This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
Sliding Joint is a sort of telescopic joint, and may be either hydraulic or else formed by means of a cork or hempen stuffing attached to the inner or sliding pipe, to allow it to work backwards or forwards in the outer one, which it keeps at the same time gas-tight.
A socket is a short tubular piece either plain, or diminishing, or reducing, with an internal thread which forms a double socket joint between two lengths of tubing when screwed on to their ends with a touch of white lead. A better joint, however, and one that is quite impervious, is made by mixing boiled linseed oil and red lead to the consistency of paint, taking off the socket, and painting over the screw ends of the tubing or barrel, then binding a thread of raw hemp round the screw, and finally screwing up all tight.
Stiff Joint is, as its name implies, a fixed joint, and in gasfitters' work signifies one that does not revolve like the ball or hang loosely like the swing joint.
Swing Joint occurs either in a swing pendant or in the ordinary single, double, or treble joint swing bracket. It is another name for the swivel joint described in the following paragraph, and is often used in piping when lateral motion is required.
This consists of a perforated plug moving in a sort of shell, and capable of turning completely round on it in one plane. It is the joint which affords motion in ordinary gas brackets. The plug is only kept in its seat by a screw, upon which too much pressure should not be permitted to fall. There are single, double, and triple swivel joints in common use, and it is somewhat singular that stops to prevent the burner from approaching too close to the wall are not more frequently attached to them. Swivel joints are liable to become more or less choked with tallow or white lead whereby the pressure of the gas at the burner is interfered with, and if the poverty of the light cannot otherwise be accounted for the joints should be taken to pieces and cleaned.
This has been described under the Smith, when it occurs in a main. In gas-fittings, whenever a new connection has to be made with a service, there is, perhaps, less chance of crippling it by inserting a proper T union in the way described under Union Joint, than by joining on the new pipe by merely soldering the two together, which is effected in much the same manner as explained under Branch Joint in Section X. The best way to insert a service into a main is to use the drill and drill clutch; then rimer, if necessary, and tap.
This, of course, is the result of good materials and good workmanship at the joint, added to careful bedding and fixing to prevent strain from disturbing the work. In the case of mains, leaky joints probably spring far more from settlement and vibration from traffic than from expansion, etc, from changing temperature. Gas pipes should not be let into walls, for, besides any corroding action that might set in, the joints are liable to injury from strains brought upon them through expansion or settlement. Small wrought-iron barrel for branches is not always approved of, because much rust sometimes accumulates in their interior and causes annoyance. Copper pipes are subject to the formation of a detonating compound within them, through the action of gas on the metal. Great care is needed in attaching heavy gasaliers, though with clean, accurately cut threads there is little chance of shearing occurring at the screw joints, the most likely accident arising from the bridging or bridgewood starting, or framing settling, and throwing a cross strain upon the stiff joints. Sometimes in screwing on a small pendant to a brass nose-bit the operation loosens the soldering at the other end, and if a short length of pipe, screwed at one end for the pendant and tinned at the other for soldering to the branch, be used instead, the same danger exists unless great care is shown in screwing up, for though there may be no leak at first, the traffic overhead may soon extend the flaw. The joints and fittings are tested by means of a small disc attached to the meter, the index of which should remain stationary, with the gas turned on at the meter but off at all the burners. The connections may likewise be tested by sucking the main or attaching an air-pump. Leakage sometimes arises from the arms of gasaliers being made with too thin metal which is still further reduced round and about the joints. To guard against this and similar dangers it might be advantageous to test fittings under a pressure equivalent to that of gas when balancing a column of water 2 in. in height, which is about double that found in gaspipes between sunset and midnight in many towns. A mixture of 7 parts of air and 1 part of gas is a fearfully explosive compound when ignited; hence the slightest odour of gas should be traced to its source and the leakage stopped, but if the odour be strong its neighbourhood ought not to be approached by a light unless there is every reason to believe that the proportion of air to gas is more than 11 to 1. Though the diffusion of gas is rapid, it would be sometimes dangerous to apply a light to the ceiling of a room when it could be held lower down with impunity.