Galvanized steel pipes are joined by two main methods: welding and threaded connection using pipe clamps and fittings. Both methods give a reliable joint when the pipe ends and the threads are prepared correctly, and both produce a recognisable set of defects when they are not. This article describes the defects seen in each method, why they occur, and the corrective action that removes them.
Welded Connection Defects
Welding is used for fixed lines where the joint must be permanent. The quality of the finished joint depends on the alignment of the two pipe mouths and on the welding technique, so most defects come from the pipe-end preparation or from the welding operation itself.
Pipe mouths not in a straight line. When the two ends meet at an oblique angle instead of square, the joint line is distorted. The remedy is to cut off a short length from the pipe head before processing, so that two clean square faces meet.
Pipe mouths not tightly butted. If the two ends do not abut fully, the molten metal fills an uneven gap and the welded joint ends up with uneven thickness. Cutting back a short section of the pipe head again gives a proper square face.
Oval openings. Handling, stacking or transport collisions can distort the pipe end into an ellipse, leaving a gap along one side of the joint. Cutting off the deformed section restores a round, matching mouth.
Sand holes at the pipe mouth. Sand holes appear when the welding technique is at fault, for example wrong current, poor arc control or contamination at the joint. They are corrected by the welder rather than by cutting the pipe.
Threaded Connection Defects
Threaded joints are used where a line must be opened for maintenance. The pipe is threaded on site or in the workshop and then tightened with a pipe clamp or a fitting. Three defects are most common.
Thread misalignment. The pipe clamp and the thread cannot make full contact, or the joint stays loose. The cure is to cut off the misaligned thread and cut a new one.
Thread and clamp mismatch. The pipe thread and the pipe clamp thread do not match, so the two parts cannot be joined at all. The clamp is replaced, or the equipment is adjusted and the thread recut.
Leakage after threading. When a newly threaded joint leaks, the cause is often insufficient wall thickness. The wall thickness of the pipe is measured against the thickness required for the threaded pipe; if the pipe is too thin for the cut, it is replaced with a heavier-wall pipe.
Choosing Between the Two Methods
Welding gives a continuous, leak-tight joint and is normally chosen for buried or fixed runs, while threaded connections are chosen where dismantling is needed or where hot work is not permitted. The preparation requirement is the same in both cases: a square, undamaged pipe end and a matched, correctly cut thread. Checking the pipe-end geometry and the wall thickness before work starts prevents most of the defects listed above, because both welding faults and threaded-joint leaks trace back to the condition of the pipe end rather than to the joining method itself.
FAQ
Q: Why does a welded pipe joint end up with uneven thickness?
Because the two pipe mouths are not tightly butted, so the weld fills an uneven gap; cutting back a short section of the pipe head gives square faces that meet properly.
Q: What causes an oval opening at a galvanized pipe end?
Handling, stacking or transport collisions distort the pipe end into an ellipse, and cutting off the deformed section restores a round mouth.
Q: What are sand holes at the welded pipe mouth?
They are voids left by a faulty welding technique, such as wrong current, poor arc control or contamination at the joint, and the welder corrects them.
Q: How is thread misalignment fixed?
The misaligned thread is cut off and a new thread is cut, so that the pipe clamp and the thread can make full contact.
Q: Why does a threaded joint leak after threading?
The wall thickness may be too thin for the cut; it is measured against the thickness required for the threaded pipe, and a heavier-wall pipe is used if necessary.
Q: What if the pipe thread and the clamp thread do not match?
The clamp is replaced or the equipment is adjusted and the thread recut, because mismatched threads cannot be joined.