In the complex pipeline system, due to the limitation of space position condition, it caused kinds of difficulties on installation work: formed pipe installation does not go up; bending over and come over, passable installation onto pipe, inevitably will caused local cold correction. According to the tube diameter and specific circumstances of the scene, the amount of cold correction is from a few millimeters to tens of mm, even to hundreds of mm. Practice has proved that the more of amount, the higher of internal stress. The residual installation stress in the piping system, is undoubtedly great hidden danger.
Since the metal hose can be bent and the bending stress generated under rated bend radius is very small, it can bring great convenience for installation. Besides, it plays a certain degree of protection effect for the safe operation of the pipeline system. This is the main reason that some pipeline systems have to use metal hoses.
A certain type of bearing lubricating oil catheter in the engine, occurred several times accident in the actual operation, because of the high installation stress.
（a）Schematic diagram of engine bearing lubrication oil conduit.
Position of bearing oil duct on the engine.
1 Air inlet;
3 Middle bearing;
4 Jet port;
（b）Reverse course to see the catheter and loading direction
Under conditions of a pressure of 0.1MPa, a temperature of 20 ℃, without corrosive media, a simulation test detected the stress at the fault location is 25.3MPa, and the average overload of ends A1、A2 direction is 83.2g while that of ends A3、A4 direction is 20.8g. The fatigue test results as follows: it only ccounted 40.9% without fracture when it reached 107 times safety cycles. It re-tested after cascading a section of metal hose under the same conditions above (see Figure 1-3). This time, the stress at the fault location dropped to 10.1MPa. The average overload of ends A1、A2 direction dropped to 11g while that of ends A3, A4 direction dropped to 205g. After multiple sets of fatigue test, the results as follows: there was no fracture when it reached more than 107 times safety cycles. The average overload of Ends A1, A2 direction and ends A3, A4 direction decreased by 85.8% and 87.8% respectively than the original, and the pass rate of installation increased from 40.9% to 100%.