|
Hydraulic Ram Pumps
A hydraulic ram (or water ram)
pump is a simple, motor less device for pumping water at low
flow rates. It uses the energy of flowing water to lift water
from a stream, pond, or spring to an elevated storage tank or to
a discharge point. It is suitable for use where small quantities
of water are required and power supplies are limited, such as
for household, garden, or livestock water supply. A hydraulic
ram pump is useful where the water source flows constantly and
the usable fall from the water source to the pump location is at
least 3 feet.
Principles of
Operation
Components of a hydraulic ram pump
are illustrated in Figure 1. Its operation is based on
converting the velocity energy in flowing water into elevation
lift. Water flows from the source through the drive pipe (A) and
escapes through the waste valve (B) until it builds enough
pressure to suddenly close the waste valve. Water then surges
through the interior discharge valve (C) into the air chamber
(D), compressing air trapped in the chamber. When the
pressurized water reaches equilibrium with the trapped air, it
rebounds, causing the discharge valve (C) to close. Pressurized
water then escapes from the air chamber through a check valve
and up the delivery pipe (E) to its destination. The closing of
the discharge valve (C) causes a slight vacuum, allowing the
waste valve (B) to open again, initiating a new cycle.
The cycle repeats between 20 and
100 times per minute, depending upon the flow rate. If properly
installed, a hydraulic ram will operate continuously with a
minimum of attention as long as the flowing water supply is
continuous and excess water is drained away from the pump.
System Design
A typical hydraulic ram pump
system layout is illustrated in Figure 2. Each of the following
must be considered when designing a hydraulic ram pump system:
-
available water source
-
length and fall of the drive
pipe for channeling water from the source to the pump
-
size of the hydraulic ram pump
-
elevation lift from the pump
to the destination
-
desired pumping flow rate
through the delivery pipe to the destination.
A hydraulic ram pump system is
designed to deliver the desired pumping flow rate for a given
elevation lift. The range of available flow rates and elevation
lifts is related to the flow quantity and velocity from the
water source through the drive pipe. The mathematical
relationship for pumping flow rate is based upon the flow rate
through the drive pipe, the vertical fall from the source
through the drive pipe, and the vertical elevation lift from the
pump to the point of use. These variables are illustrated in
Figure 2. Equation 1 is used to calculate pumping rate:
where:
Q=pumping rate in gallons per day
(gpd)
E=efficiency of a hydraulic ram pump installation, typically
equal to 0.6
S=source flow rate through the drive pipe in gallons per minute
(gpm)
L=vertical elevation lift from the pump to the destination in
feet
F=vertical fall from the source through the drive pipe in feet.
To convert the pumping rate
expressed in gallons per day (gpd) to
gallons per minute (gpm), divide by
1440. The following example illustrates an application of
Equation 1.
Example:
A hydraulic ram will be used to pump water from a stream with an
average flow rate of 20 gpm up to a water tank located 24 feet
vertically above the pump. The vertical fall through the drive
pipe in the stream to the pump is 4 feet. Assume a pumping
efficiency of 0.6. What is the maximum pumping rate from the
hydraulic ram pump?
In this example, E = 0.6, S = 20
gpm, L = 24 feet, and F = 4 feet. The resulting pumping rate, Q,
is calculated as:
The maximum pumping rate delivered
by the hydraulic ram pump operating under these conditions is
2880 gallons per day, or 2 gallons per minute.
The example shows how the pumping
rate, Q, is directly related to the source flow rate, S. If S
were to double from 20 gpm to 40 gpm, the resulting pumping rate
would also double to 5760 gpd, or 4 gpm.
The example also shows how the
pumping rate, Q, is inversely related to the ratio of vertical
elevation lift to vertical fall, L/F. If L were to double from
24 feet to 48 feet, the lift to fall ratio, L/F, would double
from 6 to 12. The resulting pumping rate would decrease by half
to 1440 gpd, or 1 gpm.
