HYDRAULIC RAM PUMP
Teferi Taye
Senior Mechanical Engineer
Energy Division, Equatorial Business Group (EBG) Plc, Addis Ababa, Ethiopia
Published in the Journal of the ESME, Vol II, No. 1, July 1998
Reprinted with ESME permission by the African Technology Forum

ABSTRACT

A hydraulic ram pump, or shortly known as a hydram, is a water lifting device that operates automatically and continuously. It lifts a small fall of water with no other external energy source, i.e., it uses water to lift water. In this paper, the history of hydram, the underlying theory of its operation and the author’s experience in the design, manufacture, and laboratory testing of such a unit is presented.

Introduction
History of Hydrams
Water Hammer & Surge Tanks
Operation of a Hydram
Practical Aspects of a Hydram Design
Author's Experience in Design, Manufacture and Laboratory Testing of a Hydram
Conclusion

INTRODUCTION

A hydram (Fig. 1) is a unique device that uses the energy from a stream of water falling from a low head as the driving power to pump part of the water to a head much higher than the supply head. With a continuous flow of water, a hydram operates automatically and continuously with no other external energy source [1].

A hydram is a structurally simple unit consisting of two moving parts: the waste valve and delivery (check) valve.  The unit also consists an air chamber and an air (snifter) valve. The operation of a hydram is intermittent due to the cyclic opening and closing of the waste and delivery valves. The closure of the waste valve creates a high pressure rise in the drive pipe.  An air chamber is necessary to prevent these high intermittent pumped flows into a continuous stream of flow. The air valve allows air into the hydram to replace the air absorbed by the water due to the high pressures and mixing in the air chamber.

Fig. 1: A Typical Hydraulic Ram Installation

 

HISTORY OF HYDRAMS

The history of hydrams goes back to more than 200 years.

 

WATER HAMMER & SURGE TANKS

To explain the principle of operation of a hydram, it greatly helps to have an insight into the function of a Surge Tank (Fig. 2) in a hydropower generation system.

Fig. 2: A Typical Installation of a Surge Tank

In hydropower generation, whenever there is an abrupt load rejection by the power system, the turbine governors regulate the water entering into the turbines in a matter of few seconds, so as to avoid change in frequency. The sudden closure of the valve creates high pressure oscillations in the penstock often accompanied by a heavy hammering sound known as a water hammer [2].

To avoid water hammer, a Surge Tank is installed between the dam and the powerhouse at the water entry of the penstock. The main function of the Surge Tank is to protect the low pressure conduit system/tunnel from high internal pressures. The Surge Tank, therefore, enables us to use thinner section conduit or tunnel, usually running for a few kilometers of length, making the system less expensive. However, unavoidably, the penstock must be designed to sustain the high pressure that will be created by water hammer, requiring the use of thick walled pipes. Here, water hammer has a negative impact. Nevertheless, this same phenomenon is used to lift water in a hydram.

 

OPERATION OF A HYDRAM

The Theoretical Pressure Rise in a Hydram

As indicated earlier, a hydram makes use of the sudden stoppage of flow in a pipe to create a high pressure surge. If the flow in an inelastic pipe is stopped instantaneously, the theoretical pressure rise that can be obtained is given by equation 1.

D H = V * C / g        (1)
Where
DH     is the pressure rise [m]
V         is the velocity of the fluid in the pipe [m/s]
C         is the speed of an acoustic wave in the fluid [m/s]
g          is the acceleration due to gravity = 9.81 m/s2

According to David and Edward [3], the speed of an acoustic wave in a fluid is given by equation 2.

C = (Ev / rho)1/2        (2)
Where
Ev         is the bulk modulus of elasticity, which expresses the compressibility of a fluid. 
            It is the ratio of the change in unit pressure to the corresponding volume change 
            per unit volume. For water, a typical value of Ev is 2.07 x 109 N/m2, and thus 
            the velocity of a pressure wave in water is C = 1440 m/s.
rho       is the density of the fluid [kg/m3]

Equation 1 represents the maximum rise possible. The actual rise will be lower than that given by equation 1, since all pipes have some elasticity and it is impossible to instantaneously stop the flow in a pipe.

Because of the head (H) created (Fig. 1), water accelerates in the drive pipe and leaves through the waste valve. This acceleration is given by equation 3.

H – f * (L / D) * V2 / (2 * g) – S (K * (V2) / (2 * g)) = (L / g) * dV/dt               (3)
Where
H         is the supply head [m]
f * (L / D) * V2 / (2 * g) is the lost head in the pipe [m]
f           is the friction factor (Darcy-Weibach Formula) [-]
S (K * (V2) / (2 * g)) is the sum of other minor head losses [m]
K         is a factor for contraction or enlargement [-]
L          is the length of the drive pipe [m]
D         is the diameter of the drive pipe [m]
V         is the velocity of the flow in the pipe [m/s]
t           is time [s]

The values of K and f can be found from standard fluid mechanics textbooks. Eventually this flow will accelerate enough to begin to close the waste valve. This occurs when the drag and pressure forces in the water equal the weight of the waste valve. The drag force Fd is given by equation 4.

