Hydraulic and Pneumatic Systems: Circuit Design and Operation

This content covers topics such as hydraulic and pneumatic systems, circuit design, and operation. It includes detailed explanations of system components, their functions, and how they work together. The content also covers system monitoring, changing fluids, and precautions to be taken. Suitable for students studying mechanical engineering, fluid mechanics, and related courses.

SOLUTION TO THE QUESTIONS
1. (a)
1- Sequence valve 1 for retraction
2- Sequence valve 2 for inlet supply
3- Directional control valve 1 (spring-operated)
4- Directional control valve 2
5- Check valve
6-Main supply line
7-Return line
8-Pilot line
6
8
7
1
2
3
3
2
1
4

(b) Directional control valves, check valves, sequence valves
DCV 1: 3 position, 4 way and spring operated
DCV2: 3 position, 4 way and spring operated
(Check figure 1 for ports numbering)
(c) The cylinder section
Control valves constituting directional control valve
Actuator –pneumatic drive
Return line
(d) How system works
-During inlet supply to cylinder, valve 2 is closed while 1 is left opened
-Air rushes through valve 1
-The check valve ensures correct sequencing is followed
-As the plunger pushes forward and the check valve closes line 1 hence valve is closed
-The compressed air exits via valve 2 and is directed to the action centre
(e) Functions of (1) and (2):
1- Regulates flow of air into cylinder for extensions
2-Regulates flow of air out of cylinder in retraction mode
2. (a) -A,+B,+C,-D and the return : +D,-C,-B,+A full cycle
(b) Electronics controller integration
The controller shown is integrated in circuit figure 2 below

3. Hydraulic circuit cascade system

4. Modified circuit for Qn 3
The additional DCV 3 will ensure in case of emergency, both cylinders retract and oil is returned
to the reservoir via the DCV 3. Meanwhile, DCV 1 and 2 are temporarily locked to isolate the
active components of the system.
5. Modified circuit for Qn 4
In this case, we use the pilot pressure valve to lock out the active components from pressure
build up hence see the figure below:

6. (a) A-Rotary actuator: Provides a means of driving the system in a rotational manner
B-3-way, 4port DCV: Regulates flow of oil in the main supply circuit
C-3-way, 4port DCV: Regulates flow of oil in the return line circuit
D- Gate valve: Determines the amount of oil allowable in the system from the reservoir
E- motor: Converts power to mechanical from electrical hence the pressure energy is derived
F- Sequence valves: For drive sequencing be it extension or retraction.
(b) The rotary actuator receives controlled actions from the motor drive via the sequence and
DCV valves. It operates two sets of drive functions of compression such that actuator delivers
power to either of the two. The valves regulate flow and pressure.
The limitation is that since the circuit has no speed control mechanism such that the actuation
speed is wholly transferred to either of the design drive points without speed regulation

7. In this case, the circuit is modified to provide a single directional pump and a means of preventing
over-speeding in the case of an overhauling load

8. (a) The two stage compressor
4
P
3 2
1
V
Process 1 -2 : Stage 1 compressor
Process 2-3: Intercooling
Process 3-4: Stage 2 compression
Process 1-4: Singe direct compression without intercooling
(b) Advantages to be gained by a multistage compressor
-Brings the compressor closer to isothermal compression hence making it more efficient
-Multistage compression saves the amount of energy expended in compressing air
-It normally delivers higher pressures per unit time
-Mostly mechanical problems are few as air temperature is controlled
-There is better balance in machine configuration
-Moisture can easily be removed
-Lower compression ratio is achieved per stage compared to single compression

-Suction and delivery valves remain cleaner as vaporization and temperature of lubricating oil is less
9 (a). Rotary vane compressor operating principle
-Firstly, these types of compressors are configured with a series of rotating vanes within the cylindrical
housing
-Eccentric mounting provides the rotor with a sliding in and out of the slots via both pressure and
centrifugal mechanisms.
-The slots are occupied with air when the vanes leave and just enter. This air would expand during
suction conditions hence increasing air volume in the slots and volume is decreased during compression
-As the vane rotates, the air in the slot get squeezed to create pressure at the exhaust port
-It should be noted that the action of suction and compression take place in a continuous fashion to
bring air to the compressed state as required.
9.(b) Reasons why oil must be injected into the machine
-For cooling purpose hence lowering mechanical damages due to increased temperature
-For proper lubrication of the moving parts
Figure 1: Rotary vane compressor
10. (a) piston diameter = 120mm
Mass = 400kg
Determining the system pressure to just raise the load

First, determine area of piston: A= 1/4x3.142xd2 = 1/4x3.142x0.12x0.12 = 0.01131m2
Force on Piston = mg= 400x 9.81 = 3924N
System Pressure = F/A = 3924/0.01131 = 346.95= 350kPa
(b) The speed in m/s = 0.6/10 = 0.06m/s
Q = A x V = 0.01131x 0.06 = 0.0006786m3/s
Into l/min = 0.0006786 x1000x60 = 40.716l/min
11. Operation of a variable displacement axial piston (swash plate) pump
-Normally it operates by translating rotary into reciprocating. It is configured such that a series of pistons
are aligned coaxially with a shaft through a swash plate to pump a fluid. As the swash plate rotates, in
one half cycles, the piston moves out of the cylinder hence resulting into increased volume while in the
second half cycle, the piston moves in and volume is decreased. The resulting reciprocation ensures
pumping of fluid is realized. Input is varied by the swash plate angle such that increased angle of swash
plate would increased the pumping capacity.
12. Positive displacement compressors
Positive displacement Compressed dynamic air
They allow air into an enclosed chamber In this case, speed is increased in a centrifugal
motion
Then volume is reduced by compression via a
plunger
Air flow is restricted such that is reduced to allow
its pressure rise
Compressed air is released intermittently They include axial to centrifugal compressors are
the most common types
Most common types are reciprocating compressor
s
13. Select and size suitable type
Pressure drop = 800lQ2/Rd5.31, substituting in the formula to find Q and using the Q value, we obtain the
power rating hence: Reciprocating compressor, with power rating of 6kW
14. Establish FAD (N.T.P)
FAD = 1.2 x6 x100000/(60x0.293x293) = 8.386kg/s
15. Difference between regenerative absorption and chemical absorption air drying
-In chemical absorption, the air enters desiccant bed of deliquescent materials including soluble salts
and shotted urea. Due to their hygroscopic nature, the condense water as air passes through them while
in regenerative absorption especially in pressure swing type, the desiccant regeneration is implemented

