There has been considerable interest generated in increasing the capacity of existing propane vessels to maximize payloads and save money over the purchase of new vessels. In as much as we are registered with the Federal DOT and also have both ASME "U" and National Board "R" stamps, we looked into the possibility of doing this work for some of our customers.
We have investigated stretching both 72" ID and 80" ID vessels. Existing 72" vessels are usually limited to 3000 to 3200 water gallon capacity, but smaller vessels of this diameter could be resized into 3000 gallon tanks. To stretch these tanks much beyond this size would necessitate the use of a longer than normal wheelbase chassis. This would obviously increase truck overall length, but would also increase chassis turning radius. On the other hand, the 80" diameter tanks can be enlarged all the way to 3500 water gallons, or even beyond if so desired.
Part 180 of the DOT regulations addresses this type of modification, listing the various requirements involved in performing this work. Here is a list of operations and documentation we would provide for each vessel modified to satisfy the appropriate regulations:
- Generate a sketch of the vessel as received, showing all of the original design parameters.
- Perform all the required calculations to enlarge the vessel including:
- ASME shell and head thickness requirements.
- ASME reinforcement calculations for installation of the new manway assembly.
- DOT relief valve calculations based on increasing the outside surface area of the vessel.
- DOT structural integrity calculations for new vessel capacity.
- Generate all NBIC Code documents including Form R-2, report of vessel alteration.
- Modify vessel by installing new shell section and manway assembly. Manway to go in rear head.
- Radiograph (x-ray) new welds as required by the ASME Code section VIII division 1.
- Pressure test tank to 1.5 times design pressure with Authorized Inspector to witness testing and sign appropriate Code documents.
- Install Alteration nameplate below original manufacturers nameplate.
- Install new vessel supports (sled runners) to suit new mounting length (full length).
- Determine that the completed vessel complies with current DOT requirements.
- Provide new Certificate of Compliance signed by Design Certified Engineer and Registered Inspector.
- Forward copies of all documentation to customer.
The estimated cost for extending an existing 80" ID vessel to 3500 water gallons is $7,500, excluding taxes and/or transportation charges that may apply. For comparison, the list price of a new 3499 water gallon propane bobtail tank, FOB the factory, is around $17,150.
Each vessel considered for stretching would be evaluated as received, before starting the modification, to check for external corrosion, additional nozzle requirements, other desired design changes, etc. Because the tank will be tested and Code Inspected, this would be an excellent time to upgrade to a float gauge, or change from a 2" threaded bottom outlet to a 3" flanged bottom outlet. Of course these changes would be at additional cost.
If you think that this approach to vessel recycling is interesting, please give us a call to discuss your questions, concerns, and needs.
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