The purpose of hydrostatic testing is to reveal defects in the vessel workmanship and to detect the presence of leaks. Note that most weld defects cannot be determined by the use of hydrostatic testing and must be determined by other forms of nondestructive testing.
Code rules permit an initial hydrostatic test pressure to be set at 1.5 times the maximum allowable working pressure (MAWP), or the design pressure to be stamped on the vessel. This value may be adequate for vessels of simple configuration used in noncorrosive services. However, for large or other complex vessels, or when a significant corrosion allowance is provided, this pressure rating can result in operating the vessel at stresses above those induced by the hydrostatic test (after corrosion has reduced the thickness of the vessel wall).
Hydrostatic test pressure should be calculated to stress the full thickness, including corrosion allowance and cladding of the strongest Category A weld to 150% of the Code allowable stress, at the test temperature, provided no component of the vessel is stressed above 90% of the minimum yield strength. The pressure should also be reduced as necessary to avoid overstressing weaker Category A welds.
Industry experience indicates that, for test purposes, an upper limit on stress equal to 90% of the minimum specified yield strength of the base metal can be used provided no excessive stress raisers are present. Common stress raisers are: improperly reinforced openings; poorly designed transitions between shell sections and between shells and heads (especially in the case of cone-to-cylinder intersections); or torispherical heads. Designs for especially large or thick vessels or unusual details should be reviewed by a stress analyst, especially in cases where test stresses approach the yield strength. Hydrostatic testing must also be conducted above the minimum design metal temperature (to avoid brittle fracture).
If certain Category A welds cannot be fully tested as outlined above without overstressing other parts, the following alterations should be made: first, heads and transition sections should be thickened; and second, Category A welds which still cannot be fully tested should be 100% radiographed.
Warning Hydrostatic or pneumatic pressure testing is dangerous and has caused many injuries. Testing, therefore, must be conducted by those trained to do so. The following guidelines are important to observe:
1. Avoid being within the vicinity of any pressure test. Let the authorized inspectors and those with direct responsibility for testing the vessel read the pressure gages and check for leaks.
2. Stay away from the vessel when it is being pressurized.
3. Never make a close-up visual examination for leaks, particularly when the vessel is being pressurized. Authorized personnel can make leak checks, after the vessel has been depressurized to design pressure, using safe methods. High pressure leaks eject high velocity jets of gas/liquid mixtures that can penetrate deep into the skin or eyes. Many persons have been blinded by visual leak checking without following proper precautions.
4. Most fabricators locate the hydro pump and its pressure gage at some nominal distance away from the vessel during the testing procedure so they need not get too close to the vessel. Stay away from the pump and its gage and tubing as well. Sometimes fittings break off or the gages leak, and anyone in the path of the leak or fittings may be injured.
5. Catastrophic failure during pressure testing is a possibility. There is a common belief that because the liquid compressibility is low, the stored energy and the energy released during a vessel failure is low. On the contrary, significant energy in hydrostatic tests comes from the compressibility of the water, the expansion of the vessel, energy stored in dissolved gases, and most significantly, from trapped air that may not have been removed. Photographs of hydrostatic tests intentionally carried to destruction indicate that nearby objects or buildings would be significantly damaged.
6. The energy of pneumatic testing is far greater than hydrotesting because of the high compressibility of air. Therefore, avoid being within the vicinity of any vessel under pneumatic testing.
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