Conveyance of Water

Laying of pipes should be done carefully so that the line is watertight, correct in line and level, and safe from damage.

Steps in pipe laying

1. Survey and setting out: the alignment and levels are marked on the ground from the drawings.
2. Excavation of trench: width = pipe diameter + 0.3–0.6 m; depth gives a cover of 0.75–1.0 m (more under roads). The trench bottom is graded as per the pipe gradient. Dewatering and timber shoring are used in loose or wet soil.
3. Bedding: a 100–150 mm layer of sand or fine gravel is laid and compacted at the bottom.
4. Handling and lowering: pipes are inspected for cracks, cleaned inside, and lowered carefully by ropes or cranes, not dropped.
5. Laying: pipes are laid from the lower end to the upper end, with sockets facing up the slope, to the correct line and grade, checked with boning rods or a level. Air valves are placed at high points and washouts at low points.
6. Jointing: the joints are made as per the pipe type: socket-spigot with lead and yarn, flanged with gasket and bolts, solvent cement for PVC, fusion for HDPE/PPR, collar or screwed for GI.
7. Anchorage: concrete thrust blocks at bends, tees, dead ends and valves to resist the thrust.
8. Testing: after joints cure, sections of about 500 m are filled with water, air is released, and a hydrostatic test is done at 1.5 times the working pressure for a few hours. Leaks are repaired.
9. Backfilling: sand around the pipe, then soil in 150–200 mm layers, compacted; no stones near the pipe.
10. Flushing and disinfection: the line is flushed and disinfected with chlorine (about 20–50 mg/L for 24 hours) before the supply starts. Finally the surface is restored.

Three materials which are commonly used for water supply pipes are cast iron / ductile iron (CI/DI), galvanised iron (GI) and polyvinyl chloride (PVC) / HDPE plastic.

Steel, concrete and asbestos-cement pipes are used for large mains but are less common in small Nepali schemes.

Why pipe joints are required

Pipes are made in short lengths (3–6 m), so joints are needed to connect them into a long line, to join fittings and valves, to change direction, and to keep the line watertight while permitting repair or replacement.

Socket and spigot (bell and spigot) joint

This is the common joint for cast iron (CI) and ductile iron (DI) pipes. One end of each pipe is a socket (bell) with an enlarged inner diameter, and the other end is a plain spigot.

     socket (bell)            spigot
   __________________    ___________
  /  ______ lead ___ \  /          \
 |  | yarn   |lead |  |<----spigot enters
  \__|_______|_____|__/
        gap about 10 mm

Making the joint

1. The spigot of one pipe is inserted in the socket of the previous pipe, and centred, leaving an annular gap of about 10 mm.
2. The gap is filled to about half its depth with spun yarn (hemp) or gasket, rammed (caulked) tightly.
3. The rest is filled with molten lead poured at once and caulked, or with a cement mortar (1 : 1) in some cases. A rubber ring can also be used (push-on joint).

Features

• Merits: easy and quick to make, flexible to a few degrees of deflection, and it takes a small settlement.
• Demerits: not suitable for pipes under high pressure without special thrust blocks; skilled labour is needed for lead jointing; it is difficult to dismantle; and the pipes are laid with the socket facing uphill.

Why joints are required

1. Pipes are made in limited lengths (3–6 m), and must be joined to form a long line.
2. To connect pipes with fittings, valves, pumps and meters, and to change direction or diameter.
3. To make the line watertight and allow small movements, and permit repair and replacement.

Expansion joint

An expansion joint lets a pipeline lengthen or shorten with temperature changes without damaging the pipe or leaking. It is used on long, exposed straight pipes (such as on bridges), near valves and pumps.

  pipe ==[ socket | packing | gland ]== pipe
                      ^ bolts
         spigot free to slide in the socket

Working

• One pipe has a plain spigot end; the next has a special socket (stuffing box) containing a rubber or lead packing.
• A gland ring pressed by bolts compresses the packing around the spigot, so the joint is watertight.
• When the temperature rises, the pipe expands and the spigot slides further into the socket; when it falls, it slides out. The packing keeps tight in both cases, so the pipe is free from temperature stress.

A pipe material for water supply should satisfy the following requirements.

