I am not familiar with ESD valves and new to its design. What are the factors that determine the torque requirements of ESD valve actuator? In case of emergency how the torque is taken care of? Are there any arrangements(protection) in actuator which ensures that there is no shearing of actuator in case of excess torque?

An ESD valve is not some special mystery design or contraption.  It is just a line spec valve with some kind of motor or pneumatic operator on it.

The torque for the valve should be the same if operated manually or by motor.

The main protection that should be considered for the operator is fire proofing to prevent damage and allow the valve to still function even when fully engulfed in flames.

Valves have torque safety factors that can vary from 1.3 up to 2, depending on project specifications, and can also vary from Break to Open, running & End to close positions.

Speed of emergency stroke operarion can also vary from 1 second total, to 1 second per inch of diameter, to 3 seconds per inch daimeter.

The Safety factor has an effect on (some) valve shaft diameter, or material, due to much higher requisites (due to increase in resistance torque) as requested torques exceed valve MAST.
Most of ESDV are low pressure pneumatic operated (unless you are in the desert on a gas pipeline, where line pressure= to actuator sizing pressure), by low pressure anything between 3 & 5 Barg is considered...a Air Filter Regulator should be included in instrumentation to ensure that supply pressure to actuator does not increase over sizing pressure...increase of pressure to a piston = increase to torque output of actuator.

The speed of operation can also cause problems, that are mostly solved by intervention on actuator & instrumentation;
QEV (Quick Exhaust Valve) are fitted on discharge to allow faster stroke.

This may cause problems to valve, when slamming shut (damaging seal/seat & possibly connections between valve & actuator...lots will argue that it's enough to set mechanical stoppers in actuator, however these are not designed to take this sort of shock...it would not be stopping only actuator stroke (fast) but would also be taking high loads caused by inertia (valve disk can be quite a heavy mass traveling at high speeds).

The actuator manufacturer needs to add a hydraulic damper to come into action during the last few degrees of closure (slowing the speed of stroke to a "rasonable" speed before it makes contact with travel stops, but still respecting stroke time requirements).

Also with ESD valves it is appropriate to have a system/device to perform Partial Stroke Testing (PST).  This verifies that the valve(API Cast Steel Valves) will at least move away from its full-open position and gives implication that it will close on command. Partial stroke is used so as not to disrupt the process-the valve is typically moved from full-open to 85% of travel, and the PST positioner records the valve's signature, compares it to a recorded baseline, and report whether or not the valve is trustworthy to shut down the plant in an emergency.  PST is usually scheduled on some sort of regular interval.

Back up to Itascot's post: Valves become progressively more difficult to move if left in place.  Even if sedimentation/plate-out of process products/corrosion damage don't happen, the seals microscopically cold-flow to conform to surface irregularities, and the break-torque increases with time.  If an ESD works the one-time it is really needed, it pays for itself.  Put all the actuator on it that the shaft/stem will take.

MORE NEWS