Singular matrix with pressure control

[dad616610]

Hello!

I'm getting a singular matrix when using pressure control with the following initial conditions: mass flow at inlet (source) and pressure at outlet (ext_grid). I tested this behavior with replacing the pressure control with pipe - no singular matrix, got solution. I also changed the initial conditions to pressure at inlet (ext_grid) and mass flow at outlet (sink) and ran with both pressure control and pipe - got solutions in both cases.

Here is a table with the results:

	flow, press	press, flow
press_control	False	True
pipe	True	True

Table legend:

• "flow, press" = inlet flow, outlet pressure; "press, flow" - inlet pressure, outlet flow
• press_control = pressure control was used; pipe = pipe was used instead of pressure control
• False = got singular matrix; True - no singular matrix. got solution

Here is the code, simulating all the configurations above

import pandapipes as pp


def main(init_conds, use_press_control):
    net = pp.create_empty_network("network", fluid="lgas")

    pp.create_junctions(net, nr_junctions=6, pn_bar=0.8, tfluid_k=300)

    pp.create_pipes_from_parameters(
        net,
        from_junctions=[0, 1, 3, 4],
        to_junctions=[1, 2, 4, 5],
        length_km=0.1,
        diameter_m=0.2,
    )

    if use_press_control:
        pp.create_pressure_control(
            net,
            from_junction=2,
            to_junction=3,
            controlled_junction=3,
            controlled_p_bar=0.5,
        )
    else:
        pp.create_pipe_from_parameters(
            net,
            from_junction=2,
            to_junction=3,
            length_km=0.1,
            diameter_m=0.2,
        )

    if init_conds == "press_flow":
        pp.create_ext_grid(net, junction=0, p_bar=0.5)
        pp.create_sink(net, junction=5, mdot_kg_per_s=0.06)
    elif init_conds == "flow_press":
        pp.create_source(net, junction=0, mdot_kg_per_s=0.06)
        pp.create_ext_grid(net, junction=5, p_bar=0.5)

    pp.pipeflow(net, friction_model="colebrook")


if __name__ == "__main__":
    from itertools import product

    for init_conds, use_press_control in product(
        ("press_flow", "flow_press"),
        (True, False),
    ):
        try:
            main(init_conds, use_press_control)
            res = "Success!"
        except pp.PipeflowNotConverged:
            res = "Fail!"
        finally:
            print(f"{init_conds = }, {use_press_control = }\n{res}")

Could I be doing something wrong?

[jkisse]

Hi @dad616610,
sorry for the late reply. Please be aware the pressure control kind of splits your network into two sections, upstream and downstream. In both sections, at least one junction needs a fixed pressure. Downstream, this is achieved by the pressure controller's controlled_junction. Upstream, you should add an ext. grid (with a pressure higher than the pressure controller's controlled_p_bar, so that the pressure gradient allows for a flow into the pressure controller).

Note that the pressure of the "uncontrolled" junctions downstream of the pressure controller is determined by the hydraulics of the network, i.e. the pressure loss due to friction etc.. If there is an additional ext_grid there, trying to impose a certain pressure on such a junction, you'll likely end up with an overdetermined network (--> will typically raise "matrix is exactly singular").

I hope this helps :)