VALVE AUXILIARIES
Figure 3 : External corrosion ( a ) drain connection of buried pipeline valve led to external leak , ( b ) sever corrosion under insulation for body vent valve .
These auxiliaries experience high pressure and , in some cases , high temperatures , in addition to being exposed to hydrocarbons . To maintain operations and prevent failures , all associated auxiliaries need to be properly designed , selected and inspected .
Associated risks
Failure to maintain healthy valve auxiliaries can lead to serious risks in operating facilities . These risks range from operational interruptions to severe outages . The primary failure modes include : 1 . External corrosion 2 . Internal corrosion 3 . Environmental cracking 4 . Third-party damage
External corrosion
External corrosion is a common failure mode in valve auxiliaries , especially in buried and insulated valve installations . The extent of the concern includes multiple factors such as soil characteristics , environmental conditions , coating specifications and selection , in addition to the material suitability of these components . Figure 3 shows an example of an external leak in a drain connection of a buried pipeline valve due to external corrosion .
Internal corrosion
Internal corrosion is another common risk in valve auxiliaries , which are considered operational dead legs : stagnated corrosive fluids within areas of accumulation can lead to accelerated localised corrosion . Figure 4 demonstrates a typical failure of valve auxiliaries due to internal corrosion . To prevent corrosion , the material of the valve auxiliary should be made of proper corrosion-resistant alloys that can withstand both internal and external environments . As a rule of thumb , valve auxiliaries should have corrosion resistance at least equal to the trim material of the main valve . In some severe services , such as buried and insulated applications , auxiliaries should be protected from external corrosion with a suitable coating system .
Environmental cracking
As these auxiliaries are exposed to corrosive fluid environments and are subject to tensile hoop stresses , they are prone to environmental cracking mechanisms . Stress corrosion cracking ( SCC ) is commonly encountered in sour environments . Ensuring proper material selection is a key factor in preventing such failures . As there is limited control in these environments , the main prevention methods involve choosing a non-susceptible material as defined in the applicable design codes .
Fatigue
Fatigue due to flow-induced vibration is another risk , especially for small-bore equalising lines .
Figure 4 : Example of internal corrosion
Mitigation methods include increasing system rigidity through greater pipe wall thickness and providing a robust support configuration .
Third-party damage
Third-party damage is another risk , especially in the valve ’ s early stages when improper handling can cause damage to valve auxiliaries . Figure 6 demonstrates examples of damage to valve auxiliaries due to improper handling and transportation . To minimise the risk of third-party damage , valve auxiliary piping should follow the valve contour as closely as possible and be rigidly connected to the valve body .
Risk mitigation methods
Mitigation starts at an early stage during valve design : Valve auxiliaries require robust and solid design in terms of material selection and corrosion resistance to withstand the designated pressure conditions and function as intended . Periodic and preventative maintenance should always be part of ensuring their
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