The hydraulic ram pump, or hydram, concept was first developed by the Mongolfier brothers in
France in 1796 (they are better remembered for their pioneering work with hot-air balloons).
Essentially, a hydram is an automatic pumping device which utilises a small fall of water to lift
a fraction of the supply flow to a much greater height; ie it uses a larger flow of water falling
through a small head to lift a small flow of water through a higher head. The main virtue of
the hydram is that its only moving parts are two valves, and it is therefore mechanically very
simple. This gives it very high reliability, minimal maintenance requirements and a long
How a hydram works
Its mode of operation depends on the use of the phenomenon called water hammer and the
overall efficiency can be quite good under favourable circumstances. More than 50% of the
energy of the driving flow can be transferred to the delivery flow.
Figure 1 illustrates the principle; initially the impulse valve (or waste valve since it is the nonpumped water exit) will be open under gravity (or in some designs it is held open by a light
spring) and water will therefore flow down the drive pipe (through a strainer) from the water
source. As the flow accelerates, the hydraulic pressure under the impulse valve and the static
pressure in the body of the hydram will increase until the resulting forces overcome the weight
of the impulse valve and start to close it. As soon as the valve aperture decreases, the water
pressure in the hydram body builds up rapidly and slams the impulse valve shut. The moving
column of water in the drive pipe is no longer able to exit via the impulse valve so its velocity
must suddenly decrease; this continues to cause a considerable rise of pressure which forces
open the delivery valve to the air-chamber.
Once the pressure exceeds the static delivery head, water will be forced up the delivery pipe.
Air trapped in the air chamber is simultaneously compressed to a pressure exceeding the
delivery pressure. Eventually the column of water in the drive pipe comes to a halt and the
static pressure in the casing then falls to near the supply head pressure. The delivery valve
will then close, when the pressure in the air chamber exceeds that in the casing.
Water will continue to be delivered after the delivery valve has closed until the compressed air
in the air chamber has expanded to a pressure equal to the delivery head. A check valve is
included in the delivery pipe to prevent return flow. When the delivery valve closes, the
reduced pressure in the hydram body will allow the impulse valve to drop under its own weight,
thereby letting the cycle start all over again. Most hydrams operate at 30-100 cycles a
The air chamber is a vital component, as apart from improving the efficiency of the process by
allowing delivery to continue after the delivery valve has closed, it is also essential to cushion
the shocks that would otherwise occur due to the incompressible nature of water. If the air
chamber fills with water completely, not only does performance suffer, but the hydram body,
the drive pipe or the air chamber itself can be fractured by the resulting water hammer. Since
water can dissolve air, especially under pressure, there is a tendency for the air in the chamber
to be depleted by being carried away with the delivery flow.
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Hydraulic ram pumps
Stage 1: Water
flows through the
Stage 3: air
chamber fills with
Stage 2: Water
Stage 4: the
forces the water
Figure 1: The hydraulic ram pump
Different hydram designs overcome this problem in different ways. The simplest solution
requires the user to stop the hydram occasionally and drain the air chamber by opening two
taps, one to admit air and the other to release water. Another method on more sophisticated
hydrams is to include a so-called snifting valve which automatically allows air to be drawn into
the base of the air chamber when the water pressure momentarily drops below atmospheric
pressure. It is important with such units to make an occasional check to see that the snifting
valve has not become clogged with dirt and is working properly.
Figure 2: The hydraulic ram pump system
Hydraulic ram pumps
This cycling of the hydram is timed by the characteristic of the waste valve. Normally it can be
weighted or pre-tensioned by an adjustable spring, and an adjustable screwed stop is generally
provided which will allow the maximum opening to be varied. The efficiency, which dictates
how much water will be delivered from a given drive flow, is critically influenced by the valve
This is because if the waste valve stays open too long, a smaller proportion of the throughput
water is pumped, so the efficiency is reduced, but if it closes too readily, then the pressure
will not build up for long enough in the hydram body, so again less water will be delivered.
There is often an adjustable bolt which limits the opening of the valve to a predetermined
amount which allows the device to be turned to optimise its performance. A skilled installer
should be able to adjust the waste valve on site to obtain optimum performance. Therefore, it
can be seen that the output of a hydram will be constant and is non-adjustable. A storage
tank is usually included at the top of the delivery pipe to allow water to be drawn in variable
amounts as needed.
Figure 2 illustrates a typical hydram installation, pumping water to a small storage tank on a
plateau. It can be seen that the supply head is created in this case by creating a weir. In some
cases a small stream is diverted to provide the water supply. If the storage tank is for drinking
water, the volume of the tank can be half the volume of water delivered by the ram pump in
one day as the water is removed form the tank in the day time by people. Oversized tanks can
add unnecessary cost to the installation.
Where greater capacity is needed, it is common practice to install several hydrams in parallel.