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.

Selection criteria in a water supply scheme

• Pressure and head: the working pressure and the surge pressure decide the class (wall thickness) of pipe. Steel or DI is used for high pressure; PVC/HDPE for medium pressure.
• Water quality and soil: aggressive soil or water favours plastic or lined pipes.
• Diameter and flow: large diameters suit DI, steel or concrete; small diameters suit GI, PE, PVC or PPR.
• Terrain: light flexible pipes (HDPE) suit hilly and landslide-prone ground; exposed crossings need GI or steel.
• Cost, availability and skill for laying and repairs; life of the scheme.

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.

Expansion joint

An expansion joint is a special joint which permits the pipe to expand and contract freely due to temperature changes, without causing leaks or stress.

• Use: on long straight lines of exposed CI/steel pipe, on bridges, and near pumps and valves.
• Construction: the spigot end slides inside a socket (stuffing box); a rubber or lead packing is compressed by a gland and bolts.

   pipe ====[ socket | packing | gland ]==== pipe
                          ^bolts
        spigot slides in and out (10-20 mm)

Flanged joint

In a flanged joint, flanges (circular plates with bolt holes) are cast, welded or screwed on the ends of the two pipes. A rubber, rubber-fabric or lead gasket is placed between the faces, and bolts and nuts are tightened evenly.

   pipe ===|F|gasket|F|=== pipe
           | |  ||  | |    F = flange, bolts hold
           bolts through the holes

• Use: to join valves, pumps, meters and other fittings; for pipes in pump houses, treatment plants and exposed lines; wherever the pipe may have to be taken apart and rejoined.
• It is strong, watertight and easy to dismantle, but costly and it cannot take any bending or movement.

Requirements of a good pipe material

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.

Polyethylene (PE / HDPE) pipe

• A flexible thermoplastic pipe in coils or lengths, joined by butt fusion, electrofusion or compression fittings.
• Light, corrosion-free, smooth (C = 140), flexible (suits hilly and uneven ground, earthquake movement) and has a long life (50 years).
• Pressure classes PN 6, 10, 16. Used widely in rural schemes of Nepal. Weak against UV when exposed unless it is black-stabilised.

Polyvinyl chloride (PVC) pipe

• A rigid plastic pipe, 6 m lengths, joined by solvent cement or rubber ring joints.
• Light, cheap, corrosion-free, smooth interior, and easy to install; used for distribution and house connections.
• Brittle with age and sunlight, low impact strength, and it softens with heat; not for hot water.

Polypropylene random copolymer (PPR) pipe

• A thermoplastic pipe joined by heat fusion (socket fusion), which gives a leak-proof joint.
• Suited for hot and cold water inside buildings; withstands up to 95 °C; non-toxic and long life.
• Costly, needs fusion tools and expands with temperature.

Requirements of a good pipe material

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.

Concrete pipe

• Plain, reinforced (RCC) or prestressed pipes; the diameter is 300 mm and above.
• Merits: cheap in large diameters, long life, does not corrode, can be cast locally, smooth interior.
• Demerits: heavy, difficult to transport and lay, may leak at joints, can be attacked by acids and sulphates, and cracks under tension and shock; used for large gravity mains and low-pressure lines (up to about 60 m head, more for prestressed).

Cast iron (CI) pipe

• Strong, durable (100 years), resists corrosion, stands high pressure and loads; the socket-spigot or flanged joints are easy.
• Heavy, costly, brittle; the interior roughens (tuberculation) so cement lining is provided.

Polypropylene random copolymer (PPR) pipe

• Light plastic pipe, joined by heat fusion to make a leak-free joint; resists corrosion and scaling; handles hot water up to 95 °C; non-toxic and long lived.
• Costly, needs fusion machine, expands on heating; not for large diameters.

Factors for selecting a pipe material

1. Working pressure, surge pressure and external load.
2. Water quality and soil conditions (corrosion, chemical attack).
3. Diameter, flow capacity and friction (smoothness).
4. Cost of material, transport, laying and maintenance.
5. Availability, size range and standard of the material.
6. Ease of laying, jointing, tapping and repair.
7. Terrain (hilly, rocky, landslide), exposed or buried line.
8. Life of the pipe and weight for handling.

