When a line weeps at test pressure the cause is rarely the sealant. It is usually thread length.
A fitting weeps during hydrostatic test. The first reflex is to add more sealing tape. The second is to pull the wrench a little harder.
Neither solves it, because the problem is usually in the thread itself.
How a tapered thread seals
The tapered threads used on pipe connections (BSPT / R series) have a 1:16 taper — the diameter narrows by 1 mm every 16 mm. The seal does not come from a gasket; it comes from the flanks of the male and female threads wedging against each other.
Which means thread length has to be right. A thread cut too short stops the fitting before it seats — the tightening load lands on the thread crests instead of the flanks. A thread cut too long lets the fitting run all the way in, and the connection bottoms out before the taper engages.
In both cases the line will not hold test pressure, no matter how much tape is wound on.
What ruins thread length
On a hand-operated die stock, the operator decides the thread length. They decide when to stop. Measure at the start of the shift, then watch what happens once the pace picks up — this is the first thing to go.
An automatic die head takes that decision away from the operator. The chasers open by themselves once the set length is reached. The tenth thread on a given diameter comes out the same as the first.
Three other common mistakes
- Cutting dry or with insufficient oil. The chaser overheats, the thread flank tears. A torn flank leaks, and chaser life halves as well.
- Not centring the pipe. If the vice and head are out of line, the thread runs deep on one side and shallow on the other.
- Leaving the burr. Internal burr left after cutting travels into the line and causes trouble at valves and pumps.
Choosing chaser sets
Chaser sets are grouped by diameter range, with different geometries for carbon steel, stainless and galvanised pipe. On galvanised pipe, a chaser that does not strip the coating prevents rust appearing later.
Do not run chasers until they are blunt. Once you start seeing tearing on the thread flank, change them; every thread cut after that point is suspect.
An easy way to check
You do not need an expensive gauge. Keep one reference fitting. Thread it on by hand and count the turns. On a thread cut to the right size, that count comes out the same every time.
When it starts to drift, stop the machine and check the setting. It is far cheaper than cutting a whole run and taking it to test.
PRODUCT PXVL-2760T Pipe Threading Machine The equipment we supply for the work described in this note