A Diagnosis & Repair Guide for Leaky Oil Tank Filler Piping
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• What causes leaks at oil tank filler piping?
• What to do about leaky oil tank vent or filler piping
• Questions & answers about how to diagnose & repair leaks at the oil tank fill or vent pipe

The pipe threads (NPT) which are cut into an oil tank top tapping intended to accept the filler pipe or vent pipe are not precisely machined - they are a bit rough.

So are the threads on most large-diameter iron piping used to screw into the tapping.

Having installed a few of these, we can testify that it requires some very careful workmanship to make a leak-tight joint, combining cleaning of the threads, use of a high quality pipe dope rated for use on petroleum products, and turning the pipe into the tapping with sufficient force to seal it without damaging threads. In sum, often the connection is simply not very tight.

• Find the exact point of oil leakage: If you see oil on the floor, check the bottom of the tank, sides and welded seams, and be sure to check the oil lines to the boiler and in particular, check the oil tank safety valve or other tank bottom fittings as there are often leaks at these valves, as shown in this photograph.
  
  Before addressing tank top filler pipe fittings as a leak source, make sure the oil leakage is from the top of the oil tank. Other tank leaks such as at a seam or at perforations indicating a rusted tank need immediate evaluation by an expert, since these could spell major trouble like a big and costly oil spill should the tank break or a fitting blow off.
• Check for more than one leak point: Make sure that you know where the seepage is coming from. There may be separate spillage outside at the filler pipe from that occurring inside at the tank top or other fill or vent pipe fittings.

• Check condition of oil piping: Make sure that the piping you're using is proper in material (steel is safer against rupture than plastic), and in diameter (2" or larger) for both vent and fill pipes. [The soldered copper fill and vent piping at the oil tank in the photo just above was unusual and may not be permitted. At the top of this page, the silver-colored (galvanized steel) oil tank vent pipe in the tank is probably too small for modern tank fill methods.]
  
  An older-installed small diameter vent or fill pipe [like the one shown at the page top photo] risks overpressuring the tank during fill-up, and risks a costly oil spill should an old tank or fill or vent line rupture. Ask your oil company to review the condition of the piping and the condition of the tank.
  
• Small oil leaks: If the seepage is trivial in amount, say simply a light stain just a few inches around the tank top, clean it off and live with it. Or re-make the joints when other plumbing work is planned. (It's quite a bit of trouble to re-make these joints, especially if the filler and vent piping have been routed through and cemented into a masonry wall.)
  
  On our oil tank which recently "developed" a seep after our oil company bought a new, improved, higher pumping pressure delivery truck, I cleaned the tank surface and tied a rag around the filler pipe at the top of the tank to try intercepting the leak before it spreads over the tank and smells up our garage. I am doubtful that this will be effective long term but changing the rag after each tank fill was easier than scrubbing off the whole side of the tank.

• Significant oil piping leaks: If the seepage is enough to wet the tank or run down the whole tank and drip onto the floor this is an unacceptable condition that needs correction.
  
  It's likely that the piping will have to be removed and (possibly new) piping installed.
  
  I am doubtful that any "band-aid" approach like pushing epoxy putty or other sealers around the existing fitting is going to work. The surfaces will be rough, rusty, and oily.
  
  If a reader or product manufacturer has a sealant that is actually effective in this application, dealing with the rust, oil, surface shapes, and fill pressure, I'll be glad to report on that product here. It would indeed be less trouble than re-piping the tank.
  Make sure the new piping is properly sealed and secured. If heating oil has been dripped on the floor, ask your oil company to clean it up and deodorize the area.

• Actual oil spills: If there is an actual oil spill, indoors or out, most state departments of environmental protection require that they be notified within 24 hours of the spill, an inspection is performed, and an expert specifies the amount of cleanup needed.