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. Strength is usually tested by
measuring permanent deformation of the container. Hydrostatic testing is the
most common method employed for testing pipes and pressure vessels. Using this
test helps maintain safety standards and durability of a vessel over time. Newly
manufactured pieces are initially qualified using the hydrostatic test.
They are then re-qualified at regular intervals using the proof pressure test
which is also called the modified hydrostatic test. Testing of pressure
vessels for transport and storage of gases is very important because such
containers can explode if they fail under pressure.
Testing procedures Hydrostatic tests are conducted under
the constraints of either the industry's or the customer's specifications, or may
be required by law. The vessel is filled with a nearly incompressible liquid -
usually water or oil - pressurised to test pressure, and examined for leaks or
permanent changes in shape. Red or fluorescent dyes may be added to the
water to make leaks easier to see. The test pressure is always considerably
higher than the operating pressure to give a factor of safety. This factor of
safety is typically 166.66%, 143% or 150% of the designed working pressure,
depending on the regulations that apply. For example, if a cylinder was rated to
DOT-2015 PSI, it would be tested at around 3360 PSI. Water is commonly used
because it is cheap and easily available, and is usually harmless to
the system to be tested. Hydraulic fluids and oils may be specified where
contamination with water could cause problems. These fluids are nearly
incompressible, therefore requiring relatively little work to develop a high
pressure, and is therefore also only able to release a small amount of energy
in case of a failure - only a small volume will escape under high pressure
if the container fails. If high pressure gas were used, then the gas would expand
to V=(nRT)/p with its compressed volume resulting in an explosion, with the
attendant risk of damage or injury. This is the risk which the testing is
intended to mitigate. Small pressure vessels are normally
tested using a water jacket test. The vessel is visually examined for defects
and then placed in a container filled with water, and in which the change in
volume of the vessel can be measured, usually by monitoring the water level in
a calibrated tube. The vessel is then pressurized for a specified period,
usually 30 or more seconds, and if specified, the expansion will be
measured by reading off the amount of liquid that has been forced into the
measuring tube by the volume increase of the pressurized vessel. The vessel is
then depressurized, and the permanent volume increase due to plastic
deformation while under pressure is measured by comparing the final volume
in the measuring tube with the volume before pressurization. A leak will give
a similar result to permanent set, but will be detectable by holding the volume
in the pressurized vessel by closing the inlet valve for a period before
depressurizing, as the pressure will drop steadily during this period if
there is a leak. In most cases a permanent set that exceeds the specified
maximum will indicate failure. A leak may also be a failure criterion, but it
may be that the leak is due to poor sealing of the test equipment. If the
vessel fails, it will normally go through a condemning process marking the
cylinder as unsafe. The information needed to specify the
test is stamped onto the cylinder. This includes the design standard, serial
number, manufacturer, and manufacture date. After testing, the vessel or its
nameplate will usually be stamp marked with the date of the successful test,
and the test facility's identification mark.
A simpler test, that is also considered a hydrostatic test but can be performed
by anyone who has a garden hose, is to pressurize the vessel by filling it with
water and to physically examine the outside for leaks. This type of test is
suitable for containers such as boat fuel tanks, which are not pressure
vessels but must work under the hydrostatic pressure of the contents. A
hydrostatic test head is usually specified as a height above the tank
top. The tank is pressurized by filling water to the specified height through a
temporary standpipe if necessary. It may be necessary to seal vents and other
outlets during the test. Examples
Portable fire extinguishers are safety tools that are required to be on hand in
almost every public building. Fire extinguishers are also highly
recommended in every home. Over time the conditions in which they are housed, and
the manner in which they are handled have an impact on the structural
integrity of the extinguisher. A structurally weakened fire extinguisher
can malfunction or even burst when it is needed the most. To maintain the quality
and safety of this product, hydrostatic testing is utilized. All critical
components of the fire extinguisher should be tested to ensure proper
function. Pipeline testing
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 allowable operating pressure 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 strength 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 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.
Testing frequency Most countries have legislation or
building code that requires pressure vessels to be regularly tested, for
example every two years for high pressure gas cylinders and every five or
ten years for lower pressure ones such as used in fire extinguishers. Gas
cylinders which fail are normally destroyed as part of the testing
protocol to avoid the dangers inherent in them being subsequently used.
These common gas cylinders have the following requirements:
DOT-3AL gas cylinders must be tested every 5 years and have an unlimited
life. DOT-3HT gas cylinders must be tested
every 3 years and have a 24-year life. DOT-3AA gas cylinders must be tested
every 5 years and have an unlimited life. in which case the cylinder meets
certain specifications and can have a 10-year hydrostatic test life).
Typically organizations such as ISO, ASTM and ASME specify the guidelines for
the different types of pressure vessels. See also
Pipeline pre-commissioning References
External links "OSHA". Retrieved 2006-10-05.