A hydraulic ram is a pump which raises water without any external power for its operation, when large quantity of water is available at a small quantity of water can be raised to a greater height with the help of hydraulic ram. It works on the principle of water hammer.
Fig shows the main components of the hydraulic ram. When the inlet valve fitted to the supply pipe is opened, water starts flowing from the supply tank to the chamber, which has two valves at B and C. The valve B is called waste valve and valve C is called the delivery valve. The valve C is fitted to an air vessel. As the water is coming into the chamber from supply tank, the level of water rises in the chamber and waste valve B starts moving upward. A stage comes, when the waste valve B suddenly closes. This sudden closure of waste valve creates high pressure inside the chamber, This high pressure forces opens the delivery valve C. The water from chamber then enters the air vessel and compresses the air inside the air vessel. This compressed air exerts forces on the water in the air vessel and small quantity of water is raised to a greater height as shown in figure.
When the water in the chamber losses its momentum the waste valve B opens in the downward direction and the flow of water from supply tank starts flowing to the chamber and the cycle will be repeated.
Let W= weight of water flowing per second into chamber
w = weight of water raised per second
h = height of water in supply tank above the chamber
H = Height of water raised from the chamber
The energy supplied by the supply tank to ram = Weight of water supplied $\times$ Height of supply water
= W $\times$ h
Energy delivered by the ram = weight of water raised $\times$ Height through which water is raised
= w $\times$ H
$\therefore $ Efficiency of the hydraulic ram,
$N= \dfrac {\text {Energy delivered by the ram}}{\text {Energy supplied to the ram}} = \dfrac {w\times H}{W\times h}$
The above expression of efficiency was given by D' Aubuisson and hence knwn as D' Aubuisson's efficiency
Rankine gave another from of the above efficiency. According to him the weight of water(w) is raised to a height of (H-h) and not H, The water is initially at a height of h from the ram and hence the water is only raised to a height equal to (H-h). Hence according to Rankie:-
Energy delivered by the ram = w$\times$ (H-h)
Energy supplied = (W-w)h
$\therefore$ Efficiency $N=\dfrac {w(H-h)}{(W-w)h}$..... Rankine's efficiency
The above two efficiencies, in terms of discharge is written as
D' Aubuisson's $N= \dfrac {q\times H}{Q\times h}$
Rankine's $N= \dfrac {q(H-h)}{(Q-q)h}$
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