Thursday, June 25, 2026

A commonly encountered system involving both material accumulation and heat input is the low pressure tank. For fixed roof tanks, a vapor headspace is typically present above the residing liquid, and without credit for any favorable response of instrumentation that may be present, any compression or expansion of that headspace, either through fluid transfer or ambient temperature changes, may result in excess pressure or vacuum within the tank.

Inbreathing. One of the more severe heat transfer events that may affect a low pressure tank is the cooling caused by a sudden rainstorm after the tank has been heated by solar radiation and ambient heat transfer. The rainwater hitting the surface of the tank provides a significant heat sink with a potentially large temperature differential, resulting in a rapid change in the temperature of the gas in the headspace. For systems where an ideal gas is present in the headspace, the heat transfer to the liquid and tank wall is ignored, and there is no condensation of the gas taking place, the maximum volumetric rate of contraction can be calculated based on the following:

where h_e is the heat transfer coefficient and T_w is the wall temperature.

The tank is typically presumed empty for this calculation; therefore, the volume of the headspace is equal to the volume of the tank. The rate of temperature change and heat transfer coefficients are the subject of some debate; nonetheless, ISO 28300:2008 provides guidance on appropriate values.² API Standard 2000, 4^th Edition used 20 BTU/hr·ft² as a maximum heat transfer rate or 100 °R/hr as a maximum temperature change rate for tanks having a starting temperature of 120°F, a starting pressure very close to atmospheric pressure, and assuming the contents of the empty tank were very similar to air.³

There is the potential for the tank to be emptying while the sudden rainstorm occurs; therefore, the effects of liquid withdrawal should also be accounted for. As a result, the venting requirement is the sum of the thermal contraction and the volumetric rate of liquid withdrawal to prevent excess vacuum. This venting requirement is commonly referred to as inbreathing for low pressure tanks.

Outbreathing. The heating of a tank by ambient heat transfer does not occur as quickly as the cooling during a sudden rainstorm; nonetheless, similar calculations on the maximum volumetric rate of expansion can be performed. ISO 28300:2008 provides guidance on appropriate values. When dealing with the overpressure of low pressure tanks, consideration is needed for any generation of vapors either through the heating of the contents or by means of the fluid entering the tank. Again, ISO 28300:2008 provides guidance on appropriate values. The heating and liquid transfer are assumed to occur simultaneously; therefore, the venting requirement is the sum of the thermal expansion, the volumetric rate of liquid input, and any gas or vapor introduction to prevent excess pressure. This venting requirement is commonly referred to as outbreathing for low pressure tanks.

Steam collapse. An additional consideration for the inbreathing of low pressure tanks may include the potential for steam collapse. In the event of a sudden rain storm when a tank is being steam cleaned or spraying cold liquid into a tank after steam cleaning but before removing the steam, the steam may condense rapidly while the tank is isolated. This steam collapse may result in the formation of a vacuum within the tank with a potentially significant inbreathing requirement to offset the vacuum; therefore, risk assessments are typically employed to determine appropriate mitigation measures.

Blog series information. This blog is part of a series on the proposed updates to the CCPS Guidelines 2nd edition §3.3 Venting Requirements for Nonreacting Cases that were removed during final editing. See the general CCPS Guidelines for Pressure Relief and Effluent Handling 2nd Edition review for more information.

 

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[1] AIChE Center for Chemical Process Safety. “CCPS Guidelines for Pressure Relief and Effluent Handling Systems”. 2nd Edition, 2017; New Jersey: John Wiley & Sons, Inc.

[2] ISO 28300:2008. Petroleum, petrochemical and natural gas industries — Venting of atmospheric and low-pressure storage tanks. International Standards Organization, Geneva, Switzerland.

[3] API Standard 2000. Venting Atmospheric and Low Pressure Storage Tanks, 4^th Edition. American Petroleum Institute, Washington DC, USA.

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