Chapter (2) Hydraulic Power (pumps)
What is the pump? • Pumps are machines which supply energy to liquid in order to move it from place to another.
• Hydraulic pumps convert mechanical energy from a prime mover (engine or electric motor) into hydraulic (pressure) energy. The pressure energy is used then to operate an actuator. • Pumps push on a hydraulic fluid and create flow. Pumps enable liquid to :1- flow from a region or a low pressure to one of high pressure. 2- flow from a low level to a higher level. 3- flow at a faster rate.
Hydraulic Pumps
Non–Positive Displacement Pumps
Positive Displacement Pumps
1- non positive displacement pumps (hydrodynamic) An example of this pump is the centrifugal pump (impeller) and axial pump (propeller). These pumps characterized by: 1- smooth continuous flow. 2- the output flow reduced as the circuit resistance increased. 3- it is not self priming, due to the clearance space between the impeller and the housing 4- it is typically used in low pressure high volume flow applications. 5- due to the output flow changes with external resistance so they are rarely used in hydraulic systems.
2- positive displacement pumps (hydrostatic) This type of pumps ejects a fixed amount of fluid per revolution of the pump shaft. It has small internal leakage, so it is suitable to hydraulic systems. It has a problem in the build up pressure that we must protect it from over pressure. These type of pumps are classified to:
Positive displacement pumps
Gear pumps
Vane pumps
Piston pumps
Positive displacement pumps have the following advantage over the non positive displacement pumps. 1- high pressure capability ( up to 10000 psi.). 2- capable to over comes the pressure resulting from mechanical
loads and resistance to flow due to friction. 3- smaller size. 4- higher volumetric efficiency. 5- small changes in efficiency 6- can operate over a wide range of pressure requirement and speed range.
Reciprocating pumps These types of pump operate by using a reciprocating piston . The liquid enters a pumping chamber via an inlet valve and is
pushed out via a outlet valve by the action of the piston or diaphragm. Reciprocating pumps are generally very efficient and are suitable for very high heads at low flows.
This type of pump is self priming as it can draw liquid from a level below the suction flange even if the suction pipe is not evacuated.
Discharge
Suction
Pump Parameters 1- Displacement Volume (VD) It is the volume of liquid suction by the pump per revolution
VD
4
D2 L
D = Diameter (Bore) L = Stroke
m
3
rev
2- Theoretical Volume Flow Rate (Qth) It is the volume flow rate of liquid suction by the pump per second
Qth VD N P
m s 3
3- Theoretical Power Consumption
Theoretical Input Power P Qth Where
P Pressure Difference across the Pump P Pexit Pinlet 4- Theoretical Torque (Tth)
Tth
Theoretical Power
P VD 2
Pump Efficiencies 1- Volumetric efficiency some of the oil at the discharge port can leak directly back toward the suction port.
This means that the actual flow rate QP is less than the theoretical flow rate Qth the
term volumetric efficiency, which equal about 90% Actual flow rate producedby the pump Volumetric efficiency 100 Theoretical flow rate
V
Qp Qth
100
2- Mechanical efficiency This indicates the amount of energy losses that occur in pumps, not taking into account the leakages. These losses include: • Friction in bearing and other moving parts and
• Energy losses due to fluid turbulence. is given by the equation,
Theoretica l Torque required to operate the pump m 100 Actual Torque delivered to Pump Tth m 100 TA
3- Overall Efficiency
If both volumetric and mechanical efficiencies are known, the overall efficiency can be computed as follows
Overall efficiency 0 m v The actual power delivered to a pump from a prime mover through a rotating shaft is known as Brake power and the actual power
delivered by a pump to a fluid is known as hydraulic power.
Theoretical Power delivered to pump Overall efficiency Actual Power delivered to pump
4- Leakage Flow Rate (QL)
QL Qth QPump QL Qth V Qth QL 1 V Qth
QL 1 V VD N P
A- Gear pumps 1- External gear pump The operation of the external gear pump is based on the carrying of fluid between the teeth of the meshing gears and the pump housing. One of the gears is connected to the drive shaft and the second gear is driven by
the meshing of the driven gear.
The operation of the pump is illustrated as following as shown in the figure. 1- vacuum is carried in the inlet port of the pump which connected to the tank. 2- oil is carried around housing in champers formed between tooth and housing.
3- then it is forced out from the output port of the pump as tooth go back into mesh.
Displacement Volume (VD) the following analysis permit us to evaluate the theoretical flow rate of a gear pump.
VD 2 b m Z sin 2
2
b =tooth height (m) m = tooth module Z = Number of teeth
= pressure angle VD = displacement volume of pump (m3 / rev).
EXTERNAL GEAR PUMP CHARACTERISTICS Typical displacements to 250 cm3/r Typical pressures to 250 bar Fixed displacement only Good speed range, limited indirect
drive capability, simple multiple assemblies Generally noisy Good contamination sensitivity Poor serviceability
Compact, low weight Low cost.
