SECTION 10—POST CONSTRUCTION ACTIVITIES
10.1 Scope
This section provides requirements and guidance for post construction activities necessary for the safe startup of
storage tank systems covered in this standard. Activities include pressure testing, purging, and cool down.
— Additional requirements for concrete structures are provided within ACI 376.
— Additional requirements for metal structures are provided in API 620.
10.2 General
All construction activities, inspections, testing, and cleaning (all sand, sludge, and standing water shall be removed) of
the tank shall be completed. All instrumentation shall be calibrated and verified prior to final closure of the tank. All
electrical systems including the foundation heating system shall be verified as operational. A drying/purging and cool
down procedure shall be prepared by the Tank System Contractor for incorporation into the detailed plant purge and
cool down procedure.
10.3 Hydrostatic and Pneumatic Testing
10.3.1 Testing of Primary Liquid and Vapor Containers
Primary liquid and vapor containers shall be hydrostatically tested and leak tested in accordance with the API 620 or
ACI-376, as applicable, and the following, and all leaks shall be repaired.
a) Primary liquid containers shall be hydrostatically tested to a minimum liquid height equal to the design liquid height
times the product design specific gravity times 1.25 or higher if so stated in the governing construction code or
standard.
b) Primary vapor containers shall be tested to an overload pressure of 1.25 times the pressure for which the vapor
space is designed.
c) Settlement monitoring during hydrostatic test in accordance with API 620, C.11 shall be a mandatory requirement.
10.3.2 Testing of Secondary Liquid Containers
Hydrostatic testing of secondary liquid containers of double and full containment tank systems is not required unless
explicitly specified by the Purchaser. If specified for specific projects, hydrostatic testing of secondary liquid containers
shall include the following:
a) hydrostatic testing of the primary container shall be completed prior to the secondary container test and shall not
be drained prior to filling and emptying the secondary container;
b) verification of primary bottom leak tightness shall be made during testing of the primary liquid container;
c) the bottom insulation system shall be protected from exposure to water during secondary container testing;
d) water test height for the secondary container shall, as a minimum, be set at a height that produces a liquid
pressure in the base of the container equivalent to 1.25 times the pressure produced to contain the full primary
container filled with product at design liquid level.
10-1
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10-2 API STANDARD 625
10.3.3 Pressure Testing of Pump Columns
10.3.3.1 Pump columns shall be pressure tested, hydrostatically or pneumatically, in accordance with the standard
used for their design (see 7.3.3).
10.3.3.2 Pump columns shall be installed prior to hydrostatic testing of the primary liquid container.
10.3.3.3 Pump column internal pressure testing shall be performed with the primary liquid container empty, and the
pump column shall be empty when the primary liquid container is hydrostatically tested.
10.3.4 Pressure Testing of Piping
Piping shall be pressure tested as required by API 620, Appendix Q or Appendix R.
10.4 Drying and Purging
10.4.1 Immediately following the hydrostatic test of the tank, residual standing water shall be removed.
10.4.2 Erection procedures shall incorporate provisions that eliminate collection of excessive moisture within the
insulation system.
Comment: Excessive free water within the insulation system can cause the insulation system to perform below its
design basis and, in the case of cellular glass load bearing insulation, could cause damage to the insulation system.
10.4.3 The dew point values in Table 10.1 may be used as an indication for when detrimental moisture has been
adequately removed. It is not necessary to lower the dew point below 32 °F (0 °C). If the recommended dew point is
reached at the end of the nitrogen purge and if the nitrogen purge is followed by a warm product purge, it is not
necessary to take subsequent readings
Comment: Excessive moisture in the tank atmosphere will be naturally removed from the gas when the gas
temperature drops below the dew point of the gas. Therefore, removal of most moisture from the gas within the tank
will be achieved through the process of nitrogen and warm gas purges discussed below.
10.4.4 A nitrogen purge shall be conducted to reduce the oxygen level in the tank to a level that will allow the
product to be introduced without creating a combustible gas mixture.
Comment: The O2 end point value in Table 10.1 is a value that is considered safe for ethylene per AGA Purging
Principles and Practice. Percent oxygen in nitrogen gas end points for all other gasses covered by this standard could
be safely set at a higher level but the dew point values listed will normally be harder to achieve than the 8 % O2 level.
10.4.5 An exception to the O2 values in Table 10.1 is ammonia storage. The O2 level achieved prior to cool down
(liquid accumulation) is recommended to be lower than the value in Table 10.1 and should be as low as practical.
Comment: Anhydrous ammonia storage is susceptible to stress corrosion cracking (SCC). Water additions have been
shown to reduce the SCC process and any free moisture exposed to ammonia vapor will combine with the ammonia.
The percent O2 at time of liquid accumulation is also important to reduce the SCC process.
10.4.6 A warm product purge to between 80 % and 90 % product gas normally follows the nitrogen purge and is
completed prior to tank cool down. If liquefied gas is introduced directly into a nitrogen environment, the initial
introduction can cause the temperature of the liquid to drop below the product design temperature and the design
metal temperature. Material selection and tank design shall consider this lower temperature if a warm product purge
is not performed.
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TANK SYSTEMS FOR REFRIGERATED LIQUEFIED GAS STORAGE 10-3
10.4.7 If the tank is not cooled down immediately following completion of the purging, a small positive pressure shall
be maintained to prevent ingress of oxygen and moisture. If the cool down is delayed more than 2 weeks, the purge
dew point end point for the annular space, as measured at the end of the nitrogen purge, shall be reduced from the
values in Table 10.1 by 5 °C to stabilize the moisture levels in the tank.
Comment: Purge dew point levels recommended for primary concrete containers are found in ACI 376.
Table 10.1—Recommended Drying and Nitrogen Purging End Points for Steel Tanks
Section Dew Point at 1 atm O2 Concentration Level
Inner tank and dome space –5 °C (+23 °F) Max. 8 % Max.
Annular space
(internally insulated double wall suspended +10 °C (+50 °F) Max. 8 % Max.
deck tanks)
Bottom insulation space No measurement necessary No measurement necessary
Annular space of a double wall double roof
No measurement necessary No measurement necessary
tank
10.5 Cool Down
10.5.1 Cool down shall be performed after the tank purge has been completed. A cool down procedure shall be
developed to provide a controlled process. During the initial introduction of liquid product, it is important to ensure that
the storage tank cools as uniformly as possible. Sharp thermal gradients can cause permanent local distortions and
potential crack growth.
10.5.2 The cool down rate for a steel primary liquid container shall be controlled to a maximum average of 5 °C/h
(9 °F/h) and shall not exceed 8 °C/h (15 °F/h) during any one hour. For thermal gradients and cool down rates
recommended for concrete primary liquid containers, see ACI 376.
10.5.3 The cool down may be considered complete when a minimum of 150 mm (6 in.) of liquid product is
maintained in the storage tank. At this point the bottom temperature elements and temperature elements in the first
3 m (10 ft) of the vertical TE array will be reading approximately product storage temperature.
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