Circulatory Systems

By 4 February 2018

Pipes and fittings, with their necessary seals, make up a circulatory system of liquid-powered equipment. Properly selecting and installing these components are very important. If improperly selected or installed, the result would be
serious power loss or harmful liquid contamination. The following is a list of some of the basic requirements of a circulatory system:

• Lines must be strong enough to contain liquid at desired working pressure and the surges in pressure that may develop in system.
• Lines must be strong enough to support the components that are mounted on them.
• Terminal fittings must be at all junctions where parts must be removed for repair or replacement.
• Line supports must be capable of damping the shock caused by pressure surges.
• Lines should have smooth interiors to reduce turbulent flow.
• Lines must have the correct size for the required liquid flow.
• Lines must be kept clean by regular flushing or purging.
• Sources of contaminants must be eliminated.

The three common types of lines in liquid-powered systems are pipes, tubing, and flexible hose, which are also referred to as rigid, semirigid, and flexible line.

Tubing. The two types of tubing used for hydraulic lines are seamless and electric welded. Both are suitable for hydraulic systems. Seamless tubing is made in larger sizes than tubing that is electric welded. Seamless tubing is flared and fitted with threaded compression fittings. Tubing bends easily, so fewer pieces and fittings are required. Unlike pipe, tubing can be cut and flared and fitted in the field. Generally, tubing makes a neater, less costly, lower- maintenance system with fewer flow restrictions and less chances of leakage. Figure 2-21 shows the proper method of installing tubing.

Knowing the flow, type of fluid, fluid velocity, and system pressure will help determine the type of tubing to use. (Nominal dimensions of tubing are given as fractions in inches or as dash numbers. A dash number represents a tube’s outside diameter [OD] in sixteenths of an inch.) A system’s pressure determines the thickness of the various tubing walls. Tubing above 1/2 inch OD usually is installed with either flange fittings with metal or pressure seals or with welded joints. If joints are welded, they should be stress-relieved.

Piping. You can use piping that is threaded with screwed fittings with diameters up to 1 1/4 inches and pressures of up to 1,000 psi. Where pressures will exceed 1,000 psi and required diameters are over 1 1/4 inches, piping with welded, flanged connections and socket-welded size are specified by nominal inside diameter (ID) dimensions. The thread remains the same for any given pipe size regardless of wall thickness. Piping is used economically in larger-sized hydraulic systems where large flow is carried. It is particularly suited for long, permanent straight lines. Piping is taper-threaded on its OD into a tapped hole or fitting. However, it cannot be bent. Instead, fittings are used wherever a joint is required. This results in additional costs and an increased chance of leakage.

Flexible Hosing. When flexibility is necessary in liquid-powered systems, use hose. Examples would be connections to units that move while in operation to units that are attached to a hinged portion of the equipment or are in locations that are subjected to severe vibration. Flexible hose is usually used to connect a pump to a system. The vibration that is set up by an operating pump would ultimately cause rigid tubing to fail.

Rubber Hose. Rubber hose is a flexible hose that consists of a seamless, synthetic rubber tube covered with layers of cotton braid and wire braid. Figure 2-22, shows cut-away views of typical rubber hose. An inner tube is designed to withstand material passing through it. A braid, which may consist of several layers, is the determining factor
in the strength of a hose. A cover is designed to withstand external abuse.

When installing flexible hose, do not twist it. Doing so reduces its lift and may cause its fittings to loosen. An identification stripe that runs along the hose length should not spiral, which would indicate twisting (Figure 2-23). Protect flexible hose from chafing by wrapping it lightly with tape, when necessary.

The minimum bend radius for flexible hose varies according to its size and construction and the pressure under which a system will operate. Consult the applicable publications that contain the tables and graphs which show the minimum bend radii for the different types of installations. Bends that are too sharp will reduce the bursting pressure of flexible hose considerably below its rated value.

Do not install flexible hose so that it will be subjected to a minimum of flexing during operation. Never stretch hose tightly between two fittings. When under pressure, flexible hose contracts in length and expands in diameter.

Teflon™-Type Hose. This is a flexible hose that is designed to meet the requirements of higher operating pressures and temperatures in today’s fluid-powered systems. The hose consists of a chemical resin that is processed and pulled into a desired-size tube shape. It is covered with stainless-steel wire that is braided over the tube for strength and protection. Teflon-type hose will not absorb moisture and is unaffected by all fluids used in today’s fluid-powered systems. It is nonflammable; however, use an asbestos fire sleeve where the possibility of an open flame exists.

Carefully handle all Teflon-type hose during removal or installation. Sharp or excessive bending will kink or damage the hose. Also, the flexible-type hose tends to form itself to the installed position in a circulatory system.

Installation. Flaring and brazing are the most common methods of connecting tubing. Preparing a tube for installation usually involves cutting, flaring, and bending. After cutting a tube to the correct length, cut it squarely and carefully remove any internal or external burrs.

If you use flare-type fittings, you must flare the tube. A flare angle should extend 37 degrees on each side of the center line. The area’s outer edge should extend beyond the maximum sleeve’s ID but not its OD. Flares that are too short are likely to be squeezed thin, which could result in leaks or breaks. Flares that are too long will stick or jam during assembly.

Keep the lines as short and free of bends as possible. However, bends are preferred to elbows or sharp turns. Try not to assemble the tubing in a straight line because a bend tends to eliminate strain by absorbing vibration and compensating for temperature expansion and contraction.

Install all the lines so you can remove them without dismantling a circuit’s components or without bending or springing them to a bad angle. Add supports to the lines at frequent intervals to minimize vibration or movement; never weld the lines to the supports. Since flexible hose has a tendency to shorten when subjected to pressure, allow enough slack to compensate for this problem.

Keep all the pipes, tubes, or fittings clean and free from scale and other foreign matter. Clean iron or steel pipes, tubes, and fittings with a boiler-tube wire brush or with commercial pipe-cleaning equipment. Remove rust and scale from short, straight pieces by sandblasting them, as long as no sand particles will remain lodged in blind holes or pockets after you flush a piece. In the case of long pieces or pieces bent to complex shapes, remove rust and scale by pickling (cleaning metal in a chemical bath). Cap and plug the open ends of the pipes, tubes, and fittings that will be stored for a long period. Do not use rags or waste for this purpose because they deposit harmful lint that can cause severe damage in a hydraulic system.