Disadvantages (Problems) of External gear pump 1- Heating, which comes from, A- leakage of oil
B- air in oil C- very high pressure 2- High noise, which comes from, A- Leakage of oil from small hollow B- vibration 3- Decrease in pressure, which comes from, Internal leakage between input and output when the pressure increase. This is due to the clearance between teeth and body increased or the teeth between the two meshing gears.
Advantage of increasing number of teeth 1- can over come higher pressure. 2- can work on higher pressure. 3- lower noise. 4- better continuity flow. 5- discharge does not increase.
2-Internal gear pump
1 6 3
5
2
-1جسم المضخة -2ترس قائذ
-3ترس منقاد -4فتحة سحب -5فتحة طرد
-6فراغ هاللي
4
Internal gear pump consists of 1- an internal gear , 2- a regular spur gear, 3- crescent shaped seal
4- external housing.
The internal gear uses two rotating gears which un-mesh at the suction side of the pump to create voids which allow atmospheric pressure to force fluid into the pump. The spaces between the gear teeth transport the fluid on either side of a crescent to the discharge side, and then the gears re-mesh to discharge the fluid. Viking's internal gear design has an outer drive gear (rotor- shown in orange) which turns the inner, driven ear
1. Liquid enters the suction port between the rotor (exterior gear) and idler (small interior gear) teeth. 2. Liquid travels through the pump between the teeth of the gears. The crescent shape divides the liquid and acts as a seal between the suction and discharge ports. 3. The pump head is now nearly flooded, just prior to forcing the liquid out of the discharge port. Intermeshing gears of the idler and rotor form locked pockets for the liquid which assures volume control. 4. Rotor and idler teeth mesh completely to form a seal from the discharge and suction ports. This seal forces the liquid out of the discharge port.
Displacement Volume (VD) The advantage of internal gear pump is that, it has a small size also it has low pressure , so it used on piston pump as a lubricated unit to it. the following analysis permit us to evaluate the volume displacement of a gear pump.
VD m b h Z
b =tooth height (m) m = tooth module Z = Number of teeth
h =gear width (m) VD = displacement volume of pump(m3 / rev).
Internal Gear Pump Characteristics Typical displacements to 250 cm3/r Typical pressures to 250 bar Fixed displacement only Good speed range Simple multiple assemblies Low noise
Good contamination sensitivity Poor serviceability Good fluid compatibility.
Advantages 1- Only two moving parts. 2- Ideal for high-viscosity liquids. 3- Constant and even discharge regardless of pressure conditions. 4- Operates well in either direction.
5- Can be made to operate with one direction of flow with either rotation. 6- Single adjustable end clearance. 7- Easy to maintain.
8- Flexible design offers application customization.
Disadvantages 1- Usually requires moderate speeds. 2- Medium pressure limitations. 3- One bearing runs in the product pumped. 4- Overhung load on shaft bearing.
3- Ring Gear pump (Gerotor Pump) The gerotor pump operates very much like the crescent shape gear pump. the inner gear rotate to around the outer gear caring oil between the teeth then at the small area it begin to push the
oil at the outlet port. The inner gear has one
teeth less than the outer gear.
The gerotor pump is a combination internal external gear pump. these pumps are having six tooth and Four tooth. 1- Two marked teeth are meshed and the tooth of the spur gear almost completely fills the cavity in the rotor (Fig A). 2- As the drive gear rotates and pulls the driven gear round, the volume of the cavity increases until at (fig C) it is at its maximum.
3- During the rotation from (fig A) to (fig C) the expanding cavity is under the inlet port and fluid is drawn into the pump. 4- As the gears continue to rotate, the cavity formed by the marked teeth moves under the outlet port. 5- As the drive gear meshes with the cavity next to the marked cavity in the rotor, its volume decreases. 6- The fluid in this cavity is forced out of the pump through the outlet port.
this process continues and provide a positive flow at the outlet.
Displacement Volume (VD)
VD bZ Am ax Am in the following analysis permit us to evaluate the volume displacement of a gear pump. Z = Number of rotor teeth b =gear width (m)
VD = displacement volume of pump(m3 / rev).
Advantages 1- High speed 2- High pressure
3- No overhung bearing loads 4- Relatively quiet operation 5- Design accommodates wide variety of materials
Disadvantages· 1- Four bushings in liquid area 2- No solids allowed 3- Fixed End Clearances
4- lobe pump This type of pump operate in a similar way of external gear pump
but both lobes are driven externally so that they do not actually contact each other, so they are quieter than other types of gear pumps. Due to the less space of contact surface of the insider lobe, the output will have greater amount of pulsation also the volumetric displacement is greater than any type of gear pumps.
1. As the lobes come out of mesh, they create expanding volume on the inlet side of the pump. Liquid flows into the cavity and is trapped by the lobes as they rotate. 2. Liquid travels around the interior of the casing in the pockets
between the lobes and the casing -- it does not pass between the lobes. 3. Finally, the meshing of the lobes forces liquid through the outlet port under pressure.
Advantages 1- Pass medium solids 2- No metal-to-metal contact 3- Superior CIP/SIP capabilities 4- Long term dry run (with lubrication to seals) 5- Non-pulsating discharge
Disadvantages 1- Requires timing gears 2- Requires two seals 3- Reduced lift with thin liquids