Pipe classification selection

FLOWTITE pipes are classified by DN-PN-SN, i.e. nominal diameter, nominal pressure, and nominal stiffness. Selecting the proper pipe class entails optimising the benefits of each of these parameters.

Selection of diameter is based on expected flow and acceptable head-loss. The expected flow depends on the service of the pipeline, while the head-loss depends on water velocity and the flow characteristics of the pipe. For pressure pipes the acceptable head loss depends on the price of energy.

The smooth bore and low friction loss make FLOWTITE pipes an excellent choice for all applications. In addition, the efficiency of continuous fibre reinforcements and the thin wall of the pipe result in a larger inner diameter, which is of even greater importance in terms of reduced head-loss.

In most cases economical flow velocity is 1-3 m/s, and most pipelines are designed for that range. However, the acceptable flow velocity for FLOWTITE pipes may be as high as 6 m/s, or even higher under certain conditions. See more about flow velocity under Product Scope on the Products tab on this website.

Please follow the Download tab on this website to Ami-Tools and the Ami-flow software for head-loss calculations. Please note that when comparing, inner diameter should always be used for head-loss calculations, since it can deviate substantially from nominal diameter, especially for some thick-walled pipes, such as thermoplastics and concrete. Contact your local APS representative or FLOWTITE pipe manufacturer for the nominal diameters in your area.

Pressure class
The pressure class of FLOWTITE pipes is determined by its long-term performance, i.e. the pressure rating is based on 50-year design strain, with appropriate safety factor. For pipes without end-thrust (uniaxial pipe) there are supplementary standardised requirements that allow the pipe to be operated at nominal pressure with additional loadings, such as full ring deflection (for buried pipe), or support stresses and beam-column deflection (for above-ground installations). A pipe without end-thrust (uniaxial pipe) is thus designed to operate at nominal pressure throughout its lifetime, with a good margin. By adhering to FLOWTITE’s installation instructions for pipes without end-thrust it is ensured that these conditions are met. See the Brochures section under the Downloads tab on this website for further installation information. To identify the nominal pressure classes in your area, please contact your local APS representative or FLOWTITE pipe manufacturer.

The pressure class of pipes with end-thrust (biaxial pipes) is determined by the pipe’s ability to operate at nominal pressure with full end-thrust for its design life (50 years), with appropriate safety margin. In industrial piping systems such pipes are, however, usually subjected to considerable loads, other than direct tensile stresses from pressure. These may include temperature loads, shear forces, torsion or bending moments and others. The designer of the piping system needs to determine these loads and compute the maximum stress at all critical points in the system. In most cases the resulting stress is of such magnitude that the allowable operating pressure is lower than the PN class of the pipe.

Since the short-term strength is considerably higher than the design strain, intermittent surge pressures are allowed up to 40% above the pressure class, as defined in AWWA M45. This applies to both pipes with end-thrust and without end-thrust.

Flowtite pipes are delivered in standardised stiffness classes. The stiffness class represents the pipe’s minimum initial specific ring stiffness (EI/D3), expressed in N/M2 per metre of pipe.

Nominal stiffness classes:

SN2500 SN5000 SN10000

Stiffness class is selected according to two parameters: (1) burial conditions, which include native soil, type of backfill and cover depth and (2) negative pressure, if appropriate. In most cases SN2500 is sufficient for all buried applications, while SN5000 is sometimes preferred. Only in exceptional cases under extreme conditions a selection of SN10000 is warranted.

FLOWTITE Installation Instructions for buried pipes (see under the Download tab in this website) provide guidelines on selection of stiffness class for buried pipes under various conditions, such as burial depth, native soil conditions, backfill type and compaction, presence of ground water, vacuum etc. These instructions are based on the specifications in AWWA M45.

FLOWTITE also provides tools to carry out static calculations for buried pipes. Please follow the Download tab on this website to Ami-Tools and the Ami-stat software, which provides calculation tools for both the AWWA M45 method and the ATV A127 method.