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

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

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

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

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

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)

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

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 (70-175 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

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

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

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 3 1000 𝑙 =0.000219 𝑚 2 𝐷≥ 4 𝜋 ∙0.000219 𝑚 2 ∙ 1000 𝑚𝑚 𝑚 2 =16.68 𝑚𝑚

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

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

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

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

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

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

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

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

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

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

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

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 18752. This has hose standards for 9 pressure classes from 500-8000 psi. See http://blog.parker.com/what-you-need-to-know-about-sae-vs-iso-specification-for-hydraulic-hoses.

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

End of Week 6/Lesson 1