WaterCAD - PSV and PRV in series

Hi,

I am having a problem with my static WaterCAD model as follows:

I have two sites, let's call them Site A and Site B. Site A is being supplied by a reservoir + pump combination. Site B is being supplied by a tank. Site A is at a lower pressure zone that Site B.

Site A is connected to Site B by a supplemental pipe connection. The flow only goes through the supplemental pipe connection from Site B to Site A (higher pressure zone to lower pressure zone) when the Site A reservoir + pump combination cannot supply a minimum pressure to the Site A system. When flow goes through the supplemental pipe connection from Site B to Site A, I also need to maintain a minimum pressure in Site B so that the supplemental flow needed for Site A does not adversely impact the pressure in Site B.

I have modeled the supplemental connection with a PSV and PRV. The PSV maintains the upstream pressure (i.e., Site B). Downstream of Site B and the PSV is the PRV. The PRV boosts the pressure in Site A. So, in order of the direction of flow, it is: Site B to PSV to PRV to Site A.

I modeled the setup step-by-step by first inserting the PRV only (no PSV). The model to runs as expected when I only have Site B to PRV to Site A. This model provided the minimum pressure needed for Site A by drawing from Site B.

Next, I inserted the PSV with a pressure setting of 0, meaning that the valve is always open, to test the setup. I run the scenario, and the model is giving me two error messages saying that 1) PSV cannot supply pressure and 2) PSV caused ill-conditioning. Any other pressure settings above 0 also gives me this error.

Any help with what I am doing incorrectly would be great.