1. Week 6/Lesson 1 – Fluid conduits
Fluid Power Engineering
Week 6/Lesson 1 – Fluid conduits

2. Fluid conduits In this lesson we shall
Learn about the different types of fluid conduits

3. Where we are now… Up to now we have covered:
The basics of fluid-power symbology – how to represent circuits and their components
Some basic circuits and sub-circuits
Pumps
Hydraulic motors
Cylinders
Valves

4. Still to come… Yet to cover in Fluid Power Engineering Hydraulics
Fluid conduits
Pressure losses conduits, fittings, valves
Ancillary devices
Fluid properties
Pneumatics – much of the same stuff as for hydraulic systems
“PLC” stands for “programmable logic controller”
Automation with PLCs
Closed-loop feedback control - basics
Hydraulic servo systems
Simulation of fluid-power systems

5. Fluid conduits
Fluid conduits are the passageways through which hydraulic fluid moves from its source (the pump) to the actuator (cylinder or motor)
These can be pipes, tubes, or hoses
Pipes are generally heavier gauge material than tubing
Tubing is more expensive, but it can be bent, and thus many fittings needed for pipes can be avoided
Hoses are used to accommodate a flexible joint
Hoses are easier to “make-up” than piping joints or tube bending
But the flexibility of hoses is also a problem in that they are a bit like balloons and introduce effective compressibility to the hydraulic fluid

6. Sizing fluid conduits Fluid conduits are designed
To withstand the maximum pressure to which they are exposed
To limit the flow velocity in the conduit
Since Q = A·v, for a given flow rate (from cylinder or motor speed), increasing A makes v smaller
This limits the pressure losses experienced in a pipe/tubing/hose run
Recommended are
vmax ≤ 1.2 m/sec (pump suction lines, to avoid cavitation)
vmax ≤ 6.1 m/sec (pressurized lines)

7. Pressure causes hoop stresses
Below left is shown a cross section of conduit with the internal pressure acting equally in all directions on the interior surface of the conduit
If we cut the conduit lengthwise in half, the pressure causes a normal stress, s , that acts on the area A = L·t
Thus the stress can be found from a force balance:
2∙𝜎∙𝐴=2∙𝜎∙𝐿∙𝑡=𝑝∙𝐷∙𝐿
𝜎= 𝑝∙𝐷 2∙𝑡
where D is the internal diameter
The wall thickness has dimension t
The tube extends dimension L into the slide, so A = L·t

8. Limiting pressure is the working pressure
This stress must be less than St , the tensile strength of the material
If we turn this around a bit, if we know the dimensions of pipe or tubing and we know the material and the safety factor, we can calculate at what pressure the conduit would burst:
𝑝 𝑏𝑟𝑠𝑡 = 𝑆 𝑡 ∙𝐷 2∙𝑡
We reduce the burst pressure by the safety factor to arrive at the working pressure
pwrkg = pbrst/SF . This is pmax for the system.
Safety factors recommended by Esposito are:
8 (0-70 bar)
6 ( bar)
4 (>175 bar)
10 if application will see pressure spikes
When designing systems, you need to be aware of the applicable codes and standards that should be used for the application for which you are designing

9. Conduit selection in practice
Select conduit Dmin to meet flow speed limit
Look in table to find pipe/tube/hose for conduit with at least this dimension
From the conduit’s material, get St
Use to get pbrst
Use the safety factor to get pwrkg
If pmax > pwrkg , increase wall thickness or get higher grade material
“Soft stop” – see cylinders, cushioning
Now reflect to ascertain that there are not any special circumstances that will make pressure rise above pwrkg
Take measures to protect against any over-pressure by including a PRV, an accumulator, or a cylinder incorporating a soft stop

10. Seamless tube sizes
The chart at right shows seamless tube sizes for carbon steel (dark blue) and stainless steel (purple)
The complete chart can be found at Parker Hannifin – Metric Seamless Tube Stop Sizes and Specifications

11. Example – Tube sizing for Q and p
A fluid conduit is to deliver 80 lpm of hydraulic oil at 70 bar. Select a carbon steel tube for this application.
Solution:
𝑄=𝐴∙𝑣
Since this is not a suction line, we limit vmax ≤ 6.1 m/sec
So 𝐴= 𝜋∙ 𝐷 2 4 ≥ 80 𝑙∙𝑠𝑒𝑐 6.1 𝑚∙𝑚𝑖𝑛 ∙ 𝑚𝑖𝑛 60 𝑠𝑒𝑐 ∙ 𝑚 𝑙 = 𝑚 2
𝐷≥ 4 𝜋 ∙ 𝑚 2 ∙ 𝑚𝑚 𝑚 2 =16.68 𝑚𝑚

