An operator attempts to close a fully-open upright manual gate valve to isolate a pump in a cooling water system that has been cooled down for maintenance.
However, the operator is unable to rotate the handwheel in the close direction.
Which one of the following could cause this condition?
A. A hydraulic lock has developed under the valve disk.
B. A hydraulic lock has developed in the valve bonnet between the valve disk and the packing gland.
C. The two halves of the valve disk have expanded and are jammed against the valve seats.
D. The valve disk has jammed against its backseat by the difference in the thermal contraction of the stem and the bonnet.
Share your understanding of this question with the correct explanation.