Polypropylene random copolymer (PPR) pipes

Advantages

• Resists corrosion and scale; smooth interior (little head loss).
• Lightweight and easy to install; joined by heat fusion to make a leak-free, homogeneous joint.
• Withstands hot and cold water (up to 70–95 °C), high pressure and long life (50 years).
• Non-toxic, hygienic, no taste or odour; good insulation against heat loss and sweating.

Disadvantages

• More costly than PVC or GI; needs special fusion machine and trained workers.
• Expands a lot with temperature, so loops or supports are needed.
• Damaged by UV light if left exposed; cannot be threaded; less suitable for large diameters.

Factors for selecting pipe materials

Cast iron versus steel pipes

PVC (polyvinyl chloride) pipes have the following advantages over other pipes.

1. Light weight: easy to transport, handle and lay, especially in hills; low labour cost.
2. Corrosion free: do not rust or get attacked by soil or water chemicals, unlike GI and CI.
3. Smooth interior: high Hazen-Williams C (about 150), so friction loss is less and the size can be smaller; no scale or tuberculation.
4. Low cost and quick to join, by solvent cement or rubber ring.
5. Long life (50 years), non-toxic and does not affect the taste or quality of water.
6. Good electrical insulation and low thermal conductivity.

Galvanised iron (GI) pipe

GI pipe is a steel pipe coated with a layer of zinc (galvanised) to protect it from rust.

• Available in sizes 15–150 mm in 6 m lengths in three classes (light, medium, heavy).
• Jointed by threaded sockets, unions, bends and tees (screwed joints).
• Merits: strong, rigid and cheap; can take high pressure; easy to cut and thread; good for exposed lines, house connections and stream crossings.
• Demerits: corrodes and scales (zinc is lost), especially in acidic or soft water, so the flow and the life (15–30 years) are reduced; heavy; limited to small sizes.

Cast iron (CI) pipe

CI pipe is cast from grey iron in lengths of 3–6 m with a socket at one end.

• Used for distribution mains of 80 mm and larger.
• Jointed by socket-spigot (lead and yarn), flanged or mechanical joints.
• Merits: very long life (100 years), resists corrosion (better than steel), stands high pressures and external loads, easy to cut and tap.
• Demerits: heavy, brittle (breaks under shock or uneven settlement), costly, and the interior roughens with tuberculation unless cement-mortar lined.

Wrought iron (WI) pipe

Wrought iron pipe is made from rolled and welded wrought iron plate.

• Merits: lighter than CI, stronger in tension, can be threaded and cut easily, withstands shocks and vibration, and is cheaper than CI for small sizes.
• Demerits: corrodes (rusts) easily unless galvanised or coated; the life is shorter; it is now replaced by galvanised steel pipe. It is joined by screwed joints.

Concrete pipe

Concrete pipes are plain (up to 600 mm), reinforced (RCC) or prestressed.

• Made in a factory by spinning or casting; sizes 150 mm to over 3 m; joints are spigot-and-socket or collar with cement mortar or rubber rings.
• Merits: cheap for large diameters, long life (75 years), does not corrode, smooth after finishing, and can be made locally.
• Demerits: heavy and difficult to transport; may crack under tension and leak at joints; attacked by acids, sulphates; cannot withstand high pressure (plain/RCC up to about 30–60 m head; prestressed higher).
• Used for large gravity mains and sewers.

Why pipe joints are required

Pipes come in short lengths (3–6 m) and must be joined to make a long, watertight line, and to connect fittings such as valves and bends.

Collar joint

A collar joint is used for plain-ended pipes, such as concrete, asbestos cement, or CI pipes without sockets. A short pipe-like sleeve, called a collar (with an inside diameter slightly more than the pipe), is slipped over the ends of the two pipes. The pipes are placed with a small gap in the middle, and the annular space on both sides is filled with cement mortar (1 : 1), yarn and lead or a rubber ring.

          collar
       ___________
  =====|=========|=====  pipe ends meet in the middle
       |_________|       gaps sealed with mortar,
     pipe 1  gap  pipe 2    yarn or rubber ring

It is simple, cheap and watertight, but rigid and not suited to high pressure.