-6. Lc = 12 (9.5 x 10-6) (1) (60) = 0.0068 in./ft. To determine the effects of thermal expansion and contraction on a piping system, one should know: 1. The design temperature conditions. 2. The size and physical properties of the pipe. 3. The layout of the system, including dimensions and the thermal movement, if any, of the terminal points. 4. The limitations on end reactions.
Two other methods that are seldom used and not normally recommended are: Expansion loops (1.5) Mechanical expansion joints (1.4)
Guides, expansion loops and mechanical expansion joints are installed in straight lines which are anchored at each end. Experience has shown that direction changes are the least expensive method of accommodating thermal expansion. Guide spacing is the next most economical method, followed by mechanical expansion joints. For small temperature changes, and piping systems which consist of short run lengths, it is usually unnecessary to make special provisions for thermal expansion. However, any system should have the capability to accommodate length changes. The methodology of direction changes in Section 1.6 solves this design criterion. Experience has shown that above ground piping systems require anchors. These anchors limit pipe movement due to vibrations and transient loading conditions. Anchors also fasten all transition points within the system. Transition points are places where pipe diameter, material, elevation or direction changes, or manufacturer changes. Anchors at transition points limit the transfer of thermal end loads from line section to line section.
Where: EL = Thermal end load, lb. Ct = Coefficient of thermal expansion, in/in/F E = Modulus of elasticity, psi (Compressive for expansion and tensile for contraction) A = Cross sectional area, in.2 Tc = Temperature change, F (Maximum operating temperature minus installation temperature for expansion. Installation temperature minus minimum operating temperature for contraction.) Example 2: For the 2-in. nominal pipe in Example 1, with a reinforced OD of 2.375-in. (minimum) and a maximum reinforced ID (d) of 2.35-in., the compressive modulus of elasticity, (EL) is 1.3 x 106 psi, and the tensile modulus of elasticity is 1.72 x 106 psi. The pipeline is installed at 75F and has a maximum operating temperature of 200F and a minimum operating temperature of 35F. The coefficient of thermal expansion is 9.5 x 10-6. Step 1 Calculate the temperature changes: Tc = 200 75 = 125F (for expansion) Tc = 75 75 = 40F (for contraction) Step 2 Calculate the cross sectional area: A = /4 (OD2 ID2) A = 0.7854 (2.3752 2.2352) = 0.507 in.2 Design calculations use only the reinforced dimensions. contribute significantly to the strength of the pipe. Step 3 Calculate the end load using Eq.2: EL = (9.5 x 10-6) (1.3 x 106) (0.507) (125) = 898 lb. (for expansion) EL = (9.5 x 10-6) (1.72 x 106) (0.507) (40) = 380 lb. (for contraction) When pipe lengths between anchors expand, the pipe undergoes compression. When contraction occurs, the pipe experiences tension. 1.2.1 Restrained End Loading. The most efficient method of handling thermal expansion in Conley Piping Systems is restrained End Loading. This is simply anchoring each end of long straight runs of pipe and guiding to prevent buckling. This method eliminates the need for expansion loops and expansion joints and results in a much more stable system. It also eliminates movement in the line and prevents excessive bending stresses on branching fittings such as Tees and Laterals. The decision to use restrained end loading is based primarily on pipe length and temperature differences. As a rule, straight runs of about 100-ft usually require an anchor at each end if there is enough temperature difference between installation and operating temperature. Short straight runs that end in natural offsets or loops use anchors often enough to stabilize the piping but allow it to grow as necessary. Also as a general rule, guides should be located at every other support point. For runs longer than 100-ft, or temperature changes greater than 100 degrees F. use mid-point anchors about every 200-ft and guide at every support. Example 3: For a run of 250-ft, use end anchors and one mid-point anchor. For a run of 400-ft, use end anchors and one mid-point anchor. For a run of 600-ft, use end anchors and one at 200 ft and one at 400-ft. Resin rich surfaces do not
Here is a brief example of restrained end loading design calculations: In the case of 3-in pipe, having a reinforced thickness of 0.15-in, the cross sectional area is about 1.5 sq. in. Assuming an installation temperature of 0 degrees and operating temperature of 180 degrees, the Delta T is 180 degrees F. This results in a compressive load of 7220 lb. The compressive stress is 4813 psi compared to an ultimate of 22,720 psi (not reported in the Conley Catalog). This results in a safety factor of 4.7. The actual load would of course be considerably less since the installation temperature will be much higher. When the restrained pipe cools to a point below the installation temperature it goes into tension. The ultimate tensile stress of filament wound pipe is approximately 30,000 psi so it would take a very large temperature drop in the pipe to generate that kind of load. With the required Anchors, guides and supports installed according to the design, the restrained end loading method of piping design is the simplest and most cost effective design for fiberglass piping systems.
