Glass Reinforced Epoxy Piping or GRE pipes are becoming a popular choice in the piping and pipeline industry due to its many advantages. The present article aims to give some basic principles and cares to be considered at the moment of the draft design of an aboveground GRE pipeline.
GRE pipe and GRP pipe differ in the used resin during bonding the glass fiber. GRE pipe used Epoxy Resin while GRP pipe used Isophthalic Resin.
The designer should evaluate if a deeper stress and strain analysis are required for the pipeline, for the supports, and for other bearing structures connected to the pipeline.
Apart from special cases, GRE pipes should be always connected to the bearing structures by means of saddles, made of steel or concrete or of other materials (GRE 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 pipelines for non-pressure 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.
GRE/GRP Pressure Class Selection
The selection of the GRE 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 GRE 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 the 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 maybe 50 Mpa in hoop direction and 30 MPa in the axial direction.
- The 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:
Vacuum Design for GRE pipes
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 mortared core.
Thermal Expansion Coefficient of GRE pipes
The approximate axial coefficient of thermal expansion (α) for a GRP pipe made by filament winding with a 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 steels.
The loads applied to expansion joints and to bearing structures are hence considerably lower in GRP pipelines.
Thermal End Loads for GRE pipes
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.
Selection of Anchor Points in GRE 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.
Use of Directional Changes or Offsets in GRE Piping
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.
Expansion Loops for Long GRE Pipelines
“U” expansion loops are provided for long straight pipeline runs, as shown in the figure (Fig. 7) below:
The recommended spacing between axial guide supports close to the expansion loop is also shown in the drawing. Other supports shall be spaced following other calculations (beam load).
Use of Expansion Joints in GRE Piping Systems
Various kinds of standard expansion joints can be used. Low stiffness expansion joints are preferable since they develop a low reaction in correspondence with relatively big displacements. GRE pipes expand more than steel pipes but have much lower thrusts.
Using stiff expansion joints would reduce the stresses in the pipe only by a little
We suggest rubber joints with one or more waves, possibly with limiting travel devices, with an activation load lower than the Pcr load, and with a working-travel equal to the total expansion.
Support Span for GRE Pipes
Horizontal pipes should be supported according to the spacing suggested by the support spacing data or according to a specific project.
Pipe span is defined as the distance between two consecutive pipe supports or anchoring devices.
The maximum span length for every pipe size and class is suggested by the Technical Department of GRP Vendor for standard pipes or according to a specific project.
The span length is limited by the following considerations:
- the maximum axial strain must not exceed the allowable value;
- the mid-span deflection has to be smaller than 1/300th of the span length and anyway not exceed 15 mm which is the minimum value.
If factor (b) is the determinant factor, then the distance between supports must not be changed by reducing the working pressure.
Often the spacing between the supports is set by other reasons, as for instance joint spacing or existing bearing structures. Normally the 6 meters half-length span is the maximum that is used, even for large diameter pipe, for which a theoretical longer span could be used. The maximum support span in meters is shown in the following table (Fig. 8), for different pipe sizes and pressure classes:
The maximum span has to be evaluated for a continuous span length when the joint can transmit axial loads.
In this case, the span is the distance LC between two supports of a pipeline, placed at a distance from the joint that in general shall not exceed the value of 1.2 m for pipe size up to DN 200 or 2 m for pipe size above DN 200.
GRE Piping Support Design rules
The following are suggested the basic rules to design and for the positioning of supports, anchors, and guides.
- Loads with linear and punctiform contacts have to be avoided, therefore curved supports that bear at least 120 degrees of the bottom part of the pipe and that have maximum bearing stress of 600 kPa have to be used. Unprotected pipes are not allowed to press against roller supports or flat supports. Do not bear any pipe directly against ridges or other points of the support’s surface. Protective sleeves have to be used in these cases.
- To protect pipes against external abrasion between the pipe and the steel collar, a PVC saddle (Fig. 9) or a protective rubber layer has to be positioned in-between. The PVC saddle is necessary when free axial sliding of the pipe must be permitted (axial guides).
- Valves and other heavy equipment must be supported independently in both horizontal and vertical directions.
- The clamps must fit firmly but must not transfer excessive force to the pipe wall. This could result in deformations and excessive wall stresses
5. Vertical runs have to be supported as shown in Fig. 9. Excessive loading in vertical runs has to be avoided. It is preferable to design a “pipe in compression” than a“ pipe in tension”. If the “pipe in tension” method cannot be avoided, take care to limit the tensile loading below the maximum tensile rate recommended for the pipe. The guiding collars will have to be installed by using the same space intervals used for horizontal supports.
Anchoring Points in GRE Piping Installations
An anchoring point must efficiently restrain the movement of the pipe against all of the applied forces. Anchors can be installed in both horizontal and vertical directions. Pipe anchors divide a pipe system into two sections and must be attached to some structure that is 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 or where fiberglass pipes join another material or a product from another manufacturer. Additional anchors are usually located on valves, pipeline changes of direction, and major branch connections.
It is a good practice to anchor long, straight runs of aboveground piping at intervals of approximately 90 m.
In any case, the correct positioning of anchor points has to be decided only after a detailed stress analysis.
The pipe must be able to expand radially within the pipe clamps.
To secure the pipe to the clamp it is suggested to apply a GRP lamination (as shown in Fig. 10 below) on each side of the clamp. If the movement of the pipe has to be restrained only in one direction, it is sufficient to apply only one overlay ring of GRP in the opposite position.