A hydrostatic test is
a way in which pressure vessels such as pipelines, plumbing, gas cylinders,
boilers and fuel tanks can be tested for strength and leaks. The test involves filling
the vessel or pipe system with a liquid, usually water, which may be dyed to
aid in visual leak detection, and pressurization of the vessel to the specified
test pressure. Pressure tightness can be tested by shutting off the supply
valve and observing whether there is a pressure loss. The location of a leak
can be visually identified more easily if the water contains a colorant.
It is essential to
identify the limits of the test process and obtainable results. There are
several types of flaws that can be detected by hydrostatic testing, such as:
o Existing flaws in the material,
o Stress Corrosion Cracking (SCC) and actual
mechanical properties of the pipe,
o Active corrosion cells, and
o Localized hard spots that may cause failure
in the presence of hydrogen.
When a pipeline is
designed to operate at a certain maximum operating pressure (MOP), it must be
tested to ensure that it is structurally sound and can withstand the internal
pressure before being put into service. Generally, gas pipelines are
hydrotested by filling the test section of pipe with water and pumping the
pressure up to a value that is higher than maximum allowable operating pressure
(MAOP) and holding the pressure for a period of four to eight hours.
sumber gambar :
http://en.wikipedia.org/wiki/File:Water_jacket_test_diagram.jpg
Hydrotesting of
pipes, pipelines and vessels is performed to expose defective materials that
have missed prior detection, ensure that any remaining defects are
insignificant enough to allow operation at design pressures, expose possible
leaks and serve as a final validation of the integrity of the constructed
system. ASME B31.3 requires this testing to ensure tightness and strength.
Buried high pressure
oil and gas pipelines are tested for strength by pressurizing them to at least
125% of their maximum operating pressure (MAOP) at any point along their
length. Since many long distance transmission pipelines are designed to have a
steel hoop stress of 80% of specified minimum yield (SMYS) at MAOP, this means
that the steel is stressed to SMYS and above during the testing, and test
sections must be selected to ensure that excessive plastic deformation does not
occur. Test pressures need not exceed a value that would produce a stress
higher than yield stress at test temperature. ASME B31.3 section 345.4.2 (c)Other
codes require a more onerous approach. BS PD 8010-2 requires testing to 150% of
the design pressure - which should not be less than the MAOP plus surge and
other incidental effects that will occur during normal operation.
Leak testing is
performed by balancing changes in the measured pressure in the test section
against the theoretical pressure changes calculated from changes in the
measured temperature of the test section. Australian standard AS2885.5
"Pipelines—Gas and liquid petroleum: Part 5: Field pressure testing"
gives an excellent explanation of the factors involved.
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