The present article aims to give some basic principles and cares to be considered at the moment of the draft design of an aboveground GRP pipeline. The article is published in two parts.
The designer should evaluate if a deeper stress and strain analysis is required for the pipeline, for the supports and for other bearing structures connected to the pipeline.
Apart from special cases, GRP pipes should be always connected to the bearing structures by means of saddles, made of steel or concrete or of other materials (GRP itself for instance), in order to distribute the loads on a length and on an angle that is able to minimize the stress concentration on the pipe/support contact points.
In nearly all aboveground applications tensile resistant couplings should be used.
Only in case of well supported pipe lines for nonpressure applications a non-tensile resistant system can be used. The forces close to elbows or other singular points such as valves, reductions or tees, can become relevant.
PRESSURE CLASS SELECTION:
The selection of the pressure class has to be made according to the following loads:
- working pressure
- surge pressure (water hammer)
- spacing of supports
- thermal load
The stress in hoop direction due to the internal pressure is calculated as shown in fig. 1:
- In GRP pipes it is important to always check the axial stress due to internal pressure since the material is anisotropic and the difference of strength in hoop and axial direction is relevant.
- The sum of stresses due to the above loads, calculated in the hoop and axial direction, has to be lower than the allowable stresses, defined for each pipe class or by a specific job.
- Approximate values for allowable stresses for a common filament wound pipe for above ground use may be 50 Mpa in hoop direction and 30 MPa in axial direction.
- High working temperature could reduce the allowable stress in GRP and consequently reduce the pressure class.
- The Code (AWWA M45) generally considers a 40% of tolerance in the allowable stresses in case of transient surge pressure based on the increased strength of fiberglass pipes for rapid strain rates.
Both the following equations (Fig. 2) have to be calculated:
The AWWA M45 standards admit a safety factor for vacuum conditions between 1.3 and 3.
For different pressure classes and the same standard pipe (55° filament winding) the approximate relation between pressure class, stiffness and vacuum resistance is resumed in the following table (Fig. 3).
For low pressure pipes with vacuum, a convenient solution can be either to provide stiffening ribs or a sandwich pipe wall structure with a mortar core.
THERMAL EXPANSION COEFFICENT:
The approximate axial coefficient of thermal expansion (α) for a GRP pipe made by filament winding with winding angle of 55°is:
α = 1.8×10−5 m/m °C
For different GRP pipe classes (with mortar core) or for different winding angles, please consult the GRP Vendor.
The total expansion (or contraction) of a pipe length ( L ) is calculated as:
ΔL =α ⋅ L ⋅ ΔT
ΔT is the temperature gradient (positive or negative) with reference to the installation temperature T0.
The thermal expansion coefficient of GRP has the same magnitude as the steel coefficient (α=1.2× 10-5 °C-1), whilst thermal end loads for restrained expansion are significantly lower, since the axial E-modulus of GRP (Ea) is around 1/20th of steel’s.
The loads applied to expansion joints and to bearing structures are hence considerately lower in GRP pipelines.
The thermal end load (F) due to constrained expansion is calculated as shown in Fig. 4:
and ID is the internal (nominal) diameter.
The thermal end load due to constrained expansion could be too big for both the stress arisen in the pipe, and for the load that the bearing structures have to support.
Considering the pipe itself, its elastic stability has to be checked. The pipe’s elastic stability depends on the pipe section, on the E-modulus and on the span between axial guide supports that is the length of free deflection.
The allowable compressive end load due to instability (Pcr) is calculated as shown in Fig. 5:
When the end loads are too big, they should be reduced by providing the system with anchor points and expansion joints, or better, by operating on the pipeline’s geometry and on the supports placement in order to let the line expand where it is not dangerous. Expansion loops can be added to the system where it is possible.
The second solution is preferable since the involved loads and thrusts are much lower than in a similar steel pipeline.
They have to be placed in such a way that pipeline expansions are forced in predetermined directions, in order to balance loads and displacements on the different expansion devices, and to minimize displacements close to dangerous locations, for example in weak branch connections or in connections that are not allowed to move. More detailed description about the anchor points are provided in part-2 (Final part) of this article.
Changes of direction in a pipeline can be used to partially absorb the line’s elongation, when close to an elbow; a branch that is free to expand is available, as shown in the following figure (Fig. 6):
The “available bending strength” is considered the remaining strength, after that all of the other stresses on the pipe have been removed, such as the stresses due to internal pressure.
Clearly any term of the equation can be obtained once that all of the other terms are known, for instance the length ΔL that can be absorbed can be found, when the length of the leg that is available is H. For more details Refer next part…..