Good pipe requirements; expansion joint

Requirements of a good pipe

1. Strength: withstands internal pressure, water hammer and external loads (soil, traffic).
2. Durability: long life, resistant to corrosion, chemical attack from soil and water, and to abrasion.
3. Smooth interior: high Hazen-Williams C value, low friction loss, and no growth of deposits.
4. Safe for water: no taste, colour, toxic or bacterial effect on the water carried.
5. Easy joints: watertight, flexible and quick to make.
6. Light and easy to handle and transport; available in many sizes and lengths.
7. Low cost, including laying and maintenance, and locally available.
8. Easy to repair, to tap for connections, and to extend later.

Expansion joint

Pipes expand when the temperature rises and contract when it falls. In a long, rigid line this movement causes stress, buckling or joint leakage. An expansion joint lets the pipe move freely along its axis.

• Purpose: to absorb the thermal expansion and contraction, and small ground settlement, without damaging the pipe or leaking.
• Use in CI pipes: provided on long straight exposed pipelines, pipes on bridges and piers, near valves and pumps, and where pipes cross building joints. Spaced at about 30–50 m.

       bolt             gland        packing
     +-----+        +---------+   ____|____
 ====|     |=======|  gland   |==|  socket  |====
 pipe  spigot slides <-->      lead/rubber ring
        spigot end moves inside the socket

• The spigot end of one pipe enters a special socket (stuffing box). A rubber or lead packing fills the annular gap and a gland pressed by bolts compresses it to keep the joint watertight.
• When the pipe lengthens or shortens, the spigot slides in the socket by 10–20 mm, while the packing keeps the water in.