Metal Pipes

Part of Water Systems

How to make, join, and use metal pipes for water supply under pressure in a resource-constrained rebuilding context.

Why This Matters

Metal pipes allow water to be carried under significant pressure — something clay and wood cannot do reliably. Pressure systems enable water to reach upper floors, supply indoor plumbing, feed elevated tanks, and serve fire hydrants. More practically, metal pipes are flexible enough to follow irregular terrain and small enough in diameter to be inconspicuous and frost-resistant when buried.

In a rebuilding scenario, early metal pipe is likely to be produced from available wrought iron using blacksmithing techniques, or salvaged from existing infrastructure. Either way, understanding how pipe is made, what its limits are, and how to join and repair it extends the useful life of available material and allows new production to begin as soon as iron-working capability exists.

Lead was historically used for pipes (the Latin word “plumbum” for lead gives us “plumbing”) but is toxic and should be avoided for drinking water. Copper and wrought iron are the historically proven alternatives; cast iron for larger mains.

Making Iron Pipe by Wrapping

Before industrial pipe-drawing machines, small-diameter iron pipe was made by wrapping iron sheet around a mandrel and welding or brazing the seam. This technique is achievable with basic blacksmithing equipment.

Material needed: Wrought iron sheet, 1.5–3 mm thick (thicker for larger diameter or higher pressure)

Mandrel: A steel or iron rod of the desired internal diameter — turned smooth or filed round. Slightly tapered (1:50) so the finished pipe slides off.

Procedure for 25 mm nominal bore pipe:

1. Cut iron sheet into strips: width = π × (25 + 2×2.5) = 94 mm for 25 mm ID, 2.5 mm wall
2. Anneal the strip (heat to orange, cool slowly) for maximum ductility
3. Clamp one end of the strip to the mandrel with a vice
4. Wrap the strip tightly around the mandrel, working progressively
5. Hammer the edges to bring them into contact along the seam
6. Heat the seam to welding temperature in a forge (bright yellow-white)
7. Hammer the seam to weld the lap joint — overlap 10–15 mm, weld through
8. Alternative: braze the seam with copper alloy filler — easier to achieve but creates a lower-strength joint
9. While hot, hammer the pipe round on the mandrel to correct any oval sections
10. Cool and drive the pipe off the mandrel (the taper helps)

Limitations: Wrapped pipe is adequate to 3–5 bar pressure for drinking water. The seam weld is the weak point. Test each pipe section by capping one end, filling with water, and pressurizing with a hand pump to 1.5× design pressure before installation.

Cast Iron Spigot-and-Socket Pipe

For larger sizes (50 mm and up), cast iron pipe cast in sand molds is the traditional solution. The spigot-and-socket joint (same principle as clay pipe) allows caulked lead or hemp-and-cement joints.

Casting procedure:

1. Make a wood pattern: a smooth cylinder with one end flared for the socket, the other end tapered (spigot). Wall thickness 8–12 mm for 75–100 mm bore.
2. Ram the pattern in a two-part sand mold, adding risers and a gate
3. Melt iron to cast temperature (bright orange, fully fluid)
4. Pour into mold, allow to cool in mold for 1–2 hours
5. Break out, clean sprue, and test the bore with a mandrel for round bore
6. The spigot end must be turned on a lathe for a consistent diameter

Jointing cast iron pipes: Traditional caulked joints:

1. Insert the spigot into the socket, center it with small hardwood wedges
2. Pack the annular space with oakum (tarred hemp rope) hammered with a caulking iron until tight
3. Fill the remaining space with melted lead, poured carefully to avoid splashing
4. After lead cools, caulk the lead tight with a hammer and caulking iron
5. The lead deforms to fill all gaps; the joint can accommodate slight angular movement

Modern alternative (no lead): Use cement mortar (1:2 cement:sand) in place of lead. Less flexible but adequate for straight runs on stable ground.

Salvage and Reuse of Metal Pipe

In an early rebuilding scenario, salvaged metal pipe from existing infrastructure is likely to be the primary source. Assessment and reuse procedures:

Assessment:

1. Check for external corrosion — light surface rust is acceptable; holes or wall loss exceeding 30% of original thickness means scrap
2. Check bore — flush with water; debris and internal corrosion reduce flow capacity
3. Test pressure — cap one end, pressurize with hand pump to 1.5× planned working pressure, hold 10 minutes
4. Check threads and couplings — if original threaded joints, test the threads for stripping

Cleaning:

• External rust: wire brush, apply linseed oil or tar pitch for protection
• Internal scale: steel wire brush on a rod, or fill with dilute acid (vinegar, 5% acetic acid), stand 2 hours, flush thoroughly with clean water
• Biological fouling: chlorinate (add bleach or HTH granules until 50 ppm residual), stand 24 hours, flush

Re-joining cut sections: If original couplings are missing, thread new sections with a thread die (standard NPT or BSP sizes are most common in legacy infrastructure). Cut threads carefully: keep the die square to the pipe, use cutting oil, check thread form with a test fitting every 2–3 turns.

Pressure Ratings and Safety

Working pressure guidelines:

Material	Bore	Wall thickness	Safe working pressure
Wrought iron, welded seam	25 mm	2.5 mm	3–5 bar
Wrought iron, seamless	25 mm	3 mm	6–8 bar
Cast iron spigot-socket	75 mm	9 mm	7–10 bar
Cast iron flanged	100 mm	11 mm	12–16 bar

Gravity supply context: Most gravity water systems operate at 1–5 bar (10–50 m head). Standard pipe thicknesses are adequate. The concern is water hammer — pressure spikes from rapid valve closure can be 3–5× static pressure. Always close valves slowly.

Signs of pipe failure approaching:

• Weeping at joints — minor initially but will worsen
• Visible bulging in the pipe wall — imminent failure under pressure
• Sound of water flowing when no tap is open — leak in the buried section
• Reduced pressure at taps — possible blockage or major leak

Corrosion Protection

Iron pipe corrodes in water and soil. Protection methods:

Internal: Coat with food-grade tar pitch or bitumen paint before installation. For drinking water, use linseed oil applied hot to the internal bore.

External: Wrap in bitumen-impregnated jute cloth (traditional) or coat with bitumen paint. For highly corrosive soils (acid peat or waterlogged ground), wrap with multiple layers.

Sacrificial anodes: A zinc or magnesium rod bolted to the pipe system at intervals (every 50–100 m) corrodes preferentially, protecting the iron. These must be replaced every 3–5 years.

The goal is 30–50 years of service life from well-protected iron pipe — achievable with initial care and periodic inspection.