Working with black pipe is virtually the same as working with galvanized pipe.
Working with black pipe is virtually the same as working with galvanized pipe. The pipe used must be new or, if reused, used previously only for gas fittings.
Black pipe threads are cut in a tapered manner referred to as National Pipe Taper (NPT). The diameter of the male-threaded pipe is smaller at the end. This is why it is easier to thread a fitting on initially, but gets more difficult with each turn. MPT and FPT refer to male and female threads cut to this standard.
All threads should have pipe joint compound applied before being fitted and all joints must be tested with leak detector solution once the installation is complete. Pipe compounds may be gray, thick paste, or white PTFE (Teflon) paste. You may also use yellow PTFE tape. White PTFE tape that’s commonly used for waterlines and hookups is not acceptable for gas line use.
After completing an installation and turning the gas on, each connection must be checked for leaks. Use leak detecting solution sprayed around each joint. If gas is leaking, the solution will bubble and foam. Do not use detergent as a leak detector—it contains corrosive chemicals that may degrade the connection. Tightening the leaky joint will mean that all subsequent joints need to be tightened, not loosened, to accommodate the new alignment. Loosening will potentially create more leaks.
Black pipe is available in a wide variety of threaded lengths. Shorter lengths are referred to as nipples. If you can’t make the standard lengths work for your application, most pipe retailers have thread cutting machines and will cut and thread pipe to length, usually for a fee. Black pipe fittings include Ts, reducers, elbows with two female-threaded ends, street elbows with a male- and a female-threaded end, couplings, and caps.
Shut off the gas by turning the handle of the nearest in-line stopcock so it is perpendicular to the gas line.
Apply an approved gas pipe thread compound liberally all over the threads.
Hand tighten the fittings on to the threaded pipe as far as you can.
After hand tightening, turn the fitting or pipe at least one full turn to tighten. In order to achieve the proper alignment, you may tighten up to two full turns, but do not overtighten. Use one pipe wrench to stabilize the fixed pipe or fitting, while using the second wrench to tighten the movable pipe or fitting.
From: The Complete Guide to Plumbing, 978-1-58923-378-2