Hydraulic Design

The aim of hydraulic design is to ensure that GRP piping systems are capable of transporting the specified fluid at the specified rate, pressure and temperature throughout their intended service life. The selection of nominal pipe diameter depends on the internal diameter required to attain the necessary fluid flow consistent with the fluid and hydraulic characteristics of the system. Fluid velocity, density of fluid, interior surface roughness of pipes and fittings, length of pipes, inside diameter of pipes, as well as resistance from valves and fittings shall be taken into account when estimating pressure losses. The smooth surface of the GRP may result in lower pressure losses compared to metal pipe.

For typical GRP installations, the mean linear velocity for continuous service of liquids is shown in table:

System Design

General

The aim of system design for GRP piping systems is to ensure that they shall perform satisfactorily and sustain all stresses and deformations during construction/installation throughout their service life. This clause identifies the service design criteria and the loads to which GRP may be subjected. In order to provide sufficient robustness during handling and installation, minimum. reinforced wall thickness of any GRP pipe component shall not be less than 3mm. Minimum reinforced wall thickness of pipe is calculated based on Hoop stress or stiffness requirement and by using ISO 14692 loading conditions.

Design of Above ground Pipe

For above ground design, calculated wall thickness shall be checked for below loading conditions.

(i) External pressure/vacuum

Pipe and fittings shall have sufficient stiffness to resist vacuum and/or external pressure loads. The design engineer shall ensure where possible, that vacuum conditions can be sustained by the selected component.

(ii) Axial Stress

Total axial stresses due to Internal Pressure, Self-mass and thermal Expansion are in compressive in nature then selected Pipe structural wall thickness needs to satisfy Shell and Euler buckling conditions as below.

(iii) Shell buckling

The axial elastic buckling stress for a cylinder in pure bending. The ratio of the buckling stress to the maximum axial stress shall be greater than 3.

(iv) Euler buckling

The ratio of buckling stress (due to constrained thermal expansion or vertical pipe runs with end compressive loads and a given length of unsupported pipe) to maximum axial stree shall be greater than 3

Design of Underground Pipe

Structural design of GRE not only involves establishing design conditions, selecting Pipe classes and corresponding Pipe properties also requires selecting Installation Parameters. It needs to withstand Soil as well as traffic loads as defined in AWWA M45.
Under Ground Design Procedure involves following steps:
• Calculating Pressure class, checking working and surge Pressure
• Calculating allowable deflection from ring bending
• Checking deflection prediction (ry/D)
• Checking buckling
• Buoyancy Calculations

Elastic Bending Radius

Since GRP pipes are more flexible than steel pipe so, elastic bending radius of GRP pipe are very critical during installation. A careful calculation is required as per standards which reduces bends required for pipline and it can accommodate any movements in trench profile.

Design Envelope

With Isotropic materials (properties of a material are identical in all direction) such as steel, the effect of combined stresses (pressure and non-pressure induced) are greatly simplified as hoop and axial strength are identical. This result gives a significant margin in allowable strength to accommodate this extra axial stress as the pressure induced. Axial stress (load) is exactly one half of hoop stress. Whereas, for anisotropic material (properties of a material are different in all direction) such as filament wound GRP pipes, hoop and axial strength are significantly different. Axial stress is not exactly half of hoop stress. Therefore hoop strength is significantly greater than the axial strength.


If the effective hoop and axial stresses, ợheff, and ợaeff, lie inside the factored long-term design envelope, then the GRP pipes and fittings designed within the acceptable limits. If the stresses lie outside this envelope, then high-rated GRP pipes and fittings, i.e. a thicker walled pipes and fittings shall be chosen and repeat the same stress calculation until it lies within the acceptable limits of factored long-term design envelope.

Thrust Block Design

Unbalanced thrust forces occur in pressure pipelines at changes in direction (i.e., elbows, wyes, tees, etc.), at changes in cross-sectional area (i.e., reducers), or at pipeline terminations (i.e., bulkheads). These forces, if not adequately restrained, may cause pipeline movement resulting in separated joints and/or pipe damage.

Thrust forces are:
(1) hydrostatic thrust due to internal pressure of the pipeline and
(2) hydro-dynamic thrust due to changing momentum of flowing fluid.

Since most pressure lines operate at relatively low velocities, the hydrodynamic force is very small and is usually ignored.

Typical examples of hydrostatic thrust are shown in Figure. The thrust in dead ends, tees, laterals, and reducers is a function of internal pressure P and cross-sectional area A at the pipe joint.
Thrust Force Definitions

Thrust Block

Concrete thrust blocks increase the ability of fittings to resist movement by increasing the bearing area and the dead weight of the fitting. Typical thrust blocking of a horizontal bend (elbow) and vertical bend (elbow) is shown in Figure:
Thrust block of a bend

Support Design

GRP piping systems can be supported using the same principles as those for metallic piping systems.
However, due to the proprietary nature of piping systems, standard-size supports will not necessarily match the pipe outside diameters. The use of elastomeric pads may allow the use of standard-size supports.
The following requirements and recommendations apply to the use of system supports.
a) Supports shall be spaced to avoid sag (excessive displacement over time) and/or excessive vibration for the design life of the piping system.
b) In all cases, support design should be in accordance with the CPI guidelines. CPI will provide the support design for above ground pipe system.
c) Valves or other heavy attached equipment shall be independently supported.
d) GRP piping should be adequately supported to ensure that the attachment of hoses at locations such as utility or loading stations does not result in the pipe being pulled in a manner that could overstress material.

Guidance for span lengths

The mentioned span length table is only for reference. However CPI design engineers will recommend the span distance and type of support after performing the stress analysis and will provide the design of support drawing accordingly.
Type of Pipe supports can be categorized into those that permit movement and those that anchor the pipe.

CPI PRODUCT RANGE

CPI Recommended Product Range