hydraulic ram pump instalation

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HYDRAULIC RAM PUMP Teferi Taye

Senior Mechanical EngineerEnergy 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 automaticallyand 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 authorsexperience 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 asthe driving power to pump part of the water to a head much higher than the supply head. With a continuousflow 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 isintermittent due to the cyclic opening and closing of the waste and delivery valves. The closure of the wastevalve creates a high pressure rise in the drive pipe. An air chamber is necessary to prevent these highintermittent pumped flows into a continuous stream of flow. The air valve allows air into the hydram toreplace the air absorbed by the water due to the high pressures and mixing in the air chamber.


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Fig. 1: A Typical Hydraulic Ram Installation

HISTORY OF HYDRAMS

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

The first hydram was built by John Whitehurst, an Englishman in 1775 . His hydram was notautomatic. The operation of the pump was controlled manually by opening and closing a stopcock;

The first automatic hydram was invented by a Frenchman, Joseph Montgolfier, in 1797. Hishydram however, suffered from a defect. The air in his air chamber was eventually gettingdissolved, causing an intensive banging in the mechanism. It was his son, Pierce FrancoisMontgolfier who designed the air or snifter valve to introduce air into the air chamber; and

A very large hydram, 300 mm in diameter is reported to have pumped 1700 l/min to a height of 43m in the USA.

WATER HAMMER & SURGE TANKS

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


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Fig. 2: A Typical Installation of a Surge Tank

In hydropower generation, whenever there is an abrupt load rejection by the power system, the turbinegovernors regulate the water entering into the turbines in a matter of few seconds, so as to avoid change infrequency. The sudden closure of the valve creates high pressure oscillations in the penstock oftenaccompanied 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 entryof the penstock. The main function of the Surge Tank is to protect the low pressure conduit system/tunnelfrom 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, requiringthe use of thick walled pipes. Here, water hammer has a negative impact. Nevertheless, this samephenomenon 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 pressuresurge. If the flow in an inelastic pipe is stopped instantaneously, the theoretical pressure rise that can beobtained is given by equation 1.

D H = V * C / g (1)

WhereDH 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/s 2

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


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C = (E v / rho)1/2 (2)

WhereEv 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 changeper unit volume. For water, a typical value of E v is 2.07 x 109 N/m

2, and thusthe velocity of a pressure wave in water is C = 1440 m/s.

rho is the density of the fluid [kg/m 3]

Equation 1 represents the maximum rise possible. The actual rise will be lower than that given by equation1, 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 wastevalve. This acceleration is given by equation 3.

H f * (L / D) * V 2 / (2 * g) (K * (V 2) / (2 * g)) = (L / g) * dV/dt (3)WhereH is the supply head [m]f * (L / D) * V 2 / (2 * g) is the lost head in the pipe [m]f is the friction factor (Darcy-Weibach Formula) [-] (K * (V

2) / (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 willaccelerate enough to begin to close the waste valve. This occurs when the drag and pressure forces in thewater equal the weight of the waste valve. The drag force F d is given by equation 4.

Fd = C d * A V * rho w * V2 / (2 * g) (4)

WhereFd is the drag force on the waste valve [N]AV is the cross sectional area of the waste valve [m

2]rho w is the density of water = 1000 kg/m

3 Cd is the drag coefficient of the waste valve [-]

The drag coefficient C d depends on Reynolds number of the flow and the shape of the object. For circulardisks, C d = 1.12.

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

(P0 / rho *g) + V 0 / (2 * g) + Z 0 - H L = (P 3 / rho *g) + V 3 / (2 * g) + Z 3 (5)WhereP

0is the pressure at point 0 equal to zero (atmospheric) [N/m 2]

P3 is the pressure at point 3 [N/m 2]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 fullystopped)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.


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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 andthe D'Aubuisson methods given by equations 11 and 12 respectively.

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

E (D'Aubuisson) = Q * H d / ((Q+Q W) * H) (12)

WhereE 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. Severalparameters relating to the operation of the hydram are best obtained experimentally. These parametersinclude [ 1]:

Drive pipe length (L); Cross-sectional area of the drive pipe (A); Drive pipe diameter (D) and thickness; Supply head (H); Delivery head (h); Friction head loss in the drive pipe;


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Friction head loss through the waste valve; Friction head loss at the delivery valve; The velocity in the drive pipe when the waste valve begins to close (V 0); The steady flow velocity (V S) through the waste valve when fully open; Valve weight (W); Valve stroke (S); Valve opening orifice area (A

0);

Valve cross sectional area (A V); and Size of the air chamber.

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

6H< L


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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, wasdesigned 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 chambermade from a 2" diameter G.I. pipe of 1 m length, and an air valve with a 1 mm diameter hole wereconstructed.

Fig. 4: Impulse Valve


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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(H d) above the waste valve. The weight of the impulse valve was kept at 2.2 kg. The pumped flow (Q) andthe wasted flow (Q W) were measured and the efficiencies of the pump for different combinations of headsand 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

Qw - - 3.9 7.5Q - - 0.3 1.34E 0 0 18 44Qw 3.4 2.4 6.5 11.5Q 4.5 1.3 2.5 2.85E 85 70 69 59Qw 11.5 10.7 8.8 15Q 8.3 6.3 2.1 1.86E 63 74 48 32Qw (not

shown)16.7 25 30

Q 10 7.1 3.1 2.37

E 48 60 28 21

Flows Q W and Q are in l/mm.

Please note: After publication of this paper, it was noticed that the test results for S=4 & H d =3, S=5 & H d =4 and S=6 & H d =4 are not correct. Contact Teferi Taye at [email protected] for more details.


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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


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Fig. 7: Head Ratio vs. Flow Ratio

Fig. 8: Head vs. Pump Discharge


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Fig. 9: Head vs. Efficiency

CONCLUSION

Ideally, different combinations of the supply and delivery heads and flows, stroke length and weight of theimpulse valve, length to diameter ratio of the drive pipe, volume of the air chamber and size of the sniftervalve, etc. should have been tried to come up with an optimum size of a hydram. However, due to a numberof reasons, such an extensive research work was not undertaken. Nevertheless, the test has shown that evena 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 torun hydrams, the potential need for this water abstraction device is quite enormous. It is, therefore, worthconsidering the further development of this technology in the country.

ABBREVIATIONS

cm 2 Square centimeterG.I. Galvanized Ironkg Kilogram1 LiterLtd. Limitedm Meterm2 Square meterm3 Cubic metermin Minutesmm MillimeterN NewtonPvt. PrivateS SecondSI System International

ACRONYMS

EBG Equatorial Business GroupUSA United States of AmericaIDRC International Development Center of CanadaRADS Research and Development ServicesEWWCA Ethiopian Water Works Construction Authority

REFERENCES

1. IDRC, February 1986, Proceedings of a Workshop on Hydraulic Ram Pump (Hydram) Technology, heldat 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.


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

hydraulic ram pump instalation

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