def heuristic_pressure_drop(inlet_phase, outlet_phase):
    """
    Return a heuristic estimate of the pressure drop [dP; in psi]. If the fluid 
    changes phase, `dP` is 1.5 psi. If the fluid remains a liquid, `dP` is 5 psi.
    If the fluid remains a gas, `dP` is 3 psi.
    
    Parameters
    ----------
    inlet_phase: str
    outlet_phase : str
        
    Returns
    -------
    dP : float
        Pressure drop [psi].
    
    """
    if ('l' in inlet_phase and 'g' in outlet_phase or 'g' in inlet_phase and
        'l' in outlet_phase):
        dP = 1.5
    elif inlet_phase == 'l':
        dP = 5
    elif outlet_phase == 'g':
        dP = 3
    return dP
