Why a control valve on Cryogenic Service cannot be mounted with actuator in horizontal position?

The working principle of the extended bonnet (named “cold box”) used in cryogenic service (below -100°C) is based on the principle that portion of LNG which evaporates and stagnates in the upper portion of the bonnet has a function of thermal insulation between the process fluid and the gland packing. In this way ice formation on valve stem is avoided. To grant this benefit the stem/actuator axis must be in upright position. Therefore valves installed on a vertical pipe (with the actuator in horizontal position) cannot work properly.