Version 2009 and Before
When flow meters (including Balancing Orifice, Metering Orifice, Venturi Tubes, and Nozzles) are sized, there exist instances where a meter can be sized in calculated mode, but can not be sized in design mode due to flashing conditions.
When flow meters are sized, the inlet pressure is used to determine if the meter is experiencing flashing. If the meter is experiencing flashing conditions, a size is not returned since, due to the flashing, the flow through the meter would not be what it was sized for.
PIPE-FLO Professional uses different values for the inlet pressure depending on whether there are calculated values available when the meter is sized. When sizing a meter in design mode, there is no calculated pressure available so PIPE-FLO uses the pressure specified by the user in the fluid zone. If the sizing is done in calculated mode, there exists a calculated pressure so PIPE-FLO uses that value for determining flashing conditions.
It is important to note the use of two different pressures will not affect the actual sizing of the meter, it will only effect whether or not the meter is evaluated as flashing. This can however cause a situation where a meter can not be sized in uncalculated mode (due to a perceived flashing condition based off of the fluid zone pressure) but can be sized in calculated mode using the more accurate, calculated pressure.
It is suggested that if a meter is unable to be sized within design mode due to flashing, calculate the system then attempt to size the meter again with the calculated inlet pressure.
Also, it is recommended that you define a liquid fluid zone pressure of about 50-75% of the maximum pressure you expect to see in the system.