Getting the right size of pipe tubing for your gas supply is critically important for safety. It is also important to prevent unnecessary gas leakage or damage caused by too much pressure in the piping system. When choosing tubing based on size considerations, you need to take into account both the minimum wall thickness of the tubes in addition to the average thickness.
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The size of steel pipe tubing is generally expressed as its outside diameter. This outside diameter of the pipe differs somewhat from the diameter of the pipe on the inside when the thickness of the pipe is taken into consideration. The pipe size is expressed as its nominal size, which is the minimum diameter of the inside tubing pipe, without the wall thickness. This number is always less than the outside diameter.
The average thickness of pipe tubing refers primarily to the actual wall thickness of the pipe. According to Supply Rite Steel Inc., this number is generally no less than 12.5 percent less than the nominal thickness of the pipe. To calculate the actual thickness of the pipe's wall, you can simply multiply the nominal thickness by 0.875. It is possible that the wall will be thicker, but it should be no less than this number.
Nominal pipe sizes generally range from 1/8 inch all the way up to 12 inches. Increments go up 1/4 inch between these two figures. The outside diameter of the pipe does not necessarily increase at a rate that is proportional to the rate of increase in the nominal size. It is possible for pipe size to exceed 12 inches, but when it does, the nominal pipe size and outside diameter are usually expressed as the same number.
Understanding the relationship between the outside diameter, nominal thickness and the average thickness of pipe tubing is important because it also directly relates to the working pressure that these tubes can withstand. As the outside diameter of the pipe increases, the allowable working pressure within the pipe decreases, unless there is a corresponding increase in pipe thickness. The thickness of wall pipes makes it possible for the larger pipes to withstand greater levels of pressure.