Steel pipe is the best pipe to use for a spud gun. It is threaded and easy to use. The only downsides are that it is heavy and expensive. Steel pipe comes in two types: black and galvanized. For a spud gun, both can be used, and the two types can be mixed.
Steel pipe is assembled by applying Teflon tape or pipe dope to the threads of the steel pipe. A fitting is then threaded onto the pipe, and a pipe wrench is used to tighten the parts. The joint can be pressurized right away for Teflon tape, or can be pressurized once the pipe dope is cured if you are using pipe dope. Steel pipe can be threaded to other threaded parts with the same process. A pressure rating chart can be found here: Pressure Ratings. Reduce the working pressure for threaded pipe by 50 percent. If you need to cut and thread steel pipe, you can cut it with a reciprocating saw (make sure it is square!) and a metal cutting blade. Threading pipe can be done with a threader. The process is similar to tapping. A threader kit is about $50 online, but you may be able to rent one.
Advantages and Disadvantages
Steel pipe has the following advantages:
- Readily available,
- Easy to assemble,
- High pressure rating,
- Rated for compressed gases,
These are the disadvantages:
Obtaining Steel Pipe
Steel pipe can be found in sizes from 1/8 inch to 2 inch at most hardware stores. Larger sizes (2.5 inch, 3 inch and 4 inch) can be found at industrial supply stores or online. Not all hardware stores carry steel pipe that is under 1/2 inch, but most do. Most hardware stores stock black iron pipe from 3/8 inch to 1 inch and galvanized from 1/8 to 2 inch. Nipples (short threaded pipe sections) are available form close (very short and used to connect two fittings) to 12 inch. Precut and threaded steel pipe is found in lengths from 18 inch to 21 feet. Usually precut pipe is only available in 1/2 inch to 1 inch. Most hardware stores will cut and thread steel pipe for you for a fee.
Fittings for steel pipe come in three materials: Cast Iron (CI), Malleable Iron (MI) and forged steel. Most fittings at a hardware store are Class 150 MI fittings, meaning that they are rated to 300 PSI. Industrial supply stores and online shops will often carry Class 300 MI fittings. These fittings are rated to 2000 PSI for sizes up to 1", 1500 PSI for 1.25" to 2", and 1000 PSI for 2.5" and higher. Forged steel fittings can be found online, and are rated to extremely high pressures and will most likely not be used for spudguns. Cast iron fittings are rated to 250 PSI for Class 125 and 500 PSI for Class 250. These fittings are heavy, brittle and expensive so they are not recommended for spudguns. Cast iron fittings are usually only found at industrial supply stores and are used mostly for steam. They are very thick to make up for the weak and brittle nature of cast iron. These are the markings found on CI and MI fittings:
- No marking and low profile: Class 150 MI.
- No marking and very thick: Class 125 CI.
- 300 WSP: Class 300 MI.
- XXXX WOG: Class 300 MI. The WOG is the pressure rating.
- 250 WSP: Class 250 CI.