Section 20 ยท Design & Build
Pipe & materials
Most ram faults that look hydraulic turn out to be plumbing: a pipe that flexes, a bore smaller than the invoice says, a joint that sucks air, a fitting that was never meant to hold pressure. This page is about the pipe you actually bolt on.
The number on the label is not the hole you get
Every pipe is sold by a nominal size, and the nominal size means different things in different pipe families. Two pipes on the same shelf, both called 63 mm, can pass measurably different amounts of water โ and everything the ram does depends on the internal diameter.
| Pipe family | Nominal size means | What to expect |
|---|---|---|
| PVC-U, PE (metric plastics) | Outside diameter | A "63 mm" pipe is 63 mm across the outside; the bore is OD โ 2 ร wall, always smaller |
| Steel, brass, iron (imperial) | Nominal bore, near the inside | "2 inch" is about 60 mm outside and about 50 mm inside; the outside is the standardised number |
| Old or reused metal | An old schedule you cannot see | Scale, wall thickness and era all shrink the bore; measure it, never trust the label |
The consequence is not academic. The drive-pipe diameter table and the calculators both want the bore โ feed them the nominal size and you will size the pump for a larger pipe than you own.
Velocity scales with the square of the bore: at 4 L/s a true 63 mm bore runs at 1.28 m/s, while a 53 mm bore reaches 1.81 m/s โ the same flow, 41% faster, and friction rises with the square of that. Same nominal size on both invoices, different pump.
Calipers on the end of a cut length, or plug gauges for awkward sizes. Feed the measured bore to the head-loss tool and the sizing calculator. A 10% error in bore is a 20% error in velocity and a 40% error in friction.
The drive pipe: stiffness is the whole point
The drive pipe is not a hose, and it is not a delivery line. It is the flywheel of a machine that works by stopping water suddenly, and its stiffness is what makes the stop sudden. A pipe wall that stretches on the pressure wave absorbs the energy the pump needs. The result is a ram that beats weakly, or not at all.
Choose first
- Steel โ the classic ram drive pipe. Rigid, tough, tolerant of being buried or walked on, and it gives the hardest hammer.
- Heavy-wall pressure-rated PVC โ acceptable on some sites. The hammer is softer because the wave speed is lower, so expect to size up the flow or shorten the beat.
Never
- Flexible hose or layflat of any kind. It swallows the shock; the ram will not beat.
- Thin-wall drainage pipe (DWV, soil pipe). It is not a pressure pipe and it will split.
- A pipe that necks down mid-run. Constant bore, always.
Wave speed is the number that separates them: roughly 1200โ1400 m/s in steel, 300โ500 m/s in pressure-rated PVC, and under 400 m/s in HDPE. Since the hammer is proportional to that speed, a soft wall does not just make the beat gentler โ it makes the pump smaller. The material table is on the physics page.
The rest of the drive-pipe rules are about keeping the water as one clean, fast-moving column:
- Length 3โ7 ร the fall โ the timing rule. Shorter and the valve cannot react before the pressure wave returns; longer and the column is sluggish. See timing and the rules of thumb.
- Monotonic fall, no high points. A single sag traps air, and air is a cushion. A high point is worse, because it collects a bubble that will never move.
- As straight as the land allows, with smooth large-radius bends. An elbow, or worse a pair of them close to the ram, scatters the wave and costs you hammer.
- Airtight, not merely watertight. Every drive-side joint sees sub-atmospheric pressure on the recoil, so a weeping thread sucks air in with each beat. Use proper joints and sealant.
- Anchored or buried. The pipe must not flex, creep or vibrate with each slam.
- A free-flow isolating valve, if you fit one at all: gate or full-bore ball. A restrictive globe valve is a deliberate blockage in the engine.
The delivery line
Downstream of the air chamber the demands are easier, and more forgiving. The chamber has already smoothed the pulse, so the delivery line sees a small, nearly steady flow โ which is why HDPE, PVC or steel all work here, and why you size it for friction rather than for hammer.
- Size for friction. Keep the head lost to friction under roughly 15% of the lift. On a long run this is where a bigger bore pays for itself, because friction falls steeply as diameter rises โ doubling the bore cuts it by roughly a factor of thirty at the same flow. See the friction budget.
- Notice what friction does to the lift. It does not waste power so much as add height: the chamber pushes against h + hf, so a badly sized delivery line quietly raises the pump's duty.
- Protect it. Bury it, or shade it from UV. Plastics embrittle in sunlight and expand in heat; a run that is fine in April can pull apart in August.
- Rate it for the hammer, not just the head. If the air chamber ever lets go, the delivery line sees the raw spike. Either rate the pipe for that case or guarantee the chamber cannot fail โ the trade-off is set out under the pressure-vessel case.
Confirm the wetted materials are suitable for potable supply โ the fittings and seals as much as the pipe. Cheap galvanised components and unknown reclaimed pipe are the usual sources of trouble; see water quality.
Pressure rating & joints
The rating printed on a plastic pipe is its nominal pressure class, in bar. It is a working rating at a reference temperature โ and it is the static head that sets the minimum you need, before any hammer.
| Class | Pressure rating | Static head it holds | Where it turns up |
|---|---|---|---|
| PN6 | 6 bar | 61 m | Light delivery lines and low tanks |
| PN10 | 10 bar | 102 m | The general-purpose choice for delivery |
| PN12.5 | 12.5 bar | 127 m | Tall static heads |
| PN16 | 16 bar | 163 m | Deep heads, and lines asked to survive a chamber failure |
h in metres. A 20 m lift is about 2 bar of static head, so PN6 has margin โ but the hammer is tens of times that, which is exactly why the air chamber, not the pipe, is the primary protection.
Joining
- Threaded (steel, brass): taper threads with PTFE tape or a suitable sealant, made up firmly. The drive side must seal against suction as well as pressure.
- Solvent weld (PVC-U): strong and cheap, but permanent โ so put unions where you will need to open the pump.
- Compression, electrofusion or butt fusion (PE): the standard for HDPE delivery runs; follow the maker's procedure exactly, because fusion joints fail as a family when the technique is wrong.
- Flanges and unions: the only joints you can reliably dismantle. Fit them either side of the ram, at the chamber, and at the waste valve.
Anchoring
The beat is a hammer, and a hammer moves things. The ram is bolted to a pad for that reason (anatomy, installation); the pipework needs the same treatment. Anchor the drive pipe where it enters the ram, support it along its length, and put thrust blocks at bends on the delivery main โ particularly where the line turns against the direction of thrust. A delivery line that ends in a floating tank can walk a tank over a season.
DWV, soil and drainage pipe are sized for fall, not pressure. They are commonly used for a homemade air chamber because they are cheap and easy to cap โ and they are a burst waiting to happen. Use pressure-rated pipe with rated caps, or a steel vessel. See build safety.
Next: the parts that move inside all this pipework โ valves & the air chamber โ or straight to the build with Build Your Own.