8/13/2019 PH-LecNotes-HH2 (AY2013-V1.0) http://slidepdf.com/reader/full/ph-lecnotes-hh2-ay2013-v10 1/13 PAGE 1 Power Hydraulics Module PH-LecNotes-HH2 (AY2013-V1.0) A pump when connected to a hydraulic circuit causes the fluid in the reservoir to flow to the circuit and is regarded as the source of hydraulic power in the circuit. However, the pump itself is driven by a prime mover like an electric motor (as in most industrial applications) or from an internal combustion engine (especially in mobile or vehicular applications). 1. FUNCTION OF A PUMP A pump converts input mechanical power from a prime mover such as an electric motor or an internal combustion engine, into hydraulic power. It must be remembered that a pump creates flow and NOT pressure. Pressure results from the resistance to the flow. A pump thus creates the flow necessary for the development of pressure. 2. TWO BROAD CLASSIFICATION OF PUMPS There are two broad classes of pumps: non-positive displacement pumps, and positive displacement pumps CHAPTER H2H YDRAULIC P UMPS
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Power Hydraul i cs Module PH-LecNotes -HH2 (AY2013-V1.0)
A pump when connected to a hydraulic circuit causes the fluid in the reservoir to flow to the circuit and isregarded as the source of hydraulic power in the circuit. However, the pump itself is driven by a prime
mover like an electric motor (as in most industrial applications) or from an internal combustion engine(especially in mobile or vehicular applications).
1. FUNCTION OF A PUMP
A pump converts input mechanical power from a prime mover such as an electric motor or an internalcombustion engine, into hydraulic power.
It must be remembered that a pump creates flow and NOT pressure. Pressure results from theresistance to the flow. A pump thus creates the flow necessary for the development of pressure.
Non-positive displacement pumps do not separate the inlet flow from the outlet flow. An example isthe centrifugal pump as shown in Figure 1(a)). Hence an increase in pressure at the outlet affectsthe inlet flowrate. Thus flowrate decreases with increase in pressure (Figure 1(b)). Such a pump isnot good for hydraulics application because its flow is pressure dependent and their use isrestricted to fluid transfer.
2.2 Positive Displacement Pumps
Positive displacement pumps are pumps where the outlet flow is separated from the inlet. When
driven by a prime mover, it creates a partial vacuum at the inlet port which enables atmosphericpressure to force fluid from the reservoir into the pump. The mechanical action of the pump thentraps this fluid within the pump cavity and transports it through the pump and forces it out into thehydraulic system. Theoretically, all the fluid that enters the inlet must be discharged. However inreality, as the pressure increases, there will be more leakage due to clearance gaps (Figure 2).
The three main types of positive displacement pumps are:
gear pumps,
vane pumps and
piston pumps.
Gear Pump
One typical gear pump is the external gear pump. It consists of a pump housing and two meshinggears (Figure 3). One gear is driven by the drive shaft attached to the prime mover. The other gearis driven by the meshing gear teeth. In this way, fluid is displaced from the inlet port to the outletport when the gears are driven. This is a fixed displacement pump.
Vane Pump
One typical vane pump is the fixed displacement balanced vane pump (Figure 4). It consists of anelliptical track ring, a rotor and vanes placed inside equally spaced slots on the rotor. Duringrotation, centrifugal force drives the vanes outward against the elliptical track where they make firmcontact and follow the contour of the track. The elliptical track permits two intake positions and twooutlet positions (see Figure 4) which are opposite each other and hence the hydrostatic forces onthe rotor are always balanced. The pump housing is specially constructed to merge the two inletpositions into one inlet port, and the two outlet positions into one outlet port.
4. FIXED DISPLACEMENT VERSUS VARIABLE DISPLACEMENT PUMPS
Displacement is the volume of fluid pushed out by the pump for every revolution that the pumpshaft turns.
Fixed displacement pumps will produce a
fixed flowrate at a certain speed of operation. Forvariable displacement pumps, the flowrate may be varied even when operating at a fixed
speed.
