Two kinds of ferrous pipe are common and used more or less interchangeably. One is galvanized steel pipe, and the other is wrought-iron pipe. The former is slightly cheaper, but good wrought-iron pipe is said to be more resistant to corrosion. However, as they look alike and their dimensions are the same, the amateur plumber need concern himself little over which he has but merely how to use it.
It is made in sizes from 1/8" to 20", the size being the inside diameter of the pipe, with an outside diameter slightly larger. These sizes are 1/8", 1/4", 3/8", 1/2", 3/4", 1", 1 1/4", 1 1/2", 2", 2 1/2", 3", 3 1/2", 4", 4 1/2", 5", and thereafter by even inches up to 20".
Steel or iron pipe can be used for all lines except underground lines, where cast iron is better. It is best for the cold-water system, but can be used for hot-water lines as well.
The only fittings for galvanized steel or wrought iron pipes which are made out of galvanized steel or wrought iron are nipples and couplings. All the various other iron fittings are made from malleable iron or cast iron. Malleable iron is cast iron that has been annealed a long time and can be bent or pounded a little. It can be broken by a sharp blow with a hammer but not as easily as cast iron. A wide variety of elbows, tees, reducers, couplings, unions and combinations are manufactured and provide for almost any contingency.
How to install
Galvanized steel pipe comes in lengths of 20 feet. This is as long as is generally necessary, and where it is not, longer pieces would be too unwieldy to handle. Pipe is threaded at each end, and threaded into fittings or couplings. If a piece of pipe is not long enough for the straight line distance, a coupling is used to connect it to the next piece of pipe. If the direction changes, a fitting is required. Pipe may be bent, but in good plumbing bending should be avoided.
Measuring threaded pipe
There are three general methods of measuring threaded pipe in plumbing installations: (1) end to end; ( 2) end to centre; (3) centre to centre. "End to end" means the measurement from one end of a piece of pipe to the other end. "End to centre" means the measurement from one end of the pipe to the centre of the fitting on the other end of the pipe. "Centre to centre" means the measurement from the centre of the fitting on one end of a piece of pipe to the centre of the fitting at the other end.
Centre to centre is the measurement most
often used by anyone who is familiar with the use of pipe. The essential
distance is from the centre of one elbow to the centre of the second elbow.
That distance will remain the same regardless of size of pipe. But the
size determines what length the pipe will be because the fittings and
thread engagement vary with the size of the pipe.
Allowance for thread engagement
Each time a pipe is threaded into a fitting there must be a certain allowance in length of the pipe for the length of the thread engagement. For instance, suppose we have a length of 1" pipe with an elbow at one end and a tee at the other, and it is 15" from the centre of the elbow to the centre of the T. The distance from the centre of the elbow to its face is1 1/2", and the distance from the centre of the T to its face is also 1 1/2". The thread I engagement of 1" pipe is 1 1/16". Thus if the centre to centre measurement is 15", the length of the pipe will be 15" Minus 1 1/2" at each end plus 1 1/16" at each end, or 13 3/8".If you are to cut a pice of pipe, it is better to thread one end and make up the fitting that is to go on that end ( see below) before figuring where to cut the other end. The reason is that in cutting threads you may cut one thread either too long or too short, so that if you cut the threads on both ends and had them only 1/8" wrong, there would be a total difference of 1/4", perhaps a troublesome error. If you have screwed the pipe into one fitting and measured from the centre of it, the other end is unlikely to be more than 1/8" off, and generally that is close enough to be satisfactory.
How to make up a screwed joint
To get a good metal-to-metal joint, you should first wipe both male and female threads clean with a wire brush after the pipe is secured firmly in a pipe vise. The male threads are on the pipe and the female threads on the inside of the fitting. Threaded pipe may be a little rusty from exposure to weather. If any of the threads have been damaged, running a tap or die over them will generally straighten them out and make the pipe usable.
Next, put a generous amount of thread lubricant, or "pipe dope" as it is generally called, on the male threads. This "dope" is not used as a cement to seal the joint, but to reduce friction while threading the fitting on. Put "dope" on the male threads only, as an excess might be squeezed into the pipe and do harm elsewhere to valve seats or faucets.
Put no dope on female threads. Start screwing on the fitting by hand, carefully feeling to see that the thread engagement is right. You can generally make about two turns by hand. Then use a wrench to tighten it further. A few turns with a wrench are enough. Do not lean hard with an oversize wrench and try to force the pipe too far into the fitting. You can mark the amount of normal thread engagement on the pipe before you start, so as to have some idea of how far to try to go.
Do not attempt to run all the male threads into the joint. The lead of the die, in cutting the threads, always leaves a few imperfect, partly-formed threads. It should never be necessary to use a lever or extension handle ( sometimes called a "hickey") on a wrench to make up a tight joint. Doing that or using an oversize wrench may stretch or crack a fitting. The only proper time to use a lever or oversize wrench is when taking down ( unscrewing) a joint that has been in place for some time and will not yield to normal pressure.
Installing pipe in angular directionsYou can use the table to determine the measurements of an offset. For example, assume that a 45° offset of 1 1/2" pipe is to be installed and that the distance the offset is to the "step-over" corresponds to the A measurement in the illustration. The A measurement in this case, centre to centre, is 20". In Table 6666, in the column headed "Offset," locate the figure 45°. Move over along that line until you come to the last column, which reads "When A equals 1, C equals" Since A is 20" instead of 1", multiply the number under C—which is 1.4142—by 20 in order to find the C measurement. The answer in this case is 28 1/4".
A malleable iron 1 1/2" 45° threaded elbow measures 1 1/2" from centre to end, and requires a 3/4" thread engagement. Taking 3/4" from 1 1/2" leaves 3/4", the allowance for one fitting in the total measurement. Take another 3/4" for the other end, or a total of 1 1/2". The 1 1/2" from 28 1/4" leaves 26 3/4", the total length of the offset piece of pipe.
Water rushing through a pipe may cause it to vibrate if the pipe is not anchored firmly, and vibration may in time cause leaks. Pipe should run up walls or along beams where it can be solidly supported, even at the cost of extra length and turns. In the cellar, for instance, a pipe should go straight up the wall to the ceiling, then turn with a 90° elbow, and go perhaps 2 feet along the ceiling to another 90° elbow, rather than being bent to go from one point to another in an arc without any firm support.
Theoretically, starting in the cellar, or wherever the water supply enters the house, you could install all pipe and fittings by simply adding one piece to another. This would be all right if it were never going to be necessary to disconnect pipes for repairs or alterations. It is much safer and easier, and good plumbing practice, to have unions at strategic points, so that you can disconnect sections of piping.
If no union exists near a point where eventually you wish to connect another outlet or have to make repairs, you can insert one at the time you do this work. Cut through the pipe with a hacksaw. Unscrew both pieces. Thread the cut end of one piece, screw on a T, a nipple, and one side of a union. Then cut the other piece of pipe, taken out, to shorten it by the distance taken up by the new fittings and nipple, thread that, and connect it to the rest with the union.