Table 1 lists maximum pumping
rates, Q, for a range of source flow rates, S, and lift to fall
ratios, L/F, calculated using Equation 1 with an assumed pumping
efficiency, E, of 0.6. To illustrate the use of Table 1,
consider a hydraulic ram system with S = 30 gpm, L = 150 feet,
and F = 5 feet. The calculated lift to fall ratio, L/F, is 30.
The resulting value for Q is 864 gpd, or 0.6 gpm.
Table 1. Maximum pumping rates
for a range of source flow rates and lift to fall ratios
assuming a pumping efficiency of 0.6.
Hydraulic ram pumps are sized
based upon drive pipe diameter. The size of drive pipe selected
depends upon the available source water flow rate. All makes of
pumps built for a given size drive pipe use about the same
source flow rate. Available sizes range from 3/4-inch to 6-inch
diameters, with drive pipe water flow requirements of 2 to 150
gpm. Hydraulic ram pumps typically can pump up to a maximum of
50 gpm (72,000 gpd) with maximum elevation lifts of up to 400
feet.
Approximate characteristics of
hydraulic ram pumps for use in selecting pumps are listed in
Table 2. The recommended delivery pipe diameter is normally half
the drive pipe diameter. For the system described in the example
above, the available source water flow rate is 10 gpm. From
Table 2, a pump with a 1-inch drive pipe diameter and a 1/2-inch
delivery pipe diameter is selected for this system.
Installation
The location of the water source in
relation to the desired point of water use determines how the
hydraulic ram pump will be installed. The length of drive pipe
should be at least 5 times the vertical fall to ensure proper
operation. The length of delivery pipe is not usually considered
important because friction losses in the delivery pipe are
normally small due to low flow rates. For very long delivery
pipes or high flow rates, friction losses will have an impact on
the performance of the hydraulic ram pump. The diameter of the
delivery pipe should never be reduced below that recommended by
the manufacturer.
To measure the available source water flow rate from a spring
or stream, build a small earthen dam with an outlet pipe for
water to run through. Place a large bucket or barrel of known
volume below the outlet pipe, and measure the number of seconds
it takes to fill the container. Then calculate the number of
gallons per minute flowing through the outlet. For example, if
it takes 30 seconds to fill a 5-gallon bucket, the available
source water flow rate is 10 gpm. The lowest flow rates are
typically in the summer months. Measure the flow rate during
this period to ensure that the year-round capacity of the system
is adequate.
Efficiency and Power
The power required to raise water is proportional to the water's
flow rate multiplied by the height through which it is lifted
(in a ram pump q x h). Similarly, the power available from
falling water is proportional to its flow rate multiplied by the
distance dropped (Q x H). A ram pump
works by transferring the power of a falling drive flow to a
rising delivery flow.
By definition Efficiency= output power/input power= qh/QH.
Efficiency is always less than 1. It is useful to know the
efficiency because we can use it to predict the delivery flow of
a system and to compare two different pumps. Rearranging the
equation above gives the formula:
Delivery flow (q)= QHn/h
To obtain a good delivery flow, the efficiency of the pump
should be high, there should be a large drive flow, and the
delivery head should not be too many times the drive head. The
value of system efficiency to put into the formula depends upon
many factors including the design of the pump and the system
being used.
Economic Factors
One of the greatest benefits of ram pump systems is that they
have extremely low running costs. There is no input of expensive
petroleum fuels or electricity, making the systems very
inexpensive to operate. The purchase cost of a pump, however, is
usually only a fraction of the capital
cost of a system: drive and delivery pipe
work are usually the most expensive parts. Ram pump
systems can be subject to economies of scale. For example, where
there is enough drive flow, having several pumps at one site
gives a lower unit cost then if the same pumps were installed at
separate sites. In situations of plentiful drive flow, buying
one large pump may be cheaper than buying several smaller ones,
although this option does have disadvantages: having a single
large pump involves a loss of system flexibility across a range
of flows and if the pump needs maintenance or fails, 100% of the
delivery is lost. With several smaller pumps, a pump can fail or
be stopped for maintenance without stopping the entire delivery
flow.
Prices of ram pumps available today vary enormously. If a pump
is imported the costs of shipping and customs duty may
significantly increase the actual cost of the pump to its users.
For more information about Ram pumps please
click here
|
