Cast iron drain pipe and PVC connection method: transition joint and sealing skills

In the renovation of old residential areas, home pipe maintenance or new house decoration, a difficult problem is often encountered - the original cast iron drain pipe should be connected with the new PVC pipe. Cast iron pipe is bulky and easy to rust, but has strong impact resistance; PVC pipe is light, corrosion-resistant, but afraid of vibration. The two materials are different, and direct socket is easy to leak. At this time, choosing the right transition joint and doing a good job of sealing becomes the key.

1. Transition joint: the core parts connecting cast iron and PVC

The connection between cast iron and PVC pipe is essentially to "adapt" the difference between the two materials with transition joints. There are three types of common joints, each suitable for different scenarios:

1. Flexible transition joint (most commonly used)

This type of joint uses a rubber ring as the sealing core, and the outer layer is equipped with a metal clamp. When installing, insert the cast iron pipe and the PVC pipe into both ends of the joint, the rubber ring will closely fit the nozzle, and then tighten the clamp to fix it. Its advantage is that it can resist vibration and adapt to thermal expansion and contraction, such as the impact of water flow in the bathroom drain pipe, or the pipe is slightly displaced due to ground settlement, and it is not easy to leak with a flexible joint. In home renovation, 80% of the scenarios choose it.

2. Rigid transition joint (low cost)

is usually made of PVC or metal, fixed by glue or thread. For example, PVC rigid joint, one end and PVC pipe are coated with special glue, and the other end is set on the cast iron pipe and locked with thread. This joint is firmly connected but not shock-resistant, suitable for scenes where the pipe position is fixed and there is no displacement - such as the rain water pipe on the balcony, or the concealed pipe in the wall.

3. Flange transition joint (for large pipe diameter)

consists of two metal flanges and rubber gaskets, which are welded to the cast iron pipe, glued to the end of the PVC pipe, and then tightened with bolts. Its sealing performance is the strongest, suitable for large pipe diameters above DN100 (such as the drainage supervisor of commercial buildings), but the installation is troublesome, professional tools are required, and the family uses less.

Second, the connection steps: the details decide not to leak

No matter which joint is selected, the installation process must be "slow work and meticulous work":

1. First deal with the nozzle

- cast iron pipe: use an angle grinder to grind off the rust layer and burrs, and then use sandpaper to smooth - rust stains will cut through the rubber ring, burrs will push up the sealing layer, be sure to grind clean; if there are cracks in the nozzle, first replenish the cast iron repair agent.

- PVC pipe: The cutting machine cuts flat, sandpaper polishes the outside of the nozzle (to increase glue adhesion), and finally wipes off the dust and oil stains with alcohol - even a little bit of ash will make the glue not stick well.

2. Choose the right joint size

The diameter of the joint should match the pipe exactly. For example, if the cast iron pipe is DN50 and the PVC pipe is also DN50, choose the DN50 joint. If the pipe diameter is different (such as cast iron pipe DN65, PVC pipe DN50), add a variable diameter joint, not a hard sleeve.

3. Installation points

- Flexible joint: The rubber ring should be set on the cast iron pipe first, and then insert the PVC pipe - make sure that both ends are inserted to the end (generally inserted 15-20cm), and then use a wrench to screw the clamp, the force should be uniform, not too dead (the clamp will leak when deformed).

- Rigid joint: PVC pipe should be "full" - apply a circle on the outside of the nozzle, and keep it still for 10 seconds after inserting the joint; the end of the cast iron pipe should be screwed to the end, and then apply a layer of sealant to reinforce.

- flange joint: the gasket should be placed straight, the bolt should be screwed diagonally - to avoid the offset of the gasket, resulting in leakage of the gap.

4. Pressure test inspection

Be sure to test the pressure after installation: open the faucet to fill the pipeline with water, or use the pressure test pump to hit 0.5MPa pressure (equivalent to 5 meters high water column), keep it for 30 minutes. If the interface seeps water, either the clamp is not tightened, or the size of the rubber ring is wrong, quickly adjust.

3. Sealing skills: avoid "hidden details" of water leakage

many people finish installing water leakage, the problem is in "small things":

- the nozzle should be "dry and clean": whether it is cast iron or PVC, the nozzle has oil stains and water stains will destroy the seal - wipe it with alcohol to ensure that it is completely dry and then installed.

- Rubber ring do not "aging": When buying a joint, check the rubber ring for cracks, hard (poor elasticity of the aging rubber ring), it is best to choose EPDM rubber (anti-aging).

- Don't "eccentric" the clamp: The clamp of the flexible joint should be aligned with the position of the rubber ring, and the two sides should use the same force when screwing, otherwise the clamp will be crooked, and the rubber ring will be squeezed to one side and leak!

- The gap should be "filled": If the mouth of the cast iron pipe is not round, or there is a gap between the joint and the pipe, fill it with sealant (such as silicone weathering glue), and then test the pressure when the glue dries.

4. Precautions: Avoid these "pits"

- Don't use "direct socket": Some people insert the PVC pipe directly into the cast iron pipe in order to save effort, without the joint - this thermal expansion and contraction will cause the gap to become larger, and it will leak in less than half a year.

- Don't choose "inferior joint": Cheap flexible joint clamps are made of iron, which are easy to rust and break; rubber rings are recycled materials and will age after a few months. Try to choose the brand (such as United Plastics, Weixing), although expensive, but can last for ten years.

- Don't forget "thermal expansion and contraction": PVC pipes will expand in summer and shrink in winter. When installing, the joint should leave 1-2cm of "expansion and contraction space". Don't insert it too full, otherwise the expansion will crack the joint.

Finally, I want to say that the connection between cast iron and PVC pipes is the core of "adaptation" - choose the right type of joint, do a good job of pipe mouth treatment, and seal the details in place, which will basically not leak. If you are not sure, find an experienced plumber to help, which is much easier than repeated maintenance. After all, the trouble of pipe leakage is far more than just wiping the floor.