Applies To Product(s): Bentley WaterGEMS Version(s): 08.11.XX.XX Environment: N\A Area: Output and Results Subarea: N\A Original Author: Jesse Dringoli, Bentley Technical Support Group Problem What does the following user notification mean? " FCV caused Ill conditioning" What causes a flow control valve to become ill-conditioned? Product: WaterGEMS, WaterCAD, HAMMER Version: 08.XX.XX.XX Area: Analysis/Computation, Problem ID#: 48411, ERRORMESSAGE Solution The "ill conditioning" user notification (ill conditioned matrix) almost always indicates that FCVs have been incorrectly configured. Typically this problem occurs when an FCV is placed in such a way that there is no other path around it to supply downstream demands. For example let's say you have an active FCV with setting of 100 gpm, with a demand of 150 gpm downstream of it and no other path around that FCV to supply the demand. You might also see this message for a PSV or a PRV. If that is the case make sure that it's possible the valve can actually control the pressure. There are cases where no matter what pressure setting or hydraulic grade value that you enter into the PSV/PRV that it can't possible work. Take the setup below. In this model PSV1 is set control the pressure at J3 to 15 psi and there is a 2500 gpm demand downstream of the PSV at the Outflow to the canal. In this situation the pump(s) head is fixed because the pump(s) works based on the head v. flow pump curve that you have defined for them. This poses a problem because that means the pressure at junction J3 is being controlled by the pump head and flow because the pumps always have to operate at fixed point on there curve to meet the 2500 gpm demand that is set on the downstream junction, so there is nothing the PSV can do to control the pressure. What you need to do in order to control the flow at the outflow to the canal junction is have some type of pressure dependent demand element there that allows the flow can change based on the pressure available. The element that you'd use for that would be the discharge to atmosphere (D2A) element, which is located right below check valve element on the layout tool bar. All you have to do for the D2A element is enter the elevation, which should be the same as the outflow to the canal elevation and enter the flow (typical) and pressure drop (typical). For example, if you entered a pressure drop of 10 psi and flow of 2500 gpm you would see where the PSV could function and allow the pumps to add more head to the system. The typical pressure drop and typical flow are used to automatically calculate a flow emitter (orifice) coefficient, which will be used during the simulation to calculate outflows. Basically, the values you entered are used in the orifice equation to determine the size of the orifice. If necessary you can use the orifice equation to do the math to figure out the typical pressure and typical flow you'd have to enter to simulate a given orifice size. Q = C A (2 g P)^0.5 Q - Discharge (cfs, cms) C - A 'discharge coefficient' (distinct from CV used elsewhere in HAMMER) which will be computed based on the typical flow/pressure A - The cross-sectional area of the opening (ft, m) g - gravitational acceleration P - Pressure head (ft, m) As you can see, once the "C" is calculated from the initial head/flow, WaterCAD can solve for other flows, as the pressure head changes during the simulation.
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