SELECTION OF TUBES AND CONDUITS IN A HYDRAULIC CIRCUIT

2

INTRODUCTION• In a hydraulic system, the fluids travel through a distribution system consisting of

conductors and fittings.

• Conductors carry fluid from the power source to the system components and eventually to the point of operation.

• So proper design of conductors and fittings is considered to be important in a hydraulic system.

• Designing of conductors and fittings should be based on various factors such as environmental conditions, material, human safety, etc.,

3

TYPES OF HYDRAULIC CONDUCTORSFor hydraulic systems, we have four major types of conductors:

•Steel pipe

•Steel tubing- Neater than steel pipe, but costs more money

•Plastic tubing- Unable to carry as much pressure as other conductors, but is light and cheap.

•Flexible hose- Used in applications where the conductor has to move, e.g. for connecting moving machinery.

4

DIFFERENCE BETWEEN PIPE AND TUBE?• Tubes can come in different shapes like square, rectangular and

cylindrical. Pipe is always cylindrical or round.

• While rigid tubes are frequently used in structural applications, copper and brass tubes can be rather flexible. Pipes are typically always rigid and resistant to bending.

• The pipe size refers to a nominal – not actual – inside pipe diameter. The dimensions provided for tubing on the other hand refer to the actual outside diameter.

• The actual outside diameter of 1¼"pipe is 1.625" – while 1¼" tube has a true 1.25" outside diameter.

5

CONDUITS• An electrical conduit is a tube used to protect and route electrical

wiring in a building or non-building structure.

• Electrical conduit may be made of metal, plastic, fiber, or fired clay. Most conduit is rigid, but flexible conduit is used for some purposes.

• Conduit is generally installed by electricians at the site of installation of electrical equipment.

• A conduit system can be made waterproof or submersible. Metal conduit can be used to shield sensitive circuits from electromagnetic interference, and also can prevent emission of such interference from enclosed power cables

6

STEEL PIPES• Pipes in general are rigid conductors of fluid and are not meant to be bent.

• Steel pipes are still extensively used in fluid power systems, although they are rapidly being supplemented by steel or plastic tubing.

• Its greatest advantage is its mechanical strength and particularly its ability to withstand abuse.

• Steel pipes are sized according to the nominal diameter that is neither the outside nor the inside diameter, while the wall thickness is specified by a schedule number.

• Great care must be taken when screwing taper threads into the body of a component, particularly if it is made of cast iron, otherwise the casting may be cracked.

7

ID, OD, WALL THICKNESS AND PIPE SIZE• Wall thickness determines the pressure rating of the

pipe.

• Schedule 40,80 and 160 are most common and are designated as low, high and very high pressure respectively.

Typical specification of a hydraulic pipe

8

STEEL TUBES• Seamless steel tubing is the most widely used material for hydraulic system

conductors.

• Main advantage of tubing is that it can be bent into any shape and thus requires fewer fittings.

• Less possibility of leaks.

• It is usual to specify hydraulic tubing by reference to the outside diameter (OD) and wall thickness.

• When bending steel tubing, it is always important to use proper tube-bending equipment with fixtures of the correct size, otherwise the pipe is flattened.

• Tubing materials also consist of Aluminium, Copper and brass; but they have the tendency to fracture.

9

PLASTIC TUBING• In general, plastic tubing is most often used in pneumatic systems,

primarily because it does not have sufficient strength to be used in most hydraulic systems.

• The plastics are compatible with most hydraulic fluids, however, and could safely be used in low-pressure applications.

• Plastic tubing is now available in polyethylene, polypropylene, polyvinyl chloride and nylon.

• Many new developments are being made in the nature of tools, fittings, quick disconnects and other devices especially for plastic fabrications.

10

TUBE SIZING• Three important terms associated with tubing are:

burst pressure, working pressure and safety factor.

• Burst pressure is the point at which a tube will rupture.

• Working pressure is the value for safe operation.

• Safety factor is the ratio between working pressure and burst pressure.

11

FLEXIBLE HOSES• Hose assemblies are primarily used to connect

fluid power systems to actuators that must be located on movable parts such as a cylinder.

• A hose is manufactured from natural and synthetic rubbers and several plastics.

• It is necessary to specify the system service pressure and the fluid that is to be used.

• Hoses are fabricated in layers of elastomers and braided fabric or braided wires. The braided fabric or wires are used to increase the strength of the hoses.

12

SELECTION OF CONDUCTORS• The working strength of conductor must be sufficient to contain fluid under all normal

operating conditions

• Sufficient reserve strength to withstand shock loads due to system operations

• The interior surface of the conductor must be as smooth as possible to minimize friction

• The pipe size must be adequate to permit design flow at a reasonable fluid velocity

• Conductors should be supported in such a way that vibrations and shocks to them are minimized

13

PRINCIPLE DIMENSIONS OF PIPE

• Outside diameter

• Inside diameter

• Wall thickness

• Length

14

CONDUCTOR SIZING PROCEDURE• Calculate minimum inside diameter based on flow rate. On the suction side of the

pump, keep the velocity below 4 ft./s to prevent cavitation; on the discharge side, use 20 ft./s to keep the flow laminar.

• Select a standard size conductor. Pick the next size up from your calculated minimum inside diameter based on the tables for tubing and pipe.

• Calculate the wall thickness. Wall thickness is outside diameter minus inside diameter, all divided by 2.

15

CONT'D..

• Calculate the burst pressure and working pressure. Use dimensions of the conductor, tensile strength of the material, and factor of safety.

• Determine if the selection is OK. Compare the working pressure with the actual line pressure. If what you can survive is more than what you have, then you’re OK. Otherwise, try different dimensions, a heavier pipe, or a different pipe material.

16

WALL THICKNESS• The pressure rating of various conductors depends on the tensile strength of the

material used and the wall thickness of the conductor

• The wall thickness and safety factors recommended by the fluid power industry standards are based on calculations using Barlow’s formula

17

BURST PRESSURE AND WORKING PRESSURE• The burst pressure is the pressure of the fluid that causes the pipe to burst

• This occurs when the tensile stress developed due to pressure (σ) equals the tensile strength (S) of the pipe material

• The working pressure is the maximum safe operating fluid pressure 

18

FAILED CONDUCTORS

19

GENERAL DESIGN CONSIDERATIONS• Keep pump inlet short & straight. straight-laminar flow and minimal pressure drop

• Seal inlet connections tightly. Good sealing prevents air bubbles from forming

• Use large return lines. These lines will reduce the back pressure in the system, and ensure laminar flow

• Seal return connections tightly. Again, good sealing prevents air bubbles from forming

• Empty the return line below the fluid level in the tank. Otherwise, you'll create air bubbles in the tank oil

20

GENERAL DESIGN CONSIDERATIONS

21

GENERAL DESIGN CONSIDERATIONS

22

CONCLUSION• Thus it essential to spend considerable amount of time in selection and design of

fluid conductors in a hydraulic circuit, as it involves the safety of the worker as well as the equipment itself.

• Design and failure criteria and general considerations to be followed in order to avoid failure have been discussed.

• Advantages and disadvantages of different types of fluid conductors in a hydraulic circuit have also been discussed.