Access fittings are special fittings installed on piping and pipelines to facilitate insertion and removal of corrosion coupons, corrosion probes, chemical injection fittings, bioprobes etc. while the line is in operation.
High pressure access fittings provide access to piping systems under full operating conditions upto 6000 psi, eliminating any need for system shutdown in order to obtain vital corrosion data. Access fittings rated up to 10,000 psi are also commercially available.
Access fittings are available in the following types
- Flareweld (weldolet type)
- Buttweld
- Socketweld
- Threaded NPT
- Flanged
Special fittings using clamped connectors e.g. Techlok or Graylok connectors are also commercially available.
Access fittings are fitted with a solid plug assembly or hollow plug assembly. Fittings for corrosion coupons utilize a solid plug assembly. Hollow plugs are used with probes such as corrosion probes, bio-probes or hydrogen probes. Hollow plugs allow the probe device to pass through the plug. Access fittings for corrosion coupons or chemical injection fittings utilize a solid plug assembly.
Access fittings for Chemical Injection
Chemical injection fittings are installed on top of piping and at points of high turbulence. Chemical injection fittings have a tee connection with isolation valve to which the chemical injection line is connected. The tee can be a NPT thread, a Nipolet or a flanged connection. The tee pipe of the chemical injection fitting should be aligned with the axis of pipe to risk accidental damage to the fitting. The first pipe support must be taken from the pipe to which the injection fitting is connected.
Weight loss Coupons
Weight loss coupons are available in the following types :
- Strip type coupons
- Multi-disc coupons
- Flush disc coupons
- Ladder strip coupons
Bio Coupons
Bacterial activity can cause corrosion in pipe work. Hence monitoring of bacterial activity is important to assess the extent of bacterial induced corrosion. A wide range of bacteria like Sulphate reducing bacteria, cessile bacteria can cause a variety of corrosion problems.
Bio coupon assembly is a tool used for monitoring bacterial activity. The bio coupon assembly comprises of five small coupons installed in a coupon housing. Installed flush with the vessel/pipe wall, bacteria will colonize these coupons as they do the inner wall. A bio coupon holder is used to fix the bio coupons into the line or vessel in conjunction with solid plugs and access fittings.
Bio coupons are retrieved at fixed intervals and sent to laboratory for further analysis in specially designed air tight containers.
Installation
Access fittings for corrosion monitoring are generally installed in the 6 o'clock position where water and debris collection can cause maximum corrosion. A clearance of about 2 metres is provided along the axis of access fitting to allow installation of isolation valve and retriever kit. The clearance required are higher for large bore pipelines and hence it is important to check the clerances with the manufacturer. Sufficient clearance (workspace) should also be provided around the fitting. This could vary between 1000mm to 1500mm.
External Thread Protectors
External threads are required on access fittings to install the isolation valve and retriever kit. Hence it is important to provide external thread protectors. Thread protector caps are available in plastic and metal construction. Protectors are intended to prevent corrosion of threads and are generally not designed to retain the full operating pressure. Some operators use heavy duty covers which are suitable for the design pressure. Pressure gauge and bleed connections are located on the external covers.