Tuesday, February 2, 2021

The Key Causes of Corrosion in Piping Systems

Several factors are operating alone or together to affect the corrosion rate occurring in a pipe and its service life in any piping system. Every piping system has distinct susceptibilities, precise corrosion threats, engineering schemes and faults, and maintenance constraints. Depending on the strength of those factors, a brand new piping system will show indications of corrosive damage in as little as two years after installation.

Among the corrosion problems, a significant proportion is linked to piping systems. Piping systems are used to carry various fluids—corrosive or not—in a very wide selection of environments. A major lifetime cost for any industrialized plant is leaking fluids and improving the piping system to limit and eradicate leaks.

Corrosion watching for any system needs basic information of its common corrosion issues, combined with the system design knowledge. Because the severity of corrosion depends on the piping system, it is necessary to understand the various piping systems resulting in corrosion and understand system-dependent corrosion.



In Piping, Corrosion Is System-Dependent

Technically no material is entirely corrosion-resistant. However, materials behave differently depending on the conditions to which they're exposed. Beneath unfavorable conditions, corrosion may be accelerated, while under ideal conditions, materials might corrode very limited amounts. Corrosion is thus not material-dependent but system-dependent. A requirement for developing a corrosion-protection strategy that might stand up to the harshest of conditions and thus prolong the lifespan of products is knowledge concerning the underlying system mandate that causes corrosion damage.

Corrosion characteristics are thus totally different between piping systems, such that they'll never be evaluated alone. For example, it has been found that condenser water has sufficiently higher corrosion than chiller water, steam condensate more than steam, and dry fire pipe corrodes considerably more than wet fire pipe.

Pipe materials and their age also play a significant role. The connecting pipes of varied materials and their electrode potential differences5 might cause galvanic corrosion and the damage of pipes, valves, and different equipment within the operation. Beneath certain circumstances in environments like saltwater with free ions, acids or bases, higher temperatures, and enough oxygen, a system can decline very quickly. A very common sequence in piping systems is Copper and low carbon steel. Such links cause higher steel corrosion than steel alone.





Common Areas of Corrosion in a Piping System

The following factors determine the rate of corrosion in a piping system:

  • The pH of the water
  • The amount of dissolved oxygen in the water
  • The biochemical structure of the water
  • The amount of galvanic corrosion from the use of unique metals used in or in contact with the piping system
  • The temp of the water
  • The velocity/pressure of the water inside the pipe
  • The elements of metals

Often, two or more different factors are the explanation for failure or corrosion problems. Some dilemmas are associated with the pipe contents' chemical treatment, whereas others are associated with the pipeline's original composition, selection of materials, and operational conditions.

What degree of corrosion activity might exist is greatly dependent upon the kind of piping system concerned. Closed systems frequently show very low corrosion and corroding activity, whereas open condenser or cooling operation loops show the highest rates. A current open method also tends to show the greatest variability in corrosion test results. This implies that wall thickness may vary at supply and return and at different areas of the pipe. This will increase the chance that any corrosion coupon testing performed at one space isn't representative of the system.

For many cooling water loops, and particularly for current open systems, dramatically altered conditions will exist at numerous points throughout the piping layout. Often, the particular causes of such corrosion modifications, like at low-flow areas or long horizontal runs, are inescapable. Similar variations in corrosion activity will exist at different areas of a fireplace protection arrangement. Therefore, corrosion regions may be foretold by merely checking the physical configuration of the piping system. The following factors contribute to corrosion in a piping method.



Physical Geometry

The physical geometry of a pipe greatly influences corrosion in a piping system. For example, lower floor areas of constant piping systems generally suffer a greater degree of corrosion and corroding activity merely owing to the settling of dirt, rust, organic material, and particulates. For several larger layouts, the flow rate decreases further from the current pumps to permit even the best particulates. Higher wall loss in several cases also exists wherever piping has been decreased in size and has less accessible wall thickness. The corrosion rate of five MPY can turn out large iron oxide volumes every year, which can settle to provide secondary corrosion issues if not removed.




Horizontal/Vertical Orientation

System-dependent corrosion may also depend on whether the pipe is in a horizontal or vertical orientation. Horizontal sections of pipe usually show a higher degree of sediment and deposit buildup, corrosion, and rusting than vertical sections. Wherever a higher-than-average corrosion rate occurs, ultrasonic testing can generally document considerably larger wall loss and corrosion on its bottom surface. Linked with low-flow conditions or the periodic loss of flow, as may happen with individual HVAC package units, horizontal piping will suffer considerably higher corrosion rates.




Bottom Sediments

Bottom sediments or deposits may also influence system-dependent corrosion of piping. Inside horizontal sections of pipe, and sometimes relying on flow rate, the lowest and lower facet wall areas typically show considerably higher metal loss due to the settlement of rust and particulates.

The presence of great wall thickness variations from high to bottom of any same horizontal pipe section may warn of an internal deposit problem.




Accidental Corrosion

Accidental corrosion on the wall of piping can also cause piping system breakdown. The net result from numerous corrosion mechanisms is generally deep and random and, in many cases, may only be determined by the metallurgic investigation.




Drainage Situations

Drainage situations have an impact on system-dependent corrosion. Piping that is drained down over the winter months or packed up and drained periodically will suffer up to 10 times more wall loss than other system areas. Such corrosion loss is usually directly proportional to the pipe's proximity to the open air.




Piping of Supply and Return

The piping of supply and return can cause system-dependent corrosion. For example, the return piping at a condenser or cooling water system typically shows a higher degree of corrosion than the supply facet, thanks to the marginally higher return water temperatures that favor corrosion activity and promote MIC. High temperatures quicken most chemical reactions.

Higher corrosion of return piping may also bring rust particulates originating from the supply pipe.




Quality of Pipe

The quality of pipe has a major impact on system-dependent corrosion. Because of the lower quality of steel pipe these days than the factory-made pipe of 50 years past, higher average corrosion rates are common. Wherever one MPY corrosion rates once existed for condenser or open water service, 3–5 MPY corrosion rates are currently expected, and 10 MPY rates aren't uncommon. Pipe made outside the U.S. also tends to be particularly prone to corrosion. 





Stagnant Flows

Stagnant areas of flow will typically develop severe corrosion from particulates' settlement and a scarcity of chemical protection. The lower flow rates existing within the distribution and run-out piping to individual package units can typically show accelerated corrosion in smaller lines that can least afford it. 




 Pipe Assembly

Although rarely a factor in the first stages of a piping system, pipe structure plays an important role in an old system. Termination gaps of various clamped type piping systems typically accumulate with particulates and microbiological agents to provide localized high corrosion and corrosion losses.

A threaded pipe will nearly always leak or fail, even without the threading method, which can be attributed to a wall loss of 50% or more. Cutting a groove into a pipe used in clamped pipe assembly, instead of rolling or swaging it, has the comparable result of substantially reducing pipe wall life. This wall loss, in addition to a high corrosion rate, can usually produce advanced failures.

The main ways to limit corrosion in piping systems are as follows:

  • Choice of proper piping materials
  • Use of corrosion-resistant exterior coating
  • Use of correct geometry of piping systems
  • Use of cathodic protection





Conclusion

In most circumstances, a piping system will incorporate a mix of potentially corrosive conditions. The corrosion problem is a multifaceted issue and is rarely simply one factor responsible for a piping failure. Therefore, reducing and preventing corrosion requires looking at all aspects that may affect the pipeline, careful inspection, and various points.

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