The water column of a certain height in the water trap is called "water seal". "Water seal" is a measure to use a certain height of hydrostatic pressure to resist the pressure change in the drain pipe and prevent the gas in the pipe from entering the room. In building drainage pipes, water seals are mainly achieved through traps. Generally speaking, the height of the water seal of the water trap shall not be less than 5cm.

PVC (Polyvinyl Chloride) pipe fittings are essential components used in plumbing systems to connect, redirect, or terminate PVC pipes. These fittings play a crucial role in creating secure and watertight connections, allowing for the efficient conveyance of fluids in various plumbing applications. Here are some common types of PVC pipe fittings:

1. Elbows: PVC elbows are fittings that allow changes in the direction of the PVC pipe. They come in various angles, with 90-degree and 45-degree elbows being commonly used. Elbows are essential for navigating around obstacles or changing the direction of the pipeline.

2. Tees: PVC tees create a T-shaped intersection, allowing the PVC pipe to branch in a perpendicular direction. They are used when a pipeline needs to split into two directions.

3. Couplings: Couplings are used to join two pieces of PVC pipe of the same diameter in a straight line. They provide a secure and straight connection.

4. Reducers: PVC reducers enable the connection of pipes with different diameters. They taper from a larger diameter to a smaller one, facilitating a smooth transition in the pipeline.

5. Adapters: PVC adapters are fittings designed to connect PVC pipes to pipes or components made from different materials. They often have different types of connections on each end.

6. Caps: PVC caps are used to seal the end of a PVC pipe, preventing the entry of debris, insects, or contaminants. They are commonly used in applications where the pipe terminates.

7. Unions: PVC unions consist of three parts: two female threaded ends and a central nut. They allow for easy disassembly of the pipeline for maintenance or repairs.

8. Crosses: PVC crosses provide a four-way intersection, allowing the pipeline to branch in both horizontal and vertical directions.

9. Bushings: PVC bushings are used to connect pipes of different diameters. They have one threaded end and one non-threaded end.

10. Plugs: PVC plugs are similar to caps but are designed to be inserted into the end of a PVC pipe. They can be threaded or unthreaded.

11. Wyes: PVC wyes are Y-shaped fittings that allow the PVC pipe to split into two directions at an angle.

12. Saddles: PVC saddle fittings are used to create branch connections on existing pipes, often for adding a new line.

These fittings are typically connected using PVC solvent cement or adhesive, creating a strong and durable joint. PVC pipe fittings are widely used in residential, commercial, and industrial plumbing systems due to their durability, corrosion resistance, and ease of installation.