Fd = Cd * AV * rhow * V2 / (2 * g)         (4)
Where
Fd         is the drag force on the waste valve [N]
AV        is the cross sectional area of the waste valve [m2]
rhow       is the density of water = 1000 kg/m3
Cd        is the drag coefficient of the waste valve [-]

The drag coefficient Cd depends on Reynolds number of the flow and the shape of the object. For circular disks, Cd = 1.12.

Applying Bernoulli's Theorem for points 0 and 3 of Fig. 1 results in equation 5.

(P0/ rho *g) + V0 / (2 * g) + Z0 - HL = (P3 / rho *g) + V3/ (2 * g) + Z3                       (5)
Where
P0         is the pressure at point 0 equal to zero (atmospheric) [N/m2]
P3         is the pressure at point 3 [N/m2]
V0           is the velocity of the fluid at point 0 equal to zero [m/s]
Z0           is the height of point 0 = H [m]
V3           is the velocity of fluid at point 3 equal to zero [m/s]  (At the instant the flow is suddenly and fully stopped)
Z3           is the height of point 3 equal to zero (datum) [m]
HL        is the head loss [m]

With the above values, equation 5 reduces to equation 6.

H - HL = P3 / rho *g    (6)

The force that accelerates the fluid can be written using Newton's Equation (equation 7).

F = m * a = rho * A * L * dV/dt               (7)
Where
F          is the accelerating force [N]
m         is the accelerated mass [kg]
a          is the acceleration of the mass [m/s2]
A         is the area of the drive pipe [m2]
L          is the length of the drive pipe [m]

The pressure (P3) at point 3, is obtained by dividing the force (F) in equation 7 by the area A.

P3 = F / A = rho * L * dV/dt                   (8)

Therefore,

P3 / rho * g = L / g * dV/dt                    (9)

From equations 6 & 9:

H - HL = L / g * dV/dt

Simplified Hydram Operation

For analysis, the pumping cycle of a hydram is divided into four main periods, based on the position of the waste valve and the average time-velocity variation in the drive pipe (Fig. 3).

  1. The waste valve is open and water starts to flow from the source and escapes through the waste valve. The flow accelerates under the effect of the supply head (H), until a velocity V0 is attained in the drive pipe;
  1. The waste valve continues to close and finally closes fully. For a good hydram design, the valve closure is rapid or instantaneous;
  1. The waste valve is fully closed and remains closed. The sudden closure creates a high pressure in the hydram and on the check valve that is in excess of the static delivery pressure. The check valve is forced open and pumping takes place until the velocity becomes zero and pumping stops, under the retarding effect of the delivery pressure head; and
  1. The delivery valve closes. The pressure near the check valve is much higher than the static supply pressure and the flow is reversed towards the supply source. This action is termed recoil. The recoil action creates a vacuum in the hydram, temporarily forcing a small amount of air to be sucked into the hydram through the air valve. The pressure on the underside of the waste valve is also reduced and together with the effect of its own weight, the waste valve opens automatically. The water in the drive pipe returns to the static supply pressure as before and the next cycle begins. The action is repeated automatically at a frequency of a few beats to more than 300 beats per minute [1].

Fig. 3: Time-Velocity Variation in Drive Pipe (Source: Ref. 1)

Efficiency of a Hydram

There are two methods commonly used to compute the efficiency of a hydram installation, the Rankine and the D'Aubuisson methods given by equations 11 and 12 respectively.

E (Rankine) = Q * h / ((Q+QW) * H)                 (11)

E (D'Aubuisson) = Q * Hd / ((Q+QW) * H)      (12)

Where
E          is the efficiency of the hydram [-] is the pumped flow [l/min]
Q          is the pumped flow [l/min]
QW      is the wasted flow [l/min]
h          is the pump head above the source [m]
H         is the supply head above the waste valve opening [m]
Hd        is the total head above the waste valve opening = (H+h) [m]

 

PRACTICAL ASPECTS OF A HYDRAM DESIGN

Hydram Parameters: The detailed mechanics of hydram operation are not well understood. Several parameters relating to the operation of the hydram are best obtained experimentally. These parameters include [1]:

Drive Pipe Length (L): The drive pipe is an important component of a hydram installation. The drive pipe must be able to sustain the high pressure caused by the closing of the waste valve. Empirical relationships to determine the drive pipe length are:

6H< L <12H                 (Europe & North America)            (13)

L = h + 0.3 (h/H)             (Eytelwein)                              (14)

L = 900 H / (N2 * D)            (Russian)                                 (15)
Where N is the Number of Beats/min

L = 150 < L/ D< 1000            (Calvert)                                  (16)

Many researchers have indicated that Calvert' s equation gives better guidelines [1].

Air chamber: It is recommended that the volume of the air chamber be approximately 100 times the volume of water delivered per cycle.

Air Valve: Experiments with different sizes indicate that the air valve size has negligible effect on a hydram operation. A small hole, less than 1 mm, is sufficient.

Waste Valve: The flow area (A0) through the waste valve should equal to or exceed the cross-sectional area of the drive pipe to avoid "chocking" of the flow.

