| 摘要 |
<p>A valve operator that is provided with a back seat which comes into use when the normally non-rising stem operator and its bearing-enclosing bonnet gland are raised, e.g. so that stem packing can be replaced, is further provided with another way to actuate the back seat, one which will come into play should the valve be subjected to heat of sufficient intensity as to call the integrity of the stem packing or other critical parts into question. The further activation mode depends on the melting of fusible material which permits the operator and its bearings to rise relative to the bonnet.</p> |
