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| Sulphuric Acid on the WebTM | Technology Manual | DKL Engineering, Inc. |
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Knowledge for
the Sulphuric Acid Industry Introduction
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Atmospheric storage tanks need to be
adequately vented to prevent the development of vacuum or pressure sufficient to distort
the roof of the tank or exceeding the design pressure as a result of filling or emptying,
and atmospheric temperature changes. Filling a tank with liquid results in the
displacement of air out of the tank. This is referred to as out-breathing.
Emptying a tank of liquid results in the atmospheric air being drawn into the tank which
referred to as in-breathing. Changes in ambient conditions will also
result in in-breathing and out-breathing. Increasing ambient temperatures will cause
the contents of the tank to heat up and expand resulting in out-breathing from the tank.
Decreasing ambient temperatures will cause the tank contents to contract resulting
in in-breathing. Changes in the local barometric pressure also resulting in
in-breathing and out-breathing. A tank vent should be located at the high
point of the tank roof to allow the entire volume of the tank to be vented. The tank
vent will not only allow the tank to breath but also allow harmful and potentially
dangerous gases to accumulate in the tank. Hydrogen gas will form as a result of the
corrosion of the carbon steel shell with concentrated sulphuric acid and must be vented
from the tank to avoid the formation of an explosive mixture. The roof of the tank should slope or curve
continuously to the high point such that no pockets are formed that will allow gas to
collect and stagnate. Atmospheric tank vents should be equipped
with a ‘goose neck’ or some other means to prevent the ingress of precipitation.
The vent opening should be equipped with a bird screen to prevent animals and
foreign objects from entering the tank. The vent should be directed away from areas
where personnel have access. Normal vents shall be sized either in
accordance with one of the following codes or standards: (1) The American Petroleum Institute Standard 2000 (1968),
Venting Atmospheric and Low-Pressure Storage Tanks (2) The vent size shall be at least as large as the filling or
withdrawal connection, whichever is larger but in no case less than 1 1/4" nominal
inside diameter. The vent on an atmospheric sulphuric acid storage tank should
be no less than 150 mm (6”) in diameter. Vent dryers are sometimes used to prevent the ingress of
moisture into a tank storing concentrated sulphuric acid. The hygroscopic nature of
concentrated sulphuric acid means that any moisture entering the tank will be quickly
absorbed into the acid. Over time the top layer of acid will become more dilute and
more corrosive. In tanks where the contents are not turned over frequently, the top
layer of weak acid will actually corrode a ring entirely around the inside of the tank.
Desiccant dryers are the most common way of preventing the
ingress of moisture into a tank. The size of the dryer required to protect a storage
tank is determined by the amount of air and the rate at which the air enters the storage
tank. Generally, the rate at which the air enters the tank is the pump-out rate;
however, there are exceptions. A large tank that is nominally empty can have air
entering the tank due to temperature and pressure changes faster than the pump-out rate.
The desiccant in the dryer should be sized to last from one to six months per
charge of desiccant. With shorter service cycles, the labour cost becomes
prohibitive and, with longer cycles, the capital costs become higher. When the dryer
desiccant capacity has reached its limit, the dryer is taken off-line and the desiccant
regenerated.
API 2000
Venting Atmospheric and
Low-Pressure Tanks 29 CFR – OSHA 1910.106
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Copyright© 2005,
2006, 2007, 2008 DKL
Engineering, Inc., All Rights Reserved |
