Over Controlled Improvements in PIPE-FLO® Professional v15
By Jeff Sines, Engineered Software, Inc.
The “Over Controlled” message in PIPE-FLO® Professional is one that historically has been difficult for users to understand and reconcile, but improvements were made in Version 15 that addressed the conditions that cause this message. Because of how 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. Improvements include new messaging to identify the specific cause of the Over Controlled configuration, highlighting the affected sections, new modes of operation for pumps, and an automatic solution for some configurations involving pumps and control valves.
New Messages and Highlighting of Over Controlled Sections
The single message (ID 104) displayed in previous versions of PIPE-FLO was replaced with individual messages that identify the exact cause of the Over Controlled configuration. ID 180 will be displayed when two or more flow control devices are configured in series, ID 181 will identify dead-ended pipes, and improperly isolating a flow control device will generate ID 182. All three messages are Level 0 messages that prevent calculations from being performed.
Along with the new messages, the section that is Over Controlled and isolated from a pressure source is highlighted to allow easy identification of the devices that may be contributing to the problem. Figure 1 shows an Over Controlled section highlighted and the devices that are involved.
Figure 1. New highlighting shown in an Over Controlled section with a valve in PRV mode.
New Operating Modes for Centrifugal Pumps and Sizing Pumps
One way to avoid or fix the Over Controlled condition caused by two or more flow control devices in series is to change the mode of operation of just one of the devices so that the pressure calculation can “jump the gap” across one of the devices. For example, changing the operating mode of the FCV or PRV Valve in Figure 1 to BPV will set the pressure at the valve inlet and establish a pressure for the middle section to allow a single solution to the network analysis. If the pump in Figure 1 is a Centrifugal Pump in VFD mode with a flow rate setting, changing it to a Fixed Speed pump will also allow the pressure to jump the gap and eliminate the Over Controlled condition.
To increase flexibility for the user to prevent Over Controlled conditions, two new modes of operation have been added to the Centrifugal Pump and Sizing Pump Operation dialogs, shown in Figure 2. The Discharge Pressure setting will allow the user to establish the pressure at the discharge of the pump and the Suction Pressure setting will set the pressure at the pump suction.
For the Centrifugal Pump device, both of these new modes of operation are considered Variable Speed modes. PIPE-FLO will not only determine the flow rate and total head of the pump, but will also calculate the speed at which the pump will operate to achieve the calculated flow and total head. For the Sizing Pump, these new operating modes will perform the same function, but because there is no pump curve data, pump speed cannot be calculated.
In addition to being highly requesting functionality for the pump devices, these two new modes of operation allow the implementation of the fourth improvement for Over Controlled configurations: an automatic solution.
Figure 2. Centrifugal Pump Operation dialog with new Discharge Pressure and Suction Pressure modes (Note that there is also a new Temperature Control operating mode that will be used with the new heat exchanger devices being added in Version 15).
Automatic Solution to Over Controlled Configurations
Adding the new operating modes to the pump devices allowed for the implementation of a unique subroutine that automatically solves some Over Controlled configurations involving pumps and valves. This automatic solution involves finding the Most Hydraulically Remote Loop, or Critical Path Load, and presenting calculated results based on the user-entered Minimum Allowable Pressure Drop across that valve.
Figure 3 shows an Over Controlled PIPE-FLO model involving two Sizing Pumps operating in parallel and four Sizing Valves set to FCV. If the operating mode of just one of the devices is changed such that pressure can “jump the gap” and reach the Over Controlled section, a single solution can be calculated. The user can do this themselves, but the results of the automatic solution shown in Figure 4 identifies FCV-1 as the most hydraulically remote valve and presents a solution that achieves the Allowable dP Min set at that valve. If the user hasn’t set the Allowable dP Min, the solution will result in zero dP across the valve and an extremely large flow coefficient (Cv). In reality, a control valve must have some pressure drop across it, so additional messaging has been added to recommend setting an Allowable dP Min at the valve.
There are also two new messages that are displayed when the automatic solution is implemented. ID 192 identifies which valve is the most hydraulically remote and ID 193 identifies one of the pumps whose results are based on the minimum dP at the remote valve. There are additional message that are displayed if the sum of the flow rates calculated at the pumps does not equal the flow rate setting, which could occur if the user entered 500 gpm total at the pumps but 400 gpm at all the valves, for example.
Figure 3. Over Controlled system prior to automatic solution.
Figure 4. Automatic solution to Over Controlled based on Allowable dP Min at the most hydraulically remote valve.
New Operating Modes Allow New Combinations of Over Controlled Configurations
With the addition of new operating modes to the pumps, there are new combinations of pumps, control valves, and flow demands that will generate an Over Controlled condition. Figure 5 shows two Over Controlled configurations involving combinations of flow demands and a Sizing Pump in the Suction Pressure and Discharge Pressure operating modes.
However, with the new messaging and highlighting of the Over Controlled sections, it is now easier for the user to see what section of the model is Over Controlled. Changing the operating mode so that the pressure can jump the gap will eliminate the condition, or replacing the flow demand with a pressure boundary will provide a pressure for PIPE-FLO to complete the network analysis of the system.
Figure 5. Over Controlled configurations with the new operating modes of the Sizing Pump and Centrifugal Pump.
Engineered Software is committed to improving the usability of its PIPE-FLO® Professional program and addressing the Over Controlled condition has been on the top of the list of improvements for some time. Tackling this difficult to understand message was made possible with the re-design that was introduced in version 12.