Hydrodynamic or nonpositive displacement pumps: Examples of this type are:

The centrifugal (impeller) and axial (propeller) pumps.­

 these provide smooth continuous flow.

 their flow output is reduced as circuit resistance is increased.
 it is possible to completely block off the outlet to stop all flow, even while the pump is running
all design speed.
 typically used for low-pressure, high-volume flow applications.
 Since there is a great deal of clearance between the rotating and stationary elements, these
pumps arc not self-priming.

This is because there is too much clearance space to seal against atmospheric pressure, and
thus the displacement between the inlet and outlet is not a positive one.
Thus the pump flow rate depends not only on the rotational speed (rpm) at which it is driven
but also on the resistance of the external system.
 As the resistance of the external system starts to increase, some of the fluid slips back into the
clearance spaces, causing a reduction in the discharge flow rate.
 This slippage is due to fluid following the path of least resis­tance.
 When the resistance of the external system becomes infinitely large (for example, a closed valve
blocks the outlet line), the pump will produce no flow and thus its volumetric efficiency
becomes zero.
For example, this dramatic drop in volumetric efficiency with increase in load resistance occurs
when using a centrifugal pump.
The operation of a such pumps starts when the fluid enters at the center of the impeller and is
picked up by the rotating impeller. As the fluid rotates with the impeller, the centrifugal force
causes the fluid to move radially outward causing the fluid to flow through the outlet discharge
port of the housing.
Character­istics of a centrifugal pumps;
 Behavior when there is no demand for fluid. ln such a case, no harm occurs to the pump, thus
no need for safety devices to prevent pump damage.
 The tips of the impeller blades merely slosh through the fluid, and the rotational speed
maintains a fluid pressure correspond­ing to the centrifugal force established.
 The fact that there is no positive internal seal against leakage is the reason that the centrifugal
pump is not forced to pro­duce flow against no demand.
Nonpositivc displacement pumps
 When demand for the fluid occurs (for example, the opening of a valve), the pressure delivers
the fluid to the source of the demand.
This is why centrifugal pumps are so desirable for pumping stations used for delivering water to
homes and factories. The demand for water may go to near zero during the evening and reach a
peak sometimes during the daytime. The centrifugal pump can readily handle these large
changes in fluid demand.
The Fig. below show the pump pressure is plotted versus pump flow. The maximum pressure is
called the shutoff head because all external circuit valves are closed and there is no flow. As the
external resistance decreases, the flow increases at the expense of reduced pressure. Because the
output flow changes significantly with external circuit resistance,
Note
Nonpositive displacement pumps
are rarely used in hydraulic systems.

Typical centrifugal
pump
pressure versus flow
 Performance curves for the radial _piston pump this pump comes in three different
sizes:

Thus, there are three

curves on two of the
graphs. Observe the
linear rela­tionship
between discharge
flow (gpm) and
pump speed (rpm}.
Also note that the
discharge flow of
these pumps is
nearly constant over
a broad pressure
range.
The volumetric and overall efficiency curves are based on a 2000-psi pump pressure
 The volu­metric efficiency is greatly affected by the following leakage losses, which can rapidly
accelerate due to wear:
1. Leakage around the outer periphery of the gears
2. Leakage across the faces of the gears
3. Leakage at the points where the gear teeth make contact

PUMP NOISE
Noise is sound that people find undesirable. For example, prolonged exposure to loud noise can result
in loss of hearing. The sounds come as pressure waves through the surround­ing air medium.
The pressure waves, which possess an amplitude and frequency, are generated by a vibrating
object such as a pump, hydraulic motor, or pipeline.
The strength of a sound wave, which depends on the pressure amplitude, is described by
its intensity. Intensity is defined as the rate at which sound energy is transmitted through a unit
area. Usually expressed (energy-transfer rate in units of decibels) in (dB).
Decibels give the relative magnitudes of two intensities to the intensity of a sound at the
threshold of hearing (the weakest intensity that the human ear can hear). One bel ( I B = 10
dB) represents a very Large change in sound intensity. Thus it has become standard practice to
express sound intensity in units of decibels.
As shown in Fig. 6-34, a sound level of 90 dB is considered very loud and is represented as the
sound level in a noisy factory. The Occupational Safety and Health Agency (OSHA) stipulates that
90 dB(A) is the maximum sound level that a person may be exposed to during an 8-hr period in
the workplace.
 ratio of the intensity under consideration to the threshold of hearing intensity. The logarithm is
used because the intensity of even a moderate sound (50 dB) is actually 100,000 (105) times the
smallest intensity that can be detected by the human ear (0 dB). Using a logarithmic scale reduces
this huge factor lo a more comprehensible one, as in:

This means that if bels are used, the intensity varies from 0 to only 12 for the entire sound range
from threshold of hearing to threshold of pain.

To increase this range by a factor of 10, the decibel is used instead of the bel per the following
equation:
Equation above can be rewritten to determine the amount that the intensity of sound increases in units of
dB if its intensity in W/m2 increases by a given factor. The applicable equation is:
Noise reduction can be accomplished as follows:
1. Make changes to the source of the noise, such as a noisy pump. Problems here include
misaligned pump/motor couplings, improperly installed pump / motor mounting plates, pump
cavitation, and excess pump speed or pressure.
2. Modify components connected to the primary source of the noise. An example is the
clamping of hydraulic piping at specifically located supports.
3. Use sound-absorption materials in nearby screens or partitions. This practice will reduce the
reflection of sound waves to other areas of the building where noise can be a problem.

Noise levels for various pump designs.

 The following rules will control or eliminate cavitation of a pump by keeping the suction pressure above
the saturation pressure of the fluid:

1. Keep suction line velocities below 5 ft/s.

2. Keep pump inlet lines as short as possible.
3. Minimize the number of fittings in the inlet line.
4. Mount the pump as close as possible to the reservoir.
5. Use low-pressure drop inlet filters or strainers. Use indicating-type filters and strainers so that they can
be replaced at proper intervals as they become dirty.
6. Use the proper oil as recommended by the pump manufacturer. Figure 6-36 shows the preferred
range of viscosities and temperatures for optimum pump operation.

PUMP SELECTION
Pumps are selected by taking into account a number of considerations for a complete
hydraulic system involving a particular application. Among these con­siderations are flow-rate
requirements (gpm), operating speed (rpm), pressurerating (psi), performance, reliability,
maintenance, cost, and noise. The selection of a pump typically entails the following sequence of
operations:
1. Select the actuator (hydraulic cylinder or motor) that is appropriate based on the loads
encountered.
2. Determine the flow-rate requirements. This involves the calculation of the flow rate necessary
to drive the actuator to move the load through a specified distance within a given time limit.
3. Determine the pump speed and select the prime mover. This, together with the flow-rate
calculation, determines the pump size (volumetric displace­ment).
4. Select the pump type based on the application (gear, vane, or piston pump and fixed or
variable displacement).
5. Select the system pressure. This ties in with the actuator size and the magni­tude of the
resistive force produced by the external load on the system. Also involved here is the total
amount of power to be delivered by the pump.
6. Select the reservoir and associated plumbing, including piping, valving, hy­draulic cylinders,
and motors and other miscellaneous components.
7. Calculate the overall cost of the system.
8. Consider factors such as noise levels, horsepower Joss, need for a heat exchanger due to
generated heat, pump wear, and scheduled maintenance service to provide a desired life of the
total system.