Hydrostatic expansion test

Hydrostatic expansion test
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Posted by: sabri ^®

01/02/2006, 07:47:36

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The material of CNG (Compressed Natural Gas) cylinders is 34CrMo4 .This cylinder quenched in 850°C – 30 minute and Tempered in 550°C – 30 minute. In this condition the min. tensile strength is 990 MPa and min. yield strength is 890 MPa.
In the volumetric expansion test the cylinder filled with water and subjected to 300 bar (30 MPa) internal pressure (150 percent of working pressure)for 5 minute duration. At this pressure the medium stress in the cylindrical part of the cylinder is Z=(P*D)/(2*t)=(30*356)/(2*7)=762MPa

The stress caused by internal pressure is 762MPa that is less than Tensile Stress of the cylinder material and according to stress-strain curve this stress is in the elastic region.

After pressurization, the volume of the cylinder increases (The cylinder volume expansion) and when the pressure removed, the cylinder’s volume decreases to a value that is greater than prime volume. This means the volume of the cylinder increases permanently.
My question is why the cylinder volume changed permanently whereas the stress applied is under tensile strength (762MPa<990MPa) ???

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Re: Hydrostatic expansion test

Re: Hydrostatic expansion test -- sabri Post Reply Top of thread Forum

Posted by: swearingen ^®

01/03/2006, 08:39:45

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There are many possible reasons to explain what happened. One is that if the ends are welded on, the residual stresses in the welds plus the stress from the internal pressure could have moved the steel into the plastic range and permanently increased the volume. There may have been residual stresses in the cylindrical part of the tank as well.

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Re: Hydrostatic expansion test
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