Most pumps are driven by electric motors running at fixed speed. It is not common to usevariable speed motor to drive fixed displacement pumps in order to obtain variable flowrate. Theflowrate can also be controlled by flow control valves. This method wastes energy because theexcess flow must return to the reservoir through the pressure relief valve.
5. ANALYSIS OF A SIMPLE FIXED POSITIVE DISPLACEMENT PUMP
A simple fixed displacement pump is shown in Figure 6. Point (1) and (2) are the pump inlet and
outlet respectively.
Since the pump converts MECHANICAL WORK to HYDRAULIC ENERGY, the input power tothe pump will be mechanical and the output power will be hydraulic.
Let
p : the pump volumetric displacement [cm3/rev]
n p : the pump speed or the speed of the prime mover [rev/min, rpm]Q1 : the pump suction flowrate or theoretical delivery at (1) [litre/min ]Q2 : the actual pump delivery at (2) [litre/min ]T p : the actual input torque at the pump shaft required to drive the pump [ Nm]T p,th : the theoretical input torque to drive the pump [ Nm]
There are some important consideration for pumps :
(a) Cavitation In a well-designed system, suction flow is the theoretical flow. In practice, the suction conditionmay become less than ideal, e.g. a suction strainer may be choked with dirt and no provision ismade to allow for by-pass flow (bad design!). Such condition will cause cavitation that will leadto physical damage to the pump in a very short time.
(b) Pressure The term "pressure" in this module shall be referred to as gauge pressure. Since hydraulicsystems work at high pressure (say 50 to 300 bars), the stated gauge pressures are useddirectly in the calculation with negligible error.
(c) Outlet Pressure, P2 The outlet pressure is proportional to the load up to the limiting pressure of the system set bythe pressure relief valve.
(d) Inlet Pressure, P1 In system design, the inlet or suction pressure always refer to the pressure just outside thepump's inlet port, and it is due to the height of the liquid of the fluid level above the inlet port (positive suction pressure) or below (negative suction pressure). Unless otherwise stated, theinlet pressure is usually assumed to be zero in the system calculation, since its magnitude issmall compare to the system's working pressure range.
(e) Pressure Rise, Pp= P2 - P1
The energy from the prime mover is used to convert low pressure inlet flow to higher pressureoutlet flow. In cases where non-zero inlet pressure is higher, the pressure rise which is Pp= P2 - P1 should be used instead of P2 , for pump's calculation.
(f) Case drain The pumping mechanism of a pump leaks continuously while in operation. This leakage flowmust be allowed to drain back to the reservoir to keep the pressure in the pump's housing low.
Figure 7 shows a hydraulic piston pump driven by an ac motor. The pump is able to deliver 60litres/min when the ac motor is rotating at 1200 rpm. The operating pressure is 120 bar and thepump’s volumetric and mechanical efficiencies are 90 % and 85% respectively. Calculate,
a) the pump’s displacement in cm3/rev
b) the mechanical power in kW provided by the ac motor
c) the torque in Nm generated by the ac motor to operate the pump
Solution:
a) The pump theoretical flowrate Q1 will be computed
min67.66
90.0
1
21
12
Q
QQ
QQ v p
Apply p pnQ 1 , and
the displacement is calculated as
min56.55min1200
min1067.66 333
1 cmrev
cm
n
Q
p
p
b) The overall efficiency is computed as 765.085.09.0 m pv po p
kW
or Watts
sm
m N
QPPower Input Mecanical
Power Input Mechanical
Power Output Hydraulic
o p
p
o p
6863.15
3.15686
765.0
601060
1012033
25
2
c) Torque can be computed from Mechanical Input Power
The speed of the pump is the speed of its prime mover. The flowrate of a pump varies directly withthe speed. The manufacturer's catalogue will show the minimum and the maximum operating
speeds. The lower the speed, the longer the pump life.
Type of fluid and fluid contamination
Pumps are designed to operate with a particular viscosity of fluid. Other types of fluids may affectthe lubrication of the pump.
Certain pumps are more tolerant of fluid contamination than others.