Storage/Loading/Unloading - Storage Tanks - Tank Venting
April 2, 2008

Introduction Design Sizing of Vents Vent Dryers         Dryer Sizing Codes and Specifications

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Introduction

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.

Design

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.

Sizing of Vents

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

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.

Dryer Sizing

At the maximum fill or pump-out rate, the dryer system including vent piping should not create a high enough differential pressure that the design pressure or vacuum of the tank is exceeded.  Therefore, it is important to size the dryer for the maximum in-breathing and out-breathing rates.

A tank that averages half full for a month will breathe 60% of its volume per month with a 11.1°C (20°F) temperature change from day to night. Each 5.6°C (10°F) increase or decrease in the temperature differential adds or subtracts another 30% to the volume. As an example, a 37.9 m³ (10,000 US gallon) tank that is 3/4 full at the beginning of the month and 1/4 full at the end of the month would breathe approximately 22.7 m³ (6,000 US gallons) per month with a 11.1°C (20°F) temperature change per day.  Add this to the 18.9 m³ (5,000 US gallons) of liquid removed from the tank, and the total breathing of the tank is approximately 41.6 m³ (11,000 US gallons) of air.

Codes and Specifications

API 2000             Venting Atmospheric and Low-Pressure Tanks

29 CFR – OSHA 1910.106