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Hydraulics Index



Hydraulics Tubing

Introduction
Fittings
Flexible Piping
Pressure losses in tubing
Tubing Information





Introduction

Note: The information on this page is limited in scope.    I do recommend for detailed information and datasheets, the links at the foot of the page to suppliers catalogues are used.


In hydostatic systems, for pipes smaller than 100mm, the piping used is generally tubing which is specifed by OD and thickness as opposed to nominal bore piping which is based on nominal diameters and schedule wall thickess.    For larger sizes as used, for example, on the return lines to the reserviors conventional nominal bore seamless piping is generally used.

The installation of the interconnecting tubing and associated fittings are very important in hydrostatic systems because it is essential to have no fluid leakage against an environment with a high degree of continuous vibration

Connections systems are available using special flanges, compression fittings, O-ring fittings, and at the highest level welded fittings.  It is also possible to use threaded fitting with sealing threads i.e "BS EN 10226 - R 3/4" or NPT and threaded fitting with engineered sealing faces on the male and female sides.    Larger diameter pipe connections can be welded but the tube systems can only be welded using special welded fittings.    It is vital, in hydraulic systems, that welded joints are only be made if inspecton of the inside and outside surfaces can be completed.

In the UK and Europe and in most countries using the metric system , the tubing used is metric OD tubing.    The size is based on nominal OD sizes e.g. 4mm, 6mm, 8mm 10mm ... And the thickness is also part of the sizing i.e a 8mm x 0,5mm tube

Hydraulic tubing is generally cold drawn seamless (CDS)/cold finished seamless (CFS).     The suppliers catalogue generally provide tables whic include the OD , thickness and the bursting strength.

General hydraulic piping is mild steel eg. CFS 360 but stainless steel eg. 316L is also popular where corrosion / chemical criteria is important.    Copper based alloys are also used for specialised applications.

The tubes have standard sizes up to 100 mm.    The tubes are supplied in lengths of 6 m, cleaned, oiled and plugged.   




Hydraulic Fittings

Hydrostatic fitting as used to make piping joints and also to connect piping to hydrostatic equipemnt eg. Pumps/ motors/ valves.   They also have to complete the following duties

1).  To connect piping / components of different sizes /standards e.g 20mm OD to 30mm nb pipe or "Pipe Thread ISO 228 - B 3/4 (BSP ext.)" to 25mm OD tubing.

2).  To enable orientation of tubing feeds, for example 90, 45, straight, or swivel fittings are available.    These are designed to be positioned at the correct angle before tightening.

3).  To enable quick and convenient connection and disconnection for maintenance/ modifications .

When a tube is connected to a manifold block / pump/motor etc the component generally has a prepared tapped hole e.g "Pipe Thread ISO 228 - G 3/4 (BSP Int.)". The hydrulic tube coupling has a male threaded end which can screwed into the component and the sealing is obtained by using either a bonded seal or an o-ring.    The bonded seal is generally a simple ring enclosing an elastomeric ring contoured to be compressed on tightening the thread.

Note: The connections below involve the make fitting being screwed down onto the seal to make the joint.    Flange connections with the same sealing arrangement are also generally available.

Connecting a length of tube to the coupling is achieved by one of a number of generic types of sealed attachment . The three main methods are shown below.

Other type of connection fitting include

..Flare fittings, are metal to metal compression seals deformed with a cone nut and pressed into a flare mating male connection.
..Beam seals are costly metal to metal seals used in specialised applications  The "Parker Dynatube" fittings use this principle.



Flexible Piping

Hydraulic pumps motors and actuators when operating tend the vibrate and are often designed to move when operating.    To allow for this, the hydraulic fluid has to be connected using flexible hoses.     There are important factors to take into account when designing systems which include flexible hoses.    Flexible hoses are generally the least reliable components in hydrostatic transmission systems for obvious reasons.     A flexible hose generally has a maximum operating/storage life of 6 years

Relevant standard
BS EN 853:1997 ..Rubber hoses and hose assemblies-Wire Braided reinforced hydraulic type -specification.

A flexible rubber hose is constructed with an extruded inside synthetic elastomer tube designed to seal and convey the hydraulic fluid in the hose.    The elastomer liner has limited strength and requires that a reinforcement layer be wound or braided around the tube in order to hold the internal pressure.    The reinforcement layer(s) are either textile or steel(or both).To protect these inner layers of the hose from the ambient conditions and scuffing , an outer synthetic elastomer cover is extruded around the reinforcement.     The outer layer can also be a wovern terylene type material.

The hoses include strict limitations on movement and temperatures.    They have minimum bend radii and are generally limited to operate between temperaturs of about -30 to 100 oC .    Higher specification hoses can be obtained to exceed these limits.

The ends of the hoses are provided with a wide range of couplings including male and female BSP/NPT ,stud pipe to note just a few.