by allowing air expand to atmospheric pressure and directing it across the wet desiccant (Normally air in
expanded form has very low water vapor concentration).
16. Diameter of a suitable air main for the distribution

First, determine pressure in mb/m (as the pressure drop scale is in mb/m) hence pressure drop=
0.3/175 x1000 = 1.71428mbars/m (Note that 1 bar is equivalent to 1000mbars) and the length of pipe
is 175m and
Based on the FAD : 6m3/s into dm3, we note that: Im3= 1000dm3 hence 6x1000= 6000dm3/s
Now, since we have the correct units of the two parameters: FAD and pressure drop, we refer to figure
3 (on piping sizing monogram) and mark the two points on the respective scales and read the other
unknown, that is, internal diameter of pipe:
-From the straight line drawn, it is being realized that extrapolation of the existing scale is inevitable
hence by extrapolation, the diameter of the pipe is obtained as: 130mm
Next, we move to figure 4

From figure 4 and with the results obtained from figure 3, we determine the diameters of the following
auxiliaries (by just reading off, where a range is provided, average value is determined):
Type of Fittings Inner pipe diameter in mm
Min. 125mm 150mm Average for an inner
pipe diameter of
130mm
Elbow fittings 35 43 39.0
Tea connectors 15 20 17.5
Diaphragm 8 10 9.0
Beds 1 1.5 1.25
17. Material for the distribution main pipe
Material Advantages Disadvantages
Steel -Withstands highest flow rates
-Great strength
-Good corrosion resistance
-Ductile materials hence bending
is possible eliminating the use of
joints
-Ideal for areas with space
restrictions
Noise and vibrations are at a
minimum
-Higher initial costs
-May be subjected to corrosion
in high acidic conditions
Copper -Combines the merits such as
good thermal conductivity,
corrosion resistance and is
durable
-Can be used in a variety of ways
-comes in standardized sizes
hence more compatible
-Lower maintenance
-is durable
Expensive to purchase and install
-Specialized skills required such
as soldering
-May suffer slight corrosion if
conditions are too acidic
Plastic -Corrosion resistant
-Cheaper
-Quick and easy to install
-No specialized skills required
-Brittle and can break easily
-Narrowed use in favorable
thermal conditions

18. –larger diameter would often deliver fluid at a relatively low speed
-smaller diameter is often accompanied by great pressure losses
-Larger diameter pipes often occupy more space than required
-Larger diameter consume more material than smaller diameter
-Water hummer is prevalent in small diameter pipes
19. Given: Q= 300l/s , converting into m3/s
Since 1m3 = 1000l and 60s is equivalent to 1min then 300/s is 0.3m3/s
Pressure drop = 0.3 bars
L = 160m
R= hydraulic radius = P’/P= 0.3/6 = 0.05
From the formula: Pressure drop is given by: P’= 800lQ2/Rd5.31
Diameter of pipe can now be determined.
Now, substituting: 0.3 x105= 800 x 160 x0.3x0.3/0.05x d5.31
0.3 x105x0.05x d5.31= 800 x 160 x0.3x0.3
D5.31 = 11520/1500= 7.68
Hence d= (7.68)1/5.31 = 1.4680m
Hence the diameter of pipe to be used is 1.5m
20. Operational difference between macro and micro oil mist lubrication
In micro oil mist lubrication, oil mist is often applied in the visible parts of rotating materials such as
bearings. This oil is often composed of larger particles and in micro oil mist, the atomized oil is
converted into small sized particles (in the range of microns) and lubricates the entire header system.
Micro ol lubrication is often used in areas where microscopic space is available hence incidences of
blockage are minimized. Besides, micro oil mist are more environmentally friendly hence it is most
preferred in areas in which pollution due to oil disposal has a great damage on the immediate
environment.
21. Possible causes of overheating on a multistage reciprocating compressor:
-Higher stage compression ratio with ineffective intercooling

-Blockage of the condensed air hence low cooling performance
-Low oil level and blocked oil passages
-Higher suction pressure due to incorrect sizing of pipes
22. System monitoring chart
DAILY SYSTEM MONITORING CHART for the Month of June
Accessories for
inspection
1 2 3 4 5 6 7 8 9 10 11 … 28 29 30
All valves
All delivery pipes
Reservoir condition
All passages
Leaking points and
seals
Normal pressures and
temperature
Others
1-
2-
3-
23. Precautions taken in changing fluid in a hydraulic system from a mineral oil based hydraulic fluid to a
fire resistant fluid:
-Determine the flashing and fire point temperature of both fluids
-Ensure moisture predisposed risk factors are minimized during transfer
-Ensure pump inlet is enlarged to avoid cavitations
-Inlet strainers are also to be changed such that finer ones (<60) are to be used
-Remove residual contamination prior to changing by undertaking system flushing
-Performance compatibility of elements with the new oil
-Prior to changing the oil, imagine all potential problems likely to be encountered and provide a
mitigation plan to ensure minimized risks in transfer