The 2017 edition of the standard provides a new modelling approach to be used for pipe stress and flexibility analyses which are based on using a standard SIF of 1.5 in combination with an equivalent fitting thickness. The latest revision also provides a standard for qualification of the SIF. Thereby a manufacturer also has the possibility of taking credit for a potentially lower SIF than 1.5 for a specific elbow design. Other important features of the ISO14692 have undergone very significant changes as well, such features are: the qualification of GRP piping components, the standard on GRP flanges, the design stress envelope, fatigue in GRP and static electricity.

10th edition of TEMA
Houmaan ashrafi

LOOP-1
An unbalanced expansion loop. Normally a pipe loop is located midspan between anchors providing uniform displacement on each side minimizing the size of the loop. In this case an unbalanced loop is chosen to reduce the loads on the vessel nozzle by placing an axial anchor that limits axial motion next to it. The trade-off is the increase in height required for the loop. The anchor and pipe loop location is a trade-off.

A pipe rack with elevated risers and expansion loops. Red pipe in the middle is the full system. Top and bottom pipes in blue are details. Motions are shown below.

Motion of pipes in the pipe rack with elevated risers. Attention is paid to optimizing the design of the loop while keeping motions and loads in the rest of the system within limits.

A tower with a guided flexible pipe connection. The guided connection allows the flexibility required in the nozzle while limiting the loads applied to the vessel nozzle. Modelling the flexibility of the nozzle and accounting for the different possible temperatures of the piping system and tower is required for successful analysis.

Pump

Stress analysis of a pump skid from storage towers to dual pumps. The temperature distribution of the piping varies depending on which pump or pumps are running. Here the pipes leading to the operating pump are hotter than the standby pipes. Multiple load cases are required to ensure the piping design is acceptable.

Types of Stresses in Piping Systems ⬇️

Primary, Secondary and Occasional Loads

From a piping stress analysis point of view the following are the main loads to be considered for the design:

⬇️ Primary load occurs from Sustained loads like dead weight, live weight, internal pressure etc. and are called non-self-limiting loads. Pressure thrust from an expansion joint is used in this article.
⬇️⬇️ Secondary loads occur from thermal expansion loads like temperature change, anchors and restraints etc. and are called self-limiting loads. Thermal expansion in a horizontal pipe loop is used in this article.
⬇️⬇️⬇️ Occasional loads occur from static wind and seismic loads and are considered to act occasionally. Seismic load on a vertical pipe loop is used in this article.

Primary Stress ⬇️

Primary Stress is generated by internal and external force and moments. Primary stress is not self limiting – even if a part moves, the load causing it does not reduce. In this example, an expansion joint without restraining hardware creates a primary stress on a pipe. 

Tied-and-Untied-Expansion-Joint

Pipe loops with tied and untied expansion joints. The foreground joint is tied – it has tie rods to prevent axial growth of the expansion joint. The background joint has no tie rods.

Expansion-joint-deflections

Deflection of the two expansion joints under pressure. The tie rods limit the expansion of the tied joint. The untied joint increases in length the same as if it is a hydraulic cylinder applying bending stresses to the pipe.

Expansion-joint-stress

Pipe stress as reported by Caesar. The untied joint is applying a bending force, which, depending on the stress level, Caesar can report as a fail. This design does not meet the expansion joint manufacturers requirements for guiding and anchoring. The pipe with the tied joint is okay.

This primary stress is caused by pressure of the fluid multiplied by the area of the pipe. It occurs all the time the system is pressurized. No matter how much the pipe displaces, the untied bellows keeps pushing on it.

Because primary stresses are not relieved by the piping moving or yielding, primary stress limits are set lower than other allowable stresses. For example, if primary stresses managed to get above the yield point, the piping would balloon out and explode.  The piping codes keep the primary stresses below the yield point by a factor of safety.

Secondary Stress ⬇️⬇️

Thermal expansion and contraction happens when a pipe heats up and cools down. The piping system must have enough flexibility to handle the expansion.

Horizontal-Loops

Two horizontal pipe loops between fixed anchors. These pipes are subject to high temperature expansion.

Horizontal-Loop-Displacement

Deflection of the piping system caused by thermal expansion. The piping must be flexible enough to handle the thermal growth.

Horizontal-Loop-Stress

Stresses generated by the deflections. Caesar II shows the larger loop to be much better able to handle the thermal expansion. The smaller loop is over-stressed.

The stress is caused by the pipe pushing against some fixed restraint. Thermal stresses are “secondary stresses” because they are self-limiting. That is, yielding or deformation of the part reduces the stress.

Higher stresses are allowed than in the primary case because of this self limiting behavior. Stresses above the yield point can be acceptable, however, over time, an accumulation of highly stressed thermal cycles can exceed the fatigue capacity of the material leading to failure. 

Occasional Stress ⬇️⬇️⬇️

Wind and seismic loads are occasional. In this example sideways seismic loads create the stress.