Hydro Test Confusion

Hydro Test API 650 Tank

Hydro Test API 650 Tank


There are three main codes when it comes to the fabrication of field erected storage tanks. When experienced crews are required to perform all services related to a fully operational and functional hydro testing North American customers’ look the NCS Fluid Handling Systems and the skilled team trained to perform all operational checks related to the testing of these tanks.

While some water management businesses will advertise the filling of a tank or multiple tanks with water as performing the Hydrostatic test, this only contributes to confusing what a fully functional test is. At NCS Fluid Handling Systems we understand the total cost of ownership of these valuable tank assets and the long-term expectations owners have for their continued performance, that is why NCS provides specialized training to all levels of the team, so that these dedicated specialists meet the specialized skills required and enhanced degree of service owners expect for pre-commissioning these value assets. Starting with temporary water diversion licensing, surveying crossing agreements, water analysis (pre-and post test), water fill and transfer, full operational checks of floating roofs, stairs, ancillary components, filtration, iron removal, erosion controlled water release to source and tank dewatering and drying. As the sole source for any or these required services NCS Fluid Handling Systems understands our clients needs fully, not just one part of a valuable overall service.


American Petroleum Institute (API) 650 was originally released in 1961 and is the international standard for tanks widely used for tanks that are designed to internal pressures of 2.5 PSI or less and store products such as crude oil, gasoline, chemicals and produced water.

API realized that the industry needed a designer specification that allowed for regional and global considerations that the API 12 series did not provide. The API-12C specification was, for lack of a better term “pre-engineered and tested” as most of the 12 series tanks are, they are standardized designs based on practical applications or predetermined size and performance parameters. So, from the foundation of API-12C the API-650 became the governing code for petroleum product storage tanks. This standard now provided designers the considerations for varying conditions that globally affect tank design, such as extreme weather variations and material performance, seismic design and zones, wind overturning, internal pressures, varying heights and diameters, material grades and thickness, non-destructive testing as well as hydro testing for all the above conditions and variations.


API 620 picks up where API-650 leaves off with regards to pressure and is the governing standard for the design and large, above ground field-assembled storage tanks that contain petroleum gases or vapors as well as other liquid products.  These storage tanks are designed to operate at 250°F or less with an operating pressure of up to 15 PSI, at which point the ASME codes take over for pressure vessel design. For the most part API 620 Tanks are typically referenced for Liquefied Natural Gas, Liquid Oxygen or other storage of chemicals which require internal design pressures more than those allowed by API 650.

The API-620 also allows for consideration of non-fully supported flat bottoms, or in simpler terms unsupported cones or bottom not fully supported reference API-620, as well may reference American Society of manufacturing Engineers (ASME) for compression sections (But this opens a whole other discussion).

API 620 also includes appendixes that cover additional requirements;

  • Appendix Q for low-pressure storage tanks for liquid hydrocarbon gases,
  • Appendix R for low-pressure storage tanks for refrigerated products,
  • Appendix S for austenitic stainless-steel tanks.

API 620 also covers requirements for examination methods, procedures, acceptance criteria, and qualification of examination personnel. API 620 examination requirements, but are not limited too, non-destructive examination (NDE) which include, visual examination (VT), magnetic particle testing (MP), liquid penetrant testing (LPT), radiographic testing (RT), ultrasonic testing (UT) and other applicable test methods may include vacuum box leak test, air-solution film leak test and Hydrostatic testing where the tank is filled with water similar to API-650 with the addition of a pneumatic pressure applied in accordance with all applicable quality and safety processes and procedures to the envelope above the shell to roof juncture.  


Like the old API-12C specification the remaining tank specification is API 12D which is utilized for the construction and field erection of welded tanks for storage of production fluids. Typically, this standard is used by upstream petroleum producers or companies. Whereas welded shop fabricated tanks used in this same service are governed by the API 12 F standard, the API 12D standard is used for field erected tanks. API 12D does not have the designer latitudes that API-650 has and uses predetermined tank sizes, dimensions, internal design pressure or vacuum pressure (also known as in and out breathing), and set criteria for accessories and testing requirements.

As a point of interest, for a tank to be a API – 12F or 12 D tank the nameplate must bear the API monogram with appropriate documentation and inspection, whereas a API-650 tank uses a complete data package referencing such things as, materials certs, as built drawings, shell and bottom elevations for Hydro test, name plate facsimile and manufacturers certificate.

The way a Hydro should be done