Pressure Drop in Tubing

The pressure drop in the interconneting tubing in hydrostatic systems is clearly a loss to the available power.    The method of calculating pressure drop / head loss is provided on webpage Pipe Flow

Providing the flow velocities are within the ranges as listed below there will be low head loss in the piping compared to normal operating pressures of 100 to 350 bar

Pump Suction...0,6 - 1,2 m/s
Delivery flow...2,0 - 5,0 m/s
Return flow...1,5 - 4,0 m/s

Considering a hydrostatic system with the pump developing a pressure of 210 barg with a flow of 150l/m in a 25m bore tube.  Assuming a fluid has a grade ISO 68 at a temperature of about 40 deg. C. This is about 50 kW of power in the pressurised flowing fluid .    In a 25mm pipe, the flow velocity would be about 5m/s and the head loss would be about 3 bar in 20m equivalent of piping.    This represents a power loss of about 0,75 kW.  . The calculations to estimate this value is found at Hydraulics_calcs- Tube power




Tubing Data

Table to show available tube sizes, with operating conditions based on mild steel piping.    Table also shows optimum flow in litres/min for suction flow, pressure flow and return flow

Pressures based on seamless cold drawn tubing with a yield strength of 235 N/mm2 with 25% min elongation

Designation O.dia wall I.dia Max Static
Pressure
Max Dyn'ic
pressure
Burst
pressure
wt/m Suction Flow
1 m/s
Pressure Flow
5m/s
Return Flow
4m/s
- mm mm mm bar bar bar kg litres/min litres/min litres/min
4x0.5 4 0.5 3 313 274 1160 0.047 0.5 2.1 1.7
4x0.75 4 0.75 2.5 409 393 1820 0.063 0.4 1.5 1.2
4x1 4 1 2 522 502 2850 0.074 0.2 0.9 0.8
5x1 5 1 3 432 416 2120 0.099 0.5 2.1 1.7
6x0.75 6 0.75 4.5 333 289 1230 0.103 1.2 4.8 3.8
6x1 6 1 4 389 374 1680 0.123 0.9 3.8 3.0
6x1.5 6 1.5 3 549 528 3050 0.166 0.5 2.1 1.7
6x2 6 2 2 692 665 0.197 0.2 0.9 0.8
6x2.25 6 2.25 1.5 757 728   0.208 0.1 0.5 0.4
8x1 8 1 6 333 289 1190 0.172 2.0 8.5 6.8
8x1.5 8 1.5 5 431 414 1860 0.24 1.4 5.9 4.7
8x2 8 2 4 549 528 3020 0.296 0.9 3.8 3.0
8x2.5 8 2.5 3 658 632   0.339 0.5 2.1 1.7
10x1 10 1 8 282 249 870 0.222 3.6 15.1 12.1
10x1.5 10 1.5 7 373 358 1380 0.314 2.8 11.6 9.2
10x2 10 2 6 478 460 2100 0.395 2.0 8.5 6.8
10x2.5 10 2.5 5 576 553 3180 0.462 1.4 5.9 4.7
10x3 10 3 4 666 641   0.518 0.9 3.8 3.0
12x1 12 1 10 235 210 760 0.271 5.7 23.6 18.9
12x1.5 12 1.5 9 353 305 1150 0.388 4.6 19.1 15.3
12x2 12 2 8 409 393 1580 0.493 3.6 15.1 12.1
12x2.5 12 2.5 7 495 476 2600 0.586 2.8 11.6 9.2
12x3 12 3 6 576 553 3200 0.666 2.0 8.5 6.8
12x3.5 12 3.5 5 651 627   0.734 1.4 5.9 4.7
14x1 14 1 12 201 182 620 0.321 8.1 33.9 27.1
14x1.5 14 1.5 11 302 265 940 0.462 6.8 28.5 22.8
14x2 14 2 10 403 343 1340 0.592 5.7 23.6 18.9
14x2.5 14 2.5 9 434 417 1760 0.709 4.6 19.1 15.3
14x3 14 3 8 507 487 2400 0.814 3.6 15.1 12.1
14x3.5 14 3.5 7 576 553 3220 0.906 2.8 11.6 9.2
14x4 14 4 6 641 616   0.986 2.0 8.5 6.8
15x1 15 1 13 188 171 590 0.345 9.6 39.8 31.9
15x1.5 15 1.5 12 282 249 980 0.499 8.1 33.9 27.1
15x2 15 2 11 376 323 1250 0.641 6.8 28.5 22.8
15x2.5 15 2.5 10 409 393 1690 0.771 5.7 23.6 18.9
15x3 15 3 9 478 460 2120 0.888 4.6 19.1 15.3
16x1 16 1 14 176 160 540 0.37 11.1 46.2 37.0
16x1.5 16 1.5 13 264 234 820 0.536 9.6 39.8 31.9
16x2 16 2 12 353 305 1170 0.691 8.1 33.9 27.1
16x2.5 16 2.5 11 386 372 1470 0.832 6.8 28.5 22.8
16x3 16 3 10 452 435 1920 0.962 5.7 23.6 18.9