This allows a choice of how many to operate at any one time so it can cater for variable supply
flows or variable demand. The size and length of the drive pipe must be in proportion to the
working head from which the ram operates. Also, the drive pipe carries severe internal shock
loads due to water hammer, and therefore normally should be constructed from good quality
steel water pipe. Normally the length of the drive pipe should be around three to seven times
the supply head. Ideally the drive pipe should have a length of at least 100 times its own
diameter. The drive pipe must generally be straight; any bends will not only cause losses of
efficiency, but will result in strong fluctuating sideways forces on the pipe which can cause it
to break loose.
The hydram body requires to be firmly bolted to a concrete foundation, as the beats of its
action apply a significant shock load. Some ram pumps should be located so that the waste
valve is located above flood water level, as the device will cease to function if the waste valve
becomes submerged. However, with the AID Foundation design of flap type waste valve the
vale should be submerged to ensure that during the recoil air will not enter through the waste
valve. Air already enters through the snifter. In this way the impact of the waste valve hitting
the waste valve stopper is also cushioned, so less wear and tear and less sound.
The delivery pipe can be made from any material capable of carrying the pressure of water
leading to the delivery tank. In all except very high head applications, plastic pipe can be
considered; with high heads, the lower end of the delivery line might be better as steel pipe.
The diameter of the delivery line needs to allow for avoiding excessive pipe friction in relation
to the flow rates envisaged and the distance the water is to be conveyed. It is recommended
that a hand-valve or check-valve (non-return valve) should be fitted in the delivery line near
the outlet from the hydram, so that the delivery line does not have to be drained if the hydram
is stopped for adjustment or any other reason. This will also minimise any back flow past the
delivery valve in the air chamber and improve efficiency. However, if the pump is known to be
reliable the working performance can be improved by removing the gate valve and non return
valves in the delivery line. The diameters of the valves are much smaller than the pipe lines
and create additional friction.
Hydraulic ram pumps
Choice of hydram design
Traditional hydram designs, such as in Figure 3, developed a century ago in Europe, are
extremely robust. They tend to be made from heavy castings and have been known to function
reliably for 50 years or more. However, although a number of such designs are still
manufactured in Europe and the USA in small numbers, they are relatively expensive,
although generally speaking the drive-pipe, delivery pipe and civil workings will be significantly
more expensive than even the heaviest types of hydram.
Lighter designs, fabricated using a welded sheet steel construction, were developed first in
Japan and are now in production in other parts of South East Asia including Taiwan and
Thailand. These are cheaper, but only likely to last a decade or so as they are made from
thinner material which will
Nevertheless they offer good
value for money and are likely
to perform reliably.
Hydrams are mostly intended
for water supply duties, in hilly
or mountainous areas,
requiring small flow rates
delivered to high heads. They
are less commonly used for
irrigation purposes, where the
higher flow rates required will
usually demand the use of
larger sizes of hydram having
6-inch or 4-inch drive pipes.
Manufacturers usually describe
Figure 3: Traditional hydram
the size of a hydram by the
supply and delivery pipe
diameters (generally given in
inches even in metric countries because of the common use of inch sizes for pipe diameters);
e.g. a 6 x 3 hydram has a 6-inch diameter drive pipe and a 3-inch diameter delivery pipe.
designs that can be
pipe fittings have
developed by aid
4), and some
have also been
such as a unit
which is being
produced in some
southern Laos from
bridges and using
Spring tension bolt
Stroke adjustment bolt
Figure 4: A ram pump made from standard pipe fittings
Hydraulic ram pumps
old propane cylinders for the air chamber. Needless to say, such devices are very low in cost
but the pipes in the end cost considerably more than the hydram. They are not always as
reliable as traditional designs, but are usually acceptably reliable with failures separated by
many months rather than days, and are easy to repair when they fail.
Table 1 indicates estimated performance for typical 4-inch x 2-inch and 6-inch x 3-inch
Hydram size in
4" X 2"
6" X 3"
Table 1: Estimated performance of hydrams
The costs of commercial hydrams are typically in the range from about £1500 for small 2-inch
drive pipe sizes up to as much as £5000 for 4-inch or 6-inch sizes. The cost of the drive pipe
can also be quite high for the larger sizes. Therefore hydrams are best suited to relatively low
flow rates and high head applications. Of course there are no fuel costs and negligible
maintenance costs associated with hydrams.
Hydraulic Ram Pumps: A Guide to Ram Pumps Water Supply Systems Jeffery, T D,
Thomas T H, Smith A V, Glover, P B, Fountain P D. Practical Action Publications, 1992
A Manual on the Hydraulic Ram for Pumping Water - Watt S B Practical Action
Renewable Energy Sources for Rural Water Supply in Developing Countries Hofkes and
Visscher - International Reference Centre for Community Water Supply and Sanitation,
The Hague, The Netherlands - 1986.
Home Made Hydraulic Ram Pumps Clemson University
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Green and Carter Rams
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