System – Natural Gas & Fuel Oil

• Overview

Natural gas systems are essential to provide fuel to boilers as well as for residential tenant appliances.  Oil is another widely used fuel.  For both, the carbon steel piping carrying the oil or gas is generally considered maintenance free since the gas which travels through the pipes is noncorrosive as well as the oil.  Oil, in fact, is applied to metal surfaces to stop corrosion.  While the presence of water or moisture in gals or fuel oil lines is possible, it is generally rare today.  Large oil storage tanks can collect water or moisture at the bottom over time, although not within the pipe itself.  Natural gas systems will have drop legs to collect any condensed moisture.

• Common Problems

Limited problems commonly exist for natural gas or fuel oil piping systems in terms of the piping itself.  Due to the typically low pressures provided, most such systems utilize standard or schedule 40 pipe.  For smaller fuel oil piping systems which are threaded, the lubricity of the oil and its ability to eventually migrate through the thread sealant results in the threaded connections being welded over.  Internal corrosion activity is virtually nonexistent for the most part, with the greatest significant risk for corrosion activity to occur externally.

• Major Threats

While water damage is a significant concern for building owners in terms of property damage, the potential for a natural gas leak raises a much greater risk toward life safety.  Natural gas is extremely flammable and raises an extreme risk if neglected.  It is common for residential buildings to feed natural gas through crawlspaces or other similar locations which are out of sight and therefore out of mind.   To further raise this threat, natural gas or fuel lines running through a mechanical room area or occupied space will always be painted for cosmetic reasons and marked according to its service.  The very same pipe traveling through more unfavorable environments such as a cellar or moisture filled crawlspace, however, will always remain bare steel.

Any pipe suspended from the ceiling of a mechanical room is generally under little to no threat of external corrosion loss.  That condition dramatically changes as the pipe moves through walls or building foundations where water infiltration is common.

In the photograph at left, years of water infiltrating through the foundation wall had severely deteriorated the outside surface of this 6 in. schedule 40 natural gas line.  The building’s response to paint over the rust product accomplished nothing, as expected, with the pipe exhibiting stratified layers of rust indicating a large wall loss over an extended area.

Called in to ultrasonically determine the condition of the pipe would have required mechanical grinding away of the rust product, which we declined for obvious reasons.  Instead, we made the recommendation to immediately replace the line through the foundation to the outside given the expectation that water reaching through the foundation to create this condition had produced an even greater impact to the pipe exposed to the outside environment.  We also suggested the alternative of running a new gas supply line into the building.

At left, approximately 100 natural gas lines of 3/4 in. steel pipe run parallel through a dirt floor crawlspace where external corrosion was noticed by a building inspector.  As expected, all pipe in the gas meter room was painted, while all pipe within a far more hostile environment was not.

Nevertheless, the surface rust was extremely minor and easily wire brushed away for ultrasonic testing to measure little to no wall loss.  With all pipe still suitable for extended service, we recommended rehabilitating the pipe by removing the rust, coating it with a rust reverter, and then painting it over with a final coat of a high solids waterproof paint.

In another more dramatic example, the failure of a fuel oil line serving the bank’s emergency generators revealed severe corrosion under the insulation installed to protect it.  Although the insulation was installed primarily to keep the heat traced oil lines warm during the winter months, it was never properly sealed at the seams and had very obvious gaps at various seams and pipe hangers.

Inspection of the failed pipe section revealed areas of massive cratering to the outside pipe surface caused by water infiltration.

Below left we show an inside view of a failed section of 1.5 in. fuel oil pipe.  Inside pipe conditions are pristine and indicate no corrosion activity whatsoever.  Yet the pipe still produced a leak.  At center, conditions of the exposed pipe showing the insulation wet and fragmenting, and rust product embedded into the insulation itself.  At right, a substantial crater caused by nothing other than poor insulation; a pinhole at its direct center.

• Testing Focus
  

Natural gas and fuel oil piping systems are rarely the main focus of any ultrasonic examination unless decades of corrosion have finally suggested to someone that a problem may exist.  Where the ultrasonic inspection of another piping system is being conducted in the vacinity of natural gas lines to the boilers, for example, CorrView will often address 1 or a few sections of natural gas pipe just to remove any concerns.  Specific areas of interest are:

• • • Bottom natural gas drip legs
    • Crawlspaces
    • Pipe showing obvious surface rust
    • Exposed fuel lines to generators
    • Outdoor insulated natural gas & fuel oil lines

If natural gas piping is known to travel through any building areas which may expose it to humidity or moisture it should be monitored and inspected for external corrosion. Should any pipe sections fail, immediate replacement should be performed.  Building owners should always consult with their professional engineers and local gas company to determine whether specific protocols are defined by local ordinances.

© Copyright 2023 – William P. Duncan, CorrView International, LLC