What is a dead leg?
A dead leg is a section of a production system (i.e. piping, tubing, etc.) that does not, rarely or intermittently see process flow. The fluid in the system becomes stagnant and can cause contamination or accelerated corrosion.
There are three main types of dead legs:
Dead legs include a multitude of piping configurations such as blanked branches, lines with normally closed block valves, lines with one end blanked, pressurized dummy support legs, stagnant control valve bypass piping, spare pump piping, level bridles, relief valve inlet and outlet header piping, pump trim bypass lines, high-point vents, sample points, drains, bleeders, and instrument connections.
Why are dead legs a concern?
Fluid is stagnant in dead legs; therefore, bacteria can establish colonies and proliferate resulting in an environment that is conducive to microbial influenced corrosion. Deposits can also form in dead legs leading to under deposit corrosion.
Corrosion rates at dead legs can be substantially higher than in the adjacent piping where the process fluid flow normally.
In addition, dead legs pose a challenge due to their potentially large number that can exist within a production system.
How can dead legs be managed?
Due to the difference in corrosion rates in dead legs relative to adjacent piping where fluid flow normally, a specific dead leg integrity management program is necessary.
Dead leg management is required in numerous industries such as Oil and Gas, Nuclear and Power Generation. A systematic approach to dead leg management is necessary, where based upon the assessed risk, inspection and mitigation strategies can be developed, planned, prioritized and implemented.
Stages of systematic management of dead legs
Accumulation of water and deposits is dependent on the orientation and configuration of the pipe work. The orientation of a dead leg may impact the susceptibility to accelerated corrosion depending on the specific corrosion mechanism. Inspection frequency should take account of the pipe materials, wall thickness and expected deterioration rates.
There are a number of inspection techniques for dead legs. These include visual inspection, thermography, profile radiography, scanning ultrasonic testing or pulsed eddy current to name but a few. The type of inspection technique selected will depend upon the specific type of dead leg and its dimensions.
The inspection frequency of dead legs varies depending on operator philosophy but API 570 offers considerable guidance.
How can AIE help with Dead Leg Management
To address dead leg corrosion, Asset Integrity Engineering (AIE) advises its clients to adopt a systematic approach to dead leg identification, risk assessment, inspection and corrosion management.
AIE specializes in providing a complete range of services for supporting its clients in dead leg management and we provide a bespoke service offering in this regard which continues to be developed and improved across our client base.