def API520_N(P1):
    """Calculates correction due to steam pressure for steam flow for use in
    API 520 relief valve sizing.

    .. math::
        K_N = \\frac{0.02764P_1-1000}{0.03324P_1-1061}

    Parameters
    ----------
    P1 : float
        Upstream relieving pressure; the set pressure plus the allowable
        overpressure, plus atmospheric pressure, [Pa]

    Returns
    -------
    KN : float
        Correction due to steam temperature [-]

    Notes
    -----
    Although not dimensional, KN varies with the units used.

    For temperatures above 922 K or 22057 kPa, KN is not defined.

    Internally, units of kPa are used to match the equation in the standard.

    Examples
    --------
    Custom example:

    >>> API520_N(1774700)
    0.9490406958152466

    References
    ----------
    .. [1] API Standard 520, Part 1 - Sizing and Selection.
    """
    P1 = P1 / 1000.0
    return (0.02764 * P1 - 1000.0) / (0.03324 * P1 - 1061)