12. Example – Tube sizing for Q and p
The example chart given only goes up to a tube ID of 12 mm. But the rest of the chart shows tube with an ID of 18 mm, which is adequate for this application.
Since pmax = 70 bar, we can use SF = 6 or 8: use 7, so pbrst = 490 bar
The first entry in the chart with D ≥ 19 mm is OD = 22 mm, t = 1.5 mm, ID = 19 mm
This has a burst pressure of 590 bar so is adequate for our application
Note that if we had used a SF = 8, the required burst pressure would be 560 bar, so we are meeting the requirement of that SF too

13. Pipes vs tubes
Pipes are not as flexible as tubes, so they cannot be bent
Pipes change directions via fittings
For pipes with OD < 32 mm, used screwed fittings
For pipes with OD ≥32 mm, used welded fittings

14. The 1.2 factor lowers the bursting stress
Thick-walled pipe
In the above section on hoop stresses, it was assumed that the conduit is thin-walled
If D/t ≤ 20, the pipe is not considered to be thin-walled and the formula for thick-walled pipe should be used
𝜎 𝑏𝑟𝑠𝑡 = 2∙𝑡∙ 𝑆 𝑡 𝐷∙1.2∙𝑡
The 1.2 factor lowers the bursting stress

15. The diameter gets larger as go toward the pipe
Pipes – tapered threads
Note that pipes have tapered threads
The diameter gets larger as go toward the pipe
One problem with tapered threads is that if you unmake and make a joint, thread moves in further each time
Pipe on this end
Pipe run needs to be flexible enough to accommodate this change in length dimensions

16. Steel tubes
Steel tubes are seamless, that is they are extruded and have no longitudinal seam
Material: SAE 1010 – soft, cold-drawn steel – St = 379 MPa
Material: AISI 4130 – more expensive, stronger – St = 517 MPa
Tubing is more flexible and can be bent
A bending tool contains bending dies to prevent tube from kinking when bent
Bending die

17. Tube fittings
For corners, rather than using fittings, the tubing is bent
But tubes do have fittings too, for instance tees for tube intersections
But tube fittings are not screwed, rather they are often flared
The nut is fitted over the tube before it is flared. Then the nut engages the fitting body and seals on the flare.
Flared tube

18. Tube fittings There are also non-flared fittings
And some fittings are sealed with O-rings
Swageloc fittings are popular; they can be made and unmade without damaging the fitting
Swagelok fitting, showing components

19. Plastic tubing
For low-pressure applications and for air, there is also plastic tubing
Often this tubing is reinforced
Of course with plastic tubing, there is no need for special bending tools to route it

20. Hydraulic hoses
Hydraulic hoses are used where flexibility is needed, for instance on a mobile application – a crane, back hoe, or tractor
Hydraulic hoses are made in layers – flexible layers and then braided reinforcement

21. Hydraulic hose fittings
Hydraulic hoses can be fitted with a variety of connection possibilities
Special tools are used to attach the hose-end to the hoses
Hoses with re-usable fittings were developed in World War II for aircraft, so that hoses damaged in combat could be quickly replaced

22. This plug seals the fluid in when the connection is disconnected
Hydraulic hose – quick disconnect
Often it is convenient to connect and disconnect hydraulic hoses with no spillage of oil
The two pieces shown are attached to hoses; then they couple together
This plug seals the fluid in when the connection is disconnected
This ring is pulled back to allow the balls to deflect outward and accept the male fitting

23. Hydraulic conduit – engineering standards
Hydraulic conduit is covered by a number of standards depending on the conduit and the industry in which it is used. ISO standards are increasingly used in lieu of American-based standards because of the global market.
SAE J1065 – Nominal Reference Working Pressures for Steel Hydraulic Tubing
ISO 7257 – Aircraft – Hydraulic tubing joints and fittings – Rotary flexure test
ISO 8575 – Aerospace – Fluid systems – Hydraulic system tubing
For hydraulic hoses, manufacturers used the SAE J517 standard for decades. Because of today’s global market, ISO standards are being used instead. For hoses, this is ISO This has hose standards for 9 pressure classes from psi. See

24. Outside learning
To better understand this subject matter, view the following videos
Don’t forget to turn the closed-captioning on to be able to understand better the details of the lectures
Watch:
Hydraulic tube bender
How to bend tubing
Flaring a tube
How to make a hydraulic hose
Assembling Swagelok fitting

25. End of Week 6/Lesson 1