Scenario Considerations:
Mixing of Hot and
Volatile Fluids
October 8, 2014
Summary. An inadvertent mixing of hot and volatile fluids can result in overpressure if the volatile fluid
is vaporized and sufficient volumetric accumulation occurs. Hot fluid temperatures exceeding the
bubblepoint temperature of the volatile fluid is the common criterion for establishing applicability of the
scenario, and cases in which that hot fluid temperature exceeds what is known as the superheat limit of
the volatile fluid are usually identified as requiring a refocus of engineering effort to prevention rather
than mitigation. For cases involving distillation systems immediately downstream, an additional
screening for the scenario applicability can be made based on the capacity of the cooling system.
Scenario Applicability. A potential cause of overpressure occasionally encountered is the mixing of hot
and volatile fluids, which may cause vaporization of the volatile fluid and subsequent overpressure of
the system.
The primary criterion used in evaluating the applicability of the overpressure scenario should be the hot
fluid temperature (Th) compared to the bubblepoint temperature of the volatile fluid (Tb): Th > Tb may
result in vaporization of the volatile fluid, thus potentially causing overpressure. There is a pressure
dependence of the bubblepoint of a volatile fluid, and the potential for overpressure would result in a
consistent basis of the system limiting pressure (i.e. the lowest MAWP in the protected system) being
selected for the evaluation of the bubblepoint temperature.
Further analysis for the applicability of the scenario is possible. There should be sufficient thermal
energy in the hot fluid to result in bringing the volatile fluid to its bubblepoint and subsequently
vaporizing the volatile fluid; therefore, an evaluation of the thermodynamics of the system is possible.
In most cases encountered, the quantity of volatile liquid is not enough to completely quench the hot
fluid and the thermodynamic assessment is commonly not performed.
In addition, there should be sufficient volumetric accumulation within the system in response to the
vaporization of the volatile fluid. Acceleration of the mixed fluid or the potential for condensation of the
volatile fluid (particularly in distillation tower systems) are other criteria that may be applied. The
acceleration criterion is commonly established for the evaluation of tube rupture scenarios, and
application of that criterion to this scenario is usually pertinent provided the downstream system can
adequately accept the vapor (which is an implicit assumption in the tube rupture acceleration analysis).
The potential for condensation of the volatile fluid involves evaluating the capability of cooling within
the system to condense the volatile fluid while still processing the normal cooling loads as well as being
able to appropriately handle the condensed volatile fluid. It is common to use a process simulator to
assess the capability to condense the volatile fluid, although an estimate may be made using the normal
operating and design duty of the cooling system. For total condensing cooling systems, the cooling
temperature must be less than the bubblepoint temperature, the volatile liquid must have an
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acceptable means for leaving the accumulator, and the design duty of the condenser must be greater
than the normal operating duty plus the energy required to condense the excess volatile fluid.
Scenarios Where Prevention May be More Appropriate. A caution is present in API Standard 521 that
in some cases the speed at which this vaporization can occur may exceed the response time of some
pressure relief devices. The criterion for which this caution applies should be the hot reservoir / fluid
temperature (Th) compared to the superheat limit of the volatile fluid (Ts): Th > Ts may result in an
“explosive” vaporization and the ability of a pressure relief valve to provide protection should be
questioned and other means to prevent or mitigate the scenario should be explored1. There is a slight
pressure dependence of the superheat limit temperature of a volatile fluid; therefore, an appropriate
pressure should be selected - the minimum operating pressure between the hot reservoir or volatile
fluid is recommended. The superheat limit can be a theoretically derived value; however, the derivation
requires an equation of state capable of extending into the meta-state regions (and appropriate
experiments to establish those equations of state). More common approaches are various experimental
techniques, a summary of which (along with data for many fluids of interest) has been published2.
The boiling point and superheat limit for water as a function of pressure is shown in the figure below.
1
H.K. Fauske, “Mechanisms of Liquid-liquid Contact and Heat Transfer Related to Fuel-coolant Interactions”, Paper
presented at Second Specialists Meeting on Sodium Fuel Interaction in Fast Reactors; Ispra, Nov 21-23, 1973.
2
C.T. Avedisian, “The Homogeneous Nucleation Limits of Liquids”, J. Phys. Chem. Ref. Data, 14(3), 1985, pp. 695729.
Considerations in the Evaluation of the Scenario for Mixing of Hot and Volatile Fluids
Page 2
Required Relief Rate Evaluation. Once the scenario is found to be credible, an evaluation of the
required relief rate is in order. For those cases in which “explosive” vaporization may occur, the
vaporization rate is very high, and specialized techniques are used. In these cases, the engineering
effort is often refocused onto prevention of the scenario in the first place, rather than on quantifying
rates.
For the remaining cases, the heat transfer rates between the intermingled fluids would provide the
theoretical basis for the rate of vaporization. Unfortunately, this is often unknown and difficult to
estimate; therefore, a common basis is to assume instantaneous vaporization, and the engineering
effort is placed on coming up with a reasonable estimate of the rate at which the fluids are introduced.
This approach is reasonable for cases involving relatively small amounts of volatile fluid entering into a
large quantity of hot fluid.
The required relief rate may then be estimated based on this introduction rate, and the relieving fluid is
the vaporized volatile fluid, consistent with other similar estimation techniques, at the relief pressure
and often superheated to the hot fluid temperature. Further analysis may be performed, particularly for
distillation tower systems, in which the volatile fluid is introduced into the distillation tower simulation,
and the excess volumetric accumulation within that system is used as the basis for the required relief
rate. This analysis is analogous to the detailed analysis used when evaluating the loss of heat in series
fractionation overpressure scenario.
Conclusion. An inadvertent mixing of hot and volatile fluids can result in overpressure, and extremely
rapid vaporization can occur in cases in which hot fluid/material temperature exceeds the superheat
limit of the volatile fluid. This ‘explosive’ vaporization necessitates a refocus on prevention rather than
mitigation. After this white paper was written, a serendipitous presentation was given at the Spring
2015 meeting of the API CRE Subcommittee on Pressure Relieving Systems that outlined even more
information on this topic, and also proposed a work item for update of API Standard 521 to reflect some
of these concepts. We will be keeping a lookout for updates like this in the next edition of API Standard
521 in order to understand how they can impact design and operation and to find ways to continually
improve process safety systems.
Considerations in the Evaluation of the Scenario for Mixing of Hot and Volatile Fluids
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