Flow in a pipe occurs as a result of pressure drop and always travels from a high pressure location (usually a pump or charged accumulator) to a lower pressure location (such as occurs when a valve opens). When there is no flow in a circuit, then the pressure is equal throughout the circuit. Note a hydraulic circuit consists of tubing and components connected together to perform some specific function. These hydraulic circuits generally have a pressure source which route fluid to a specific function, such as the landing gear. A pipe cross section is shown in Figure 1.
Figure 1 Pipe Cross Section
Generally speaking, fluid flow behaves much like flow in an electrical circuit with a power supply, ground path and wires/resistances. In this analogy, pressure drop is equivalent to voltage drop, fluid flow is equivalent to current, and pipe geometry and flow viscosity combine to form the resistance. Flow is divided up between branches depending upon the comparative resistance between each branch – branches (pipes) with low resistance get more flow and branches (pipes) with high resistances get less flow. Pipes that are closed at one end get zero flow. Flow is proportional to pressure (or voltage) drop through the pipe.
For design purposes we can use simplified equations to analyze performance and to size pipes. The equation to use depends on whether the flow is laminar or turbulent. Laminar or turbulent flow is determined using Reynolds number, which is a dimensionless number that is a function of pipe geometry, fluid properties, and fluid velocity. Analysis of piping networks can also be accomplished using 1st order pipe models in a simulation model.
In reality though, the specific behavior of flow of hydraulic fluid in pipes is very complex. The complexity is a result of (i) non-steady fluid properties, (ii) the fact that hydraulic fluid has mass, is compressible and stores energy, and (iii) valves/actuators are changing the piping configuration during operation. This end result is pressure waves of varying frequencies and magnitudes within the pipes, which cause unforeseen behavior in hydraulic systems. Determination of these pressure waves and their behaviors is very difficult, usually requiring detailed analysis using some form of Navier Stokes or exhaustive testing. Analysis of pressure wave effects in hydraulic systems is tedious and time consuming. A common approach in industry is to first use simplified equations to size pipes. Secondly, pipe network models using 1st order pipe equations are constructed for analyzing flow sharing between hydraulic circuits and to validate sufficient pressure is available at each actuator/motor/servo component. Lastly, high frequency (pressure wave) effects can then be evaluated for components or subsystems that may be susceptible to pressure waves. There are some simple checks that can be utilized and also more complex models for pressure wave analysis.
The design of hydraulic piping systems is a tradeoff between keeping pressure drop/flow velocities low while minimizing weight and cost. Low fluid velocities lead to reduced pressure drop and thus higher performance at the downstream component. Additionally, low fluid velocities reduce the magnitude of pressure waves and water hammer. However, low fluid velocity requires larger diameter pipes and therefore higher weight and cost. In aerospace, the goal is to minimize pipe diameters as much as possible while still ensuring adequate performance of the component(s) and that pressure waves can be tolerated by the system without leading to premature failures in the piping and connectors.
Hydraulic pipes (tubing) are usually made from stainless steel, aluminum or titanium. For high pressure tubing, stainless steel is strong, handles installation pre-stresses well (connections never seem to match up perfectly) and is easy welded. Steel is inexpensive and more resistant to damage than Titanium. The downside of steel is weight due to its approximate density of 0.28 lb/in3. Consequently, many manufacturers use titanium, which has an approximate density of 0.16 lb/in3, on high pressure lines. Titanium is strong and weighs much less than steel for comparable stress levels. Titanium is expensive. The other downside of titanium, which has not shown to be a critical problem, is that titanium is not as receptive to welding as steel and requires better welding controls to obtain a good weld. Also, titanium is not as forgiving when it comes to installation pre-stresses, unless wall thicknesses are increased (which increases costs). Aluminum tubing is low cost, light weight with an approximate density of 0.10 lb/in3, and relatively easy to form, but does not have the strength properties for high pressure lines (unless wall thicknesses are large). Aluminum tubing is typically used in low pressure return lines. One disadvantage of aluminum tubing is that it is easier to damage when compared to steel and titanium tubes. Aluminum tubing material properties are affected at higher temperatures, generally above 200°F; hence, they cannot be used in a fire zone or other high temperature region of the hydraulic system. Due to these characteristic some aircraft use Titanium tubing for all return tubes.
Hydraulic tube installations require robust connectors (fittings) and tubing supports that allow for some motion of the hydraulic tubes. Motion occurs in hydraulic lines due to aircraft bending and flexure as well as hydraulic pressure spikes. All hydraulic line installations utilize pipe bends and flexible clamps (such as cushioned p-clamps) to accommodate for movement of the hydraulic lines. If pipes are rigidly attached, then premature tubing failure will result. In cases, where movement of the hydraulic line is significant or the component is moving around a pivot point, hydraulic hoses are used. Connectors for hydraulic tubing are either permanent swaged fittings or standard AN/MS type fittings. Swaged fittings require repair equipment and procedures, but are often used today because of their lifetime reliability and light weight. Several manufacturers supply swaged fittings and swaging equipment. AN fittings require tubes to be flared to 37 degrees on the ends, which does add to the cost of the tubing (swaged fittings do not require flaring). MS fittings also incorporate a flareless design where the sleeve bites into the tube. AN/MS fittings need to be selected based on their pressure rating and are readily available as a standard part.