The transmitter should be isolated from the process before the equalizing valve is opened.
A five valve manifold is shown below:
If the equalizing valve is opened before the process in (blocking) valves are closed (isolating the transmitter from the process), then whatever differential pressure that the process develops will create a process flow from the high side, through the equalizing valve and back to the process through the low side.
The (blue) valve added to the diagram below only shows the equalizing function, a connection from high side to low side, but the absence of the blocking valves applies to the issue; the red arrows show the wet leg draining through an opened equalizing valve in a level application.
Likewise, the diagram below shows the effect of steam flowing through an open equalizing valve on a 3 valve manifold. The absence of the vent valves on a 5 valve manifold does not alter the effect of steam flow when the process is not blocked before the equalizing valve is opened: