"TWO PHASE" HYDRAULICS
DISCLAIMER: The following is a simplified explanation of the phenomena of "two phase" flow as traditionally applied to various liquefied compressed gases used in fire suppression systems. It is not intended to be a rigorous discussion NOR IS SUFFICIENT INFORMATION PROVIDED TO PERMIT THE READER TO CALCULATE THE FLOW PARAMETERS NEEDED TO DESIGN FIRE SYSTEMS. For further information or to report any inaccuracies which you may find in the material presented herein, contact Guardian Services, Inc. by e-mail.
Many fire fighting agents used for "special hazards" application are gases at room temperature and pressure. For a number of reasons, not the least of which is economy of space, these gases are stored under pressure in a liquid form. Among the fire fighting agents stored as liquefied compressed gases are carbon dioxide, Halon 1301, Halon 1211, FM200, CEA410, and FE13. Some of these agents are super pressurized in their storage containers with nitrogen.
When the liquid agent is discharged from a storage container into a pipe, liquid agent enters the pipe propelled by its own vapor pressure. Some of the agent will vaporize as it extracts heat from the pipe. In most systems, this initial vaporization lasts only a short time -- usually a few seconds. Then a relatively steady flow begins. As the liquid travels away from the storage container, its pressure drops and some of the liquid is changed to vapor. The resulting flow in the pipe is a "two phase" mixture of liquid and vapor agent.
Dr. James Hesson reported on this phenomena for liquid carbon dioxide. His work is the basis of the flow calculation method published in NFPA 12, Standard for Carbon Dioxide Extinguishing Systems. In the early 1970's doing research for Chemetron Fire Systems, Vic Williamson and Tom Wysocki adapted the Hesson method to predict pressure drop in systems using nitrogen-superpressurized Halon 1301. Their work is the basis for the flow calculation method set forth in NFPA 12A, Standard on Halon 1301 Fire Extinguishing Systems.
For fluids which can be considered incompressible, the rate of pressure drop for constant mass flow through a pipe of constant diameter and roughness is constant. Compressible fluids such as HFC227ea exhibit a variable rate of pressure drop for constant mass flow through a pipe of constant diameter and roughness. Many liquefied compressed gases used for fire suppression exhibit "two-phase" compressible flow characteristics.
In order to simplify pressure drop calculation and obtain more consistent flow characteristics, flow rates should be kept high enough to insure a turbulent mixture of the flowing liquid and vapor phases. Even when flow is highly turbulent, tests have indicated that some separation of the liquid and vapor phases occurs when flow is divided at tee junctions. This phenomenon makes it difficult (but not impossible) to accurately predict the exact quantity of agent which will discharge from each nozzle in an "unbalanced" piping network.
For details on the theory of two-phase flow, request the article on Two Phase Flow of Liquefied Compressed Gases.
