When modeling a piping system in PIPE-FLO® Professional, gathering key hydraulic performance data about each device and configuring them as they exist or will exist in the real-world system is critical. Flexibility is built into PIPE-FLO® with many devices having multiple operating modes to allow the user to mimic the various ways particular devices are actually used. For example, control valves can be set to one of four operating modes: flow control (FCV), back pressure control (BPV), pressure regulating (PRV), or set to a manual position. Additionally, centrifugal pumps can be set to fixed speed or variable speed with a specified flow rate.
When choosing an operating mode for each device, it is important to look at the system as a whole and understand how PIPE-FLO® interprets the operating modes of each device. One of the least understood messages in PIPE-FLO® is the over-controlled message, which is generated when the system is configured in such a way that it isolates a portion of the system from a pressure source. In this case, an infinite number of solutions will satisfy the Conservation of Energy. This message and its causes are detailed below to aid in understanding, preventing, and correcting this condition.
Primary Causes of Over-controlled Configurations
Due to the way PIPE-FLO® “sees” each device in its various operating modes, there are several piping system configurations that can cause the over-controlled condition. When users intentionally or unintentionally configure the piping system model and create an over-controlled condition, a portion of the system is isolated from a pressure source, resulting in an infinite number of solutions that could satisfy the Conservation of Energy, so PIPE-FLO® cannot arrive at a single solution.
Multiple Flow Rate Devices in Series
The most common cause of over-controlled is having two devices in series that set the flow rate at the inlet and outlet of a portion of the system. This condition could be due to a sizing pump in series with a sizing valve in a flow control valve (FCV) mode of operation, as shown in Figure 1.
During the network analysis, PIPE-FLO® can easily calculate the pressure at the pump suction and at the outlet of the valve, as well as the pressure drop across the pipeline between the two devices at the set flow rate. But there are an infinite number of solutions possible for the calculated pressure at the pump discharge and valve inlet.
For example, if the pressure drop for the pipeline between the devices in Figure 1 is 10 psi, one possible solution could be the pump discharge pressure at 50 psig and the valve inlet pressure at 40 psig. Another solution could be 100 psig at the pump discharge and 90 psig at the valve inlet. Any combination of two values of pressure that result in a pressure drop of 10 psi is a possible solution.
Figure 1. Over-controlled due to two flow rate devices in series.
A flow control device does not necessarily need to be in a flow rate mode of operation to create the over-controlled condition. Sizing valves and control valves with a pressure setting in the pressure regulating valve (PRV) or back pressure valve (BPV) operating mode may create an over-controlled condition depending on how the system is configured.
Figure 2 shows an Over-controlled configuration with a BPV in series with a Flow Demand. Figure 3 shows how sizing valves or control valves set to FCV and PRV in series can create an over-controlled condition.
Sometimes the size of the piping system model makes it hard to see why the over-controlled message is displayed. Figure 4 shows a somewhat more complex over-controlled branching system involving pumps and control valves in FCV and PRV operating modes.
Figure 2. Over-controlled due to a BPV and Flow Demand in series.
Figure 3. Over-controlled due to FCV and PRV control valves in series.
Figure 4. Over-controlled branching system.
Flow rate device in series with dead ended pipe
Two other common causes of the over-controlled message is having a dead ended pipe connected to the inlet or outlet of a pump or valve with a flow rate setting, as shown in Figure 5, and improperly isolating a flow control device, as shown in Figure 6. Again, there are an infinite number of solutions that will satisfy the Conservation of Energy.
Figure 5. Over-controlled caused by dead-ended pipe connected to a flow rate device.
Figure 6. Over-controlled caused by improperly isolating a flow rate device.