\pgfdeclareshape{valve}{
    \savedanchor{\centerpoint}{
        \pgfpointorigin
    }
    \savedanchor{\east}{
        \pgfpoint{0.5cm}{0}
    }
    \savedanchor{\north}{
        \pgfpointscale{0.5}{\pgfpoint{0}{0.5cm}}
    }
    \savedanchor{\northeast}{
        \pgfpoint{0.5cm}{0.25cm}
    }    


    \anchor{center}{
        \centerpoint
    }
    \anchor{east}{
        \east
    }
    \anchor{west}{
        \east
        \pgfpoint{-\pgf@x}{0}
    }
    \anchor{north}{
        \north
    }
    \anchor{south}{
        \north
        \pgfpoint{0}{-\pgf@y}
    }
}