Section 08 · Deep Dives
Cascades & multiple rams
Because a ram spills most of the water it uses, a hillside with a big flow is really a hillside with several potential pumps on it — one for each drop the water passes. Used deliberately, a second ram turns yesterday's waste stream into today's delivered water.
Three different reasons to use more than one ram
"Multiple rams" gets used loosely, and the three arrangements behave quite differently. Get the purpose straight before designing the pipework, because the arithmetic is very different in each case.
| Arrangement | Purpose | Does the output change? |
|---|---|---|
| Parallel (array) | Share one large drive flow; add redundancy; build incrementally as demand grows | No — outputs add, but so do the inputs. You split the flow rather than creating water |
| Cascade (waste recovery) | Put the spill from one ram to work on a second drop further down the hill | Yes — you extract more delivered water from the same source, because you are using flow that was being thrown away |
| Staged lift (chaining) | Reach a total lift greater than one ram can manage from the available fall | Yes, downwards — each stage pays its own efficiency, so the combined efficiency is the product |
There is also a family of separation arrangements, where a plentiful but dirty stream provides the drive while a clean supply is lifted for drinking — a schematic of which is discussed in the case studies. Those change where the water comes from, not how the machine works.
Rams in parallel
Parallel is the boring, reliable option and it is chosen far more often than it should be for the right reasons — redundancy and serviceability — and occasionally for the wrong one, namely the belief that two rams make more water than one. They do not: they divide the same drive flow.
- The split is the design. Divide the drive flow between the units, and give each ram its own correctly proportioned drive pipe: short pipe for a small ram, long pipe for a large one. Two rams sharing a single undersized drive pipe will fight each other and neither will beat properly.
- Keep the drive pipes similar. Mismatched lengths and diameters give the rams different natural beat rates, which can either be useful or chaotic depending on how bad the mismatch is.
- Stagger the valve weights slightly. If several rams slam in unison, their pulses add and the whole array shudders on its foundation. A small difference in beat rate spreads the pulses out.
- Provide isolation. A valve on each drive pipe means you can service one ram while the rest keep supplying water. That single detail is the main practical benefit of an array.
- Add up honestly. Total delivered flow is the sum of the individual outputs. If each ram gets half the flow, expect half the output each — no synergy, no penalty.
Cascades: recovering the waste stream
This is where a second ram earns its place. A ram that lifts a third of its drive flow to three times the fall is spilling two-thirds of the water back into the channel — and on a hillside that spill keeps falling. Site a second ram lower down, on that continuing drop, and the water being thrown away becomes the drive flow for the next pump.
Site: 3 m of fall available at the first ram, 10 L/s of drive flow, water needed 9 m up the hillside.
One ram: q = 0.6 × 10 × 3 / 9 = 2.0 L/s delivered. About 8 L/s spills away.
Two rams in parallel: the same 10 L/s split 5/5 gives 1.0 L/s each, so 2.0 L/s — identical output, twice the hardware. Useful for serviceability, not for yield.
A cascade: if the hillside offers a second 3 m drop below the first ram, the 8 L/s of spill drives a second ram at the same 3 m fall and 9 m lift, adding 0.6 × 8 × 3 / 9 = 1.6 L/s. Total delivery rises to 3.6 L/s — an 80% increase in delivered water from the same stream, for the cost of one more ram and its pipe.
The example shows the two ideas clearly. Parallel rearranges the same input; a cascade finds a second input that was being discarded. Where the terrain keeps dropping and the water is plentiful, cascading is usually the best value available anywhere in ram pumping.
What a cascade demands
- A second drop. The cascade needs its own fall for the second ram. On a long hillside there usually is one, because the spill continues downhill — but the second ram must sit lower than the first, and the water must fall to it.
- Hydraulic independence. Each ram needs its own drive pipe, sized and proportioned for its own flow, with its own air chamber. Do not attempt to drive the second ram from a tapping on the first ram's valve body: the two machines will couple, the timing will fight, and both will lose output.
- Separate forebays. The second ram wants a small settling tank at the head of its drive pipe, not a direct connection to the first ram's outfall, so that it receives a steady, air-free supply.
- A drainage plan below the last ram. The final spill must leave the site cleanly — the scour problem on Reliability applies to every machine in the chain, not just the first.
- Independent serviceability. You should be able to isolate and service each ram without stopping the others. In a cascade that also means the downstream ram should be able to keep running, or be safely stopped, while the upstream one is opened.
Staging lift is the arrangement to avoid
The obvious extension — use one ram's delivered water to drive a second ram higher up — sounds appealing and rarely works well. Because a ram needs a fall rather than a pressure supply, staging lift means building an intermediate tank to fall from, and then paying the efficiency penalty a second time. The losses multiply:
Two stages at 60% each leave 36% overall, before any friction in the extra pipework. A single ram sized for the whole lift is almost always better; chain rams only when the terrain leaves no alternative.
Choosing between them
- The fall and flow suit one ram, and you need the output: use one ram. Simplest, cheapest, most reliable.
- You need redundancy, or want to build in stages: parallel array, with isolation valves and staggered valve weights.
- You have plenty of flow, a long hillside, and hillside still dropping below the first ram: cascade, and recover the spill.
- You need more lift than the fall will give: first re-check the lift ratio against the common mistakes, then consider a cascade with an intermediate tank before chaining rams directly.
Parallel divides, a cascade multiplies — but only because it finds flow that was already being wasted. If you have a stream and a hillside, walk down it and count the drops the water passes on its way out of your catchment. Each one is a drive head, and the ram is the only pump that can turn every one of them into delivered water.
The single-ram case is the normal one, and it starts at Design & Sizing. The underlying arithmetic is derived on The Physics.