Definition:
Pipeline corrosion systems need to be monitored to provide a check on the effectiveness of corrosion control methods. Monitoring corrosion also provides the basis for remedial measures in the event of deficiencies being identified in these methods. Corrosion monitoring access fittings commonly known as corrosion coupons are used in hydrocarbon production systems to provide the corrosivity of the product. The device can be inserted and removed safely without the need for costly shutdowns or temporary isolations.
Applications:
Access fittings are not only limited to piping and pipeline systems, but their application extends to Chemical and petrochemical, heavy manufacturing, cooling systems, sand monitoring systems, and cooling systems. They are used to insert or remove devices like corrosion probe, bio-probes, erosion probes, chemical injection, fittings, sacrificial and impressed current anodes, electrical resistance probes, corrosion coupons, hydrogen probes, thermowell, linear polarization probes.
Shape and Style:
Access fittings come in various styles including:
- Welded
- Buttweld
- Socket weld
- Flare weld
- Flanged
- API flange
- ANSI flange
- Threaded
Components of access fittings:
The assembly of an access fitting has three main parts:
- Protective cover: Protects the external threads of the access fitting body.
- Plug: It is the carrier of the installed device. It may be solid or hallow depending on the application. The plug assembly screws into the body of the access fitting and seals the fitting bore to contain line pressure.
- Access fitting body: The specialized pipe fitting which is permanently attached to the process plant vessel or pipework.
Installation and Retrieval:
To perform the online installation and retrieval of the plug assembly, components like retrieval tools and service valves are required. The service valve contains the line pressure when the plug assembly is removed. Several access fitting assemblies can be maintained using one service valves and one retrieval tool.
Solid Vs Hollow Plug Assembly:
Access fittings can be installed with a solid or hollow plug assembly. For example, access fittings for corrosion coupons and chemical injection use a solid plug assembly, whereas hydrogen probes, bio-probes, and corrosion probes use hollow plug assemblies.
Other Monitoring Methods:
- In some systems, monitoring the actual pipe-wall thickness is a monitoring option.
- Other means of monitoring include the following:
- Linear Polarization Resistance (LPR) Probes
- Galvanic probes
- Field Signature Monitoring (FSM) and
- Acoustic methods
- Corrosion Probes and mass loss coupons are used to measure corrosion-inhibitor residuals, bacterial activity or to detect possible changes in the process that might affect the corrosivity.
- Probe installation location shall be based on the probability of failure in combination with criticality of the system.