Delivery (Check) Valve: 1.45 cm2 of area for every liter of water to be delivered through the valve is recommended.

Supply Head (H):  With simple weighted impulse valves, the supply head should not exceed 4 m, otherwise the valve will be closing so rapidly and frequently that no useful work will be done. In such a case, the valve should be assisted by a spring to regulate its closure.

 

AUTHOR'S EXPERIENCE IN DESIGN, MANUFACTURE AND LABORATORY TESTING OF A HYDRAM

A hydram was designed, manufactured and laboratory tested by the Research and Development Services (RADS) of the then Ethiopian Water Works Construction Authority (EWWCA), where the author was working as a member of the research team. The design was mainly based on references 1 & 4.

The hydram was constructed from off-the-shelf materials, mostly from commercial pipe fittings. Watt [4] gives a ratio of drive pipe length (L) to diameter (D) ranging between limits of 150 and 1000. The drive pipe chosen for the pump was of diameter 1 1/4" x 8000 mm length G.I. pipe, giving a ratio of L/D of 250, which falls within the recommended range. The impulse valve (Fig. 4), a vital part of the hydram, was designed in such a way that its weight (W) and stroke (S) could be varied depending on the supply head (H). A simple non-return valve (Fig. 5) having a rubber flapper backed with a steel disk, an air chamber made from a 2" diameter G.I. pipe of 1 m length, and an air valve with a 1 mm diameter hole were constructed.

Fig. 4: Impulse Valve

Fig. 5: A Non-Return Valve

A lab test was carried out on this hydram for different settings of the stroke (S) and the total delivery head (Hd) above the waste valve. The weight of the impulse valve was kept at 2.2 kg. The pumped flow (Q) and the wasted flow (QW) were measured and the efficiencies of the pump for different combinations of heads and strokes, based on the D'Aubuisson method were calculated and the result was as shown on Table 1.

Table 1: Test Results of the Hydram Prototype

S (mm)

Hd (m)

3

4

5

6

4

Qw

-

-

3.9

7.5

Q

-

-

0.3

1.3

E

0

0

18

44

5

Qw

3.4

2.4

6.5

11.5

Q

4.5

1.3

2.5

2.8

E

85

70

69

59

6

Qw

11.5

10.7

8.8

15

Q

8.3

6.3

2.1

1.8

E

63

74

48

32

7

Qw

(not shown)

16.7

25

30

Q

10

7.1

3.1

2.3

E

48

60

28

21

Flows QW and Q are in l/mm.

Please note: After publication of this paper, it was noticed that the test results for S=4 & Hd=3, S=5 & Hd=4 and S=6 & Hd=4 are not correct.  Contact Teferi Taye at ebg.tech@telecom.net.et for more details.

 

Characteristics Curves of the Prototype

The following characteristic curves of the hydram prototype were drawn for a constant supply head (H) of 2 m, impulse valve weight of 2.2 kg and a drive pipe diameter of 1 1/4".

Fig. 6: Stroke vs. Efficiency

Fig. 7: Head Ratio vs. Flow Ratio

Fig. 8: Head vs. Pump Discharge

Fig. 9: Head vs. Efficiency

 

CONCLUSION

Ideally, different combinations of the supply and delivery heads and flows, stroke length and weight of the impulse valve, length to diameter ratio of the drive pipe, volume of the air chamber and size of the snifter valve, etc. should have been tried to come up with an optimum size of a hydram. However, due to a number of reasons, such an extensive research work was not undertaken. Nevertheless, the test has shown that even a simple hydram which is not based on a casting technology can deliver a reasonable flow and efficiency of 85%. Ethiopia being endowed with a number of perennial rivers and streams with sufficient gradients to run hydrams, the potential need for this water abstraction device is quite enormous. It is, therefore, worth considering the further development of this technology in the country.

ABBREVIATIONS

cm2     Square centimeter
G.I.      Galvanized Iron
kg        Kilogram
1          Liter
Ltd.     Limited
m         Meter
m2       Square meter
m3       Cubic meter
min      Minutes
mm      Millimeter
N         Newton
Pvt.     Private
S          Second
SI         System International

ACRONYMS

EBG                 Equatorial Business Group
USA                United States of America
IDRC               International Development Center of Canada
RADS             Research and Development Services
EWWCA       Ethiopian Water Works Construction Authority

REFERENCES

1.  IDRC, February 1986, Proceedings of a Workshop on Hydraulic Ram Pump (Hydram) Technology, held at Arusha, Tanzania, May 29-June 1, 1984, International Development Research Center (IDRC), IDRC-MR1O2e R.

2.  Dnadhar, M.M and Sharma, K.N, 1979, Water Power Engineering, Vikas Publishing House Pvt. Ltd. India.

3.  David, J.P. and Edward, H.W., 1985, Schaum's Outline of Theory and Problems of Fluid Mechanics and Hydraulics, SI (Metric) Edition, McGraw-Hill Book Company, Singapore.

4.  Watt, S.B., 1982, Manual on a Hydraulic Ram for Pumping Water, Intermediate Technology Publication Ltd. London.

 

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