Forces that cause touchpoint damage and techniques for mitigation.
The focus is on above ground piping, not buried piping. Above ground pipes exist in petrochemical or refining complexes but can also be found as transmission pipelines between processing facilities.
Above ground piping has to be supported in place often by steel or concrete beams and it’s the pipe-to-support interface (often referred to as the pipe touchpoint or CUPS) that is the area of concern. The touchpoint or CUPS will sustain damage over time, reducing the remaining wall thickness of the piping which was originally designed to safely contain and transport pressurized contents.
Any reduction in the wall thickness compromises the integrity of the entire system and depending on the criticality of the particular line, a single leak can require a single unit or an entire plant to be shut down for safety reasons until the leak is mitigated, and can cost operators many millions of dollars in lost revenue in a single day.
To compensate for this known damage mechanism, a Corrosion Allowance is designed into all piping systems based on a certain design life, however many facilities are operating well beyond their intended design lives therefore additional maintenance is required to recondition and extend the life of piping infrastructure, and because maintenance is significantly more cost-effective than replacing the entire system either during a shutdown or turnaround.
The challenge with performing repairs at the touchpoint is that it’s impossible to access this region unless the pipe is lifted away from its support. Lifting provides free access to perform inspection and repairs, enabling operators to recondition the line for another period of service.
Various forces in piping systems and atmospheric conditions are responsible for the damage sustained by piping systems which are generally classified as either CUPS, TPC or CPC (Corrosion Under Pipe Supports, Touchpoint Corrosion or Contact Point Corrosion). The major culprits are explained below:
2.1 Erosion
Piping systems are subject to heating and cooling cycles due to fluctuations in daily temperatures, seasonal changes, and the start/stop of hot or cold product flow through the piping. These temperature changes cause thermal expansion and contraction, forcing the line to move on top of the supports and erode the pipe's outer wall material. Anchoring the pipe to the support will stop this erosive force however piping is designed to have flexibility in certain sections to accommodate this thermal expansion that would otherwise exert tremendous loads onto equipment and structures causing significant damage.
A lot of installed piping has no protection against erosion. Instead, pipes are often left to rest directly on the support. It takes very little movement over a long period of time for the pipe to slowly erode the outer wall of the pipe thereby reducing it’s minimum wall thickness.
Pipe shoes can be used to prevent erosion against the piping. They are fixed to the pipe and act as sacrificial material but they are primarily used to elevate a pipe to accommodate a layer of soft insulating material that would otherwise be crushed against the support.
Pipe shoes are either welded on or clamped, but both types introduce new challenges around the touchpoint. Welded shoes are prone to stress cracking and installing them is a challenge. If fitted before the pipe is fitted you risk a location error whereas welding them in after the pipe is installed requires lifting the line. Moreover, clamped shoes provide new locations for corrosion against the pipe which is typically under a layer of insulation. Not directly visible and to access or replace the shoe the line must once again be lifted.
2.2 Oxidation
Humidity and moisture cause corrosion, especially in areas that are shaded with poor air circulation. This occurs in pipe touchpoint areas where accumulated moisture isn’t evaporated by direct sunlight.
This damage mechanism is accelerated when there is a coating failure (often caused by erosion) at the pipe touchpoint which leaves bare metal exposed to these oxidizing conditions. This phenomenon gave rise to the industry term "Corrosion Under Pipe Supports" (CUPS), recognized as one of the leading causes of piping failures. Over time, the definition of CUPS has expanded beyond corrosion under pipe supports to include all damage mechanisms at pipe touchpoints described here.
2.3 Galvanic Corrosion
Dissimilar metals in contact with one another such as a stainless steel pipe resting on a carbon steel support causes galvanic corrosion which is when one metal causes another metal to corrode and break down.
For corrosion to initiate, three components are required: an anode (consisting of one metal), a cathode (composed of a different metal), and an electrolyte (commonly water).
Certain metals exhibit greater propensity to donate electrons, while others display a stronger inclination to accept additional electrons. Consequently, when these dissimilar metals are exposed to environments abundant in electrolytes, electron transfer occurs from one metal to the other.
The removal of electrons by a metal leads to the onset of galvanic corrosion, which causes the metal to rust.
2.4 Liquid Hammer
Within the piping, there exist internal forces generated by the flow of either gas or liquid passing through it. Similar to how a car traveling on a freeway possesses momentum determined by its mass and velocity, a fluid coursing through a pipe also carries momentum. Any sudden disruption in the flow can result in the transfer of a substantial amount of energy to the piping, leading to an occurrence known as liquid hammer or pipe hammer.
These events induce significant movement along the line, and in more severe instances, can cause complete destruction and rupture of the piping system. Even less severe hammer events can displace the pipes without causing rupture, but they may result in pipe shoes detaching from their supports or pipes becoming dislodged from their guides. Rectifying this requires line lifting to reposition and restore the line to its original configuration.
2.5 Ground Movement
The final force we investigate, though less common, can pose a significant financial burden due to the remediation that’s required: ground movement impacting the spatial alignment of the structures supporting the piping system.
Piping systems consist of intricate networks of pipes and equipment. Within this system there are fixed points often where the pipes tie into equipment, and there are areas of flexibility such as pipe expansion loops. These systems are designed as a single unit and a lot of engineering goes into the exact placement of pipe supports, guides and anchors to ensure the piping has the flexibility between the fixed points to expand and contract with changing temperatures, operating conditions etc. This is done to prevent components from becoming over stressed.
Movement of one or more supports in relation to another not only displaces the line but introduces stresses on the piping and connections which altogether can cause flanges to leak and support structures to become loaded past their design limits.
The image below shows a pipe rack that was built on a hillside in an area that receives a lot of rainfall year round. During the initial construction there was no consideration for ground movement so the pipe supports or the structure supporting the pipelines was slowly moving over time.
Replacing the entire system would require a lengthy shutdown which was not an option so the challenge was to build a new pipe support structure around the existing one, and transfer the weight of the pipes from the old structure to the new one. This can only be achieved by lifting all the pipes across a single support simultaneously.
Once the lines are lifted, the old support can be cut out allowing the new support to be installed at the correct elevation without obstruction.
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