Bulletin #:001_08

Date:04-06-2008

Category:2

Subject:Operating Pressures Greater than Fluid Critical Pressure

Program affected:PIPE-FLO Professional, PIPE-FLO Stock, and Flow of Fluids

Approved by:RWL

Background

When either a set pressure or calculated pressure in a system is greater than the critical pressure of the fluid, PIPE-FLO returns a warning stating, "Warning: fluid changes state." This only applies to fluids from the installed tables, the warning does not occur with custom fluids for this condition. 

If a fluid is in a liquid state and at a temperature between its freezing point and critical temperature, it will change to a solid state if a sufficiently high pressure is reached. The pressure at which this happens is well above the fluid's critical pressure. With water this change of state occurs at pressures in excess of 50,000 psia, much greater than water's critical pressure of 3198 psia. Clearly, there will be few if any systems that ever reach this high of pressure and the warning will likely not apply when water is being used. Water is the extreme case and some fluids may change from liquid to solid at much lower pressures. Therefore it is necessary for the user to assess for themselves whether a change of state will occur in there system.

In the case of a gas that is at a temperature above its critical temperature, it will become a supercritical fluid if the operating pressure exceeds the critical pressure.  Many gases have very low critical temperatures and systems commonly operate at temperatures above the critical temperature of the gas.  The change of state warning is also returned in this case.

 PIPE-FLO fluid tables do not contain property data for pressures above the critical pressure of a fluid.  Therefore, when a system is operating at a pressure above its fluid's critical pressure the calculated table fluid properties may not be valid for the system operating conditions. 

Condition

For fluid zones using an installed fluid table, if a set pressure or calculated pressure in the system is greater than the fluid critical pressure the program returns a change of state warning.

Workaround

Since the operating pressure of the system is above the fluid's critical pressure and therefore beyond the valid range of the PIPE-FLO fluid table property calculations, the fluid should be specificied as a custom fluid with properties applicable to the operating conditions of the system.