STEEL VESSELS METAL STRESSED STATE INSPECTION
BY METAL STRESS INDICATORS IN-01M AND IN-02

 

A steel vessels (fig.1) and industrial liquid containers have large surface of cover and exposed permanently to different factors causing metal to complex plane (two-axial) stressed state. Inspection of plane stressed state of metal by convenient acoustic and magnetic techniques requires making a lot of measures, on the basis of ones the directions of principal stresses; their numeric values etc. are required to be calculated for each test point. The performance of these stressed state inspection tecqniques are not high, so at present application of ones still basically not came out from scientific laboratories.

Fig.1. Steel vessel for the gas condensate storage

 

Main tasks of metal stress inspection are the durability of structure determination and its residual service life evaluation. There are 4 techniques for durability and residual service life of complex industrial structures calculation, based on the different fundamental conditions. Each of ones is applicable only for certain type of steel parts; however all of them converge to common approach: an evaluation of the durability should be carried out on the value of effective мmechanical stress of metal, which is determined on the basis of the main on the components of principal stresses of metal and Poisson coefficients, as particularly described here.

Resulting experimental investigations carried out in cooperation with Ufa State Petroleum Technological University on the specimens of steel vessels working under pressure (see figures 2 and 3) the readouts of IN-01m and IN-02 the indicators turned out to corresponds uniquely to effective value of mechanical stress of metal. In the practice it means that the evaluation of durability and residual service file of metal structures could be provided directly on readouts of the indicators.

 

Fig.2. The bench for experimental investigation of stressed state of steel vessels metal under pressure

1 – oil pump, 2 – pressure gage, 3 – steel vessel investigated

 

   

Fig.3. Change of thin-wall steel vessel stressed state measured by IN-02 the scanning metal stress indicator before and after applying the load of 5 MPa internal pressure value

 

Lets consider an example of the stressed state inspection of the condensate collector presented on figure 1 (the material made from is 9MnSi5 steel). The facility given was exposed to direct flame for long time as result of fire occurred.

In order to take a decision for the possibility of following service of condensate collector the inspection of its metal stressed state was required necessarily, which is carried out by the metal stress indicators for two stages:

1. Express inspection of all 100% surface area of condensate collector by IN-02 scanning indicator (figure 4) to detect of local areas with intenvive stressed state of metal immediately and to evaluate a shape of its distribution;

Fig.4. Scanning the surface of steel condensate collector by IN-02 the metal stress indicator

 

Fig.5. Investigation of the local area with high-stressed metal on condensate collector by IN-01m the mechanical stress indicator

 

On the results of condensate collector express inspection by IN-02 the metal stress indicator the shape of metal stressed state distribution obtained is turned out extremely non-uniform and the local areas of high-stressed metal are detected, which are reflected in the surges of the metal stress indicator readouts.

2. An investigation of areas with high-stressed metal detected by IN-01m the mechanical stress indicator in order to evaluate quantitatively the durability of condensate collector durability (figure 5).

Fig.6. Stressed state of 3-th and 4-th metal sheets of condensate collector

 

According to IN-01m the metal stress indicator readouts an effective stress in local areas of high-stressed metal detected is exceeded the yield limit of 9MnSi5 steel for 9…32% (see to the figure 6), which corresponds to deep plastic state of the metal is close to the failure. So the steel condensate collector inspected had not approved for service would be followed.