The user defined fluid temperature is used to calculate the fluid's vapor pressure, which is then compared to the user defined fluid pressure. If fluid pressure is greater than the vapor pressure, the fluid is a liquid and the corresponding liquid properties are calculated. Fluid pressure does not affect the liquid's density for incompressible fluids, only temperature is used to calculate the density.

The user defined fluid pressure is also used in the calculation of the maximum flow rate at which choked flow occurs in a designed control valve (ie, one that has Cv data entered). The user defined fluid pressure is only used for the initial Qmax calculation since the inlet pressure of the control valve is unknown for the first iteration of calculations.  All of the subsequent and the final Qmax calculations use the valve's calculated inlet pressure.

For relatively low pressure systems (less than about 200 psig), it does not matter what pressure the user enters, the calculation of Qmax is accurate. For high pressure systems (above about 200 psig), if the liquid pressure defined by the user is less than about 50% of the actual liquid pressure at the inlet of the control valve, PIPE-FLO's initial estimate for Qmax will be  much smaller than it  should be. 

Conclusion 

Due to the possibility of inaccuracy in the calculation for Qmax for a designed control valve, we recommend that you enter a fluid pressure greater than about 50% of the maximum pressure you expect to see in the system you are modeling.