18x1 18 1 16 157 143 510 0.419 14.5 60.3 48.3
18x1.5 18 1.5 15 235 210 780 0.61 12.7 53.0 42.4
18x2 18 2 14 313 274 1040 0.789 11.1 46.2 37.0
18x2.5 18 2.5 13 392 335 1320 0.956 9.6 39.8 31.9
18x3 18 3 12 409 393 1830 1.111 8.1 33.9 27.1
20x1.5 20 1.5 17 212 191 570 0.684 16.3 68.1 54.5
20x2 20 2 16 282 249 920 0.888 14.5 60.3 48.3
20x2.5 20 2.5 15 353 305 1220 1.079 12.7 53.0 42.4
20x3 20 3 14 373 358 1450 1.258 11.1 46.2 37.0
20x3.5 20 3.5 13 426 410 1720 1.424 9.6 39.8 31.9
20x4 20 4 12 478 460 2080 1.578 8.1 33.9 27.1
22x1.5 22 1.5 19 192 174 590 0.758 20.4 85.1 68.1
22x2 22 2 18 256 228 850 0.986 18.3 76.3 61.1
22x2.5 22 2.5 17 320 280 1040 1.202 16.3 68.1 54.5
22x3 22 3 16 385 329   1.406 14.5 60.3 48.3
25x2 25 2 21 226 202 670 1.134 24.9 103.9 83.1
25x2.5 25 2.5 20 282 249 920 1.387 22.6 94.3 75.4
25x3 25 3 19 338 294 1050 1.628 20.4 85.1 68.1
25x4 25 4 17 394 379 1520 2.072 16.3 68.1 54.5
25x4.5 25 4.5 16 437 420 1780 2.275 14.5 60.3 48.3
25x5 25 5 15 478 460 2120 2.466 12.7 53.0 42.4
28x1.5 28 1.5 25 151 139 450 0.98 35.3 147.3 117.8
28x2 28 2 24 201 182 620 1.282 32.6 135.7 108.6
28x2.5 28 2.5 23 252 224 770 1.572 29.9 124.6 99.7
28x3 28 3 22 302 265 920 1.85 27.4 114.0 91.2
28x4 28 4 20 403 343   2.368 22.6 94.3 75.4
28x5 28 5 18 434 417   2.836 18.3 76.3 61.1
30x2 30 2 26 188 171 620 1.381 38.2 159.3 127.4
30x2.5 30 2.5 25 235 210 770 1.695 35.3 147.3 117.8
30x3 30 3 24 282 249 920 1.998 32.6 135.7 108.6
30x4 30 4 22 376 323 1250 2.565 27.4 114.0 91.2
30x5 30 5 20 409 393 1580 3.083 22.6 94.3 75.4
35x2 35 2 31 161 147 470 1.628 54.3 226.4 181.1
35x2.5 35 2.5 30 201 182 620 2.004 50.9 212.1 169.7
35x3 35 3 29    720   47.6 198.2 158.5
35x4 35 4 27 322 281 960 3.058 41.2 171.8 137.4
35x5 35 5 25 403 343   3.699 35.3 147.3 117.8
35x6 35 6 23 419 403   4.291 29.9 124.6 99.7
38x2.5 38 2.5 33 186 168 550 2.189 61.6 256.6 205.3
38x3 38 3 32 223 200 660 2.589 57.9 241.3 193.0
38x4 38 4 30    970   50.9 212.1 169.7
38x5 38 5 28 371 319 1350 4.069 44.3 184.7 147.8
38x6 38 6 26 390 375   4.735 38.2 159.3 127.4
38x7 38 7 24 446 429   5.352 32.6 135.7 108.6
42x2 42 2 38 134 124 390 1.973 81.7 340.2 272.2
42x3 42 3 36    580   73.3 305.4 244.3
42x4 42 4 34 269 238 850 3.749 65.4 272.4 217.9
50x6 50 6 32      57.9 241.3 193.0
50x9 50 9 32 437    9.1 57.9 241.3 193.0
 
Useful Links

  1. E4 Training- Hydraulic Calculators.. A number of useful calculators and notes.
  2. Hydra Products. Calculators.. Equipment supplier .A number of useful calculators and notes.
  3. Hydraulics Technical Library.. Online hydraulics library containing application formulas, product data, maintenance tips and "how-to" tutorials.
  4. How stuff works Hydraulics.. Clear useful basic information..
  5. Eaton .. A supplier of hydraulic equipment..
  6. Lift Systems...High capacity lifting using hydraulics
  7. Hydraulics...A detailed document on all aspects of Hydraulics
  8. IHS service -Pipe sizes...Downloadable paper titled Determining Tube Size for Hydraulic Systems.
  9. Parker...Parker flexible hose catalogue. Extremely informative
  10. Betabite Hydraulics...Betabite Hydraulic Fittings and couplind -Extremely useful
  11. Primary FP...KR type coulings price lists and datasheets
  12. Swagelok UK ...A large range of information catalogues and datasheets

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Last Updated 25/01/2013