Modeling and Troubleshooting Flow Control Valves
Flow control valves (FCVs) maintain the flow rate in a line to a fixed value and calculate the differential pressure across the control required to regulate the flow rate. When troubleshooting FCV installations, it is helpful to know how PIPE-FLO internally models them.
A schematic of how PIPE-FLO internally models FCVs is shown below:
The following points about FCVs should be noted:
- The direction of flow through the FCV is determined from the direction of the connecting pipelines.
- PIPE-FLO models a FCV by creating two nodes in the system where the valve is installed (refer to the figure). A flow demand value equal to the set flow rate is taken out of the first node (the inlet node) and an equal flow demand is set entering the second node (the outlet node).
- The program will calculate the valve inlet pressure based on the upstream conditions, and calculate the valve outlet pressure based on the downstream conditions.
- If the inlet pressure is greater than the outlet pressure, the control valve differential pressure is displayed (if selected in the Drawing Options).
- If the calculated inlet pressure is less than the outlet pressure, a warning is generated stating that the "Flow rate cannot be achieved", and a message is displayed in the Fly-by Viewer stating that the "Valve is operating like a pump with a total head" displayed in ( ). These results are invalid and indicate that the upstream pressure needs to be higher to achieve the desired flow rate. If a pump is installed upstream, the pump is undersized and is not generating enough head.
- If Cv data is entered for the control valve, a "Flow exceeds range of supplied curve" error can be generated if the set flow rate would result in the valve being less than 5% open or more than 100% open.
- For a control valve with Cv data, a "Choked Flow" error message will be generated if the set flow rate of the control valve is greater than the calculated Qmax of the valve. Choked flow can occur when the fluid pressure at the vena contracta of the valve drops significantly below the fluid vapor pressure and the valve's Liquid Pressure Recovery Factor (FL) is low enough that the fluid pressure does not rise back above the vapor pressure. The calculated results are invalid because the flow rate (and therefore the head loss and dP) of the system is based on the set flow rate of the control valve and not the maximum flow rate that is attainable with the valve.