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.   

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

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

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.

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

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

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