Figure 1 Typical expansion joint installation The equation for calculating the allowable activation force is: Pcr = 2 Ec I/ LG2 Where: Pcr = Critical buckling force of pipe, lb. Ec = Compressive modulus of eleasticity, psi I = Moment of inertia, in. 4 LG = Support spacing interval, in. (From Tables) Example 4: Compute the critical buckling force for the 2-in. nominal pipe: Pcr = 3.14162 (1.3 x 106) (0.337)/[(7.5) (12)]2 Pcr = 534 lb. Note: For large diameter piping, the pressure thrust must be considered. Pressure thrust is the design pressure times the area of the expansion joint. In all applications, the activation force of the expansion joint must not exceed the thermal end loads developed by the pipe. The cost and limited motion capability of expansion joints makes them impractical to use in many applications. In these cases, loops, guide spacing, or short lengths of flexible hose can handle thermal expansion. The expansion joint needs an anchor on one side for proper operation. (Eq. 3)
Two guides on both sides of each expansion loop ensure proper alignment. The recommended guide spacing is four and fourteen nominal pipe diameters, the same as for expansion joints in Paragraph 1.4. Additional guides or supports should be located so maximum spacing interval is not exceeded. See the Loop Leg Length Table and calculations found in the Conley Product Data and Engineering Guide.
Figure 3
Figure 4
Rule #4 Support heavy equipment independently. Valves, and other heavy equipment, must be supported independently in both horizontal and vertical directions (Figure 4). Rule #5 Avoid excessive bending. When laying pipe lines directly on the surface, take care to ensure there are no excessive bends that would impose undue stress on the pipe. Rule #6 Avoid excessive loading in vertical runs. Support vertical pipe runs as shown in Figure 5. The preferred method is to design for pipe in compression. If the pipe in tension method cannot be avoided, take care to limit the tensile loadings below the recommended maximum tensile rating of the pipe. Install guide collars using the same spacing intervals used for horizontal lines (Figure 5).
Figure 5 1.7.1 Guides. The guiding mechanism must be loose to allow free axial movement of the pipe. However, the guides must be attached rigidly to the supporting structure so the pipe moves only in the axial direction (Figure 6). All guides act as supports and must meet the minimum requirements for supports, anchors, and guides presented in Section 1.7. When frequent thermal cycles, vibration, or pulsating loadings affect the pipe, it needs protection at all contact points. This is normally accomplished by bonding to the pipe wall a wear saddle made of steel or one-half of a section of the same pipe or coupling.
1.7.2 Anchors. An anchor must restrain the movement of the pipe against all applied forces. Pipe anchors divide a piping system into sections. They attach to structural material capable of withstanding the applied forces. In some cases, pumps, tanks, and other similar equipment function as anchors. However, most installations require additional anchors where pipe sizes change and where fiberglass pipe joins another material or a product from another manufacturer. Additional anchors usually occur at valve locations, changes in direction of piping runs and at major branch connections. To minimize stress on saddles and laterals caused by bending side runs, anchor the pipe on either side of the saddle or anchor the side run.
