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Why Perform a Hydrostatic Test on a Storage Tank?

January 31, 2018

New tank construction and major alterations to existing storage tanks have many testing requirements that are intended to confirm the integrity of the equipment.  Hydrostatic testing, as the name implies, is essentially testing a piece of equipment by filling it with water.  API-650, the standard to which many new oilfield storage tanks are constructed and API-653, the standard for tank repair have different requirements for hydrostatic testing.  The proximity of the equipment to a water source, the client and the nature of certain repairs can also have an effect on the necessity of a hydrostatic test. 

Why Perform a Hydrostatic Test on a Storage Tank?

Following completion of a new storage tank, one of the final mechanical tests recommended is a hydrostatic test.  The first and most obvious reason to fill the tank with water is to ensure the tank has no leaks.  The tank is filled with water at a prescribed rate and then examined after 24 hours to ensure there are no cracks, pinholes, or other discontinuities in the welds.  The second purpose of the test is to confirm the mechanical strength of the tank.  If there are weak spots they will be evident when the tank is full of water, and far less detrimental to workers and the environment than if the tank is full of a potentially toxic substance.  The final, and most critical observation made during a hydrostatic test is the integrity of the foundation that supports it.  The foundation is one of the leading causes for tank damage.  While the tank is full of water settlement can be observed and measured in a controlled environment confirming its ability to withstand the weight of a full tank and provide baseline measurements for comparison over time.

What if I Have No Water?

Depending on the size of your storage tank a very large quantity of clean water can be required to hydrostatic test.  Construction sites are sometimes remote and trucking or pumping in litres and litres of water is not feasible, not to mention having a suitable location for drainage following the test.  API-650 does have options available to the owner to forego the hydrostatic test if additional, alternate testing is performed.  The first test that is required is an alternative leak test of the tank. This can be achieved by performing a penetrating oil test or an air pressure test. Both of these tests are commonly used to find leaks in storage tanks. The second requirement is that the tank must receive additional radiographic examination to capture all weld intersections in the shell. These intersections are critical in the overall seam strength and although you’re increasing the amount of radiography by four times, it is essential in proving integrity that would have been otherwise tested by the hydrostatic test. The last item that API-650 requires is that the owner designs the foundation to a minimum weight of 110 percent of the product weight or to a least the equivalent weight of the tank filled with water.  If all these alternate methods are completed the hydrostatic test can be waived and the equipment remains in compliance with the standard.  Settlement of the foundation can still be observed during the first fill with product and baseline measurements taken.

API-653 Major Alterations

Following what is considered a “major alteration” by API-653 a hydrostatic test is also recommended.  Similar to above, the most obvious reason for this is to ensure there are no leaks in the new welds and the overall integrity of the repair area.  With consideration there are no two repairs situations alike, there are many factors that can affect the requirement of a hydrostatic test and the steps to be taken vary if the client is interested in waiving the test, or the testing environment is undesirable.  Independent engineering review, additional testing requirements and analysis can be completed to confirm if the repair situation is eligible for exemption from hydrostatic testing. 

Elite has participated in the hydrostatic testing, performance of alternate testing and assistance with major alteration testing on many new storage tank builds and a vast array of repairs.  With our experience and partners in industry, we would be happy to explore your next project and what testing methods suit your needs.