🧿1. Sealing Principle of Socket-and-Spigot Connections

Socket-and-spigot connections (especially those with rubber ring seals) rely primarily on the elastic deformation of the rubber ring to fill the gap between the pipe socket and spigot, thereby achieving a seal. Ideally, the spigot is inserted into the socket.
The rubber ring is compressed, generating a continuous, uniform rebound force, tightly adhering to the metal surface and blocking water flow.

Causes of Leakage Under 10 kg Pressure

2.If leakage occurs under 10 kg (1.0 MPa) pressure, it indicates an imperfect seal. Possible causes include:

Improper installation of the rubber ring: It may be slightly twisted, partially not fully seated in the groove, or the initial compression may be insufficient.

Uneven joint clearance: The socket or spigot may have slight manufacturing tolerances, ovality, or minor surface defects (such as burrs, dirt, or scratches), resulting in localized gaps. Excessive lubricant or the wrong lubricant can cause the rubber ring to fail to secure properly during installation, causing it to "blow" or "shift" under pressure.

In this case, the 10 kg water pressure finds these weak seals and successfully forces water through these tiny channels, creating a visible leak.

3.Why does the leak stop when the pressure is increased to 20 kg?

When the pressure is increased to 20 kg (2.0 MPa), the entire system changes:

  1.The rubber seal is further compressed and deformed.

Higher pressure pushes the rubber ring more forcefully toward the leaking gap.

Rubber is an elastic polymer material that deforms more under higher pressure. The tiny gap that was not completely filled under 10 kg pressure is filled and blocked by the "extraordinary" elastic deformation of the rubber ring under 20 kg pressure, thus closing the leak path.

  2.Elastic deformation of the pipe and fittings themselves (a key factor)
Pipes (such as ductile iron pipes, steel pipes, and even some thick-walled plastic pipes) and sockets are all elastomers. Under high internal pressure, the entire piping system will experience a slight expansion.
The male end of the spigot will experience a slight radial expansion (becoming slightly thicker) due to internal pressure.
The female end of the bell will also experience a slight radial expansion due to internal pressure.
This expansion process has a key effect: it reduces the clearance between the spigot and the bell, even creating a tighter "interference fit." This effectively provides an additional "holding force" to the rubber ring from the outside, greatly enhancing the sealing effect.

  3.Changes in the System's Overall Quality
Higher pressure will "press" the various components of the pipe joint (spigot, rubber ring, and bell) together more tightly, potentially eliminating minor misalignments caused by installation stress or insufficient external support, and ensuring a better fit between the sealing surfaces.

A simple analogy: It's like gently pushing on a loose door. The door will leak (leakage under 10 kg pressure). But if you push harder, the door frame and door body will slightly deform due to the impact, tightening the seal even more tightly (leakage under 20 kg pressure).

Important Warning and Subsequent Action Recommendations

While this phenomenon can be theoretically explained, it is by no means a problem that can be ignored; rather, it is a red flag.

Abnormal Condition: A qualified socket connection should pass a single pressure test at the design pressure (even 1.5 times the design pressure) and have leakage within the permitted range. This "leakage at low pressure, no leakage at high pressure" situation indicates a flaw in the initial installation.

Potential Failure Risks:

Poor Pressure Cycling Reliability: When the system pressure drops from 20 kg to the operating pressure (e.g., 8-10 kg) during routine operation, elastic deformation recovers and leakage is likely to reoccur.

Fatigue Failure: Excessive compression and deformation of the rubber ring under high pressure may accelerate its aging and shorten its service life. After multiple pressure cycles, it may suddenly fail during a pressure drop.

Safety Hazard: This indicates a potential risk in the joint. In the event of water hammer, earthquakes, or other external loads, it may be the weakest link and the first to fail.

🧿The Correct Action:

Immediate Pressure Relief: Reduce the pipeline pressure to zero.
Inspection and Repair: Identify the leaking connection, disassemble it, and carefully inspect the socket, spigot, and rubber ring for damage, dirt, or signs of improper installation.
Reinstall: Clean the contact surfaces, use the correct lubricant, and reinstall the rubber ring and connection according to specifications.
Retest the pressure: Slowly increase the pressure from zero and observe for leaks throughout the entire pressurization process (especially when reaching 10 kg). Ensure that the connection remains leak-free at the test pressure.

In the case that the water supply cannot be stopped, or the pipe is too large to be reinstalled, a large-diameter pipe repair clamp can be used for repair. For example, a DN1800 socket pipe repair clamp corresponding to the DN1800 socket pipe can be used to wrap the entire socket for repair to achieve the effect of plugging the leak.