Figure 7 shows typical anchors. A full assortment of anchors, supports, and guides can be found in Appendix A. Operating experience with piping systems indicates that it is good practice to anchor long straight runs of pipe above ground periodically. These anchors prevent pipe movement due to vibration or water hammer. One anchoring method features a clamp placed between anchor sleeves or a set of anchor sleeves and a fitting. The sleeves bonded on the pipe prevent movement in either direction. Sleeve thickness must equal or exceed the clamp thickness. To achieve this, it is often necessary to bond two sleeves on each side of the clamp. The anchor sleeve must be at least one pipe diameter in length and cover 180 of circumference. Anchors act as supports and guides and must meet minimum requirements for supports as set fourth in this section. 1.7.3 Supports. To prevent excessive pipe deflection due to the pipe and fluid weight, support horizontal pipe at intervals determined by the Span Tables found in the Conley Product Data and Engineering Guide. 1.7.4 Supporting of Valves and Process Instrumentation. Conley manufactures standard reducing tees and saddles with flanged and threaded connections for adding process instrumentation, vent, drain and sampling valves, etc. to piping runs. All heavy equipment attached to fiberglass piping, whether mounted in-line or branch connected, must be supported independently of the piping system in order to avoid excess bending moments. Valves, and other heavy equipment, must be supported independently in both horizontal and vertical directions. The heavy weight of valves and other instrumentation can cause excessive bending moments, especially on branch connections, and the forces from actuating devices such as valves can transmit forces at the branch connection that exceed the allowable bending moments. As stated in all Conley Piping Specifications, spacer rings must be used on fiberglass flanges when mating with raised face steel flanges, or with wafer style butterfly valves, to achieve a flat mating surface. In some cases, steel backing rings must be used with fiberglass flanges to distribute the bolt torque loading to prevent bending and cracking of the fiberglass flange. Most branch connections for instrumentation are small size connections such as or , and heavy instrumentation such as valves must be supported independently. Conley Corporation/Tulsa, Oklahoma 8
Supports can be attached directly to valves and other instrumentation and connected to structural steel or floor type supports. In some cases where the valve or instrumentation cannot be easily supported, supports can be provided for the flange or piping immediately adjacent to the valve or instrument.
1.8 Bending.
The minimum bending radius for fiberglass pipe normally results from a design stress that is one eighth of the ultimate short term bending stress. Certain fittings, such as saddles and laterals, are more susceptible to bending failure than other types. See the Minimum Bend Radius Tables found in the Conley Product Data and Engineering Guide.
Where: At = Average wall temperature, F Ti = Inside wall temperature, F Tt = Heat tracing temperature, F At = (95 + Tt)/2 = 210F Tt = 325F Step 2 For Criteria 2, the following equation is used: Tt = TR + 25 Where: TR = Maximum rated temperature of pipe, F Tt = 210 + 25 = 235F Step 3 The maximum tracing element temperature is the lesser of the values calculated using Eqs. 4 and 5. In this case the value is 235F. The maximum tracing element temperature using this methodology applies only to applications involving flowing, non-stagnant, fluid conditions. For stagnant conditions, the maximum allowable trace element temperature is the chemical resistance temperature of the pipe. For this example Tt < 100F. Step 4 For Criteria 3, it is necessary to check the manufacturers published data to determine recommended chemical resistance of the pipe for this application. This value is compared with the inside wall temperature, Ti. The published value must be greater than Ti. For Example 5 then the maximum allowable trace temperature is then 100F. (Eq. 5)
1.11 Limitations.
The limitations and design values for fiberglass pipe are different from those normally used for steel pipes. The following outline presents various considerations and limitations that differ somewhat from those used with steel product. Most design pressures or stresses are 25% of ultimate failure pressure or stress. Exceptions are internal pressure and bending stress as noted below. 1.11.1 Design pressure or stress. Design stresses for pipe internal pressure come from ASTM D 2992. The internal operating pressure for fittings generally are based on one third of the ultimate short term failure pressure (ASTM D 1599). These values typically become the design allowables for the pipe. 1.11.2 Modulus of elasticity. There is more than one modulus of elasticity for fiberglass pipe because the end product is an anisotropic composite material. The tensile, bending and compressive moduli differ significantly and one should take care to use the correct value. Precise values for the moduli for specific conditions of loading should come from the manufacturer. 1.11.3 Allowable tensile or compressive loads. Typically the allowable design stress is 25% of the ultimate short term failure loads. These stress values can be used with the minimum reinforced wall thickness (area) to calculate the allowable maximum loads. 1.11.4 Bending loads. Ultimate beam stress is determined by using a simple beam with a concentrated load applied to the center to achieve short term failure. The allowable design stress is then established by apllication of an 6:1 factor of safety to the ultimate failure value. The 6:1 factor is selected to compensate for combined loading which occurs in piping applications. Conley Corporation/Tulsa, Oklahoma 10
Excerpts from Stringfellow, William D. ed. Fiberglass Pipe Handbook. Fiberglass Pipe Institute. Second edition 1992. Chapter 6.
Conley Corporation
2795 East 91st Street Tulsa, Oklahoma 74137 www.conleyfrp.com 800.331.5502
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