Relief requirements for boiling systems with heat transfer
For heat input to two phase systems with a phase change, the vaporization of the liquid forms the basis for the required relief rate, ignoring the expansion of the individual phases. For these systems, the hydrodynamics within the container should be considered. The relief requirements discussed below are for heating or cooling of a constant volume container containing a boiling two-phase system.
Relief requirements for boiling systems with complete disengagement. For containers having two phases with a phase change occurring in response to the heat input, the vent stream is expected to be all vapor throughout the venting incident in the vast majority of nonreactive cases when the relief device is a pressure relief valve and is installed in the vapor space. Liquid swell out the top due to accumulated bubbles can occur for a time if the container starts out essentially liquid-full. For atmospheric tanks, droplet entrainment at less-than-full levels can theoretically cause excessive pressure accumulation2,3. The predicted consequences of an initial period of two-phase venting become of less concern as the design pressure and overpressure allowance increase, even for vessels that begin liquid-full.4
Present industry experience shows that pressure relief valves and vents sized for all-vapor venting are adequate even if some initial two-phase venting is predicted.4,5 Thus, present practice is to use the all-vapor venting basis for relief system sizing; however, for the purpose of effluent system design, any period of two-phase venting may need to be accounted for even if the two-phase venting is not the basis for vent sizing. For rupture disk systems, the blowdown of the vessel that occurs when the rupture disk is activated can causes significant superficial velocities within the container being protected, which typically results in two-phase venting, even when the rupture disk is mounted in piping connected to the vapor space of the container. For cases in which the relief device is mounted in piping connected to the liquid space of the container, the required relief rate is based on the displacement of that liquid by the vapor generated.
The recommended general approach for all-vapor venting is developed to satisfy the basic requirement that the minimum required vent rate equals the vaporization rate at the design value of pressure accumulation in order to control the pressure rise. The venting requirement for a single-component fluid with a constant heat input is:
Where W is the required relief rate, Q is the heat input, Hgf is the latent heat of vaporization, vgf is the change in specific volume between the liquid and gas phases, and vg is the specific volume of the gas phase. The properties are evaluated at the maximum venting pressure condition. Note this equation can be used for multicomponent fluids by replacing the latent heat parameter by an effective heat of vaporization that is the heat required to vaporize the liquid mixture at the given equilibrium vapor composition.
At moderate pressure levels, the liquid specific volume is small with respect to that of the vapor. Thus, the specific volume factor in the equation above is typically close to unity. It is for this reason that the specific volume factor is not shown in many guidelines, including API Standard 521 5 and API Standard 2000 6.
The CCPS Guidelines 2nd edition1 §188.8.131.52 provides additional information regarding an alternative form of the calculation near the critical region. It is here that API Standard 521 5 §184.108.40.206.5.2 recommends the use of a minimum latent heat value of 50 BTU/lb.
Relief requirements for boiling systems with partial disengagement. For the majority of nonreactive systems, the evaluation of partial disengagement tends to be more engineering effort than can be justified given the estimation techniques being employed to derive relief requirements. While the vessel hydrodynamics should be considered when evaluating the relieving requirements, for those instances where complete disengagement is not expected, the homogeneous vessel venting techniques, as discussed below, are commonly employed. The CCPS Guidelines 2nd edition1 §3.2.9 provides the details needed to perform partial liquid-vapor disengagement calculations.
Relief requirements for boiling systems with homogeneous vessel venting. A conservative value of vent rate for nonreactive systems experiencing external heat input is obtained by presuming that the bubbles formed at the wall will uniformly froth up the entire contents and the relief device vents this homogeneous mixture. The maximum venting requirement is based on the dynamics of the system response to the heat input, which may be constant or variable, and the history of the venting through the relief device (that is, the amount of mass removed from the vessel at a given point of time).
The CCPS Guidelines 2nd edition1 §220.127.116.11 present the different cases for constant heat input7, variable heat input from an isothermal heat source8, and variable heat input from a non-isothermal heat source 8.
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.
 AIChE Center for Chemical Process Safety. “CCPS Guidelines for Pressure Relief and Effluent Handling Systems”. 2nd Edition, 2017; New Jersey: John Wiley & Sons, Inc.  Fisher HG, Forrest HS. “Protection of storage tanks from two-phase flow due to fire exposure.” Process Safety Progress, 14(3): 183-199.  Forrest HS. “Emergency relief system design for fire exposure with consideration of multiphase flow.” Proceedings of the International Symposium on Runaway Reactors and Pressure Relief Design. Boston, MA, 2-4 Aug 1995: 604-630.  Simpson LL. “Fire exposure of liquid filled vessels.” Process Safety Progress. 22(1): 27-32.  American Petroleum Institute. “API Standard 521-Pressure-relieving and Depressuring Systems”. 6th Edition, April 2014.  American Petroleum Institute. “API Standard 2000-Venting Atmospheric and Low-pressure Storage Tanks”. 7th Edition, March 2014.  Leung JC. “Simplified vent sizing equations for emergency relief requirements in reactors and storage vessels.” AIChE Journal. 32(10): 1622—1634.  D’Alessandro R, Hertel C. “Simplified pressure relief requirements for indirectly heated equipment containing a homogenous fluid.” Proceedings of the AIChE Spring Meeting and Global Congress on Process Safety. 2007, paper 83f.