Section 25 · Design & Build
Safety: pressure, water & ice
A ram is a gentle machine with one violent component. Almost every way a ram scheme can hurt someone comes from stored energy or from water moving where people are — and both are things you decide about at the drawing stage, not after.
Stored pressure
The air chamber is the only part of a ram that stores enough energy to kill. Everything else about the machine is slow, cold and made of water at modest pressure; the chamber is a gas volume held at pressure, and gas holds energy in a way that water does not.
- Pressure-rated vessel, rated caps. Never thin-wall drainage pipe, however convenient it is to cap. See pipe & materials.
- Never pressure-test with compressed air. A gas-filled vessel stores its energy and vents it explosively; a water-filled one cannot. Test hydrostatically, with water, always.
- Never stand over or beside a beating chamber, and never lean on one to listen to it.
- Never weld, braze or patch a pressure vessel — including a small steel tank. Replace it.
- Fit a relief valve on the delivery side so a blocked line cannot raise the pressure without limit — the pressure-vessel case.
Testing with water
The whole art of pressure testing is to make the stored energy as small as possible while you prove the vessel. Water is nearly incompressible: a water-filled vessel at pressure contains a little energy, while an air-filled one at the same pressure contains a great deal. That single fact is why the standard test is hydrostatic, and why pneumatic testing of anything but purpose-made equipment is a specialist activity.
A 20 m delivery head is only about 2 bar of static pressure; a hydrostatic test at 1.5 × that is roughly 3 bar — an easy, safe test with water, and a frightening one with air. Test with the gauge on, the vessel behind cover if it is unproven, and nobody in line with a cap or fitting.
Two details that catch people out. The relevant pressure is not the static head. It is the static head plus the hammer, which is tens of times larger for a fraction of a second — so a vessel that holds the head comfortably can still be over its rating every beat (chamber design). And an old vessel is an unknown vessel. A second-hand cylinder or air receiver with no legible rating should be scrapped rather than adopted, however tempting it is on a budget.
Water, ice & the site
The waste jet
Every second, the waste valve throws a pulse of water out at the full drive-pipe velocity. It is a high-pressure, intermittent jet that carries grit, and it is exactly at the height of a person kneeling to tune the valve. Wear eye protection when starting and tuning — that is the whole of the risk, and the whole of the cure.
Ice
Freezing water expands by about 9% and there is no pipe rating that survives it. Ice does three separate things on a ram site, and the third is the one that surprises people:
- A full, sealed pipe splits — usually at a fitting, which is the weakest point.
- Ice on the structure makes a wet concrete pad and its steps genuinely dangerous underfoot.
- An ice dam at the outfall backs the waste stream up until water pools against the foundation, which is the scour problem arriving on schedule every winter (cold climates).
Drain before hard frost, keep the outfall steep and generous, and treat the shelter — not the water — as the thing to insulate.
Water you can fall into
The intake, the forebay and any diversion weir are places where someone can end up in moving water, and a low weir with a hydraulic jump below it is a trap. Keep the works small, keep them visible, keep them clear of the path people actually use, and keep children away from them — the clack is fascinating, the jet is better, and neither is a toy. Fence or cover what you can; a timber cover over the ram does double duty, taming the noise and removing the temptation.
What the machine does to its own site
- Scour. A ram can be undermined where the bed moves or the waste water is allowed to pond. Inspect after every flood (failure modes).
- Slips and falls. Wet concrete, algae, ice and a steep bank are a more realistic injury than any pressure event. Site the pad where it can be reached and worked on safely.
- Noise. A ram's clack measures roughly 60–75 dB at close range — fine for a field, wearing over a working day, and a genuine nuisance close to a house. Hearing protection while working on it, and a sound cover or pit for the neighbours (noise at purchase).
- Livestock. Heavy animals lean on things and drink from anything; protect the pipework and keep the pump out of the stocking area.
Working routines & checklist
Building and installing
- Lifting. A drive pipe is heavy, unwieldy and often handled on a slope. More than one person, or a mechanical aid, and never a lone lift on a bank.
- Plumbing. Two pipe wrenches slipping on a seized thread is the classic DIY injury — brace the work, keep fingers clear of the jaw, and wear gloves.
- Cutting and solvent welding. Sharp cut edges, and solvent adhesives are volatile: ventilate, and keep them away from the stream you are about to drink from.
- Excavation. Any trench deep enough to be in is deep enough to need shoring. Keep trench depth to the minimum the pipe requires.
Servicing
Isolate, depressurise, then verify — in that order, every time.
Stop the beats
Close the delivery valve, then hold or block the waste valve open so the water flows through without slamming (the recharge procedure).
Release the stored pressure
Open the chamber drain and any isolating valve, and let the delivery side discharge. The chamber holds gas under pressure even with the pump stopped.
Verify, do not assume
Confirm zero on a gauge before opening a joint. A valve left closed somewhere upstream is the usual reason a "depressurised" system is not.
Water tanks
A covered tank is a confined space, and the hazards in it — depleted oxygen, or accumulated gas — are invisible and have no smell. Do not enter one to clean it. Design the tank to be cleaned from outside: a removable lid, hatches, a sloping floor, a washout drain and a long-handled brush do everything a person climbing in would do, and none of it needs a permit or a second rescuer.
Before you start the machine — a quick check
Pressure-rated parts only, tested with water and never with air. Drain before frost. Keep people out of the jet, the intake and the tank. Wear eye protection, take the pressure off before opening anything, and give the pump a cover — for the noise, for the children, and for the rain. Do that, and a ram is about as dangerous as a garden tap, which is roughly what it is.
Building or servicing it for the first time? Read build safety alongside this page, then work through commissioning.